Tag Archives: supersymmetry

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?

Higgs – About Time

Most newspaper reports of the recent discovery of a new fundamental boson (let’s agree to call it the Higgs, shall we?) mentioned the long time delay between Higgs postulating the particle and physicists detecting it. That got me to wondering, this morning, whether the time delay was particularly long.

Peter Higgs was the first to postulate the existence of a fundamental scalar particle that might be detectable. He did this in 1964. (Several physicists, at around the same time, argued that a fundamental scalar field was required to give other particles mass; you can read all about that elsewhere.) The point is, it took experimental physicists until 2012 – that’s 48 years – to find the particle and prove that it existed. Is 48 years a long time in this context?

Well, Wolfgang Pauli postulated the existence of the electron neutrino in 1930; it took until 1956 before it was discovered – a lag of 26 years between theory and experiment. There was a similar gap of about a quarter of a century between theorists postulating the existence of the top neutrino and experimenters finding it. (The muon neutrino was discovered a mere 14 years after it was postulated.) So it seems that neutrinos, which are notoriously difficult to study, were found much more quickly than the Higgs.

What about quarks? Well, the bottom and top quarks were postulated in 1973 by Kobayashi and Maskawa; the b quark was found in 1977 (a mere four year later) and the t quark was found in 1995 (a gap of 22 years). So the quark sector was cleared up fairly quickly too.

The W and Z bosons turned up in experiments in 1983, 15 years after Glashow, Salam and Weinberg told people to expect them.

So it would seem that the Higgs is indeed something of a standout amongst the elementary particles: it took almost twice as long to find the Higgs as it did to find any of the other fundamental particles that theorists posited. Personally I’m hoping that the LHC will turn up evidence for a supersymmetric particle. Although supersymmetry itself has a long history, going back to the 1970s, the first realistic supersymmetric version of the Standard Model didn’t arrive until 1981, with work by Georgi and Dimopoulos. If we take 1981 as the starting date, then, it won’t be until 2029 that the Higgs record for a delay between postulation and experiment is broken.