CERN at ICHEP day three

Day three at the International Conference on High Energy Physics in Valencia, Spain, saw the end of the parallel sessions

Day three saw a wealth of results from the ALICE experiment, an observation of Z bosons in proton-lead collisions, and prospects for going beyond the Standard Model.

In heavy-ion physics, all four LHC experiments study proton-lead collisions, while  ALICE, ATLAS and CMS study lead-lead collisions. ALICE, dedicated to this field of research, presented a wide range of results from proton-proton, proton-lead and lead-lead collisions. ALICE studies all these kinds of collisions to understand the primordial state of matter known as Quark Gluon Plasma (QGP) - in which quarks and gluons are unbound. QGP existed just after the birth of the Universe. ALICE studies how it behaves, how it would have evolved in to the matter that populates the Universe today and the nature of the strong interaction that governs its behaviour. Among new results from ALICE were measurements of the flow of phi-mesons, which have a mass close to that of the proton and so allow researchers to establish to what extent particle mass rather than particle type is relevant for understanding the properties of the hot and dense medium of QGP. ALICE also presented results on the suppression of the lightest mesons containing charm quarks. Taken in conjunction with similar measurements from the CMS experiment, which looked at heavier charm-mesons, these results allow the influence of QGP on the propagation of composite particles to be modelled.  While most of these results conform to expectations, the behaviour of the heaviest charm-mesons remains a riddle. CMS and ATLAS presented other results on QGP, ranging from how the QGP medium breaks up jets of emerging particles to the production of W and Z bosons in proton-lead collisions.

The LHCb collaboration also announced its first observation of Z bosons produced in high-energy proton-lead collisions. These bosons will also help study the properties of QGP. Z bosons are produced from the direct collisions of quarks and gluons bound inside atomic nuclei. By tracking the momentum of these Z bosons, researchers can measure the ratio of momenta carried by the quarks and gluons in the lead nuclei, giving them a better understanding of how quark-gluon plasma is produced.

Today also saw presentations of results in the search for supersymmetry and physics beyond the Standard Model. The wealth of new Higgs measurements has enabled scientists to place further restrictions on the possible hiding places of new particles and new physical processes. The experimental Higgs data has enabled theorists to develop more comprehensive models that incorporate all the newly measured properties. These models include predictions of the existence of possible heavier or alternate Higgs bosons that would lead to new physics.

The ATLAS collaboration presented a measurement slightly above the theoretical prediction of the production rate of W boson pairs. This process is a very important background for many new physics searches and measurements of the Higgs boson. The high value compared to the theoretical prediction could be due to missing pieces in the theoretical calculation, or to new physics contributing in an unknown way. The measured rate agrees with an earlier CMS result. Looking for such deviations is one way researchers look for flaws in the Standard Model. If such a deviation were confirmed to high statistical significance, it would help lead to a theory beyond the Standard Model. Although deviations at the level so-far observed come and go, this will be something to watch for when the LHC resumes in 2015. Meanwhile, theorists are revisiting their calculations.

ATLAS presented a precise measurement of the rate of proton-proton collisions. This measurement used the "ALFA" detectors for the first time. These are mounted 240m from the interaction point on both sides of the ATLAS detector. By measuring outgoing protons in the ALFA stations, the overall rate of proton-proton collisions has been measured with a precision of just 1.4%. Understanding of this rate is a fundamental test of the strong interaction, and is important for our understanding of the rate of multiple collisions in the LHC detectors.

Direct searches for physics beyond the Standard Model have yet not revealed signs of new particles. But all the searches presented at the conference have pushed the limits on the masses of such particles higher. The ATLAS and CMS experiments also showed very detailed studies revealing that supersymmetric particles twice as heavy as the current limits will be within reach when the LHC resumes at higher energy in 2015 and at higher luminosity by 2020.

For more information, visit:

http://cern.ch/alice

http://atlas.ch/

http://cern.ch/cms

http://cern.ch/lhcb-public