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

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.