Flavor Changing Neutral Currents and the Third Family

We consider a Two Higgs Doublet Model with Flavor Changing Scalar Neutral Currents arising at the tree level. All the most important constraints are taken into account and the compatibility with the present Electroweak measurements is examined. The Flavor Changing couplings involving the third family are not constrained to be very small and this allows us to predict some interesting signals of new physics. (This paper relies on some work done in collaboration with D. Atwood (CEBAF) and A. Soni (BNL)). to appear in the Proceedings of the XXXIst Rencontres de Moriond, “Electoweak Interactions”, Les Arcs, France, March 1996. All processes involving Flavor Changing Neutral Currents (FCNC) are suppressed in the Standard Model (SM) because they are forbidden at the tree level. Some of them end up having a measurable, although small, branching fraction since they are enhanced at the loop level by the presence of a top quark in the loop. This is the case of some radiative B-meson decays, like those induced at the parton level by b → sγ (Br(B → Xsγ) ∼ 10 [1]). However, a similar enhancement cannot take place for the up-type FC transitions and therefore this can be a good place to look for evidence of new physics. Moreover, the outstanding nature of the top quark (with its huge mass, mt ∼ 175 GeV) should induce us to reexamine our theoretical prejudices about the existence of Flavor Changing Scalar Interactions (FCSI), expecially for the top quark itself. Probing the top-charm and top-up flavor changing vertex consequently deserves a special attention. We will present a theoretical model in which FCSI can be generated at the tree level with a given hierarchy and discuss some possible experimental environments in which definite bounds on the top quark FC couplings can be put. We will consider a Two Higgs Doublet Model (2HDM) with allowed FCNC in the scalar sector, the so called Model III [2]. In fact, in models with a non-minimal Higgs sector, e.g. in the 2HDM, FCSI arise readily at the tree level. In order to avoid the severe constraints from K−K̄ and B− B̄ mixing, it was originally proposed [3] to forbid all FCSI by imposing a suitable discrete symmetry acting on the quark and the scalar fields [4]. However, as later realized by many authors [5], it is possible to remove the ad hoc discrete symmetry and satisfy the constraints by chosing an adequate ansatz on the FC couplings. In particular, it was observed that the necessary hierarchy on the FC couplings between fermions and scalars is provided by the mass parameters of the fermion fields themselves [5]. If this is the case, then the top quark FC couplings can be greatly enhanced with respect to the first and second generation ones. In some recent papers [6, 7, 8], we have analyzed in detail this kind of 2HDM and studied the possible phenomenological implications that large FC top couplings can have. Due to the theoretical and experimental interest of this analysis, we want to provide a brief but comprehensive description of Model III and of the most important constraints that affect its FCSI. Given the constrained Model, we will proceed to the discussion of some clean experimental environments in which signals from FCSI can be detected. Let us focus on the quark Yukawa interactions only and write the corresponding Yukawa Lagrangian for a 2HDM in the following very general form [2] LY = η ijQ̄iφ̃1Uj + η ijQ̄iφ1Dj + ξ ijQ̄iφ̃2Uj + ξ ijQ̄iφ2Dj + h.c. (1) where φi for i = 1, 2 are the two scalar doublets, while η U,D ij and ξ U,D ij are the non diagonal coupling matrices. By a suitable rotation of the fields we chose the physical scalars in such a way that only the ηU,D ij couplings generate the fermion masses, i.e. such that