Search for lepton flavor violation via the intense high-energy muon beam

A deep inerastic scattering process \mutau is discussed to study lepton flavor violation between muons and tau leptons. In supersymmetric models, the Higgs boson mediated diagrams could be important for this reaction. We find that at a muon energy ($E_{\mu}$) higher than 50 GeV, the predicted cross section significantly increases due to the contribution from sea $b$-quarks. The number of produced tau leptons can be $\mathcal{O}(10^4)$ at $E_{\mu}$= 300 GeV from $10^{20}$ muons, whereas $\mathcal{O}(10^2)$ events are given at $E_{\mu}= 50$ GeV.Comment: Contribution to the 6th International Workshop on Neutrino Factories & Superbeams(NuFact04), Jul. 26-Aug. 1, 2004, Osaka Univerisity, Osaka, Japan, talk given by S.K., to appear in the Proceedings, 3 pages, 4 figure

Mass bounds of the lightest CP-even Higgs boson in the two-Higgs-doublet model

The upper and the lower bounds of the lightest CP-even Higgs-boson mass are discussed in the two-Higgs-doublet model (2HDM) with a softly-broken discrete symmetry. They are obtained as a function of a cut-off scale $\Lambda$ ($\leq 10^{19}$ GeV) by imposing the conditions in which the running coupling constants neither blow up nor fall down below $\Lambda$. In comparison with the standard model (SM), although the upper bound does not change very much, the lower bound is considerably reduced. For $\Lambda = 10^{19}$ GeV and $m_t = 175$ GeV, the mass is not bounded from below in Model I of the 2HDM, whereas in Model II the mass less than about 100 GeV is excluded because of the $b \to s lighter than others, the lower bound is given by about 100 GeV for Model I and II, which is smaller by about 40 GeV than the corresponding lower bound in the SM

Large Solar Neutrino Mixing in an Extended Zee Model

The Zee model, which employs the standard Higgs scalar ($\phi$) with its duplicate ($\phi^\prime$) and a singly charged scalar ($h^+$), can utilize two global symmetries associated with the conservation of the numbers of $\phi$ and $\phi^\prime$, $N_{\phi,\phi^\prime}$, where $N_\phi+N_{\phi^\prime}$ coincides with the hypercharge while $N_\phi-N_{\phi^\prime}$ ($\equiv X$) is a new conserved charge, which is identical to $L_e-L_\mu-L_\tau$ for the left-handed leptons. Charged leptons turn out to have $e$-$\mu$ and $e$-$\tau$ mixing masses, which are found to be crucial for the large solar neutrino mixing. In an extended version of the Zee model with an extra triplet Higgs scalar (s), neutrino oscillations are described by three steps: 1) the maximal atmospheric mixing is induced by democratic mass terms supplied by $s$ with $X$=2 that can initiate the type II seesaw mechanism for the smallness of these masses; 2) the maximal solar neutrino mixing is triggered by the creation of radiative masses by $h^+$ with $X$ = 0; 3) the large solar neutrino mixing is finally induced by a $\nu_\mu$-$\nu_\tau$ mixing arising from the rotation of the radiative mass terms as a result of the diagonalization that converts $e$-$\mu$ and $e$-$\tau$ mixing masses into the electron mass.Comment: RevTex, 10 pages including one figure page, to be published in Int. J. Mod. Phys. A (2002

Phenomenology of Higgs bosons in the Zee-Model

To generate small neutrino masses radiatively, the Zee-model introduces two Higgs doublets and one weak-singlet charged Higgs boson to its Higgs sector. From analyzing the renormalization group equations, we determine the possibile range of the lightest CP-even Higgs boson ($h$) mass and the Higgs boson self-couplings as a function of the cut-off scale beyond which either some of the coupling constants are strong enough to invalidate the perturbative analysis or the stability of the electroweak vacuum is no longer guaranteed. Using the results obtained from the above analysis, we find that the singlet charged Higgs boson can significantly modify the partial decay width of $h \to \gamma \gamma$ via radiative corrections, and its collider phenomenology can also be drastically different from that of the charged Higgs bosons in the usual two-Higgs-doublet models.Comment: Added a paragraph and a figure in Section V, corrected typos, added references. (RevTeX, 45 pages, 16 figures included.) To appear in Physical Review

Sum rules for e+e−→W+W−e^+e^- \to W^+W^- helicity amplitudes from BRS invariance

The BRS invariance of the electroweak gauge theory leads to relationships between amplitudes with external massive gauge bosons and amplitudes where some of these gauge bosons are replaced with their corresponding Nambu-Goldstone bosons. Unlike the equivalence theorem, these identities are exact at all energies. In this paper we discuss such identities which relate the process $e^+e^- \to W^+W^-$ to $W^\pm\chi^\mp$ and $\chi^+\chi^-$ production. By using a general form-factor decomposition for $e^+e^- \to W^+W^-$, $e^+e^- \to W^\pm \chi^\mp$ and $e^+e^- \to \chi^+\chi^-$ amplitudes, these identities are expressed as sum rules among scalar form factors. Because these sum rules may be applied order by order in perturbation theory, they provide a powerful test of higher order calculations. By using additional Ward-Takahashi identities we find that the various contributions are divided into separately gauge-invariant subsets, the sum rules applying independently to each subset. After a general discussion of the application of the sum rules we consider the one-loop contributions of scalar-fermions in the Minimal Supersymmetric Standard Model as an illustration.Comment: 37 pages, including 16 figure