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New physics against the odds

Large Hadron Collider
Scientists working at the Large Hadron Collider have found some unusual results that potentially contradict the current and established theory of elementary particles. But don’t jump to conclusions yet. As Professor Tim Gershon from the Department of Physics explains, there is still a long way to go before we re-write the rule book.

It has been reported that scientists have evidence of new physics discovered at the Large Hadron Collider. Researchers have detected particles behaving in ways that can’t be described by the Standard Model – the established theory of particle physics.

That evidence sheds light on a process that happens within a fraction of a microsecond, as tiny subatomic particles decay into even smaller ones. In the decays under study, particles called leptons appear to be emitted in proportions that don’t fit with our understanding of the subatomic universe. It could be a chance occurrence, but it could also be a tantalising sign of something unexpected at work.

The researchers involved are remaining cautiously optimistic. The evidence may be tantalising, but the implications to science are so great that a very high bar needs to be reached to say for certain.

“We would expect, if the Standard Model is correct, that we might get a deviation like this one or two times in every thousand experiments. If you were backing a horse with those odds, you’d be pretty happy about that.” Says Professor Tim Gershon from the Department of Physics at the University of Warwick. “But thousands of different measurements have been made at the Large Hadron Collider, so it is also possible that this is just a fluke.”

A spotter’s guide to fundamental particles

The deviation that scientists have detected corresponds to an imbalance in the ratio of rates at which pairs of electrons and muons are emitted during the decay of particles called B mesons. This doesn’t fit with the prediction of the Standard model, the theory that describes elementary particles, where equal decay rates to electrons and muons are expected.

Struggling with the terminology? In simple terms:

  • Mesons are made up of quarks and anti-quarks, but are very unstable and decay extremely quickly into lighter particles
  • The LHCb (Large Hadron Collider beauty) experiment is specifically designed to study the decays of mesons containing a ’beauty’ quark, called a B meson.
  • B mesons can decay in many different ways. In the processes being studied here, they emit pairs of particles called leptons: either an electron or a muon, together with their corresponding antiparticle.
  • The Standard Model tells us that these leptons are identical except for their masses, which means that electrons and muons should be produced in B meson decays at an equal rate. This is called lepton universality.

To allow scientists to study these fundamental particles, the Large Hadron Collider at CERN fires two proton beams in opposite directions around its underground beam pipe, speeding them up before smashing them together. New types of particles, including B mesons, can be produced in these collisions, but these decay extremely rapidly. To study them, sophisticated detectors are used to identify the particles produced in their decays, and then work backwards to reconstruct the initial decaying particle.


A thing of beauty

The Large Hadron Collider beauty experiment, or LHCb, looks for particles containing the beauty or b quark. Stretching for 20 metres along the LHC beam pipe, it was where this latest exciting measurement was made.

Professor Tim Gershon leads a group at the University involved in a number of different activities at LHCb, including studying these types of decays. He says: “We look for the production of these particles in a process in which a particle called the B-meson, containing the b quark, decays to another type of particle called a K meson, emitting a pair of leptons, either an electron and anti-electron, or a muon and an anti-muon.

“We essentially count how often B-mesons decay by those two different processes and compare the rate at which they happen. In the Standard Model, because the interactions are the same, we expect those processes to happen at the same rate. What we are seeing is that the process with muons happens less often than the process with electrons, with a ratio of 85% and an uncertainty just under 5%.”

That could mean that the Standard Model doesn’t adequately explain these particles, as long as this fluctuation didn’t occur by chance. “It would be a major breakthrough if confirmed,” says Professor Gershon. “If we can pin down these measurements and find out what is responsible for them, that is going to give us a completely new perspective on nature at its most fundamental level.”

Model standards

Professor Gershon was the chair of the internal review committee that scrutinised the results. The process involved checking over a thousand pages of documentation to ensure that the team’s analysis had met the high standard expected by all 1,000 plus members of the LHCb collaboration.

“We take this very, very seriously so that when we do put results out, everyone knows the quality they can expect from an LHCb publication. This analysis is no exception,” he says.

So what might explain what physicists are seeing at LHCb?

“There’s quite a large number of theories, but the two main ones involve new force mediators. One is a type of particle called a Z-prime, similar to the Z-boson of the Standard Model except more massive and with different couplings to electrons and muons. The other is a leptoquark, where a lepton and a b quark may couple together. That would be something that has no analogy in the current Standard Model – it would be completely new,” states Professor Gershon.

Against the odds

The next step will be to look at existing data to see if there’s evidence that supports these results in similar B meson decays. Longer term, it will be possible to make even more precise measurements when the Large Hadron Collider goes back online next year after its long shutdown. For this next data-taking period, LHCb itself will go online with an upgraded detector that will record data at a rate of up to five times higher than previously. So within a few years it should be possible to make significantly more precise measurements.

Professor Gershon adds: “One of the reasons that we’re excited is that we’ve had accumulating evidence for violation of lepton universality over several years, with precision gradually improving as a number of new measurements have come out. This new measurement is the first one that goes over the three standard deviation threshold, or ‘three-sigma’, for when we would claim evidence of a new effect.

“Five sigma is the gold standard for claiming that we’ve discovered something new. In fact, some of my colleagues may even want us to be beyond five sigma because this is such a major result. But with the new data that will be available in the next few years, the prospects for significant progress are very exciting.”



22 April 2021


Prof Tim GershonProfessor Tim Gershon is an expert in flavour physics and CP violation; neutrino and heavy quark physics. He leads the Warwick Elemenatry Particle Physics group's activity on LHCb.

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