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MoEDAL zeroes in on magnetic monopoles

abelchio — Fri, 26 Apr 2024 08:45:58 +0000

MoEDAL zeroes in on magnetic monopoles

The MoEDAL detector (Image: CERN)

The late physicist Joseph Polchinski once said the existence of magnetic monopoles is “one of the safest bets that one can make about physics not yet seen”. In its quest for these particles, which have a magnetic charge and are predicted by several theories that extend the Standard Model, the MoEDAL collaboration at the Large Hadron Collider (LHC) has not yet proven Polchinski right, but its latest findings mark a significant stride forward. The results, reported in two papers posted on the arXiv preprint server, considerably narrow the search window for these hypothetical particles.

At the LHC, pairs of magnetic monopoles could be produced in interactions between protons or heavy ions. In collisions between protons, they could be formed from a single virtual photon (the Drell–Yan mechanism) or the fusion of two virtual photons (the photon-fusion mechanism). Pairs of magnetic monopoles could also be produced from the vacuum in the enormous magnetic fields created in near-miss heavy-ion collisions, through a process called the Schwinger mechanism.

Since it started taking data in 2012, MoEDAL has achieved several firsts, including conducting the first searches at the LHC for magnetic monopoles produced via the photon-fusion mechanism and through the Schwinger mechanism. In the first of its latest studies, the MoEDAL collaboration sought monopoles and high-electric-charge objects (HECOs) produced via the Drell–Yan and photon-fusion mechanisms. The search was based on proton–proton collision data collected during Run 2 of the LHC, using the full MoEDAL detector for the first time.

The full detector comprises two main systems sensitive to magnetic monopoles, HECOs and other highly ionising hypothetical particles. The first can permanently register the tracks of magnetic monopoles and HECOs, with no background signals from Standard Model particles. These tracks are measured using optical scanning microscopes at INFN Bologna. The second system consists of roughly a tonne of trapping volumes designed to capture magnetic monopoles. These trapping volumes – which make MoEDAL the only collider experiment in the world that can definitively and directly identify the magnetic charge of magnetic monopoles – are scanned at ETH Zurich using a special type of magnetometer called a SQUID to look for any trapped monopoles they may contain.

In their latest scanning of the trapping volumes, the MoEDAL team found no magnetic monopoles or HECOs, but it set bounds on the mass and production rate of these particles for different values of particle spin, an intrinsic form of angular momentum. For magnetic monopoles, the mass bounds were set for magnetic charges from 1 to 10 times the fundamental unit of magnetic charge, the Dirac charge (g_D), and the existence of monopoles with masses as high as about 3.9 trillion electronvolts (TeV) was excluded. For HECOs, the mass limits were established for electric charges from 5e to 350e, where e is the electron charge, and the existence of HECOs with masses ranging up to 3.4 TeV was ruled out.

“MoEDAL’s search reach for both monopoles and HECOs allows the collaboration to survey a huge swathe of the theoretical ‘discovery space’ for these hypothetical particles,” says MoEDAL spokesperson James Pinfold.

In its second latest study, the MoEDAL team concentrated on the search for monopoles produced via the Schwinger mechanism in heavy-ion collision data taken during Run 1 of the LHC. In a unique endeavour, it scanned a decommissioned section of the CMS experiment beam pipe, instead of the MoEDAL detector’s trapping volumes, in search of trapped monopoles. Once again, the team found no monopoles, but it set the strongest-to-date mass limits on Schwinger monopoles with a charge between 2g_D and 45g_D, ruling out the existence of monopoles with masses of up to 80 GeV.

“The vital importance of the Schwinger mechanism is that the production of composite monopoles is not suppressed compared to that of elementary ones, as is the case with the Drell–Yan and photon-fusion processes,” explains Pinfold. “Thus, if monopoles are composite particles, this and our previous Schwinger-monopole search may have been the first-ever chances to observe them.”

The MoEDAL detector will soon be joined by the MoEDAL Apparatus for Penetrating Particles, MAPP for short, which will allow the experiment to cast an even broader net in the search for new particles.

abelchio Fri, 04/26/2024 - 10:45

Byline

Ana Lopes

Publication Date

Fri, 04/26/2024 - 10:24

ALICE gets the green light for new subdetectors

ckrishna — Thu, 18 Apr 2024 12:52:06 +0000

ALICE gets the green light for new subdetectors

Two detector upgrades of ALICE, the dedicated heavy-ion physics experiment at the Large Hadron Collider (LHC), have recently been approved for installation during the next long shutdown of the LHC, which will take place from 2026 to 2028. The first one is an upgrade of the innermost three layers of the Inner Tracking System (ITS3), and the second is a new forward calorimeter (FoCal), optimised for photon detection in the forward direction of the ALICE detector.

High-energy collisions of heavy ions like lead nuclei at the LHC recreate quark–gluon plasma: the hottest and densest fluid ever studied in a laboratory. Besides studying the properties of quark–gluon plasma, the ALICE programme covers a broad array of topics involving strong interaction, such as determining the structure of nuclei and the interactions between unstable particles, as presented in "A journey through the quark-gluon plasma and beyond".

Inner Tracking System (ITS3)

ALICE’s current Inner Tracking System, installed for the ongoing LHC run, is the world’s largest pixel detector to date, with 10 m2 of active silicon area and nearly 13 billion pixels. The new Inner Tracking System, ITS3, builds on the successful use of monolithic active pixel sensors and takes this concept to the next level.

“ALICE is like a high-resolution camera, capturing intricate details of particle interactions. ITS3 is all set to boost the pointing resolution of the tracks by a factor of 2 compared to the current ITS detector,” said Alex Kluge and Magnus Mager, the project leaders of ITS3. “This will strongly enhance the measurements of thermal radiation emitted by the quark–gluon plasma and provide insights into the interactions of charm and beauty quarks when they propagate through the plasma.”

The ITS3 sensors are 50 µm thick and as large as 26×10 cm2. To achieve this, a novel stitching technology was used to connect individual sensors together into a large structure. These sensors can now be bent around the beampipe in a truly cylindrical shape. The first layer will be placed only 2 mm from the beampipe and 19 mm from the interaction point. It can now be cooled by air instead of water and has a much lighter support structure, significantly reducing the materials and their effect on the particle trajectories seen in the detector.

Forward Calorimeter (FoCal)

The FoCal detector consists of an electromagnetic calorimeter (FoCal-E) and a hadronic calorimeter (FoCal-H). FoCal-E is a highly granular calorimeter composed of 18 layers of silicon pad sensors, each as small as 1×1 cm2, and two additional special layers with pixels of 30×30 μm2. FoCal-H is made of copper capillar tubes and scintillating fibres.

“By measuring inclusive photons and their correlations with neutral mesons, and the production of jets and charmonia, FoCal offers a unique possibility for a systematic exploration of QCD at small Bjorken-x. FoCal extends the scope of ALICE by adding new capabilities to explore the small-x parton structure of nucleons and nuclei,” said Constantin Loizides, project leader of FoCal at the ALICE collaboration.

The newly built FoCal prototypes have recently been tested with beams in the CERN accelerator complex, at the Proton Synchrotron and Super Proton Synchrotron, demonstrating their performance in line with expectations from detector simulations.

The ITS3 and FoCal projects have reached the important milestone of completing their Technical Design Reports, which were endorsed by the CERN review committees in March 2024. The construction phase of ITS3 and FoCal starts now, with the detectors due to be installed in early 2028 in order to be ready for data taking in 2029.

ckrishna Thu, 04/18/2024 - 14:52

Byline

ALICE collaboration

Publication Date

Thu, 04/25/2024 - 10:00

CERN's edge AI data analysis techniques used to detect marine plastic pollution

ptraczyk — Mon, 22 Apr 2024 14:28:09 +0000

CERN's edge AI data analysis techniques used to detect marine plastic pollution

Earth Observation (EO) and particle physics research have more in common than you might think. In both environments, whether capturing fleeting particle collisions or detecting transient traces of ocean plastics, rapid and accurate data analysis is paramount.

On this Earth Day, as we reflect on our responsibility to reduce plastics for the benefit of our society and all life on our planet, we are excited to present a new EU project, Edge SpAIce. It applies CERN’s cutting-edge AI technology to monitor the Earth’s ecosystems from space in order to detect and track plastic pollution in our oceans.

“In particle physics, the trigger system plays a critical role by swiftly determining which data from the particle detector should be retained, given that only a small fraction of the 40 million collision snapshots taken each second can be recorded. As the data influx at the Large Hadron Collider (LHC) has grown significantly over the years, physicists and computer scientists are continually innovating to upgrade this process - and this is where AI technology comes in,” says Sioni Summers, a CERN physicist working on the CMS experiment at the LHC, who is supervising this work.

Edge SpAIce is a collaborative endeavour involving CERN, EnduroSat (BG) and NTU Athens (GR) and coordinated by AGENIUM Space. Its aim is to develop a specially designed on-board system for satellites that will make it possible to acquire and process high-resolution pictures using a DNN (Deep Neural Network). The system will use the “edge AI” approach, in which data is processed in near real-time directly on the satellite, mirroring the efficient filtering of LHC data in particle detectors at CERN. This means that it is not necessary to transmit all of the captured data back to Earth but only the relevant information - in this case, the presence of marine plastic litter. The system will also be deployed on FPGA hardware developed in Europe, which will improve competitiveness. This could open the door for a whole new market for EO services and applications.

As modern life increasingly relies on technology, the solution that the project offers adeptly addresses the growing demand for data processing and the rapid expansion of EO satellites. By eliminating the need for heavy processing in Earth-based data centres, it not only reduces the carbon footprint but also helps to relieve the burden on these facilities. The innovative approach holds potential for broader applications in domains such as agriculture, urban planning, disaster relief and climate change. Additionally, this technology will provide environmental scientists and policymakers with invaluable data for targeted clean-up operations. It will advance our understanding of plastic pollution patterns, thereby enhancing our capacity to address environmental challenges effectively.

“AGENIUM Space is thrilled to have found synergies with CERN in developing innovative solutions for our planet’s future,” said Dr Andis Dembovskis, a business development executive with AGENIUM Space.

The Edge SpAIce project exemplifies how creative thinking by partners across diverse fields can lead to a collaborative knowledge transfer project that tackles major societal challenges. To discover how other CERN knowledge transfer and innovation projects are making a positive impact on the environment, please visit: https://kt.cern/environment

ptraczyk Mon, 04/22/2024 - 16:28

Byline

Marzena Lapka

Publication Date

Mon, 04/22/2024 - 16:23

SHiP lève l’ancre pour explorer le secteur caché

cmenard — Thu, 18 Apr 2024 14:00:27 +0000

SHiP sets sail to explore the hidden sector

The SHiP (Search for Hidden Particles) collaboration was in high spirits at its annual meeting this week. Its project to develop a large detector and target to be installed in one of the underground caverns of the accelerator complex has been accepted by the CERN Research Board. Thus, SHiP plans to sail to explore the hidden sector in 2031. Scientists hope to capture particles that interact very feebly with ordinary matter – so feebly, in fact, that they have not yet been detected.

This group of hypothetical particles includes dark photons, axions and axion-like particles, heavy neutral leptons and others. These particles, which could be among the dark matter, particles, are predicted by several theoretical models that extend beyond the Standard Model, the current theory describing elementary particles and the forces that unite them.

Although very solid, the Standard Model does not explain certain phenomena. The particles predicted by the Model – in other words, the ordinary matter that we know – account for just 5% of the Universe. The rest is thought to be unknown matter and energy, which scientists refer to as dark matter and dark energy. Their effects can be observed in the Universe, but their nature is a mystery that a growing number of experiments are trying to uncover.

This is where SHiP comes in. The idea is simple: the more particles that are produced, the greater the chances of finding feebly interacting particles. A high-intensity proton beam from the Super Proton Synchrotron (SPS) accelerator will be repeatedly sent to a target, a large metal block, producing a vast number of particles. Among them, scientists hope to find particles from the hidden sector. Thanks to the very high beam intensity, SHiP will be more sensitive than the existing experiments.

Another special feature of SHiP is that its detectors will be placed several tens of metres away from the target in order to detect relatively long-lived particles and eliminate “background noise”, in other words, particles such as muons that could interfere with the detection of long-lived particles. The experiment is equipped with a magnet system to divert the flow of muons and a large 50 m-long chamber in which the particles of interest can decay into known particles.

The experiment therefore complements the large LHC experiments, whose detectors surround the collision point and are unable to study the feebly interacting particles that travel several tens of metres before transforming. Theoretical models predict that the lower their mass and the weaker their coupling (the intensity of the interaction), the longer the lifetime of these particles. SHiP will therefore be sensitive to particles with relatively low masses.

In their journey through the detector, these particles could either disintegrate into known particles or collide with an atom of ordinary matter, which would also produce particles. The SHiP detectors have been designed to detect their signals.

Beyond the hypothetical dark-matter particles, SHiP will also study neutrinos which, despite being known particles of the Standard Model, are difficult to intercept and still hold many mysteries.

The target and the experiment will be installed in an existing underground cavern at CERN and supplied by a beam line from the SPS, CERN’s second largest accelerator, which supplies several experiments and pre-accelerates particles for the LHC.

The target is a complex device that is more like a beam dump than a conventional fixed target. Under study for several years, it is a 1.5-metre-thick block made of several different metals in order to produce the specific particles required by SHiP and fitted with a cooling and shielding system.

Part of the SHiP collaboration during its annual meeting, which was held at CERN this week. (Image: Marina Cavazza/CERN)

cmenard Thu, 04/18/2024 - 16:35

Byline

Corinne Pralavorio

Publication Date

Fri, 04/19/2024 - 14:05

CMS releases Higgs boson discovery data to the public

abelchio — Tue, 09 Apr 2024 12:29:59 +0000

CMS releases Higgs boson discovery data to the public

CMS event display of a candidate Higgs boson decaying into two photons, one of the two decay channels that were key to the discovery of the particle. (Image: CERN)

As part of its continued commitment to making its science fully open, the CMS collaboration has just publicly released, in electronic format, the combination of CMS measurements that contributed to establishing the discovery of the Higgs boson in 2012. This release coincides with the publication of the Combine software – the statistical analysis tool that CMS developed during the first run of the Large Hadron Collider (LHC) to search for the unique particle, which has since been adopted throughout the collaboration.

Physics measurements based on data from the LHC are usually reported as a central value and its corresponding uncertainty. For instance, soon after observing the Higgs boson in LHC proton–proton collision data, CMS measured its mass as 125.3 plus or minus 0.6 GeV (the proton mass being about 1 GeV). But this figure is just a brief summary of the measurement outcome, a bit like the title of a book.

In a measurement, the full information extracted from the data is encoded in a mathematical function, known as the likelihood function, that includes the measured value of a quantity as well as its dependence on external factors. In the case of a CMS measurement, these factors encompass the calibration of the CMS detector, the accuracy of the CMS detector simulation used to facilitate the measurement and other systematic effects.

A likelihood function of a measurement based on LHC data can be complex, as many aspects need to be pinned down to fully understand the messy collisions that take place at the LHC. For example, the likelihood function of the combination of CMS Higgs boson discovery measurements, which CMS just released in electronic format, has nearly 700 parameters for a fixed value of the Higgs boson mass. Among these, only one – the number of Higgs bosons found in the data – is the physics parameter of interest, while the rest model systematic uncertainties.

Each of these parameters corresponds to a dimension of a multi-dimensional abstract space, in which the likelihood function can be drawn. It is hard for humans to visualise a space with more than a few dimensions, let alone one with many. The new release of the likelihood function of the CMS Higgs boson discovery measurements – the first likelihood function to be made publicly available by the collaboration – allows researchers to get around this problem. With a publicly accessible likelihood function, physicists outside the CMS collaboration can now precisely factor in the CMS Higgs boson discovery measurements in their studies.

The release of this likelihood function, as well as that of the Combine software, which is used to model the likelihood and fit the data, marks a new milestone in CMS’s decade-long commitment to fully open science. It joins hundreds of open-access publications, the release of almost five petabytes of CMS data on the CERN open-data portal and the publication of its entire software framework on GitHub.

Find out more on the CMS website.

abelchio Tue, 04/09/2024 - 14:29

Byline

CMS collaboration

Publication Date

Tue, 04/16/2024 - 10:50

ProtoDUNE’s argon filling underway

ckrishna — Fri, 12 Apr 2024 08:15:38 +0000

ProtoDUNE’s argon filling underway

CERN’s Neutrino Platform houses a prototype of the Deep Underground Neutrino Experiment (DUNE) known as ProtoDUNE, which is designed to test and validate the technologies that will be applied to the construction of the DUNE experiment in the United States.

Recently, ProtoDUNE has entered a pivotal stage: the filling of one of its two particle detectors with liquid argon. Filling such a detector takes almost two months, as the chamber is gigantic – almost the size of a three-storey building. ProtoDUNE’s second detector will be filled in the autumn.

ProtoDUNE will use the proton beam from the Super Proton Synchrotron to test the detecting of charged particles. This argon-filled detector will be crucial to test the detector response for the next era of neutrino research. Liquid argon is used in DUNE due to its inert nature, which provides a clean environment for precise measurements. When a neutrino interacts with argon, it produces charged particles that ionise the atoms, allowing scientists to detect and study neutrino interactions. Additionally, liquid argon's density and high scintillation light yield enhance the detection of these interactions, making it an ideal medium for neutrino experiments.

Interestingly, the interior of the partially filled detector now appears green instead of its usual golden colour. This is because when the regular LED light is reflected inside the metal cryostat, the light travels through the liquid argon and the wavelength of the photons is shifted, producing a visible green effect.

The DUNE far detector, which will be roughly 20 times bigger than protoDUNE, is being built in the United States. DUNE will send a beam of neutrinos from Fermi National Accelerator Laboratory (Fermilab) near Chicago, Illinois, over a distance of more than 1300 kilometres through the Earth to neutrino detectors located 1.5 km underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota.

Watch a short time-lapse video of protoDUNE being filled with liquid argon:

ckrishna Fri, 04/12/2024 - 10:15

Byline

Chetna Krishna

Publication Date

Fri, 04/12/2024 - 10:30

Searching for new asymmetry between matter and antimatter

abelchio — Thu, 11 Apr 2024 09:41:56 +0000

Searching for new asymmetry between matter and antimatter

The LHCb detector seen in 2018 during its opening (Image: CERN)

Once a particle of matter, always a particle of matter. Or not. Thanks to a quirk of quantum physics, four known particles made up of two different quarks – such as the electrically neutral D meson composed of a charm quark and an up antiquark – can spontaneously oscillate into their antimatter partners and vice versa.

At a seminar held recently at CERN, the LHCb collaboration at the Large Hadron Collider (LHC) presented the results of its latest search for matter–antimatter asymmetry in the oscillation of the neutral D meson, which, if found, could help shed light on the mysterious matter–antimatter imbalance in the Universe.

The weak force of the Standard Model of particle physics induces an asymmetry between matter and antimatter, known as CP violation, in particles containing quarks. However, these sources of CP violation are difficult to study and are insufficient to explain the matter–antimatter imbalance in the Universe, leading physicists to both search for new sources and to study the known ones better than ever before.

In their latest endeavour, the LHCb researchers have rolled up their sleeves to measure with unprecedented precision a set of parameters that determine the matter–antimatter oscillation of the neutral D meson and enable the search for the hitherto unobserved but predicted CP violation in the oscillation.

The collaboration had previously measured the same set of parameters, which are linked to the decay of the neutral D meson into a positively charged kaon and a negatively charged pion, using its full data set from Run 1 of the LHC and a partial data set from Run 2. This time around, the team analysed the full Run-2 data set and, by combining the result with that of its previous analysis, excluding the partial Run-2 data set, it obtained the most precise measurements of the parameters to date – the overall measurement uncertainty is 1.6 times smaller than the smallest uncertainty achieved before by LHCb.

The results are consistent with previous studies, confirming the matter–antimatter oscillation of the neutral D meson and showing no evidence of CP violation in the oscillation. The findings call for future analyses of this and other decays of the neutral D meson using data from the third run of the LHC and its planned upgrade, the High-Luminosity LHC.

Other neutral D meson decays of interest include the decay into a pair of two kaons or two pions, in which LHCb researchers observed CP violation in particles containing charm quarks for the first time, and the decay into a neutral kaon and a pair of pions, with which LHCb clocked the speed of the particle’s matter–antimatter oscillation. No avenue should be left unexplored in the search for clues to the matter–antimatter imbalance in the Universe and other cosmic mysteries.

Find out more on the LHCb website.

abelchio Thu, 04/11/2024 - 11:41

Byline

Ana Lopes

Publication Date

Thu, 04/11/2024 - 17:00

The next-generation triggers for CERN detectors

ckrishna — Thu, 11 Apr 2024 09:45:16 +0000

The next-generation triggers for CERN detectors

The experiments at the Large Hadron Collider (LHC) require high-performance event-selection systems – known as “triggers” in particle physics – to filter the flow of data to manageable levels. The triggers pick events with distinguishing characteristics, such as interactions or collisions of particles recorded in particle detectors, and make them available for physics analyses. In just a few seconds, the complex system can determine whether the information about a given collision event is worth keeping or not.

The ATLAS and CMS experiments use triggers on two levels. The first trigger runs in sync with the rate of particle bunches colliding in the detectors, deciding in less than 10 microseconds which data to keep. Events that pass the first-level trigger move on to the second high-level trigger for further selection. The selected events are then sent to the CERN Data Centre, where the data is copied, stored and eventually made available to scientists around the world for data analysis.

In preparation for the High-Luminosity LHC (HL-LHC), the ATLAS and CMS detectors are being upgraded with finer spatial and timing granularity, which will result in more data for each collision. The principle is the same as taking a picture with a camera with more pixels: the resulting file will be bigger because the image contains more detail, and the picture will be of higher quality. To prepare for the data deluge expected when the LHC enters the high-luminosity era, scientists need to develop new strategies for more sophisticated event processing and selection.

The key objective of the five-year Next-Generation Triggers (NextGen) project is to get more physics information out of the HL-LHC data. The hope is to uncover as-yet-unseen phenomena by more efficiently selecting interesting physics events while rejecting background noise. Scientists will make use of neural network optimisation, quantum-inspired algorithms, high-performance computing and field-programmable gate array (FPGA) techniques to improve the theoretical modelling and optimise their tools in the search for ultra-rare events.

The foundations of the NextGen project were laid in 2022 when a group of private donors, including former Google CEO Eric Schmidt, visited CERN. This first inspiring visit eventually evolved into an agreement with the Eric and Wendy Schmidt Fund for Strategic Innovation, approved by the CERN Council in October 2023, to fund a project that would pave the way for the future trigger systems at the HL-LHC and beyond: NextGen was born.

NextGen will collaborate with experts in academia and industry. The work builds on the open-science and knowledge-sharing principles embedded in CERN's institutional governance and modus operandi. The project includes a work package dedicated to education and outreach, a unique multi-disciplinary training programme for NextGen researchers and targeted events and conferences for the wider community of scientists interested in the field. The intellectual property generated as part of the NextGen Triggers project, owned by CERN, will be released and shared under open licences in compliance with the CERN Open Science Policy.

The NextGen Triggers project will mark a new chapter in high-energy physics, leveraging upgraded event-selection systems and data-processing techniques to unlock a realm of discoveries.

ckrishna Thu, 04/11/2024 - 11:45

Byline

Antonella Del Rosso

Publication Date

Thu, 04/11/2024 - 12:00

ATLAS provides first measurement of the W-boson width at the LHC

abelchio — Wed, 10 Apr 2024 10:03:51 +0000

ATLAS provides first measurement of the W-boson width at the LHC

View of an ATLAS collision event in which a candidate W boson decays into a muon and a neutrino. The reconstructed tracks of the charged particles in the inner part of the ATLAS detector are shown as orange lines. The energy deposits in the detector’s calorimeters are shown as yellow boxes. The identified muon is shown as a red line. The missing transverse momentum associated with the neutrino is shown as a green dashed line. (Image: ATLAS/CERN)

The discovery of the Higgs boson in 2012 slotted in the final missing piece of the Standard Model puzzle. Yet, it left lingering questions. What lies beyond this framework? Where are the new phenomena that would solve the Universe's remaining mysteries, such as the nature of dark matter and the origin of matter–antimatter asymmetry?

One parameter that may hold clues about new physics phenomena is the “width” of the W boson, the electrically charged carrier of the weak force. A particle’s width is directly related to its lifetime and describes how it decays to other particles. If the W boson decays in unexpected ways, such as into yet-to-be-discovered new particles, these would influence the measured width. As its value is precisely predicted by the Standard Model based on the strength of the charged weak force and the mass of the W boson (along with smaller quantum effects), any significant deviation from the prediction would indicate the presence of unaccounted phenomena.

In a new study, the ATLAS collaboration measured the W-boson width at the Large Hadron Collider (LHC) for the first time. The W-boson width had previously been measured at CERN’s Large Electron–Positron (LEP) collider and Fermilab’s Tevatron collider, yielding an average value of 2085 ± 42 million electronvolts (MeV), consistent with the Standard-Model prediction of 2088 ± 1 MeV. Using proton–proton collision data at an energy of 7 TeV collected during Run 1 of the LHC, ATLAS measured the W-boson width as 2202 ± 47 MeV. This is the most precise measurement to date made by a single experiment, and — while a bit larger — it is consistent with the Standard-Model prediction to within 2.5 standard deviations (see figure below).

This remarkable result was achieved by performing a detailed particle-momentum analysis of decays of the W boson into an electron or a muon and their corresponding neutrino, which goes undetected but leaves a signature of missing energy in the collision event (see image above). This required physicists to precisely calibrate the ATLAS detector’s response to these particles in terms of efficiency, energy and momentum, taking contributions from background processes into account.

However, achieving such high precision also requires the confluence of several high-precision results. For instance, an accurate understanding of W-boson production in proton–proton collisions was essential, and researchers relied on a combination of theoretical predictions validated by various measurements of W and Z boson properties. Also crucial to this measurement is the knowledge of the inner structure of the proton, which is described in parton distribution functions. ATLAS physicists incorporated and tested parton distribution functions derived by global research groups from fits to data from a wide range of particle physics experiments.

The ATLAS collaboration measured the W-boson width simultaneously with the W-boson mass using a statistical method that allowed part of the parameters quantifying uncertainties to be directly constrained from the measured data, thus improving the measurement’s precision. The updated measurement of the W-boson mass is 80367 ± 16 MeV, which improves on and supersedes the previous ATLAS measurement using the same dataset. The measured values of both the mass and the width are consistent with the Standard-Model predictions.

Future measurements of the W-boson width and mass using larger ATLAS datasets are expected to reduce the statistical and experimental uncertainties. Concurrently, advancements in theoretical predictions and a more refined understanding of parton distribution functions will help to reduce the theoretical uncertainties. As their measurements become ever more precise, physicists will be able to conduct yet more stringent tests of the Standard Model and probe for new particles and forces.

Comparison of the measured W-boson width with the Standard-Model prediction and with measurements from the LEP and Tevatron colliders. The vertical grey band illustrates the Standard-Model prediction, while the black dots and the associated horizontal bands represent the published experimental results. (Image: ATLAS/CERN)

abelchio Wed, 04/10/2024 - 12:03

Byline

ATLAS collaboration

Publication Date

Wed, 04/10/2024 - 11:57

CERN pays tribute to Peter Higgs

cmenard — Wed, 10 Apr 2024 07:33:08 +0000

CERN pays tribute to Peter Higgs

Peter Higgs has passed away at the age of 94. An iconic figure in modern science, Higgs in 1964 postulated the existence of the eponymous Higgs boson. Its discovery at CERN in 2012 was the crowning achievement of the Standard Model (SM) of particle physics – a remarkable theory which explains the visible universe at the most fundamental level.

Alongside Robert Brout and François Englert, and building on the work of a generation of physicists, Higgs postulated the existence of the Brout-Englert-Higgs (BEH) field. Alone among known fundamental fields, the BEH field is “turned on” throughout the universe, rather than flickering in and out of existence and remaining localized. Its existence allowed matter to form in the early universe some 10^-11 s after the Big Bang, thanks to the interactions between elementary particles (such as electrons and quarks) and the ever-present BEH field. Higgs and Englert were awarded the Nobel prize for physics in 2013 in recognition of these achievements.

“Besides his outstanding contributions to particle physics, Peter was a very special person, an immensely inspiring figure for physicists around the world, a man of rare modesty, a great teacher and someone who explained physics in a very simple yet profound way,” said CERN’s Director-General Fabiola Gianotti, expressing the emotion felt by the physics community upon his loss. “An important piece of CERN’s history and accomplishments is linked to him. I am very saddened, and I will miss him sorely.”

Peter Higgs’ scientific legacy will extend far beyond the scope of current discoveries. The Higgs boson – the observable “excitation” of the BEH field which he was the first to identify – is linked to some of most intriguing and crucial outstanding questions in fundamental physics. This still quite mysterious particle therefore represents a uniquely promising portal to physics beyond the SM. Since discovering it in 2012, the ATLAS and CMS collaborations have already made impressive progress in constraining its properties – a painstaking scientific study which will form a central plank of research at the LHC, high-luminosity LHC and future colliders for decades to come, promising insights into the many unanswered questions in fundamental science.

(Video: CERN)

cmenard Wed, 04/10/2024 - 09:33

Publication Date

Wed, 04/10/2024 - 09:21

Large Hadron Collider reaches its first stable beams in 2024

ndinmore — Fri, 05 Apr 2024 08:28:16 +0000

Large Hadron Collider reaches its first stable beams in 2024

LHC Page 1 showing the first stable beams of 2024 (Image: CERN)

On Friday 5 April, at 6.25 p.m., the LHC Engineer-in-Charge at the CERN Control Centre (CCC) announced that stable beams were back in the Large Hadron Collider, marking the official start of the 2024 physics data-taking season. The third year of LHC Run 3 promises six months of 13.6 TeV proton collisions at an even higher luminosity than before, meaning more collisions for the experiments to take data from. This will be followed by a period of lead ion collisions in October.

Before the LHC could restart, each accelerator in the CERN complex had to be prepared for another year of physics data taking. Beginning with Linac4, which welcomed its first beam two months ago, each accelerator has gone through a phase of beam commissioning in which it is gradually set up and optimised to be able to control all aspects of the beam, from its energy and intensity to its size and stability. During this phase researchers also test the accelerator’s performance and address any issues before it is used for physics. Following Linac4, which contains the source of protons for the beam, each accelerator was commissioned in turn: the Proton Synchrotron Booster, the Proton Synchrotron, the Super Proton Synchrotron, and finally the LHC from 8 March until 5 April. The whole complex is now ready for data taking.

Back to the CCC. While stable beams are the goal, the CCC engineers must first take several steps to achieve them. First, they must inject the beams into the LHC from the previous accelerators in the chain. Then begins the ramp-up process, which involves increasing the beam energy up to the nominal energy of 6.8 TeV. The next step – shown as “flat top” on LHC Page 1 – is where the energy in the beams is consistent, but they’re not quite ready yet. In order to achieve stable beams, the circulating beams must then be “squeezed” and adjusted using the LHC magnets. This involves making the beams narrower and more centred on their paths, and therefore more likely to produce a high number of collisions in the detectors. Only after the squeezing and adjustment has been completed can stable beams be declared and the experiments around the LHC begin their data taking.

Watch a video explaining the process from first injection to stable beams:

Delphine Jacquet and Georges Trad, both engineers in charge of the LHC, explain how the LHC beams work from the injections of protons to stable beams. (Video: CERN)

Did you know?

Although the solar eclipse on 8 April will not affect the beams in the LHC, the gravitational pull of the moon, like the tides, changes the shape of the LHC because the machine is so big. Read more here.

ndinmore Fri, 04/05/2024 - 10:28

Byline

Naomi Dinmore

Publication Date

Fri, 04/05/2024 - 18:30

FASER measures high-energy neutrino interaction strength

cmenard — Thu, 04 Apr 2024 12:46:29 +0000

FASER measures high-energy neutrino interaction strength

Operating at CERN’s Large Hadron Collider (LHC) since 2022, the FASER experiment is designed to search for extremely weakly interacting particles. Such particles are predicted by many theories beyond the Standard Model that are attempting to solve outstanding problems in physics such as the nature of dark matter and the matter-antimatter imbalance in the Universe. Another goal of the experiment is to study interactions of high-energy neutrinos produced in the LHC collisions, particles that are nearly impossible to detect in the four big LHC experiments. Last week, at the annual Rencontres de Moriond conference, the FASER collaboration presented a measurement of the interaction strength, or “cross section”, of electron neutrinos (ν_e) and muon neutrinos (ν_μ). This is the first time such a measurement has been made at a particle collider. Measurements of this kind can provide important insights across different aspects of physics, from understanding the production of “forward” particles in the LHC collisions and improving our understanding of the structure of the proton to interpreting measurements of high-energy neutrinos from astrophysical sources performed by neutrino-telescope experiments.

FASER is located in a side tunnel of the LHC accelerator, 480 metres away from the ATLAS detector collision point. At that location, the LHC beam is already nearly 10 metres away, bending away on its circular 27-kilometre path. This is a unique location for studying weakly interacting particles produced in the LHC collisions. Charged particles produced in the collisions are deflected by the LHC magnets. Most neutral particles are stopped by the hundreds of metres of rock between FASER and ATLAS. Only very weakly interacting neutral particles like neutrinos are expected to continue straight on and reach the location where the detector is installed.

The probability of a neutrino interacting with matter is very small, but not zero. The type of interaction that FASER is sensitive to is where a neutrino interacts with a proton or a neutron inside the detector. In this interaction, the neutrino transforms into a charged “lepton” of the same family – an electron in the case of an ν_e, and a muon in the case of a ν_μ – which is visible in the detector. If the energy of the neutrino is high, several other particles are also produced in the collision.

The detector used to perform the measurement consists of 730 interleaved tungsten plates and photographic emulsion plates. The emulsion was exposed during the period from 26 July to 13 September 2022 and then chemically developed and analysed in search of charged particle tracks. Candidates for neutrino interactions were identified by looking for clusters of tracks that could be traced back to a single vertex. One of these tracks then had to be identified as a high-energy electron or muon.

Event displays identified by the FASER collaboration as candidates for an νe (left) and a νμ (right) interacting in the detector. Invisible here, the neutrinos arrive from the left and then interact to create multiple tracks spraying out to the right (coloured lines), one of which is identified as a charged lepton (labelled). (credit: FASER collaboration)

In total, four candidates for an ν_e interaction and eight candidates for a ν_μ interaction have been found. The four ν_e candidates represent the first direct observation of electron neutrinos produced at a collider. The observations can be interpreted as measurements of neutrino interaction cross sections, yielding (1.2^+0.9_−0.8) ×10⁻³⁸ cm2 GeV⁻¹ in the case of the ν_e and (0.5 ± 0.2) × 10⁻³⁸ cm2 GeV−1 in the case of the ν_μ. The energies of the neutrinos were found to be in a range between 500 and 1700 GeV. No measurement of the neutrino interaction cross section had previously been made at energies above 300 GeV in the case of the ν_e and between 400 GeV and 6 TeV in the case of the ν_μ.

The results obtained by FASER are consistent with expectations and demonstrate the ability of FASER to make neutrino cross-section measurements at the LHC. With the full LHC Run 3 data, 200 times more neutrino events will be detected, allowing much more precise measurements.

The CMS experiment at CERN measures a key parameter of the Standard Model

angerard — Wed, 03 Apr 2024 09:01:04 +0000

The CMS experiment at CERN measures a key parameter of the Standard Model

Last week, at the annual Rencontres de Moriond conference, the CMS collaboration presented a measurement of the effective leptonic electroweak mixing angle. The result is the most precise measurement performed at a hadron collider to date and is in good agreement with the prediction from the Standard Model.

The Standard Model of Particle Physics is the most precise description to date of particles and their interactions. Precise measurements of its parameters, combined with precise theoretical calculations, yield spectacular predictive power that allows phenomena to be determined even before they are directly observed. In this way, the Model successfully constrained the masses of the W and Z bosons (discovered at CERN in 1983), of the top quark (discovered at Fermilab in 1995) and, most recently, of the Higgs boson (discovered at CERN in 2012). Once these particles had been discovered, these predictions became consistency checks for the Model, allowing physicists to explore the limits of the theory’s validity. At the same time, precision measurements of the properties of these particles are a powerful tool for searching for new phenomena beyond the Standard Model – so-called “new physics” - since new phenomena would manifest themselves as discrepancies between various measured and calculated quantities.

The electroweak mixing angle is a key element of these consistency checks. It is a fundamental parameter of the Standard Model, determining how the unified electroweak interaction gave rise to the electromagnetic and weak interactions through a process known as electroweak symmetry breaking. At the same time, it mathematically ties together the masses of the W and Z bosons that transmit the weak interaction. So, measurements of the W, the Z or the mixing angle provide a good experimental cross-check of the Model.

The two most precise measurements of the weak mixing angle were performed by experiments at the CERN LEP collider and by the SLD experiment at the Stanford Linear Accelerator Center (SLAC). The values disagree with each other, which had puzzled physicists for over a decade. The new result is in good agreement with the Standard Model prediction and is a step towards resolving the discrepancy between the latter and the LEP and SLD measurements.

“This result shows that precision physics can be carried out at hadron colliders,” says Patricia McBride, CMS spokesperson. “The analysis had to handle the challenging environment of LHC Run 2, with an average of 35 simultaneous proton-proton collisions. This paves the way for more precision physics at the High-Luminosity LHC, where five times more proton pairs will be colliding simultaneously.”

Precision tests of the Standard Model parameters are the legacy of electron-positron colliders, such as CERN’s LEP, which operated until the year 2000 in the tunnel that now houses the LHC. Electron-positron collisions provide a perfect clean environment for such high-precision measurements. Proton-proton collisions in the LHC are more challenging for this kind of studies, even though the ATLAS, CMS and LHCb experiments have already provided a plethora of new ultra-precise measurements. The challenge is mainly due to huge backgrounds from other physics processes than the one being studied and to the fact that protons, unlike electrons, are not elementary particles. For this new result, reaching a precision similar to that of an electron-positron collider seemed like an impossible task, but it has now been achieved.

The measurement presented by CMS uses a sample of proton-proton collisions collected from 2016 to 2018 at a centre-of-mass energy of 13 TeV and corresponding to a total integrated luminosity of 137 fb⁻¹, meaning about 11 000 million million collisions!

The mixing angle is obtained through an analysis of angular distributions in collisions where pairs of electrons or muons are produced. This is the most precise measurement performed at a hadron collider to date, improving on previous measurements from ATLAS, CMS and LHCb.

angerard Wed, 04/03/2024 - 11:01

Publication Date

Wed, 04/03/2024 - 16:00

CERN and the Swiss Arts Council announce the artists selected for the sixth edition of Connect

angerard — Tue, 02 Apr 2024 14:09:16 +0000

CERN and the Swiss Arts Council announce the artists selected for the sixth edition of Connect

Connect is an art residency programme launched by Arts at CERN and Pro Helvetia in 2021. Dedicated to Swiss-based artists working at the intersection of science and artistic research, this research-led residency invites them to come to CERN to explore ideas and develop new work.

The selected duo is composed of Robin Meier Wiratunga and Vimala Pons. Meier Wiratunga is an artist and composer who seeks to understand how humans, insects and objects think. Collaborating closely with scientific researchers, Meier Wiratunga's work blends machine learning with insights from animal intelligence, creating constellation scores where musical patterns emerge as ‘thinking tools'. Vimala Pons is an actress who has worked in independent and auteur cinema.

Meier Wiratunga and Pons will dedicate their residency at CERN to developing their proposal titled Guided Meditations for the End of the Universe. This project aims to delve into cosmological theories about the end of the Universe to transform them into meditative and embodied experiences. Employing an anthropological approach to engage with science, the artists intend to employ spoken voice recordings, electronic music, light environments and sonified data from particle events.

Now in its sixth edition, Connect has become a pivotal platform for Swiss-based artists to expand their artistic practice in dialogue with the field of physics and science at CERN. This collaboration framework between Arts at CERN and Pro Helvetia will continue through the next year with an iteration in Chile and India, sustaining its mission to foster interactions and dialogue between artistic and scientific communities.

“We find ourselves in a challenging yet exciting time, as we witness the emergence of a strong and vibrant infrastructure devoted to the integration of artistic activity within the sciences,” says Mónica Bello, Curator and Head of Arts at CERN. “This is a transformational and promising development that presents unprecedented opportunities for cultural innovation. Connect is evidence of these dynamics, and I am proud to see that our partnership with Pro Helvetia is advancing further, bringing in new residents with different perspectives and backgrounds and strengthening the confluence of art and science in Switzerland.”

“This sixth edition of Connect continues to reaffirm the significance of the interface between art, science and technology. The quality and diversity of applications received illustrate the rich interdisciplinarity inherent in this dynamic field, and the extent to which it is part of current artistic practice. We are happy to foster dialogue and innovation at this remarkable intersection and are very pleased about the ongoing collaboration with Arts at CERN,” explains Philippe Bischof, Director of Pro Helvetia.

The jury of Connect was formed by Mónica Bello, Curator and Head of Arts at CERN; Giulia Bini, Head of Program and Curator of “Enter the Hyper-Scientific” at EPFL Lausanne; and Federica Martini, Head of the CCC - Critical Curatorial Cybermedia Master at HEAD Genève.

angerard Tue, 04/02/2024 - 16:09

Publication Date

Wed, 04/03/2024 - 14:00

AMS’s second new life

ckrishna — Tue, 26 Mar 2024 12:40:37 +0000

AMS’s second new life

In 2011, the Alpha Magnetic Spectrometer (AMS) was installed on the International Space Station (ISS). Since then, it has recorded more than 200 billion cosmic ray events and, while most of their sources are known, a few signatures in the data could point to dark matter. The detector’s latest upgrade will enable scientists to investigate this further.

AMS collects cosmic ray particles that reach Earth coming either directly from the Sun or from far-away sources such as stars ending in supernovae or black holes. Most of the cosmic rays that AMS detects are protons, but heavy nuclei like iron or silicon also reach the detector. However, one signature is particularly intriguing. AMS has detected an unusually high flux of positrons – the antimatter partners of electrons. Positrons and other antimatter particles are rare in the Universe and hence not expected to be seen in the observed data at the strength found by AMS. Their origin is not yet confirmed; they could come from pulsars (fast rotating remnants of stars that emit regular signals), a yet-unknown astrophysical source or dark matter. The observed positron flux fits very well with dark matter models. But in order to investigate this more accurately, the AMS collaboration is now working on refurbishing the detector.

The main upgrade will be a new detector layer with a higher number of silicon strips that will increase the acceptance of recording infalling particles by 300%. “By 2030, AMS will extend the energy range of the positron flux and reduce the error by a factor of two compared with current data,” says AMS spokesperson Sam Ting (MIT). This will allow the detector to investigate the positron signature even further.

A second important addition will be three new radiative surfaces. Because AMS is exposed to direct sunlight, it was painted white to reflect excess heat and remain at operational temperatures. After 13 years in the demanding conditions of space, the paint has degraded and, to compensate for this, the new radiators will keep AMS cool again.

Astronauts training at NASA’s “Neutral Buoyancy Lab” on a full-scale ISS model submerged under water where they learn to mount the new AMS upgrade parts (Image: Corrado Gargiulo/NASA)

Currently, all the parts of the new upgrade, including electronics and hardware, are being built as “validation” and “qualification” models. If they pass all the tests happening at CERN, INFN Perugia and IABG in Germany, the final flight model will go into production. Astronauts are already training with the prototypes in space-like environments on Earth. In 2026, when the upgrade is launched, the astronauts will mount the new detector parts onto AMS during spacewalks. “Everything is going very, very fast,” says chief engineer Corrado Gargiulo (CERN). “This is a requirement, otherwise we arrive too late at the ISS for the upgrade to make sense.” Indeed, the mission now has an end date. NASA has scheduled the deorbiting of the ISS for 2030 and, until then, AMS will have plenty of cosmic ray events to record to explore the positron signature.

A mock-up detector for the next AMS upgrade, which will be installed during the next anticipated spacewalk for AMS. Another part of the upgrade includes a large power distribution system (PDS) radiator to restore AMS’s optimal thermal performance. (Image: Chetna Krishna/CERN)

ckrishna Tue, 03/26/2024 - 13:40

Byline

Sanje Fenkart

Publication Date

Tue, 04/02/2024 - 10:00

CERN to change name for 70th Anniversary

ndinmore — Wed, 27 Mar 2024 18:02:52 +0000

CERN to change name for 70th Anniversary

The Organization’s new logo. The correct usage of the branding can be found on the Design Guidelines website.

Update: Did you enjoy our April Fools’ day story? CERN is indeed celebrating its 70th anniversary, but without changing its name. For the complete CERN70 anniversary events and programme of activities, visit cern.ch/cern70.

Since its inception in 1954, CERN has grown from a small group of physicists from a handful of countries to a thriving international hub for science and technology. This is why, on the occasion of the Laboratory’s 70th Anniversary, the time has come for the name to be adapted to reflect its new role in society. The new name, the Network of Experiments for Research and Development in Society, will come into force on 1 October 2024, at the Laboratory’s 70th birthday party.

The acronym CERN was borne from an intergovernmental meeting of UNESCO in Paris in December 1951. This is when the first resolution concerning the establishment of a European Council for Nuclear Research (in French Conseil Européen pour la Recherche Nucléaire, or CERN) was adopted. Two months later, an agreement was signed establishing the provisional Council – and the name “CERN” stuck. However, today, our understanding of matter goes much deeper than the nucleus, and “CERN” is now widely viewed across the scientific community as an outdated and exclusionary name.

“The word “nuclear” doesn’t really reflect the full breadth of scientific research we do here,” says Noah Lott, Head of Rebranding at the Organization. “Network of Experiments for Research and Development in Society indicates the range of particle physics, computing, engineering and technology research that takes place at the Laboratory, as well as its impact on society.”

“We are also no longer just a European organisation, as we have grown to a global community encapsulating more than 80 countries,” adds Ivana Reed, spokesperson for international relations at the Laboratory. “We feel that everyone, no matter who they are, will feel accepted and proud to be associated with NERDS.”

A dedicated working group – the Decision for a Unified Moniker Board – was created in 2021 to assess potential options for the new name. “NERDS is so much more memorable and inclusive than CERN,” explains Wirall Geex, president of the working group. “Since we are a global network of varied experiments, we hope the new name will remove negative stereotypes about the type of people who work at the Organization,” he adds, pushing up his glasses.

NERDS was chosen by the working group out of a list of names put forward by the CERN community. Among the high contenders were A Large International Experimental Network (ALIEN), the High Energy Laboratory for Physics (HELP), and the initial frontrunner, the Global Organization for Discovery (GOD). However, following a spirited debate in the working group, GOD was discarded on account of unfortunate echoes of the “God particle” – the controversial name accidentally given to the Higgs boson in 1993.

NERDS looks forward to continuing to be at the forefront of scientific research for the next 70 years and beyond.

ndinmore Wed, 03/27/2024 - 19:02

Byline

Naomi Dinmore

Publication Date

Mon, 04/01/2024 - 09:00

CERN and STFC support environmentally sustainable physics

katebrad — Wed, 27 Mar 2024 09:29:19 +0000

CERN and STFC support environmentally sustainable physics

CERN Director-General, Fabiola Gianotti, and STFC Executive Chair, Mark Thomson, sign a new agreement to support the development of more sustainable particle accelerators (Image: CERN)

On 22 March, CERN and the UK’s Science and Technology Facilities Council (STFC) signed a new agreement to collaborate on the research and development of advanced new technologies to make future particle accelerators significantly more sustainable.

Minimising the environmental impact of particle physics activities and ensuring their sustainability and energy efficiency is one of the key recommendations in the last update of the European Strategy for Particle Physics, published in 2020.

“CERN is fully committed to fostering sustainability across its existing and forthcoming projects, actively engaging in a variety of initiatives,” explains Mike Lamont, CERN Director for Accelerators and Technology. “These include sourcing renewable energy, implementing heat recovery schemes and forging collaborations with industry to explore innovative applications of sustainable technology, such as high-power electricity distribution in various contexts. Our philosophy in this regard aligns well with that of the STFC and we look forward to exploiting the potential of this collaboration – together we are stronger.”

The agreement will act as a framework to better direct CERN and STFC’s funding, expertise and technological investment to minimise environmental impact. It provides guidance and recommendations that consider the entire lifecycle of accelerator facilities, from design and construction to operation and decommissioning.

The agreement also outlines a proposal for STFC to establish a new Centre of Excellence in Sustainable Accelerators (CESA) at the Daresbury Laboratory in the UK. CESA would conduct original research in sustainable accelerator technologies and train accelerator scientists, technicians and engineers in the skills required to develop new accelerators with sustainability at the heart of the design.

For more details, see the UKRI website.

katebrad Wed, 03/27/2024 - 10:29

Publication Date

Wed, 03/27/2024 - 10:28

The delicate balance of lepton flavours

abelchio — Tue, 26 Mar 2024 09:33:12 +0000

The delicate balance of lepton flavours

In a talk at the ongoing Rencontres de Moriond conference, the ATLAS collaboration presented the result of its latest test of a key principle of the Standard Model of particle physics known as lepton flavour universality. The precision of the result is the best yet achieved by a single experiment in decays of the W boson and surpasses that of the current experimental average.

Most elementary particles can be classed into groups or families with similar properties. For example, the lepton family includes the electron, which forms the negatively charged cloud of particles surrounding the nucleus in every atom, the muon, a heavier particle found in cosmic rays, and the tau-lepton, an even heavier short-lived particle only seen in high-energy particle interactions.

As far as physicists know, the only difference between these particles is their mass, as generated through their different strengths of interaction with the fundamental field associated with the Higgs boson. In particular, a remarkable feature of the Standard Model is that each lepton type, or “flavour”, is equally likely to interact with a W boson, the electrically charged carrier of the weak force that is one of the four fundamental forces of nature. This principle is known as lepton flavour universality.

High-precision tests of lepton flavour universality, as obtained by comparing the rates of decay of the W boson into an electron and an electron neutrino, into a muon and a muon neutrino or into a tau-lepton and a tau neutrino, are therefore sensitive probes of physics beyond the Standard Model. Indeed, if lepton flavour universality holds, these decay rates should be equal (within negligible mass-dependent corrections).

This can be tested by measuring the ratios of the W boson’s rates of decay into the different lepton flavours. One of the challenges associated with such measurements at the Large Hadron Collider (LHC) is the collection of a pure (“unbiased”) sample of W bosons. In a paper released by Nature Physics in 2021, ATLAS reported the world’s most precise measurement of the ratio of the W boson’s rate of decay into a tau-lepton versus its rate of decay into a muon, demonstrating that collision events in which a pair of top quarks is produced provide an abundant and clean sample of W bosons.

In a recent paper, ATLAS released a new measurement, this time addressing the ratio of the W boson’s rate of decay into a muon versus its rate of decay into an electron. While the combination of all previous measurements showed that this ratio is within about 0.6% of unity, corresponding to equal decay rates, there was still room for improvement.

The new ATLAS result is based on a study of its full dataset from the second run of the LHC, collected between 2015 and 2018. The analysis looked at over 100 million top-quark-pair collision events. The top quark decays promptly into a W boson and a bottom quark, so this sample provides 100 million pairs of W bosons. By counting the number of these events with two electrons (and no muon) or two muons (and no electron), physicists can test whether the W boson decays more often into an electron or a muon.

However, it's not that simple. The Z boson, the electrically neutral carrier of the weak force, can also decay into a pair of electrons or muons, leaving a similar experimental signature to that of a top-quark pair. Since the combined mass of the leptons in Z-boson events clusters around the Z-boson mass of 91 GeV, this background process can be estimated and subtracted.

Moreover, as a result of measurements conducted in the 1990s at CERN’s Large Electron–Positron (LEP) collider, the LHC’s predecessor, and at the Stanford Linear Collider (SLC), the ratio of the Z boson’s rate of decay into two muons versus its rate of decay into two electrons is known to be equal to unity within 0.3%. Thus, in this ATLAS analysis, the Z boson’s decay rate ratio was determined as a reference measurement, allowing researchers to reduce uncertainties coming from the reconstruction of electrons and muons. Additionally, as many measurement uncertainties are similar in the events with two electrons and those with two muons, they were found to have only a minor effect on the measured decay rate ratio.

The final result from this new ATLAS analysis is a ratio of 0.9995, with an uncertainty of 0.0045, perfectly compatible with unity. With an uncertainty of only 0.45%, the result is more precise than all previous measurements combined (see figure below). For now, lepton flavour universality survives intact.

Measurements of the ratio of the W boson’s rate of decay into a muon versus its rate of decay into an electron. The new ATLAS result is shown in the last row as an open blue circle. Previous measurements are shown above using solid symbols, and the Particle Data Group average of all previous results is shown using a black diamond. (Image: ATLAS/CERN)

abelchio Tue, 03/26/2024 - 10:33

Byline

ATLAS collaboration

Publication Date

Tue, 03/26/2024 - 10:17

First observation of photons-to-taus in proton–proton collisions by CMS

sandrika — Mon, 25 Mar 2024 15:27:20 +0000

First observation of photons-to-taus in proton–proton collisions by CMS

In March 2024, the CMS collaboration announced the observation of two photons creating two tau leptons in proton–proton collisions. It is the first time that this process has been seen in proton–proton collisions, which was made possible by using the precise tracking capabilities of the CMS detector. It is also the most precise measurement of the tau’s anomalous magnetic moment and offers a new way to constrain the existence of new physics.

The tau, sometimes called tauon, is a peculiar particle in the family of leptons. In general, leptons, together with quarks, make up the “matter” content of the Standard Model (SM). The tau was only discovered in the late 1970s at SLAC, and its associated neutrino – the tau neutrino – completed the tangible matter part upon its discovery in 2000 by the DONUT collaboration at Fermilab. Precise research for the tau is rather tricky though, as its lifetime is very short: it remains stable for only 290·10^-15 s (a hundred quadrillionth of a second).

The two other charged leptons, the electron and the muon, are rather well studied. A lot is also known about their magnetic moments and their associated anomalous magnetic moments. The former can be understood as the strength and orientation of an imaginary bar magnet inside a particle. This measurable quantity, however, needs corrections at the quantum level arising from virtual particles tugging at the magnetic moment, deviating it from the predicted value. The quantum correction, referred to as anomalous magnetic moment, is of the order of 0.1%. If the theoretical and experimental results disagree, then this anomalous magnetic moment, a_l , opens doors to physics beyond the SM.

The anomalous magnetic moment of the electron is one of the most precisely known quantities in particle physics and agrees perfectly with the SM. Its muonic counterpart, on the other hand, is one of the most investigated ones, into which research is ongoing. Although theory and experiments have mostly agreed so far, recent results give rise to a tension that requires further investigation.

For the tau, however, the race is still going. It is especially hard to measure its anomalous magnetic moment, a_τ, due to the tau’s short lifetime. The first attempts to measure a_τ after the tau’s discovery came with an uncertainty that was 30 times higher than the size of the quantum corrections. Experimental efforts at CERN with the LEP and LHC detectors improved the constraints, reducing the uncertainties to 20 times the size of the quantum corrections.

In collisions, researchers look for a special process: two photons interacting to produce two tau leptons, also called a di-tau pair, which then decay into muons, electrons, or charged pions, and neutrinos. So far both ATLAS and CMS have observed this in ultra-peripheral lead–lead collisions. Now, CMS reports on the first observation of the same process during proton–proton collisions. These collisions offer a higher sensitivity to physics beyond the SM as new physics effects increase with the collision energy. With the outstanding tracking capabilities of the CMS detector, the collaboration was able to isolate this specific process from others, by selecting events where the taus are produced without any other track within distances as small as 1 mm. “This remarkable achievement of detecting ultra-peripheral proton–proton collisions sets the stage for many groundbreaking measurements of this kind with the CMS experiment,” said Michael Pitt, from the CMS analysis team.

This new method offers a new way to constrain the tau anomalous magnetic moment, which the CMS collaboration tried out immediately. While the significance will be improved with future run data, their new measurement places the tightest constraints so far, with higher precision than ever before. It reduces the uncertainty from the predictions down to only three times the size of the quantum corrections. “It is truly exciting that we can finally narrow down some of the basic properties of the elusive tau lepton,” said Izaak Neutelings, from the CMS analysis team. “This analysis introduces a novel approach to probe tau g-2 and revitalises measurements that have remained stagnant for more than two decades,” added Xuelong Qin, another member of the analysis team.

Further material: 3D interactive version of the event display with all tracks here.

sandrika Mon, 03/25/2024 - 16:27

Publication Date

Mon, 03/25/2024 - 17:00

Brazil becomes Associate Member State of CERN

angerard — Fri, 22 Mar 2024 08:06:54 +0000

Brazil becomes Associate Member State of CERN

Brazil has become the first Associate Member State of CERN in the Americas, following official notification that the country has completed its internal approval procedures in respect of the agreement signed in March 2022 granting it that status and of the Protocol on Privileges and Immunities of the Organization. The starting date of Brazil’s status as an Associate Member State is 13 March 2024.

Formal cooperation between CERN and Brazil started in 1990 with the signature of an International Cooperation Agreement, allowing Brazilian researchers to participate in the DELPHI experiment at the Large Electron–Positron Collider (LEP). Over the past decade, Brazil’s experimental particle-physics community has doubled in size. At the four main Large Hadron Collider (LHC) experiments alone, about 200 Brazilian scientists, engineers and students collaborate in fields ranging from hardware and data processing to physics analysis.

Today, Brazilian institutes participate in all the main experiments at the LHC – ALICE, ATLAS, CMS and LHCb and their ongoing and planned upgrades – as well as in ALPHA at the anti-proton decelerator. They are also involved in experiments at ISOLDE, ProtoDUNE at the Neutrino Platform and instrumentation projects such as Medipix. Following on from their participation in the RD51 collaboration, Brazilian teams are also contributing to setting up the DRD1 and DRD3 R&D collaborations for future detectors. Brazilian nationals also participate very actively in CERN training and outreach programmes.

Beyond particle physics, CERN and Brazil’s National Centre for Research in Energy and Materials (CNPEM) have also been formally cooperating since December 2020 on accelerator technology R&D and its applications.

As an Associate Member State, Brazil is entitled to appoint representatives to attend meetings of the CERN Council and the Finance Committee. Its nationals are eligible to apply for limited-duration staff positions and CERN’s graduate programmes, and its industry is entitled to bid for CERN contracts, increasing opportunities for industrial collaboration in advanced technologies.

angerard Fri, 03/22/2024 - 09:06

Publication Date

Fri, 03/22/2024 - 17:00