Tracking new physics—horse or zebra?

If you hear hoof beats, common sense says the cause is more than likely a horse. Yet, the possibility still exists that you're actually hearing a zebra. Physicists at LHCb are applying that same logic to an unusual finding in a recent analysis of the B meson.

Around one in every million B mesons decays into an excited kaon and two muons. The decay can occur in several different ways, so physicists classify them in what they call bins. The Standard Model predicts precisely the probability of the angles of these particle decays in each bin. The experiment can measure this probability, so it is an observable. Any difference between the measured observable and prediction could indicate new physics.
Nicola Serra of LHCb, one of the analysts of the B meson decay data from 2011, and his colleagues found such a difference.

"Most of the observables we measured in this analysis were close to Standard Model expectations, but a particular observable showed a sizable discrepancy," he says.

On the 'sigma' scale that physicists use to describe the certainty of a result, Serra's discrepancy between the expected and the measured result scored 3.7 sigma - there could be evidence for new physics but they need more data to confirm it. When they considered the probability of seeing that particular deviation with all of the data from the entire analysis, the sigma level dropped to 2.8 sigma, translating to a half a percent chance that the discrepancy is caused by statistical fluctuation. (The gold standard for a discovery is 5 sigma.)