by JON BUTTERWORTH
The LHCb experiment at the CERN Large Hadron Collider (LHC) today released an update of a measurement that many of us physicists have been keeping a close eye on for quite some time.
Konstantinos Petridis from the University of Bristol gave a presentation at CERN, more-or-less in parallel with a presentation at the Moriond meeting (which is sadly online-only this year, absent the usual ski-breaks) by Razvan-Daniel Moise of Imperial College London. There is also a preprint available on arXiv https://arxiv.org/abs/2103.11769.
The reason for “keeping an eye” is that the result could be the first sign of a new fundamental force, or particle. Something beyond our “Standard Model” of particle physics.
The experiment measures the decays of B-hadrons, particles containing bottom quarks. Quarks make up the protons and neutrons inside every atomic nucleus, but those are “up” and “down” quarks. The bottom quark is one of their cousins, and is much heavier.
This means B-hadrons need something like the collisions at the LHC to produce them (that’s the “b” in LHCb). It also means they are unstable, because the b-quark inside them will decay to less massive particles.
One type of particles that can be produced in these decays is a lepton. In this case, either an electron, or their heavier cousin, the muon. The Standard Model makes a very firm prediction that both these decays should be equally likely. The measurement shows that the decay to pairs of muons only happens about 85% as often as the decay to pairs of electrons.
Of course, the devil is in the uncertainties.
This discrepancy first appeared a few years ago, and LHCb have been taking and analysing more data since. With the latest tranche of data, the central value – the 85% – hardly moved, but the uncertainty shrank a bit, pushing the “significance” above the arbitrary-but-well-established “3 sigma” level at which we traditionally declare “evidence” for something funny going on. They have 3.1 sigma, in fact, which means that if the Standard Model is correct, you would expect an anomaly like this to happen about twice in 1000 experiments.
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