b --> s gamma using a Sum of Exclusive Modes

TL;DR

This study measures the inclusive radiative decay b->s gamma using a semi-inclusive approach that sums 12 exclusive final states to reconstruct the photon-energy spectrum from the hadronic-mass spectrum. The hadronic-mass spectrum is interpreted with the Kagan-Neubert HQET framework to extract heavy-quark parameters mb and lambda_1 and to determine the inclusive branching fraction. The analysis yields a first moment constraint on LambdaBar and, after fits, an inclusive branching fraction of B(b->s gamma) = (4.3 ± 0.5 (stat) ± 0.8 (syst) ± 1.3 (model)) × 10^-4, with mb around 4.79 GeV/c^2 and lambda_1 in a negative range; results are compatible with prior measurements and provide HQET constraints valuable for Vcb and Vub extractions. The work demonstrates a precise semi-inclusive methodology for flavor-changing neutral current processes at BaBar and informs future determinations of CKM elements.

Abstract

This paper describes preliminary results on the inclusive process b --> s gamma obtained from 20.7 fb^-1 of data recorded with the BaBar detector during 1999-2000. Signal event yields are found from a combination of twelve exclusive decay channels after subtracting continuum and BBbar backgrounds. Cross-feed from incorrectly reconstructed b --> s gamma events is also removed. Branching fractions in bins of hadronic mass are calculated using corrected Monte Carlo signal efficiencies; this is equivalent to measuring the gamma energy spectrum. We measure the first moment of the gamma energy spectrum constraining the HQET parameter LambdaBar = (0.37 +/- 0.09 (stat) +/- 0.07 (syst) +/- 0.10 (model)) GeV/c^2. A fit to the hadronic mass spectrum gives B(b --> s gamma) = (4.3 +/- 0.5 (stat) +/- 0.8 (syst) +/- 1.3 (model))x 10^-4 for the inclusive branching fraction. We also constrain the HQET parameter lambda_1.

Figures (8)

• Figure 1: Expected continuum and $B\overline{ B}{}\xspace$ background events in 20.7 $\hbox{,fb}^{-1}$ from Monte Carlo (MC) simulation as a function of the hadronic mass.
• Figure 2: The fits to the data in bins of $M_{Had}$ between 0.8 and 2.4 ${\mathrm{\,Ge V\!/}c^2}$.
• Figure 3: Signal yields (left plot) and weighted partial branching fractions (right plot), calculated with corrected efficiencies. Statistical errors only are shown.
• Figure 4: Branching fraction as a function of $M_{Had}$. The errors are purely statistical.
• Figure 5: Branching fraction as a function of $E_{\gamma}$. The errors are purely statistical.