How long can beauty and charm live together?

The LHCb collaboration has recently made the world’s most precise measurement of the lifetime of the B⁺_c meson – a fascinating particle that has both beauty and charm.

The heavy flavours of beauty and charm are produced in proton–proton collisions at the LHC in quark–antiquark pairs. The resulting hadrons usually contain the original pair, as in the case of quarkonia, or a single heavy quark bound to the abundantly produced light quarks. However, in rare cases, a c quark and a b antiquark combine into a B⁺_c. Since the top quark, t, decays too quickly to form hadrons, this is the only meson composed of two particles carrying different heavy flavours. As such, it offers a unique laboratory to test theoretical models of both the strong interaction, which accounts for its production, and the weak interaction, via which the meson has to decay. Indeed, the lifetime of the B⁺_c meson is one of the key parameters that provide a test-bench for theoretical models. Knowledge of the lifetime is also essential to develop selection algorithms and to improve the accuracy of the branching-fraction determination for most B⁺_c decay modes.

Following initial investigation at the Tevatron, the B⁺_c meson is being studied extensively at the LHC. In particular, the LHCb collaboration has already published several observations of new decay channels and the world’s most precise determination of the B⁺_c mass. Now, the collaboration has achieved the world’s most precise measurement of the lifetime by studying the semileptonic decays B⁺_c → J/ψμν, with the subsequent decay J/ψ → μ⁺ μ^–. The particle identification capabilities of LHCb allow a high-purity sample to be selected for these three-muon decays without any requirement on the decay time, therefore not biasing the measured lifetime. Using the data sample collected in 2012, about 10,000 signal decays were selected – the largest sample of reconstructed B⁺_c decays to have ever been reported.

The challenge with semileptonic decays is that the B⁺_c kinematics is not completely reconstructed, because of the impossibility of detecting the neutrino. This effect can be corrected on a statistical basis, although at the cost of introducing an uncertainty owing to the theoretical model of the decay used for the correction. LHCb developed a technique to constrain this model-dependence using data and found that the corresponding systematic uncertainty is small.

The result for the lifetime is 509±15 fs. This is twice as precise as the current world-average from the Particle Data Group, obtained combining measurements by the CDF and D0 experiments at the Tevatron, and opens the door for a new era of precision B⁺_c studies.