Charged Higgs effects on exclusive semi-tauonic <Emphasis Type="Italic">B </Emphasis> decays

We study effects of charged Higgs boson exchange in the B semileptonic decays /~ ~ D(*)z~. Both branching ratio and z polarization are examined. We use the recent experimental data on semileptonic B decays and the heavy quark effective theory in order to reduce theoretical uncertainty in the hadronic form factors. Theoretical uncertainty in the branching ratio is found to be rather small and that in the z polarization is almost negligible. Their measurements will give nontrivial constraints on the charged Higgs sector. The evidence for top quarks [1] leaves the Higgs sector as the only missing part of the standard model (SM). In the minimal SM, we have one Higgs doublet, which gives a neutral scalar particle as a physical state. An extension of the Higgs sector is an interesting possibility for new physics beyond the SM. One of the most attractive possibilities is the supersymmetric extension of the SM [2]. In the minimal supersymmetric SM, we have to introduce two Higgs doublets in order to cancel the anomaly and to give the fermions masses. Another important possibility is CP violation in the Higgs sector [3, 4]. It is known that CP can be broken in the Higgs sector if we have two or more Higgs doublets. Since the existence of one or more charged Higgs bosons is an inevitable consequence of the multi-Higgs-doublet extensions of the SM, the search for their effects is one of the key points in the quest for new physics. The most stringent experimental bound on the charged Higgs boson mass at present is mn>260GeV, given by the measurement of the inclusive radiative b quark decay b ~ s 7 [5]. This bound was found for the two-Higgs-doublet model of the "SUSY-type" Higgs couplings to fermions, called Model II [6]. Since this process takes place via l-loop diagrams, the bound may be changed depending on details of the model considered. If the model contains new particles other than the charged Higgs boson which contribute to the b ~ s7 process, the above lower bound may be modified. This is indeed the case in the minimal supersymmetric SM [7]. From this point of view, it is worthwhile to investigate charged Higgs boson effects in tree level processes which are less dependent on the other sectors of the multi-Higgs-doublet models. In this paper, we study effects of the charged Higgs boson on the branching ratio and the z polarization of the processes B ~ D(*)~ . These processes are expected to be much more sensitive to the charged Higgs sector than the semileptonic K decay processes because the Higgs couplings to fermions are proportional to the fermion mass. The 1% level branching ratio of these modes expected in the SM will give of order 106 semi-tauonic B decay events at the planned B factories. In N-Higgs-doublet models, we have N 1 physical charged Higgs bosons. Their couplings to quarks and leptons are described by the following interaction Lagrangian [8] : N YH = (2x/2GF) 1/2 ~ LVrMMDDR + Y,URMvVKMDL i=1 7 + ZiNLMEERJH~+ h.c.. (1) Here Hi + is the i-th lightest physical charged Higgs boson, UL(R) = (U, C, t)T(R), DL(R) = (d, s, b)T(R), NL = (Ve, V,, VO T, ER = (e, #, "C) r, (2) My = diag.(re,, mc, mr), Mo = diag.(ma, ms, mb), ME = diag.(me, m,, m,), (3) represent the quark and lepton fields and their masses respectively, and V/~M is the Kobayashi-Maskawa matrix. Note that the K M matrix which appears in the charged current mixing appears in the above Lagrangian. This is a consequence of the natural flavor conservation [9] which we implicitly assumed in Eq. (1) in order to suppress flavor changing neutral Higgs interactions.