D0 provides precise results on B⁰_s oscillations

The D0 collaboration at Fermilab has published the first direct two-sided bound on the oscillation frequency of the B⁰_s, the meson comprising a strange quark (s) and a bottom antiquark (Bbar). The result is consistent with what is expected from the Standard Model, within a 90% confidence level. The phenomenon in which a B⁰_d meson (with a down quark, d, instead of the strange quark) converts into its antiparticle Bbar⁰_d is well established, and its oscillation frequency Δm_d has been measured precisely (Heavy Flavour Averaging Group 2006). The value of the corresponding measure of the oscillation frequency of a B⁰_s meson into its antiparticle Bbar⁰_s – Δm_s – was until now much more poorly known.

The D0 collaboration is an international team of 700 physicists from 90 institutions and 20 countries working at Fermilab’s Tevatron, which provides high-energy proton-antiproton collisions for two experiments, D0 and CDF. The data for the D0 result were taken from 1 fb^-1 of total collision data, yielding more than a billion events (Abazov et al. 2006). The 90% confidence level means that the result does not qualify as a discovery, although it does provide a very strong indication. However, according to Rob Roser, co-spokesperson of CDF, within the next month or so the CDF collaboration should provide a result with greater precision.

The D0 result already provides some interesting constraints on supersymmetry. “The D0 value of 17 < Δm_s < 21 ps^-1 limits the contributions to the oscillation process that could be made by supersymmetric particles,” explains D0 co-spokesperson Terry Wyatt, from the University of Manchester. “The basic idea is that supersymmetric particles may be exchanged in the box diagrams that are responsible for B⁰_s mixing.” Several theoretical models of supersymmetry predict a much faster oscillation of B⁰_s, and the D0 result now disfavours these models.