Measurement of the $B^0$ lifetime and flavor-oscillation frequency using hadronic decays reconstructed in 2019-2021 Belle II data

Abstract

We measure the $B^0$ lifetime and flavor-oscillation frequency using $B^0\to D^{(*)-}π^+$ decays collected by the Belle II experiment in asymmetric-energy $e^+e^-$ collisions produced by the SuperKEKB collider operating at the $Υ(4S)$ resonance. We fit the decay-time distribution of signal decays, where the initial flavor is determined by identifying the flavor of the other $B$ meson in the event. The results, based on $33000$ signal decays reconstructed in a data sample corresponding to ${190}\text{fb}^{-1}$, are $τ_{B^0} = {1.499}\pm{0.013}\pm{0.008}\;\text{ps}$, $Δm_d = {0.516}\pm{0.008}\pm{0.005}\;\text{ps}^{-1}$, where the first uncertainties are statistical and the second are systematic. These results are consistent with the world-average values.

Figures (3)

• Figure 1: Distributions of $\Delta E$ (top) and $C$ (bottom) in data (points) and the fit model (curves and stacked shaded regions).
• Figure 2: Top: Distribution of $\delta t$ in simulated data (points) and distribution modeled by the response function from the fit to the simulated data (curves) and from the fit to the experimental data (shaded). The shaded area accounts for the statistical and systematic uncertainties on the parameters of the response function. Bottom: distribution of the pull, defined as the difference between the event count in each bin and its value predicted by the fit, divided by the Poisson uncertainty.
• Figure 3: Distribution of $\Delta t\xspace_{\ell}$ in data (points) and the fit model (lines) for opposite-flavor candidate pairs (red) and same-flavor pairs (blue) and their asymmetry (black).