Exclusive radiative B-meson decays within minimal flavour violating 2HDMs

Abstract

In the "Higgs basis" for a generic 2HDM, only one doublet gets a nonzero vacuum expectation value and, under the criterion of minimal flavour violation, the other one is fixed to be either colour-singlet or colour-octet, referred respectively as the type-III and the type-C model. Both of them can naturally avoid large FCNC transitions and provide very interesting phenomena in some low-energy processes. In this paper, we study their effects on exclusive radiative B-meson decays due to the exchange of colourless or coloured charged Higgs. It is found that, while constraints from the branching ratios are slightly weaker than the ones from the inclusive $B\to X_{s} \gamma$ decay, the isospin asymmetries in exclusive decays provide very complementary bounds on the model parameters. As the two models predict similar corrections to the dipole coefficient $C_7^{\rm eff}$ whereas similar magnitudes but with opposite signs to $C_8^{\rm eff}$, the branching ratios cannot discriminate the two models, and we have to resort to the direct CP and isospin asymmetries of $b\to s$ processes, which are more sensitive to $C_8^{\rm eff}$. Due to the CKM factors $|\lambda_u^{(d)}|\sim|\lambda_t^{(d)}|$, the terms proportional to $\lambda_u^{(d)}$ make the observables of $b\to d$ processes exhibit a different dependence on the possible new physics phase. In addition, correlations between the various observables in the exclusive $B\to V\gamma$ and the inclusive $B\to X_{s,d}\gamma$ decays are investigated, which could provide further insights into the models with more precise experimental measurements and theoretical predictions for these decays.