Measurement of the direct $CP$ asymmetry in $\bar{B}\rightarrow X_{s+d}\gamma$ decays with a lepton tag

TL;DR

This paper measures the direct $CP$ asymmetry in radiative $ar{B} ightarrow X_{s+d} ext{gamma}$ decays using a large data sample, finding results consistent with the Standard Model predictions.

Contribution

First measurement of the direct $CP$ asymmetry in $ar{B} ightarrow X_{s+d} ext{gamma}$ decays with a comprehensive dataset from Belle detector.

Findings

Measured $A_{CP}$ as a function of photon energy threshold.

Result at $E^{*}_{ ext{gamma}} ext{ ≥ 2.1 GeV}$ is $(2.2 extpm 3.9 extpm 0.9) ext{ extperthousand}$.

Results are consistent with Standard Model expectations.

Abstract

We report the measurement of the direct $CP$ asymmetry in the radiative $\bar{B}\rightarrow X_{s+d}\gamma$ decay using a data sample of $(772 \pm 11)\times 10^6$ $B\bar{B}$ pairs collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The $CP$ asymmetry is measured as a function of the photon energy threshold. For $E^{\rm{*}}_{\gamma} \geq 2.1~{\rm{GeV}}$, where $E^{\rm{*}}_{\gamma}$ is the photon energy in the center-of-mass frame, we obtain $\mathcal{A}_{CP}(\bar{B}\rightarrow X_{s+d}\gamma)= (2.2 \pm 3.9 \pm 0.9) \%$, consistent with the Standard Model prediction.