Measurements of the B -> X_s gamma Branching Fraction and Photon Spectrum from a Sum of Exclusive Final States

TL;DR

This BaBar analysis measures the inclusive ${\cal B}(b \rightarrow s \gamma)$ for $E_\gamma>1.9\,\mathrm{GeV}$ by summing 38 exclusive ${X_s}$ final states, achieving a precise result and providing a photon-energy spectrum with moments to extract heavy-quark parameters. The study employs a sophisticated signal model anchored in the Kagan–Neubert framework, detailed fragmentation corrections, and a multi-component fit to the beam-energy-substituted mass $m_{ES}$ to separate signal from backgrounds and cross-feed. The photon spectrum is found to be well described by both shape-function and kinetic schemes, yielding $m_b$ around $4.65$–$4.68\,\mathrm{GeV}$ and $\mu_\pi^2$ in the range $0.12$–$0.32\,\mathrm{GeV}^2$, with strong anti-correlation between the two parameters. Isospin asymmetry is measured to be consistent with zero, and the results are compatible with semileptonic $B$ decays, reinforcing SM predictions and providing inputs for $|V_{ub}|$ extractions.

Abstract

Using 88.9 million BB events collected by the BaBar detector at the Y(4S), we measure the branching fraction for the radiative penguin process B -> X_s gamma from the sum of 38 exclusive final states. The inclusive branching fraction above a minimum photon energy E_gamma > 1.9 GeV is BF (b -> s gamma) = (3.27 +/- 0.18 (stat.) +0.55/-0.40 (syst.) +0.04/-0.09 (theory)) 10^-4. We also measure the isospin asymmetry between B^- -> X_s ubar gamma and B^0bar -> X_s dbar gamma to be Delta_0- = -0.006 +/- 0.058 (stat.) +/- 0.009 (syst.) +/- 0.024 (B^0bar / B^-). The photon energy spectrum is measured in the B rest frame, from which moments are derived for different values of the minimum photon energy. We present fits to the photon spectrum and moments which give the heavy-quark parameters m_b and mu_pi^2. The fitted parameters are consistent with those obtained from semileptonic B -> X_c l nu decays, and are useful inputs for the extraction of Vub from measurements of semileptonic B -> X_u l nu decays.

Figures (7)

• Figure 1: Efficiency for correctly reconstructing a signal event in one of the 38 final states as a function of hadronic mass. Note that this efficiency does not include the missing fractions of $B\rightarrow\xspace X_{s}\gamma$ final states.
• Figure 2: Fits to the $M(X_s)=1.4-1.5\,\mathrm{\,Ge V}\xspace$ bin for (a) correctly reconstructed signal Monte Carlo events; (b) simulated continuum background; (c) simulated cross-feed and hadronic $B$ decay backgrounds, where the contributions from the peaking (dotted) and combinatorial (dashed) backgrounds are shown separately; (d) on-peak data, where the contributions from the signal (dotted-dashed), continuum background(dotted), peaking $B$ background (long-dashed), and combinatorial $B$ background (dashed) are shown separately.
• Figure 3: On-peak data fit to the full $M(X_s)$ range, $M(X_s)=0.6-2.8\,\mathrm{\,Ge V}\xspace$ bin, where the contributions from the signal shape (dotted-dashed), fixed continuum ARGUS shape (dotted), peaking background shape (long-dashed), and combinatorial $B\overline{ B}{}\xspace$ background shape (dashed) are shown separately.
• Figure 4: Missing fractions with systematic errors as a function of the hadronic mass.
• Figure 5: The hadronic mass spectrum (a), and the photon energy spectrum (b). The data points are compared to theoretical predictions (histograms) obtained using the shape function (solid line) and kinetic (dashed line) schemes.