/ 98 09 59 0 v 1 3 0 Se p 19 98 SCALAR - MEDIATED FLAVOR - CHANGING NEUTRAL CURRENTS MARC SHER

The simplest extension of the standard model involves adding a scalar doublet–the so-called two-Higgs model. In general, the additional scalar will mediate tree-level flavor-changing neutral currents. Although one can arbitrarily impose a discrete symmetry to avoid these, it isn't necessary to do so; reasonable assumptions about the size of the flavor-changing couplings can make them sufficiently small as to avoid problems in the kaon sector. However, these same assumptions give much larger effects in the third-family case. We discuss the model, the " reasonable assumptions " on the size of the couplings, and examine phenomenological bounds on the couplings, showing that the most promising signatures are from Bs → µτ , µ → eγ, etc. We then include the newest result which shows potentially significant effects on the anomalous magnetic moment of the muon. One of the simplest, and most-often studied, extensions of the standard model is the " two-Higgs doublet " model, in which an additional scalar doublet is included. A potentially serious problem with such models is the existence of tree-level flavor-changing neutral currents. Although one can eliminate such currents with a discrete symmetry, such a symmetry is ad hoc, and may have cosmological problems. In this talk, I'll point out that the problem of flavor-changing neutral currents (FCNC) may not be as serious as often believed, and that reasonable assumptions about the structure of Yukawa coupling matrices allow sufficient suppression of FCNC couplings between the first two generations, while simultaneously predicting much bigger effects in coupling the second and third generations. The phenomenological bounds will be reviewed and the most promising decays examined. Finally , a new bound, from the anomalous magnetic moment of the muon, will be discussed. It is easy to see why FCNC do not appear in the standard model. The Yukawa interactions in the standard model are of the form (focusing on the neutral fields only) L Y = − h ij √ 2 ψ i ψ j φ (1) After spontaneous symmetry breaking, this gives a mass matrix M ij = h ij √ 2 φ (2) Clearly, when the mass matrix is diagonalized, the Yukawa coupling matrix will also be diagonalized, and thus there will be no neutral flavor-changing couplings involving the Higgs boson. In the two doublet model, this is not the case. The Yukawa interactions are of the form