Measurement of the time-integrated mixing probability $\chi_d$ with a semileptonic double-tagging strategy and $34.6 {\rm fb}^{-1}$ of Belle II collision data

TL;DR

This paper reports the first measurement of the B meson mixing probability $$ using Belle II data, employing a novel semileptonic double-tagging method at 10.58 GeV, achieving results consistent with previous measurements.

Contribution

The study introduces a new semileptonic double-tagging strategy to measure $$, providing a direct and precise determination with Belle II data.

Findings

Measured $ = 0.187  0.010  0.019$

Results agree with previous measurements

Demonstrates effectiveness of the novel methodology

Abstract

We present the first measurement of the time-integrated mixing probability $\chi_d$ using Belle II data collected at a center-of-mass (CM) energy of 10.58 GeV, corresponding to the mass of the $\Upsilon$(4S) resonance, with an integrated luminosity of $34.6 {\rm fb}^{-1}$ at the SuperKEKB $e^+ e^-$ collider. We reconstruct pairs of B mesons both of which decay to semileptonic final states. Using a novel methodology, we measure $\chi_d = 0.187 \pm 0.010 \text{ (stat.)} \pm 0.019 \text{ (syst.)}$, which is compatible with existing indirect and direct determinations.