Abstract
We analyse the constraints and some of the phenomenological implications of a class of two Higgs doublet models where there are flavour-changing neutral currents (FCNC) at tree level but the potentially dangerous FCNC couplings are suppressed by small entries of the CKM matrix $V$. This class of models have the remarkable feature that, as a result of a discrete symmetry of the Lagrangian, the FCNC couplings are entirely fixed in the quark sector by $V$ and the ratio $v_2 /v_1$ of the vevs of the neutral Higgs. The discrete symmetry is extended to the leptonic sector, so that there are FCNC in the leptonic sector with their flavour structure fixed by the leptonic mixing matrix. We analyse a large number of processes, including decays mediated by charged Higgs at tree level, processes involving FCNC at tree level, as well as loop induced processes. We show that in this class of models one has new physical scalars beyond the standard Higgs boson, with masses reachable at the next round of experiments.
Highlights
A discrete symmetry leading to natural flavour conservation (NFC) [7]
A very interesting alternative to NFC is provided by the so-called BGL models [14,15,16], where there are non-vanishing flavour-changing neutral currents (FCNC) at tree level, but they are naturally suppressed as a result of an exact symmetry of the Lagrangian, which is spontaneously broken by the vevs of the neutral Higgs
In the previous section we have presented a large set of relevant observables that can constrain the different BGL models, excluding regions of the parameter space {tan β, mH±, mR, mI } where the New Physics (NP) contributions are not compatible with the available experimental information
Summary
We consider the extension of the SM consisting of the addition of two Higgs doublets as well as three right-handed neutrinos. This leads, in a natural way, to a very strong suppression in the FCNC entering in the “dangerous” ∆S = 2 processes contributing to K0 − K 0 transitions In this variant of BGL models, the couplings entering in the tree level ∆S = 2 transition are proportional to |VtdVt∗s| leading to a λ10 suppression in the Higgs mediated ∆S = 2 transition, where λ ≈ 0.2 denotes the Cabibbo parameter. With this strong suppression even light neutral Higgs, with masses of the order 102 GeV are allowed. We impose present constraints from several relevant flavour observables, as specified
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