Measurement of $CP$ asymmetry in $B_s^0 \to D_s^{\mp} K^{\pm}$ decays

TL;DR

This paper reports the measurement of $CP$ asymmetry parameters in $B_s^0 o D_s^{ m ext{ extminus}} K^{ m extplus}$ decays using LHCb data, leading to a new estimate of the CKM angle $oldsymbol{ m extgamma}$ and evidence for $CP$ violation.

Contribution

First measurement of $CP$-violating parameters in $B_s^0 o D_s^{ m extminus}} K^{ m extplus}$ decays, enabling a new determination of the CKM angle $oldsymbol{ m extgamma}$.

Findings

Measured $CP$-violating parameters with statistical and systematic uncertainties.

Estimated $ m extgamma$ angle as $(128_{-22}^{+17})^ ext{ extdegree}$.

Provided $3.8\sigma$ evidence for $CP$ violation in these decays.

Abstract

We report the measurements of the $CP$-violating parameters in $B_s^0 \to D_s^{\mp} K^{\pm}$ decays observed in $pp$ collisions, using a data set corresponding to an integrated luminosity of $3.0\,\text{fb}^{-1}$ recorded with the LHCb detector. We measure $C_f = 0.73 \pm 0.14 \pm 0.05$, $A^{\Delta \Gamma}_f = 0.39 \pm 0.28 \pm 0.15$, $A^{\Delta \Gamma}_{\overline{f}} = 0.31 \pm 0.28 \pm 0.15$, $S_f = -0.52 \pm 0.20 \pm 0.07$, $S_{\overline{f}} = -0.49 \pm 0.20 \pm 0.07$, where the uncertainties are statistical and systematic, respectively. These parameters are used together with the world-average value of the $B_s^0$ mixing phase, $-2\beta_s$, to obtain a measurement of the CKM angle $\gamma$ from $B_s^0 \to D_s^{\mp} K^{\pm}$ decays, yielding $\gamma = (128\,_{-22}^{+17})^\circ$ modulo $180^\circ$, where the uncertainty contains both statistical and systematic contributions. This corresponds to $3.8\,\sigma$ evidence for $CP$ violation in the interference between decay and decay after mixing.