Measurement of Branching Fraction and First Evidence of CP Violation in B0 --> a1+-(1260) pi-+ Decays

TL;DR

This paper reports the measurement of the branching fraction and provides the first evidence of CP violation in B0 decays to a1(1260) pi, using a large data sample from the Belle experiment, with significant findings on mixing-induced CP violation.

Contribution

It presents the first evidence of CP violation in B0 --> a1(1260) pi decays and measures the branching fraction using the full Belle dataset, including an upper limit on a related decay mode.

Findings

Evidence of mixing-induced CP violation at 3.1 sigma

Dominance of a1+-(1260) decay rate without the spectator quark at 4.1 sigma

No significant direct CP violation observed

Abstract

We present a measurement of the branching fraction and time-dependent CP violation parameters in B0 --> a_1^+-(1260) pi^-+ decays. The results are obtained from the final data sample containing 772x10^6 BBbar pairs collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We also measure an upper limit on the product branching fraction for a possible decay with the same final state B0 --> a_2^+-(1320) pi^-+. In a time-dependent measurement to extract CP asymmetries, we find evidence of mixing-induced CP violation in B0 --> a_1^+-(1260) pi^-+ decays with a 3.1 sigma significance and the rate where the a_1^+-(1260) does not contain the spectator quark is found to dominate the rate where it does at the 4.1 sigma level. However, there is no evidence for either time and flavor integrated direct CP violation or flavor-dependent direct CP violation.