$B_s \to \mu^+ \mu^-$ in a Two-Higgs-Doublet Model with flavour-changing up-type Yukawa couplings

Abstract

We present a Two-Higgs-Doublet Model in which the structure of the quark Yukawa matrices is governed by three spurions breaking the flavour symmetries of the quark Yukawa sector. The model naturally suppresses flavour-changing neutral current (FCNC) amplitudes in the down-type sector, but permits sizable FCNC couplings in the up sector. We calculate the branching ratio of $B_s \to \mu^+ \mu^-$ to leading and next-to-leading order of QCD for the case with FCNC couplings of the heavy neutral Higgs bosons to up-type quarks and verify that all counterterms follow the pattern dictated by the spurion expansion of the Yukawa matrices. We find correlations between $B_s \to \mu^+ \mu^-$, $b\to s\gamma$, and the Higgs masses. The $B_s - \bar B_s$ mixing amplitude is naturally suppressed in the model but can probe a portion of the parameter space with very heavy Higgs bosons.