Determining the lifetime of the Bs

25 August 2022

A report from the LHCb experiment.

LHCb figure 1

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.

LHCb figure 2

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. 

Further reading

LHCb Collab. 2022 arXiv:2206.03088.

LHCb Collab. 2016 Phys. Lett. B 762 484.

LHCb Collab. 2014 Phys. Rev. Lett. 112 111802.

LHCb Collab. 2014 Phys. Lett. B 736 446.

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