Determination of the Branching Fraction for Inclusive Decays B -> X_{s} gamma

TL;DR

This work measures the inclusive branching fraction ${\cal B}(B \rightarrow X_s \gamma)$, a rare radiative penguin process, using a fully inclusive BaBar approach with a lepton tag from the other $B$ to suppress backgrounds. The analysis relies on off-resonance data to subtract continuum and MC-controlled $B\overline{B}$ backgrounds, with control samples to constrain modeling uncertainties, and defines a signal region of $2.1< E^{*}_{\gamma}<2.7$ GeV. The final result, ${\cal B}(B \rightarrow X_s \gamma) = (3.88 \pm 0.36_{stat} \pm 0.37_{syst} \pm^{0.43}_{0.23}_{\mathrm{model}})\times 10^{-4}$, is consistent with Standard Model predictions and constrains possible new-physics contributions to the electromagnetic penguin amplitude. The analysis demonstrates how inclusive photon-tagged techniques, careful background estimation, and model-dependent corrections can yield a precise probe of rare $B$ decays and guide future improvements with larger data samples.

Abstract

We present a preliminary determination of the inclusive branching fraction for the rare radiative penguin transition B -> X_{s} gamma. The measurement is based on a data sample of 60 million BB pairs collected between 1999 and 2001 with the BaBar detector at the PEPII asymmetric-energy e+e- B Factory at SLAC. We study events containing a high-energy photon from one B (or Bbar) decay and a tagging primary lepton from the decay of the other B meson. By this means, we are able to reduce a significant component of the background without introduction of model dependent uncertainties in the event selection efficiency. We determine the branching fraction BR(B->X_{s} gamma)=3.88 +-0.36(stat.)+-0.37(sys.) +0.43-0.23(model.)x10^{-4}, which is consistent with Standard Model predictions and provides a constraint on possible new physics contributions to the electromagnetic penguin amplitude in B decays.

Figures (5)

• Figure 1: The energy distribution, in the $焇{(4S)}$ center of mass, of simulated photon candidates, after "photon quality" cuts designed to reduce backgrounds from $\pi^0$s, $\eta$s and hadrons, but before other selection requirements. Shown are $B \rightarrow\xspace X_{s} \gamma$ signal (dark shading), $B\overline{ B}{}\xspace$ background (grey shading) and continuum background (unshaded), all normalized to $$ 54.6 $\hbox{,fb}^{-1}$ . The high-end tail for $B\overline{ B}{}\xspace$ is mainly due to residual hadrons.
• Figure 2: Left: The reconstructed $E^{*}_{\gamma}$ distribution expected from Monte Carlo simulation after the selection criteria. The $B \rightarrow\xspace X_{s} \gamma$ signal assuming $$$\cal B$($B \rightarrow\xspace X_{s} \gamma$)=3.45 10^-4 (dark shading), $B\overline{ B}{}\xspace$ background (grey shading) and continuum background (unshaded) are normalized to $$ 54.6 $\hbox{,fb}^{-1}$ . Right: The generated $E^{*}_{\gamma}$ spectrum before cuts (arbitrary normalization) for different values of the $b$ quark mass $m_{b} \xspace$, using the model of Kagan and Neubert bib:smtheory. Our signal region is defined for the corresponding reconstructed quantity as $$ 2.1 < $E^{*}_{\gamma}$ < 2.7$\mathrm{\,Ge V}$.
• Figure 3: Left: The $E^{*}_{\gamma}$ distribution for the $\pi^0\xspace$ and $\eta$ anti-veto control sample for data (points) compared to the Monte Carlo prediction (solid line). Right: Corresponding distribution for the hadron anti-veto control sample.
• Figure 4: The $E^{*}_{\gamma}$ distribution of on-resonance data (solid points) compared to background expectation. All errors are statistical only (including, just for this figure, $B\overline{ B}{}\xspace$ background statistics). Bin-by-bin systematic uncertainties and correlations have not yet been studied; the systematics quoted in the text apply only to an integral measurement from 2.1 to 2.7$\mathrm{\,Ge V}$.
• Figure 5: The B A B A R measurement compared to previous experiments bib:otherexps and to theoretical predictions bib:smtheory.