Measurement of the $B\to X_s\gamma$ branching fraction and photon energy spectrum using the recoil method

We present a measurement of the branching fraction and photon-energy spectrum for the decay $B\to X_s\gamma$ using data from the BABAR experiment. The data sample corresponds to an integrated luminosity of $210{\mathrm{fb}}^{-1}$, from which approximately 680 000 $B\overline{B}$ events are tagged by a fully reconstructed hadronic decay of one of the $B$ mesons. In the decay of the second $B$ meson, an isolated high-energy photon is identified. We measure $\mathcal{B}(B\to X_s\gamma )=(3.66\pm {0.85}_{\mathrm{stat}}\pm {0.60}_{\mathrm{syst}})\times {10}^{-4}$ for photon energies $E_{\gamma }$ above 1.9 GeV in the $B$ rest frame. From the measured spectrum we calculate the first and second moments for different minimum photon energies, which are used to extract the heavy-quark parameters $m_{\mathrm{b}}$ and ${\mu }_{\pi }^2$. In addition, measurements of the direct $CP$ asymmetry and isospin asymmetry are presented.

Figure 1

Fits to the distribution of the beam-energy-substituted mass ${\mathrm{m}}_{\mathrm{ES}}$ for two $E_{\gamma }$ regions. The dashed curve shows the CB term and the dotted curve is the ARGUS term, corresponding to $B$ and non-$B$ events, respectively; the solid curve is their sum. (a) $1.6\mathrm{GeV}<E_{\gamma }<1.7\mathrm{GeV}$ for the charged $B$ sample. (b) $2.3\mathrm{GeV}<E_{\gamma }<2.4\mathrm{GeV}$ for the neutral $B$ sample. (c) The measured numbers of $B$ events as a function of photon energy. The points are from data; the histogram is from a $B\overline{B}$ MC sample which excludes the signal decay $B\to X\gamma$.

Figure 2

The partial branching fractions $(1/{\Gamma }_B)(d\Gamma /d{E}_{\gamma })$ with statistical (inner) and total (outer) uncertainties.