Precision measurement of CP violation and branching fractions in $B^{\pm} \to K^0_{\mathrm{S}} h^{\pm}$ $(h = \pi, K)$ decays and search for the rare decay $B_c^{\pm} \to K^0_{\mathrm{S}} K^{\pm}$

TL;DR

This paper reports the most precise measurements to date of CP asymmetries and branching fractions in specific B meson decays, using LHCb data at 13 TeV, and searches for a rare Bc decay with no significant signal found.

Contribution

First precise measurements of CP asymmetries and branching ratios in $B^{ m ext{±}}$ decays to $K^0_S h^{ ext{±}}$, and the first search for the rare $B_c^{ m ext{±}}$ decay in this channel.

Findings

CP asymmetry in $B^{ m ext{±}} o K^0_S ext{π}^{ ext{±}}$ is measured as -0.028 ± 0.009 ± 0.009.

Branching fraction ratio ${ m rac{ ext{Br}(B^{ m ext{±}} o K^0_S K^{ ext{±}})}{ ext{Br}(B^{ m ext{±}} o K^0_S ext{π}^{ ext{±}})}}$ is 0.055 ± 0.004 ± 0.002.

No significant signal for $B_c^{ m ext{±}} o K^0_S K^{ ext{±}}$ decay; upper limit set at 0.015 (0.016) at 90 ext{–}95\% CL.

Abstract

The decay $B^{\pm} \to K^0_{\mathrm{S}} \pi^{\pm}$, with a $CP$ asymmetry expected to be close to zero in the Standard Model, is theoretically clean and sensitive to potential new physics. An analysis of the decays $B^{\pm} \to K^0_{\mathrm{S}} \pi^{\pm}$ and $B^{\pm} \to K^0_{\mathrm{S}} K^{\pm}$ is performed using proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of $13\,\mathrm{TeV}$, corresponding to an integrated luminosity of $5.4\,\mathrm{fb}^{-1}$. The \CP asymmetries are determined to be ${\cal A}^{CP}(B^{\pm} \to K^0_{\mathrm{S}} \pi^{\pm})=-0.028\pm 0.009\pm 0.009$ and ${\cal A}^{CP}(B^{\pm} \to K^0_{\mathrm{S}} K^{\pm})=0.118\pm 0.062 \pm 0.031$, and the branching fraction ratio is measured to be ${\cal B}(B^{\pm} \to K^0_{\mathrm{S}} K^{\pm})/{\cal B}(B^{\pm} \to K^0_{\mathrm{S}} \pi^{\pm})=0.055\pm 0.004 \pm 0.002$, where the first uncertainties are statistical and the second are systematic. These results are the most precise measurements of these quantities to date. A search for the rare decay $B_c^{\pm} \to K^0_{\mathrm{S}} K^{\pm}$ is also performed. No significant signal is observed, and the upper limit on the product of the branching fraction ratio ${\cal B}(B_c^{\pm} \to K^0_{\mathrm{S}} K^{\pm})/{\cal B}(B^{\pm} \to K^0_{\mathrm{S}} \pi^{\pm})$ and the fragmentation-fraction ratio $f_c/f_u$ is set to be 0.015 (0.016) at the 90\% (95\%) confidence level.