A report from the LHCb experiment.
As the LHCb experiment prepares for data taking with an upgraded detector for LHC Run 3, the rich harvest of results using data collected in Run 1 and Run 2 of the LHC continues.
A fascinating area of study is the quantum-mechanical oscillation of neutral mesons between their particle and antiparticle states, implying a coupled system of two mesons with different lifetimes. The phenomenology of the Bs system is particularly interesting as it provides a sensitive probe to physics beyond the Standard Model. A Bs meson oscillates with a frequency of about 3 × 1012 Hz, or on average about nine times during its lifetime, τ. In addition, a sizeable difference between the decay widths of the heavy (ΓH) and light (ΓL) mass eigenstates is expected. Measuring the lifetime of a CP-even Bs-decay mode determines τL = 1/ΓL.
LHCb has recently released a new and precise measurement of this parameter, making use of Bs → J/ψη decays selected from 5.7 fb–1 of Run 2 data. The study improves the previous Run 1 precision by a factor of two. Due to the combinatorial background, the reconstruction of the η meson via its two-photon decay mode is a particular challenge for this analysis. Despite this, and even with the modest energy resolution of the calorimeter leading to a relatively broad mass peak overlapping partially with the signal from the B0 → J/ψη decay, a competitive accuracy has been achieved. By exploiting the latest machine-learning techniques to reduce the background and the well understood LHCb detector, the Bs → J/ψη decay is observed (figure 1), and τL is extracted from a two-dimensional fit to the mass and decay time.
The analysis finds τL = 1.445 ± 0.016 (stat) ± 0.008 (syst) ps, which is the most precise measurement of this quantity. Combined with the LHCb Run 1 study of this and the Bs → Ds+ Ds– decay mode, τL = 1.437 ± 0.014 ps, which agrees well both with the Standard Model expectation (τL = 1.422 ± 0.013 ps) and the value inferred from measurements of Γs and ΔΓs in Bs → J/ψφ decays. Further improvement in the knowledge of τL is expected both by considering other CP-even Bs decays to final states containing η or η′ mesons, the Bs → Ds+ Ds– dataset collected during Run 2 and from the upcoming Run 3.