Measurement of the Direct $CP$ Asymmetry in $b\to s\gamma$ Decays

We describe a measurement of the direct $CP$ asymmetry between inclusive $b\to s\gamma$ and $\overline{b}\to \overline{s}\gamma$ decays. This asymmetry is expected to be less than 0.01 in the standard model, but could be enhanced up to about 0.10 by new physics contributions. We use a sample of $89\times {10}^6$ $B\overline{B}$ pairs recorded with the BABAR detector at SLAC PEP-II, from which we reconstruct a set of 12 exclusive $b\to s\gamma$ final states containing one charged or neutral kaon and one to three pions. We measure an asymmetry of $A_{CP}(b\to s\gamma )=0.025\pm 0.050\left(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\right)\pm 0.015\left(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\right)$, corresponding to an allowed range of $-0.06<A_{CP}(b\to s\gamma )<+0.11$ at $90\%$ confidence level.

Figure 1

Monte Carlo simulations of the four contributions to the beam-energy substituted mass distribution of events selected as $b\to s\gamma$, with the corresponding fits: (a) signal, (b) continuum, (c) $B\overline{B}$ decays, and (d) cross feed. The plots are normalized to the luminosity of our data sample.

Figure 2

Fits to the beam-energy substituted mass distributions in data events for: (a) all $b\to s\gamma$, (b) all $\overline{b}\to \overline{s}\gamma$, (c) lepton-tagged $b\to s\gamma$, and (d) lepton-tagged $\overline{b}\to \overline{s}\gamma$ decays. Contributions are shown from peaking crystal ball (dot-dashed), fixed continuum ARGUS shape (dotted) and free $B\overline{B}$ and cross-feed ARGUS shape (dashed).