An N=2 SUSY Gauge Model for Dynamical Breaking of the Grand Unified SU(5) Symmetry

We construct an extension of the recently proposed dynamical model for the breaking of SU(5)$_{\rm GUT}$ gauge symmetry, in which a pair of massless chiral supermultiplets for Higgs doublets are naturally obtained. We point out that a model at a specific point in the parameter space of superpotential is regarded as a low-energy effective theory of an N=2 supersymmetric gauge model for the strongly interacting hypercolor sector.Comment: 7 page

Dimension-six Proton Decays in the Modified Missing Doublet SU(5) Model

Dimension-five operators for nucleon decays are suppressed in the modified missing doublet (MMD) model in the supersymmetric SU(5) grand unification. We show that nonrenormalizable interactions decrease the unification scale in the MMD model which increases the nucleon decay rate of dimension-six operators by a significant amount. We find that the theoretical lower bound on the proton life time \tau(p \to \e^+ \pi^0) is within the observable range at SuperKamiokande.Comment: 9 pages, Latex, 1 Postscript figure

Limit on the Color-Triplet Higgs Mass in the Minimum Supersymmetric SU(5) Model

In the minimum supersymmetric SU(5) GUT, we derive the upper limit on the mass of the color-triplet Higgs multiplets as \mhc\leq 2.4\times 10^{16}~\GEV (90 \% C.L.) taking all possible corrections into account in a renormalization group analysis. If the above upper limit is compared with a limit on \mhc from the negative search for the proton decay; \mhc \geq 2.0\times 10^{16}~\GEV (in which effects of the larger top-quark mass are included), the minimum supersymmetric SU(5) GUT is severely constrained

Yukawa Matrix for the Neutrino and Lepton Flavour Violation

We estimate the magnitude of Lepton Flavour Violation (LFV) from the phase of the neutrino Yukawa matrix. In the minimal supersymmetric standard model with right-handed neutrinos, the LFV processes l_i \to l_j \gamma can appear through the slepton mixing, which comes from the renormalization group effect on the right-handed neutrino Yukawa interaction between the Grand Unified Theory scale and the heavy right-handed neutrino mass scale. Two types of phases exist in the neutrino Yukawa matrix. One is the Majorana phase, which can change the magnitude of the LFV branching ratios by a few factor. The other phases relate for the size of the Yukawa hierarchy and its phase effect can change the LFV branching ratios by several orders of magnitude.Comment: Talk given by K. Tsumura at NuFact04, Osaka, Japan, July 26 - August 1,2004 - 3 pages, 2 figure

Search for Lepton Flavor-Violating "tau -> mu gamma" decay

We search for the lepton flavor-violating "tau -> mu gamma" decay using 29.7 million tau pairs accumulated by the Belle experiment. The main background sources are found to be tau pairs with "tau -> mu nu nu" decay and radiative dimuon events. One event is observed in the signal region, while 2.5 +- 0.6 background events are expected. A preliminary upper limit Br(tau -> mu gamma) < 6 x 10^{-7} at the 90% confidence limit is obtained.Comment: Invited talk at the Seventh International Workshop on Tau Lepton Physics (TAU02), Santa Cruz, Ca, USA, Sept 2002, 6 pages, LaTeX, 13 eps figure

Exact Event Rates of Lepton Flavor Violating Processes in Supersymmetric SU(5) Model

Event rates of various lepton flavor violating processes in the minimal supersymmetric SU(5) model are calculated, using exact formulas which include Yukawa vertices of lepton-slepton-Higgsino. We find subtlety in evaluating event rates due to partial cancellation between diagrams. This cancellation typically reduces the event rates significantly, and the size of the reduction strongly depends on superparticle mass spectrum.Comment: 11pages, 8 figures. Fig.5 where the mu-e conversion rates in nuclei was shown was incorrect due to an error in our numerical computation.It is replaced in this corrected version. All conclusions remain unchange

A Constraint on Yukawa-Coupling Unification from Lepton-Flavor Violating Processes

We present a new constraint on a lepton mixing matrix $V$ from lepton-flavor violating (LFV) processes in supersymmetric standard models with massive neutrinos. Here, we assume Yukawa-coupling unification $f_{\nu 3}\simeq f_{\rm top}$, in which $\tau$-neutrino Yukawa coupling $f_{\nu 3}$ is unified into top-quark Yukawa coupling $f_{\rm top}$ at the unification scale $M_*\simeq 3\times 10^{16}$ GeV. We show that the present experimental bound on $\mu \to e \gamma$ decay already gives a stringent limit on the lepton mixing (typically $V_{13}<0.02$ for $V_{23}=1/\sqrt{2}$). Therefore, many existing neutrino-mass models are strongly constrained. Future improvement of bounds on LFV processes will provide a more significant impact on the models with the Yukawa-coupling unification. We also stress that a precise measurement of a neutrino mixing $(V_{MNS})_{e3}$ in future neutrino experiments would be very important, since the observation of non-zero $(V_{MNS})_{e3}$, together with negative experimental results for the LFV processes, have a robust potential to exclude a large class of SUSY standard models with the Yukawa-coupling unification.Comment: 12 pages, 3 figure

A Detailed Study of the Gluino Decay into the Third Generation Squarks at the CERN LHC

In supersymmetric models a gluino can decay into tb\tilde{\chi}^{\pm}_1 through a stop or a sbottom. The decay chain produces an edge structure in the m_{tb} distribution. Monte Carlo simulation studies show that the end point and the edge height would be measured at the CERN LHC by using a sideband subtraction technique. The stop and sbottom masses as well as their decay branching ratios are constrained by the measurement. We study interpretations of the measurement in the minimal supergravity model. We also study the gluino decay into tb and \tilde{\chi}^{\pm}_2 as well as the influence of the stop left-right mixing on the m_{bb} distribution of the tagged $tb$ events.Comment: revtex, 20 pages in PRD format, 35 eps file

Atmospheric Neutrino Oscillation and Large Lepton-Flavour Violation in the SUSY SU(5) GUT

The atmospheric neutrino anomaly reported by the super-Kamiokande collaboration suggests existence of a large flavour violating Yukawa coupling in the lepton sector. We discuss lepton-flavour violation at low energies in the framework of the supersymmetric SU(5)_{GUT} model with right-handed neutrinos. We find that for a wide range of parameter space suggested from the atmospheric neutrino anomaly, if the tau-neutrino Yukawa coupling is as large as that of top quark, lower bounds of the branching ratios of mu -> e gamma, tau -> mu gamma, and the mu-e conversion rate on $^{48}_{22}Ti$ become around 10^{-14}, 10^{-9}, and 10^{-16}, respectively. These reaction rates may be in the region accessible by near future experiments.Comment: 13 pages, 4 figure