Abstract
In models with more than one Higgs doublet, the Yukawa couplings of the d-type (u-type) quarks are changed by a factor A/sub d/ (A/sub u/), which is often a ratio of vacuum expectation values. Abbott, Sikivie, and Wise showed that the K/sub L/-K/sub S/ mass difference places stringent constraints on A/sub u/ (A/sub u/< or approx. =10 for a charged-Higgs-boson mass of m/sub W/); however, this bound is very sensitive to the charged-Higgs-boson mass and to small Kobayashi-Maskawa mixing angles. We note that if the enhancement is greater than 4--5, then the top quark's Yukawa coupling will exceed its strong coupling, even if m/sub t/approx.35 GeV. The effects of this enhancement on the e/sup +/e/sup -/ cross section, on energy levels, leptonic widths, and decay rate of t-quarkonium, and on top-quark decays are discussed; the former two effects are sensitive to neutral-scalar masses only, the latter to the charged-Higgs-boson mass; all of the effects are insensitive to small mixing angles. We also note that the best limit on A/sub d/ comes from recent data on the semileptonic branching ratio of b quarks.
Highlights
In models with more than one Higgs doublet, the Yukawa couplings of the d-type (u-type) quarks are changed by a factor Ad (A„), which is often a ratio of vacuum expectation values
Sikivie, and Wise showed that the KL-Kq mass difference places stringent constraints on (A„(10 for a charged-Higgs-boson mass of m~); this bound is very sensitive to the charged-Higgs-boson mass and to small Kobayashi-Maskawa mixing angles
Two-doublet models have increased in popularity; the simplest supersymmetric models and models with a Peccei-Quinn symmetry all contain two doublets
Summary
— enhancement is greater than 4 5, the top quark's Yukawa coupling will exceed its strong coupling, even if m, -35 GeV The effects of this enhancement on the e+e cross section, on energy levels, leptonic widths, and decay rate of t-quarkonium, and on top-quark decays are discussed; the former two effects are sensitive to neutral-scalar masses only, the latter to the charged-Higgs-boson mass; all of the effects are insensitive to small mixing angles. As discussed by ASW, there are two ways to couple the Higgs doublets to quarks without very large flavor-changing neutral currents: (i) couple one doublet, Ni, to all quarks and the other, @2, to no quarks (using a discrete symmetry) or (ii) couple @& to u-type quarks and 42 to d-type quarks The latter choice occurs automatically in all supersymmetric theories and in all models with Peccei-Quinn symmetry, we will concentrate on that choice (it would be easy to generalize our results to the other choice). The value of R is given by [in the modified minimalsubtraction (MS ) scheme]
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