Hunting season at the LHC

Like hunters following the tracks of their prey, physicists compare real collision data with simulations of what they expect to see if a new particle is produced and decays in their detectors. (Supersymmetry simulation image: the CMS collaboration)

With the LHC now back smashing protons together at an energy of 13 TeV, what exotic beasts do physicists hope to find in this unfamiliar corner of the natural world?

Among the top priorities for the LHC experiments this year is the hunt for new particles suspected to lurk at the high-energy frontier: exotic beasts that do not fit within the Standard Model of particle physics and could lift the lid on an even deeper theory of nature’s basic workings.

New particles predicted by specific models of physics beyond the
Standard Model (Image: Daniel Dominguez, with permission from
Hitoshi Murayama)

Following the discovery of the Higgs boson five years ago, which was the final missing piece of the Standard Model of particle physics, physicists have good reason to expect that new particle species lie over the horizon. Among them is the mystery of what makes up dark matter, why the Standard Model particles of matter weigh what they do and come in three families of two, and, indeed, why the Higgs boson isn’t vastly heavier than it is – that is, why it isn't so heavy that it could have ended the evolution of the universe an instant after the Big Bang.

Casting the net wide

These outlandish prey are just a few of the known unknowns for physicists. To ensure that no corner of the new-physics landscape is left unturned, the LHC experiments also employ a model-independent approach to search for general features such as pairs of high-energy quarks and leptons or for unexplained sources of missing energy.

Their most elusive quarry might not light up their detectors at all, forcing the LHC exploration teams to adopt stealth approaches, such as making ultra-precise measurements of known Standard Model processes and seeing if they diverge from predictions. While physicists are hoping for a clear shot at any new particle species – a distinctive “bump” in the data that can only be explained by the presence of a new, heavy particle – they could be faced with a mere rustling in the undergrowth or other indirect signs that something is awry. This quest is not just the preserve of all of the LHC experiments, but also of numerous other experiments at CERN that are not linked to the LHC.

Either way, physicists exploring this uncharted territory of the high-energy frontier have to take extreme care not to get tricked by numerous Standard Model doppelgängers or be teased by inconclusive statistics. Even after an exotic new beast has been snared statistically and it seems that the LHC experiments have a discovery on their hands, so begins the task of identifying what the beast really is: a mere mutant or close relative of a species we already know? Or the first glimpse of a new subatomic kingdom?

Ranging from the bizarre to the mind-boggling, and in no particular order, below is a summary of some of the quantum creatures that are in the LHC experimentalists’ sights this year.

Supersymmetric particles

What?

For more than 40 years, physicists have been beguiled by a hypothetical symmetry of space–time called supersymmetry (SUSY), which would imply that every particle in the Standard Model has a partner called a “sparticle”. Given that these have not yet been seen, they must be heavier than the standard version.

Why?

Considered by many to be mathematically beautiful, SUSY can settle some of the technical problems with the Standard Model and suggests ways in which the fundamental forces may be unified. The lightest SUSY particle is also a good candidate to explain what makes up dark matter.

How?

SUSY could reveal itself in many ways in the LHC’s ATLAS and CMS experiments, for instance in events in which much of the energy is carried away by massive, weakly interacting sparticles. Like previous colliders, the LHC has so far found no evidence for supersymmetry, which rules out the existence of certain types of sparticles below a mass of 2 TeV.

 

Higgs siblings

What?

The Standard Model demands just one type of Higgs boson, and so far it seems that the observed Higgs particle fits the requirements. However, many theories suggest that this standard Higgs is one of a wider family of Higgs particles with slightly different properties – SUSY predicts no less than five of them.

Why?

Since the Higgs boson, which gives the Standard Model particles their masses, is a fundamentally different “scalar” object compared to all other known particles, it could open the door to new physics domains.

How?

Exotic cousins of the Higgs have different electrical charges and other properties, especially their mass, forcing them to decay differently to the standard Higgs in ways that should be relatively easy to spot.

New vector bosons 

What?

At the quantum level, nature’s fundamental forces are mediated by elementary particles called vector bosons: the neutral photon for electromagnetism, and the neutral Z or charged W bosons for the weak nuclear force responsible for radioactive decay. In principle, additional vector bosons – known as W’ and Z’ – could exist, too.

Why?

Finding such particles would constitute the discovery of a fifth force of nature, radically changing our view of the universe and extending the structure of the Standard Model.

How?

Experimental signatures of new vector bosons, which presumably are heavier than the W and Z, otherwise they would have been spotted by now, range from direct production in ATLAS and CMS to more subtle signs of lepton flavour violation in LHCb. 

Extra dimensions

What?

The possible existence of additional dimensions of space beyond the three we know of was put forward in the late 1990s to nurse some of the Standard Model’s ills. In this picture, the entire universe could merely be a 3D “brane” floating through a higher-dimensional bulk, to which the Standard model particles are forever shackled while leaving the force of gravity to propagate freely in the bulk, or there could be additional microscopic dimensions at extremely small scales.

Why?
If true, it would allow physicists to study gravitons and other gravitational phenomena in the lab, as it would shift the scale of quantum gravity by many orders of magnitude, right down to the TeV scale where the LHC operates.

How?

The presence of extra dimensions could produce a clear missing-energy signal in the ATLAS and CMS detectors and lead to “resonances”, like notes on a guitar string, that correspond to invisible relatives of the hypothetical carrier of gravity: the graviton.

Quantum black holes

What?

If extra dimensions exist, implying gravity is stronger than we thought, it is possible for very light black-holes to exist – mathematically resembling a conventional astrophysical black hole but trillions and trillions of times lighter. Such a state is predicted to evaporate more or less as soon as it formed and therefore poses no danger. After all, if such creatures are created at high energies, then they are also created all the time in collisions between cosmic rays and the upper atmosphere without doing any apparent harm.

Why?
The discovery of a miniature black hole would revolutionise physics and accelerate efforts to create a quantum theory of gravity that unites quantum mechanics with Einstein’s general theory of relativity.

How?

Miniature black holes would decay or “evaporate” instantly into other particles, revealing themselves as events containing multiple particles.

Dark matter

What?

The Standard Model, while passing every test on Earth, can only account for 5% of the matter observed in the universe as a whole. It is presumed that the dark matter known to exist from astronomical observations is made of some kind of particle, perhaps a supersymmetric particle, but precisely which type is a still a mystery.

Why?
In addition to explaining a large fraction of the universe, the ability to study dark matter in the laboratory would open a rich and fascinating new line of experimental study.

How?

Dark matter interacts very weakly, if at all, via the standard forces, and would leave a characteristic missing-energy signature in the ATLAS and CMS detectors.

Leptoquarks

What?

The Standard Model contains two basic types of matter: quarks, which make up protons and neutrons; and leptons, such as electrons and neutrinos. Leptoquarks are hypothetical particles that are a bit of both, allowing quarks and leptons to transform into one another.

Why?
Leptoquarks appear in certain extensions of the Standard Model, in particular in attempts to unify the strong, weak and electromagnetic interactions.

How?

Since they are expected to decay into a lepton and a quark, searches at the LHC look for characteristic bumps in the mass distributions of decay products. 

Quark substructure

What?

All the experimental evidence so far indicates that the six types of quarks we know of are indivisible, but history has shown us to be wrong on this front with other particles, not least the atom. Exploring matter at smaller scales, it is natural to ask: are quarks really the smallest entities, or do they possess components inside them?

Why?

If found, quark substructure would prove that there is a whole new layer of the subatomic world that we do not yet know about. The existence of “preons” has been postulated to give an explanation at a more fundamental level to the table of elementary particles and forces, with the aim of replicating the successful ordering of the periodic table.

How?

The experimental signature of the compositeness of quarks can be the detection of the decay of a quark in an excited state into ordinary quarks and gluons, which will in turn produce two streams of highly-energetic collimated particles called jets.

Heavy sterile neutrinos

What?

The Standard Model involves three types of light neutrinos – electron, muon and tau neutrinos – but several puzzles, such as the very small mass of regular neutrinos, suggest that there might be additional, sterile neutrinos, much heavier than the regular ones.

Why?

If found, a heavy sterile neutrino can help solve the problem of matter-antimatter asymmetry in the universe. It could also be a candidate for dark matter, in addition to accounting for the small masses of the regular, non-sterile neutrinos, which cannot be otherwise explained in the framework of the Standard Model.

How?

The mass of sterile neutrinos is theoretically unknown, but their presence could be revealed when they “oscillate” into regular, flavoured neutrinos.

Long-lived particles

What?

New particles produced in a particle collision are generally assumed to decay immediately, almost precisely at their points of origin, or to escape undetected. However, many models of new physics include heavy particles with lifetimes large enough to allow them to travel distances ranging from a few micrometres to a few hundred thousand kilometres before decaying into ordinary matter.

Why?

Heavy, long-lived particles can help explaining many of the unsolved questions of the Standard Model, such as the small mass of the Higgs boson, dark matter, and perhaps the imbalance of matter and antimatter in the universe.

How?

Long-lived particles could appear like a stream of ordinary matter spontaneously appearing out of nowhere (“displaced vertices”). Other ways to search for them include looking for a large “dE/dx”, long time of flight or tracks disappearing in the detector.