Sensitivity to charged scalars in B → D (*) τν τ and B → τν τ decays

Abstract

We analyze the recent experimental evidence for an excess of τ -lepton production in several exclusive semileptonic B-meson decays in the context of two-Higgs-doublet models. These decay modes are sensitive to the exchange of charged scalars and constrain strongly their Yukawa interactions. While the usual Type-II scenario cannot accommodate the recent BaBar data, this is possible within more general models in which the charged-scalar couplings to up-type quarks are not as suppressed. Both the B → D ( * ) τ ν τ and the B → τ ν τ data can be fitted within the framework of the Aligned Two-Higgs-Doublet Model, but the resulting parameter ranges are in conflict with the constraints from leptonic charm decays. This could indicate a departure from the family universality of the Yukawa couplings, beyond their characteristic fermion mass dependence. We discuss several new observables that are sensitive to a hypothetical charged-scalar contribution, demonstrating that they are well suited to distinguish between different scenarios of new physics in the scalar sector, and also between this group and models with different Dirac structures; their experimental study would therefore shed light on the relevance of scalar exchanges in semileptonic b → c τ −ν τ transitions.

Sterile neutrinos in leptonic and semileptonic decays

“…Similarly, the authors of ref. [81] found regions in the parameter space of the Aligned 2HDM able to accommodate the R(D * ) measurement, although in conflict with the constraints from leptonic charm decays.…”

“…7 It is worth mentioning that the BaBar excess of events in B → D * τ ν decays, revealing a 3.4 σ deviation from the SM prediction [80], cannot be accommodated by contributions from charged Higgs bosons in the context of type-II Two Higgs Doublet Model (2HDM) [81]. In fact, BaBar excludes the latter contributions as an explanation of the deviations in R(D) and R(D * ) from their SM predictions [82].…”

Status of B to D(*) τ ν decays in two-Higgs-doublet models

“…Table 2: SM predictions and experimental values for the different observables considered in the analysis, see Ref. [10] for details. The first uncertainty corresponds to the statistical error, and the second, when present, to the theoretical error.…”

“…[4,10,[15][16][17][18]. These observables can be useful to disentangle different kinds of new physics in b → cτν transitions and more importantly to our discussion, they can be used to test for charged scalar contributions independently of the flavour structure of its fermionic couplings.…”

“…The processes considered are listed in Table 2, a more detailed discussion of the results presented here can be found in Refs. [9,10].…”