The LHCb experiment has recently made the most precise measurement yet of the asymmetry in oscillations between the matter and antimatter versions of Bs mesons. The measurement exploits the full LHCb data set recorded during Run 1 of the LHC and is consistent with the Standard Model prediction.

Subtle quantum mechanics effects allow the Bs meson, which contains a strange quark and a beauty antiquark, to spontaneously transform into its own antiparticle, BS, in which the quark-antiquark assignment is reversed. Due to quantum interference effects, in the Standard Model this transition occurs at almost exactly the same rate as the reverse process, with the asymmetry between them being predicted to be two parts in a hundred thousand. Finding an asymmetry that is significantly different from this value would suggest that particle-antiparticle oscillations can be indirectly affected by the presence of heavy new particles, as are predicted in new physics models.

Many of the oscillations can occur within the finite lifetime of the Bs mesons, and an asymmetry would therefore appear as a difference in the numbers of Bs and BS meson decays observed by LHCb. Semi-leptonic decays into a charmed hadron, a muon and a neutrino are particularly suited, and the LHCb data set contains around two million of them. The challenge is to avoid being fooled by fake sources of asymmetry due to small imperfections in the detector. Novel methods have been developed to control these sources based on extensive use of the rich samples of signals with charm and charmonium decays.

The final measured asymmetry is 0.45±0.26±0.20%, which is a factor of two more precise than the next-best measurement from the D0 experiment at Fermilab (see figure). The 13 TeV data that are now being recorded will provide an increased rate of Bs and Bd mesons, which will enable LHCb to probe far smaller asymmetries and enhance its sensitivity to possible new physics effects.