Observation of B+ --> K1(1270)+ gamma

TL;DR

This paper reports the first observation of the radiative decay B+ --> K1(1270)+ gamma with a significant branching fraction, using data from the Belle detector, and sets limits on related decay modes.

Contribution

The study provides the first measurement of the branching fraction for B+ --> K1(1270)+ gamma decay, confirming its existence and quantifying its rate.

Findings

Branching fraction for B+ --> K1(1270)+ gamma is (4.3 +- 0.9) x 10^-5.

No significant signal for B+ --> K1(1400)+ gamma was observed.

Upper limit set for B+ --> K1(1400)+ gamma at 1.5 x 10^-5.

Abstract

We report the observation of the radiative decay B+ --> K1(1270)+ gamma using a data sample of 140 fb-1 taken at the Upsilon(4S) resonance with the Belle detector at the KEKB e+e- collider. We find the branching fraction to be Br(B+ --> K1(1270)+ gamma) = (4.3 +- 0.9(stat.) +- 0.9(syst.)) x 10-5 with a statistical significance of 7.3sigma. We find no significant signal for B+ --> K1(1400)+ gamma and set an upper limit Br(B+ --> K1(1400)+ gamma) < 1.5 x 10-5 at the 90% confidence level. We also measure inclusive branching fractions for B+ --> K+ pi+ pi- gamma and B0 --> K0 pi+ pi- gamma in the mass range 1 GeV/c^2 < M(K+(0)pi+pi-) < 2 GeV/c^2.