LHCb and supersymmetry

The Bs meson consists of a strange quark and a bottom antiquark, and once it is produced it quickly decays. Very, very rarely it decays into a muon and an antimuon.

The Standard Model of particle physics predicts the rate of the decay of a Bs meson into muons: for every billion Bs mesons that are produced, about 3 of them will decay into a muon-antimuon pair. (The actual figure is 3.54 plus or minus 0.3.)

LHCb, the Large Hadron Collider beauty experiment, has been studying the decay of Bs mesons. (The beauty quark is another name for the bottom quark. Either way, we’re talking here about b quarks.) The experiment says that, for every billion Bs mesons that are produced, about 3 of them decay into a muon-antimuon pair. (The actual figure is 3.2 plus or minus 1.5.) You can find the paper from this CERN webpage. The team hasn’t claimed this as a discovery: the result is at a 3.5 sigma level, which means that there is about a 1-in-4300 chance that the LHCb would see the same bump in their data just due to random chance. But the result is certainly intriguing.

Bs meson decay to muon antimuon

In May 2012 the LHCb collaboration saw this decay of a Bs meson into a muon-antimuon pair (Credit: CERN, LHCb)

Why should this matter? Isn’t it just another case of the Standard Model being proved right? After all, with the discovery of a Higgs (and we’ll soon know for sure whether it’s the Higgs) the Standard Model is on firmer ground than ever. Well, that’s the whole point! The measurement does agree with the Standard Model. But the decay of the Bs into a muon-antimuon pair is believed to be sensitive to physics beyond the Standard Model. In particular there are several models of supersymmetry which, if they were realised in nature, would have the effect of increasing the rate of Bs decay into muons: in these models LHCb should see more than 3 muon-antimuon pairs per billion Bs decays. If the LHCb result stands, then several models of supersymmetry would appear to be ruled out.

Several recent articles have reported the LHCb finding as a significant blow to the whole idea of supersymmetry. Those articles are, I believe, wrong.

  • First, as the LHCb collects more data it’s possible that deviations from the Standard Model prediction will become evident. Let’s wait and see.
  • Second, there are other models of supersymmetry that aren’t affected by this result. What’s happening here is that the LHC is narrowing the range in which supersymmetry can be found, just as it narrowed the range where a Higgs could be found – and then found it.
  • Third, if the LHCb data confirms the Standard Model then the result poses challenges for all ideas for physics beyond the Standard Model. It’s not just supersymmetry that physicists are investigating here, after all.

The result, if confirmed, does raise one disturbing prospect. Perhaps the LHC may not see physics beyond the Standard Model, even when it starts to run at its highest energies. Supersymmetry could still be a phenomenon that applies at really high energies, but we wouldn’t be able to test it with machines such as the LHC. How frustrating would that be?