Novel Tests of Lepton Universality Boost Evidence for ‘New Physics’

by johnsmith

Physicists from CERN’s LHCb (Large Hadron Collider beauty) Collaboration have carried out new tests of lepton universality, one of the basic principles of the Standard Model of particle physics. This principle states that the Standard Model treats the three charged leptons — electrons, muons and taus — identically, except for differences due to their different masses. The new measurements exhibit the same coherent pattern of deviations from lepton universality as seen in previous results from the LHCb Collaboration.

The decay of a B0 meson into a K0 and an electron-positron pair in the LHCb detector, which is used for a sensitive test of lepton universality in the Standard Model. Image credit: CERN.

The decay of a B0 meson into a K0 and an electron-positron pair in the LHCb detector, which is used for a sensitive test of lepton universality in the Standard Model. Image credit: CERN.

The Standard Model describes all the known particles that make up the Universe and the forces that they interact through.

It has passed every experimental test to date, and yet physicists know it must be incomplete.

It does not include the force of gravity, nor can it account for how matter was produced during the Big Bang.

It also contains no particle that could explain the mysterious dark matter.

As a result, physicists have long been hunting for signs of physics beyond the Standard Model that might help us to address some of these mysteries.

One of the best ways to search for new particles and forces is to study particles known as beauty quarks.

These are exotic cousins of the up and down quarks that make up the nucleus of every atom.

Beauty quarks don’t exist in large numbers in the world around as they are incredibly short-lived — surviving on average for just a trillionth of a second before transforming or decaying into other particles.

However, billions of beauty quarks are produced every year by CERN’s Large Hadron Collider and are recorded by a purpose-built detector called LHCb.

The way beauty quarks decay can be influenced by the existence of undiscovered forces or particles.

In March, the LHCb Collaboration released results showing evidence that beauty quarks were decaying into particles called muons less often than to their lighter cousins, electrons.

This is impossible to explain in the Standard Model, which treats electrons and muons identically, apart from the fact that electrons are around 200 times lighter than muons.

As a result, beauty quarks ought to decay into muons and electrons at equal rates.

Instead, the LHCb physicists found that the muon decay was only happening around 85% as often as the electron decay.

The difference between the LHCb result and the Standard Model was about three units of experimental error, or ‘3 sigma’ as it is known in particle physics. This means there is only around a one in a thousand chance of the result being caused by a statistical fluke.

Assuming the result is correct, the most likely explanation is that a new force that pulls on electrons and muons with different strengths is interfering with how these beauty quarks decay. However, to be sure if the effect is real more data is needed to reduce the experimental error.

Only when a result reaches the ‘5 sigma’ threshold, when there is less than a one in a million chance of it being due to random chance, will particle physicists start to consider it a genuine discovery.

“The fact that we’ve seen the same effect as our colleagues did in March certainly boosts the chances that we might genuinely be on the brink of discovering something new. It’s great to shed a little more light on the puzzle,” said Dr. Harry Cliff, a physicist at the Cavendish Laboratory.

The new result examined two new beauty quark decays from the same family of decays as used in the previous result.

The team found the same effect — the muon decays were only happening around 70% as often as the electron decays.

“The excitement at the Large Hadron Collider is growing just as the upgraded LHCb detector is about to be switched on and further data collected that will provide the necessary statistics to either claim or refute a major discovery,” said Professor Val Gibson, also from the Cavendish Laboratory.

The team’s paper will appear in the journal Physical Review Letters.


R. Aaij et al. (LHCb Collaboration). 2021. Tests of lepton universality using B0→K0Sℓ+ℓ− and B+→K∗+ℓ+ℓ− decays. Phys. Rev. Lett, in press; arXiv: 2110.09501

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