P. Odier

Measurement of the BS0 lifetime

Studies of Bs –B 0 s mixing provide important tests of the Standard Model (SM) of particle physics. In the SM, mixing occurs via box diagrams. Extensions to the SM may introduce additional CP-violating phases that alter the value of the Bs –Bs mixing weak phase, φs , from that of the SM [1]. The Bs system exhibits a sizeable difference in the decay widths L and H, where L and H refer to the light and heavy Bs mass eigenstates, respectively. The effective lifetime, τeff , of a Bs meson decay mode is measured by approximating the decay time distribution, determined in the Bs rest system, by a single exponential function. For final states that can be accessed by both Bs and Bs mesons the effective lifetime depends on their CP components and is also sensitive to φs [2,3]. In this analysis τeff is determined for the CP-even Bs → J/ψη decay mode. As φs is measured to be small [4,5] the mass eigenstates are also CP eigenstates to better than per mille level and τeff measured in Bs → J/ψη decays is equal, to good approximation, to the lifetime of the light Bs mass eigenstate, τL ∝ −1 L . In the SM τL is predicted to be 1.43 ± 0.03 ps [6]. Measurements of τL have previously been reported by LHCb in the Bs → D+s D−s and Bs → K +K − decay modes [7,8]. The latter is dominated by penguin diagrams, which could arise within and beyond the SM and gives rise to direct CP violation. This then leads to a different τeff, when compared to measurements in the Bs → D+s D−s and Bs → J/ψη decays which are mediated by tree diagrams. Improved precision on the effective lifetimes τL and τH will enable more stringent tests of the consistency between direct measurements of the decay width difference s = L − H measured in Bs → J/ψφ decays and those inferred using effective lifetimes. The measurement of the effective Bs → J/ψη lifetime presented in this Letter uses 3 fb−1 of data collected in pp collisions at centre-of-mass energies of 7 TeV and 8 TeV during 2011 and 2012 using the LHCb detector. The J/ψ meson is reconstructed via the dimuon decay mode and the η meson via the diphoton decay mode. The presence of only two charged particles in the final state minimizes systematic uncertainties related to the tracking system.