New results cast light on semileptonic B_s asymmetry

The LHCb experiment has made the most precise measurement to date of the asymmetry a^s_sl, which is a measure of a flavour-specific matter–antimatter asymmetry in B-mesons and a test for physics beyond the Standard Model.

In 2010, and with an update in 2011, the Fermilab DØ collaboration reported an asymmetry in the semileptonic decays of B mesons decay into muons, which they observed in the number of events containing same-sign dimuons. The most recent result, using almost the full DØ data sample of 9 fb^–1, gives an asymmetry of about –1%, and differs by 3.9 σ from the tiny value predicted within the framework of the Standard Model (Abazov et al. 2011). If confirmed, it would indicate the presence of new physics.

Same-sign dimuons can be produced from the decay of pairs of neutral B mesons, which can mix between their particle and antiparticle states. Owing to the inclusive nature of the DØ measurement, the asymmetry, denoted A^b_sl, is a sum of contributions from the individual asymmetries in the B_d and B_s meson systems, a^d_sl and a^s_sl respectively. It is shown as the diagonal band in the plane of those asymmetries in the figure. The individual asymmetries characterize CP-violation in B-meson mixing, similar to the parameter ε_K in the neutral kaon system.

One of the highest priorities in flavour physics has been to measure a^d_sl and a^s_sl separately to establish if there is a disagreement with the Standard Model – and, if so, whether it occurs in the B_d or B_s system. Previous measurements of a^d_sl by the BaBar and Belle collaborations working at the ϒ(4S) resonance and of a^s_sl in an independent analysis by DØ have not been sufficiently precise to answer this question.

The new result from LHCb, based on the full 2011 data sample of 1.0 fb^–1, and first presented at ICHEP2012 (p53), provides the most precise measurement to date of a^s_sl. The analysis uses B⁰_s→D^–_sμ⁺X (and charge conjugate) decays, with D^–_s→φπ^– and relies on excellent control of asymmetries in the μ^± trigger and reconstruction. The result, a^s_sl = (–0.24 ± 0.54 ± 0.33)%, which is shown as the horizontal blue band in the figure, is consistent with the Standard Model prediction (LHCb collaboration 2012). Updated results from DØ on both a^d_sl and a^s_sl, which were also presented at ICHEP2012, continue to leave the situation unclear; more precise measurements are needed (Stone 2012). With the recently announced extension of proton running at the LHC for 2012, the LHCb collaboration expects to more than triple its data sample, so updates on this topic will be most exciting.