There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
Abstract
The standard model of particle physics describes the fundamental particles and their
interactions via the strong, electromagnetic and weak forces. It provides precise
predictions for measurable quantities that can be tested experimentally. The probabilities,
or branching fractions, of the strange B meson (B(s)(0)) and the B0 meson decaying
into two oppositely charged muons (μ+ and μ−) are especially interesting because of
their sensitivity to theories that extend the standard model. The standard model predicts
that the B(s)(0) →µ+µ− and B(0) →µ+µ− decays are very rare, with about four of the
former occurring for every billion mesons produced, and one of the latter occurring
for every ten billion B0 mesons. A difference in the observed branching fractions
with respect to the predictions of the standard model would provide a direction in
which the standard model should be extended. Before the Large Hadron Collider (LHC)
at CERN started operating, no evidence for either decay mode had been found. Upper
limits on the branching fractions were an order of magnitude above the standard model
predictions. The CMS (Compact Muon Solenoid) and LHCb (Large Hadron Collider beauty)
collaborations have performed a joint analysis of the data from proton–proton collisions
that they collected in 2011 at a centre-of-mass energy of seven teraelectronvolts
and in 2012 at eight teraelectronvolts. Here we report the first observation of the
B(s)(0) → µ+µ− decay, with a statistical significance exceeding six standard deviations,
and the best measurement so far of its branching fraction. Furthermore, we obtained
evidence for the B(0) → µ+µ− decay with a statistical significance of three standard
deviations. Both measurements are statistically compatible with standard model predictions
and allow stringent constraints to be placed on theories beyond the standard model.
The LHC experiments will resume taking data in 2015, recording proton–proton collisions
at a centre-of-mass energy of 13 teraelectronvolts, which will approximately double
the production rates of B(s)(0) and B0 mesons and lead to further improvements in
the precision of these crucial tests of the standard model.