Determining the lifetime of the B_s

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 B_s system is particularly interesting as it provides a sensitive probe to physics beyond the Standard Model. A B_s 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 (Γ_H) and light (Γ_L) mass eigenstates is expected. Measuring the lifetime of a CP-even B_s-decay mode determines τ_L = 1/Γ_L. 

LHCb has recently released a new and precise measurement of this parameter, making use of B_s → 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 B_s → J/ψη decay is observed (figure 1), and τ_L is extracted from a two-dimensional fit to the mass and decay time.

The analysis finds τ_L = 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 B_s → D_s⁺ D_s^– decay mode, τ_L = 1.437 ± 0.014 ps, which agrees well both with the Standard Model expectation (τ_L = 1.422 ± 0.013 ps) and the value inferred from measurements of Γ_s and ΔΓ_s in B_s → J/ψφ decays. Further improvement in the knowledge of τ_L is expected both by considering other CP-even B_s decays to final states containing η or η′ mesons, the B_s → D_s⁺ D_s^– dataset collected during Run 2 and from the upcoming Run 3.