New Strategy for B_s Branching Ratio Measurements and the Search for New Physics in B^0_s -> mu^+ mu^-

TL;DR

This paper proposes a new method to measure the B_s -> mu^+ mu^- decay rate more precisely, enhancing the search for new physics by reducing uncertainties in fragmentation function ratios.

Contribution

It introduces a novel strategy using specific B meson decays to improve the precision of B_s decay measurements and the sensitivity to new physics signals.

Findings

Allows detection of new physics at twice the Standard Model rate with 5 sigma significance.

Provides a method to determine fragmentation function ratios with 20% theoretical precision.

Improves the new-physics reach of LHCb by a factor of two.

Abstract

The LHCb experiment at CERN's Large Hadron Collider will soon allow us to enter a new era in the exploration of B_s decays. A particularly promising channel for the search of "new physics" is B^0_s -> mu^+ mu^-. The systematic key uncertainty affecting the measurement of this -- and in fact all B_s-decay branching ratios -- is the ratio of fragmentation functions f_d/f_s. As the presently available methods for determining f_d/f_s are not sufficient to meet the high precision at LHCb, we propose a new strategy using \bar B^0_s -> D_s^+ pi^- and \bar B^0_d -> D^+ K^-. It allows us to obtain a lower experimental bound on BR(B^0_s -> mu^+ mu^-) which offers a powerful probe for new physics. In order to go beyond this bound and to determine f_d/f_s with a theoretical precision matching the experimental one it is sufficient to know the SU(3)-breaking correction to a form-factor ratio from non-perturbative methods at the level of 20%. Thanks to our strategy, we can detect new physics in B^0_s -> mu^+ mu^- at LHCb with 5 sigma for a branching ratio as small as twice the Standard-Model value, which represents an improvement of the new-physics reach by about a factor of two with respect to the current LHCb expectation.