Measuring beta_s with Bs -> K0(*) K0bar(*) -- a Reappraisal

TL;DR

This paper reevaluates the accuracy of measuring the Bs mixing phase beta_s using B decays, highlighting potential large theoretical errors and proposing methods to better estimate SU(3) breaking effects.

Contribution

It critically re-examines the CPS method for extracting beta_s, showing that theoretical errors can be significantly larger than previously estimated.

Findings

Theoretical error in beta_s extraction can reach up to 18 degrees.

Large SU(3) breaking effects can impact the measurement accuracy.

Knowledge of SU(3) breaking reduces the theoretical uncertainty.

Abstract

The Bs-Bsbar mixing phase, beta_s, can be extracted from Bs -> K0(*) K0bar(*), but there is a theoretical error if the second amplitude, Vub* Vus P'uc, is non-negligible. Ciuchini, Pierini and Silvestrini (CPS) have suggested measuring Puc in Bd -> K0(*) K0bar(*), and relating it to P'uc using SU(3). For their choice of the direct and indirect CP asymmetries in Bd -> K0(*) K0bar(*), they find that the error on beta_s is very small, even allowing for 100% SU(3) breaking. In this paper, we re-examine the CPS method, allowing for a large range of the Bd -> K0(*) K0bar(*) observables. We find that the theoretical error in the extraction of beta_s can be quite large, up to 18 degrees. This problem can be ameliorated if the value of SU(3) breaking were known, and we discuss different ways, both experimental and theoretical, of determining this quantity.