Measurement of the CP-violating phase $\phi_s$ and the $B^0_s$ meson decay width difference with $B^0_s \to J/\psi\phi$ decays in ATLAS

TL;DR

This paper reports a precise measurement of the CP-violating phase and decay width differences in B_s mesons using ATLAS data, confirming Standard Model predictions with improved statistical and systematic uncertainties.

Contribution

First combined measurement of $ au_s$, $eta_s$, and decay parameters in B_s decays from ATLAS, with enhanced precision and constraints on the decay width difference.

Findings

Measured $eta_s$ consistent with Standard Model

Decay width difference $ au_s$ precisely determined

Results agree with theoretical predictions

Abstract

A measurement of the $B^0_s$ decay parameters in the $B^0_s \to J/\psi\phi$ channel using an integrated luminosity of 14.3 $\mathrm{fb}^{-1}$ collected by the ATLAS detector from 8 TeV $pp$ collisions at the LHC is presented. The measured parameters include the $CP$-violating phase $\phi_s$, the decay width $\Gamma_s$ and the width difference between the mass eigenstates $\Delta\Gamma_s$. The values measured for the physical parameters are statistically combined with those from 4.9 $\mathrm{fb}^{-1}$ of 7 TeV data, leading to the following: \begin{eqnarray*} \phi_s & = & -0.090 \pm 0.078\;\mathrm{(stat.)} \pm 0.041\;\mathrm{(syst.)\;rad} \\ \Delta\Gamma_s & = & 0.085 \pm 0.011\;\mathrm{(stat.)} \pm 0.007\;\mathrm{(syst.)\;ps}^{-1} \\ \Gamma_s & = & 0.675 \pm 0.003\;\mathrm{(stat.)} \pm 0.003\;\mathrm{(syst.)\;ps}^{-1}. \end{eqnarray*} In the analysis the parameter $\Delta\Gamma_s$ is constrained to be positive. Results for $\phi_s$ and $\Delta\Gamma_s$ are also presented as 68% and 95% likelihood contours in the $\phi_s-\Delta\Gamma_s$ plane. Also measured in this decay channel are the transversity amplitudes and corresponding strong phases. All measurements are in agreement with the Standard Model predictions.