Time-dependent measurement of the γ angle in the B⁰_s →D^±_s K^∓ decay at LHCb

This document describes the measurement of the weak phase γ in the B⁰_s → D^±_s K^∓ decay performed by the LHCb experiment. The γ angle is the worst measured among the CKM unitarity triangle angles. The B⁰_s → D^±_s K^∓ mode allows to measure γ in a very clean way since it decays through tree-level diagram avoiding the problem of the penguin pollution. The analysis is performed on the 2011 data sample collected by LHCb at the center of mass energy of 7TeV, which corresponds to an integrated luminosity of 1 fb⁻¹. The relevant observables to be measured through the decay time dependence are the CP-violating coefficients C_f , S_f , S_¯f , A^ΔΓ_f , AΔ^Γ¯_f since they depend on the γ angle. We find the CP observables to be: C_f = 0.53±0.25±0.04, S_f = −1.09±0.33±0.08, S_¯f = −0.36±0.34±0.08, A^ΔΓ_f = 0.37 ± 0.42 ± 0.20, A^ΔΓ_¯f = 0.20 ± 0.41 ± 0.20, where the uncertainties are statistical and systematic, respectively. We use these observables to perform the first measurement of γ in the B⁰_s → D^±_s K^∓ decay mode, finding γ = (115⁺²⁸ ₋₄₃)^◦ modulo 180^◦ at 68% CL where both the statistical and systematic uncertainties are included.