Aug 26, 2011
CMS and LHCb pull together in search for rare decay
In this issue, news from the LHC experiments focuses on a few highlights at the first big summer conference.
The first major conference since the LHC started to deliver significant luminosities provided the opportunity for the experiments to begin to work together on certain results. CMS and LHCb joined forces in just this way in their search for the decay Bs→μ+μ–. This rare decay mode is suppressed in the Standard Model, which predicts a branching ratio of (3.2 ± 0.2) × 10–9. It has recently gained much attention, with a preliminary measurement from the CDF experiment at Fermilab indicating a possible excess of events over the Standard Model expectation.
Now LHCb and CMS have combined their results based on 0.34 fb–1 and 1.14 fb–1 of proton–proton collisions, respectively, at a centre-of-mass energy of 7 TeV. The observed candidates in both experiments are consistent with the expectation from the sum of backgrounds and Standard Model signal. The combination results in an upper limit on the branching ratio for Bs→μ+μ– of less than 1.1 × 10–8 at 95% confidence level (CL), which improves on the limits obtained by the separate experiments and represents the best existing limit on this decay. Enhancement of the branching ratio by more than 3.4 times the Standard Model prediction is excluded at 95% CL. However, there remains room for a contribution from new physics, so the experiments will press ahead with this search, as the data flood in from the LHC.