Unitarity Triangle Without Semileptonic Decays

TL;DR

This paper proposes a novel approach to constraining the Unitarity Triangle that avoids semi-leptonic decays, using alternative measurements like epsilon_K, BR(B -> tau nu), and Delta M_Bs, revealing persistent tensions with the Standard Model.

Contribution

The paper introduces a new method for constraining the Unitarity Triangle that bypasses the challenging semi-leptonic decay calculations.

Findings

Tensions with the Standard Model remain at ~1.8 sigma.

Improving B -> tau nu measurements enhances the method's effectiveness.

Refinements in lattice calculations of f_Bs can significantly improve constraints.

Abstract

Use of semi-leptonic decays has become standard in constraining the Unitarity Triangle. Bearing in mind that precise calculations of these are very challenging, we propose an entirely new approach. In particular the |Vcb| + epsilon_K constraint, which depends extremely sensitively on Vcb in the traditional method, is replaced by the interplay between epsilon_K, BR(B ->tau nu) and Delta M_Bs. It is found that even in this method tensions with the Standard Model persist at the ~1.8 sigma level. Furthermore, improvements on the B -> tau nu branching ratio and on the lattice determination of f_Bs Bs^(1/2) can increase the effectiveness of this method significantly.