$b\to sγ$ Branching Fraction and CP Asymmetry

TL;DR

This work measures the inclusive branching fraction for the radiative decay $b \rightarrow s \gamma$ and searches for CP asymmetry using the CLEO detector at CESR. It combines two robust background suppression strategies—a neural-network–based shape analysis and a pseudo-reconstruction with a chi-square–driven flavor tag—to extract the signal in the photon energy window $2.1 < E_\gamma < 2.7$ GeV and uses off-resonance data to subtract continuum backgrounds. The result is $\mathcal{B}(b \rightarrow s \gamma) = (3.15 \pm 0.35_{\text{stat}} \pm 0.32_{\text{syst}} \pm 0.26_{\text{model}})\times 10^{-4}$, compatible with the SM prediction, with a conservative 95% CL range of $(2.0-4.5)\times 10^{-4}$. The CP asymmetry is found to be consistent with zero, with a 90% CL limit of $-0.09 < \mathcal{A} < 0.42$, placing constraints on new physics scenarios. The analysis demonstrates effective background control and sets the stage for exploiting the full data set and potential lepton-tagging to further improve precision.

Abstract

We present preliminary results on $b \to s γ$ from the CLEO experiment. An updated result on the branching fraction is reported at $\cal{B}$$(b \to s γ) = (3.15 \pm 0.35 \pm 0.32 \pm 0.26)\times 10^{-4}$, where the first uncertainty is statistical, the second is systematic, and the third for model dependence. We also describe a new analysis performed to search for CP asymmetry in $b \to s γ$ decays. We observe no such asymmetry and set conservative limits at $-0.09 <$ $\cal{A}$ $< 0.42$.

Figures (1)

• Figure 1: Photon energy spectra. (a) On-resonance data (points), luminosity scaled off-resonance data (double hatched), and $B\bar{B}$ background (single hatched). (b) Background-subtracted data (points) and Monte Carlo prediction of the shape of the $b \rightarrow s \gamma$ signal from a spectator model calculation ($<m_b> = 4.88 \text{ GeV}/c^2$) and Fermi momentum set at 250 MeV/$c$. Only statistical uncertainties are shown.