Probing Left-handed Slepton Flavor Mixing at Future Lepton Colliders

It has been argued in the literature that the search for the slepton oscillation phenomenon can be a powerful probe of intergenerational mixing between sleptons, once sleptons are found at future colliders. In this article we estimate possible reach of future lepton colliders in probing left-handed slepton flavor mixing, especially mixing between the first and third generations, on which constraints imposed by other processes like $\tau \to e \gamma$ are very weak. $e^+e^-$ collider is suitable for this purpose, since it can produce, if kinematically allowed, sleptons of the first generation via t-channel, in addition to s-channel. Utilizing e^+e^- \to \tau e + 4jets + \E signal at $e^+e^-$ linear collider with integrated luminosity L=50 fb^{-1}(500 fb^{-1}) it may be possible to reach mixing angle $\sin 2\theta_{\tilde{\nu}} \gtrsim 0.06 (0.04)$ and mass difference $\Delta m_{\tilde{\nu}} \gtrsim 0.07 (0.04)$ GeV for sneutrinos in the first and third generations at the statistical significance of 5 \sigma.Comment: 27 pages, 6 figures. A new section added. Conclusion unchanged. To appear in Phys. Rev.

Direct Detection of Electroweak-Interacting Dark Matter

Assuming that the lightest neutral component in an SU(2)L gauge multiplet is the main ingredient of dark matter in the universe, we calculate the elastic scattering cross section of the dark matter with nucleon, which is an important quantity for the direct detection experiments. When the dark matter is a real scalar or a Majorana fermion which has only electroweak gauge interactions, the scattering with quarks and gluon are induced through one- and two-loop quantum processes, respectively, and both of them give rise to comparable contributions to the elastic scattering cross section. We evaluate all of the contributions at the leading order and find that there is an accidental cancellation among them. As a result, the spin-independent cross section is found to be O(10^-(46-48)) cm^2, which is far below the current experimental bounds.Comment: 19 pages, 7 figures, published versio

Democratic (S)fermions and Lepton Flavor Violation

The democratic approach to account for fermion masses and mixing is known to be successful not only in the quark sector but also in the lepton sector. Here we extend this ansatz to supersymmetric standard models, in which the K\"ahler potential obeys underlying S_3 flavor symmetries. The requirement of neutrino bi-large mixing angles constrains the form of the K\"ahler potential for left-handed lepton multiplets. We find that right-handed sleptons can have non-degenerate masses and flavor mixing, while left-handed sleptons are argued to have universal and hence flavor-blind masses. This mass pattern is testable in future collider experiments when superparticle masses will be measured precisely. Lepton flavor violation arises in this scenario. In particular, \mu \to e \gamma is expected to be observed in a planning future experiment if supersymmetry breaking scale is close to the weak scale.Comment: 22 pages, 2 figure

Search for neutrinoless tau decays tau -> 3l and tau -> l K0S

Neutrinoless tau-lepton decays into either three leptons (tau- -> l1- l2 l3) or one lepton and one K0S meson(tau- -> l- K0S) where lepton l means either an electron or muon, have been searched for using 48.6 fb^{-1} of data collected with the Belle detector at the KEKB e+e- collider. No evidence for candidate decays are found in any channel. Therefore we set 90% confidence level upper limits on the branching fraction for 8 different decay modes. These limits are more stringent than those set previously and reach to the 10^{-7} level.Comment: Invited talk at the Seventh International Workshop on Tau Lepton Physics (TAU02), Santa Cruz, Ca, USA, Sept 2002, 7 pages, LaTeX, 3 eps 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

Not Even Decoupling Can Save Minimal Supersymmetric SU(5)

We make explicit the statement that Minimal Supersymmetric SU(5) has been excluded by the Super-Kamiokande search for the process $p \to K^{+} \overline{\nu}$. This exclusion is made by first placing limits on the colored Higgs triplet mass, by forcing the gauge couplings to unify. We also show that taking the superpartners of the first two generations to be very heavy in order to avoid flavor changing neutral currents, the so-called ``decoupling'' idea, is insufficient to resurrect the Minimal SUSY SU(5). We comment on various mechanisms to further suppress proton decay in SUSY SU(5). Finally, we address the contributions to proton decay from gauge boson exchange in the Minimal SUSY SU(5) and flipped SU(5) models.Comment: 8 pages, 4 figure

Slepton mass-splittings as a signal of LFV at the LHC

Precise measurements of slepton mass-splittings might represent a powerful tool to probe supersymmetric (SUSY) lepton flavour violation (LFV) at the LHC. We point out that mass-splittings of the first two generations of sleptons are especially sensitive to LFV effects involving $\tau-\mu$ transitions. If these mass-splittings are LFV induced, high-energy LFV processes like the neutralino decay {\nt}_2\to\nt_1\tau^{\pm}\mu^{\mp} as well as low-energy LFV processes like $\tau\to\mu\gamma$ are unavoidable. We show that precise slepton mass-splitting measurements and LFV processes both at the high- and low-energy scales are highly complementary in the attempt to (partially) reconstruct the flavour sector of the SUSY model at work. The present study represents another proof of the synergy and interplay existing between the LHC, i.e. the {\em high-energy frontier}, and high-precision low-energy experiments, i.e. the {\em high-intensity frontier}.Comment: 11 pages, 5 figures. v2: added discussion on backgrounds, added references, version to be published on JHE

A New Parametrization of the Seesaw Mechanism and Applications in Supersymmetric Models

We present a new parametrization of the minimal seesaw model, expressing the heavy-singlet neutrino Dirac Yukawa couplings $(Y_\nu)_{ij}$ and Majorana masses $M_{N_i}$ in terms of effective light-neutrino observables and an auxiliary Hermitian matrix $H.$ In the minimal supersymmetric version of the seesaw model, the latter can be related directly to other low-energy observables, including processes that violate charged lepton flavour and CP. This parametrization enables one to respect the stringent constraints on muon-number violation while studying the possible ranges for other observables by scanning over the allowed parameter space of the model. Conversely, if any of the lepton-flavour-violating process is observed, this measurement can be used directly to constrain $(Y_\nu)_{ij}$ and $M_{N_i}.$ As applications, we study flavour-violating $\tau$ decays and the electric dipole moments of leptons in the minimal supersymmetric seesaw model.Comment: Important references adde

Muon anomalous magnetic moment and lepton flavor violation in MSSM

We give a thorough analysis of the correlation between the muon anomalous magnetic moment and the radiative lepton flavor violating (LFV) processes within the minimal supersymmetric standard model. We find that in the case when the slepton mass eigenstates are nearly degenerate, $\delta a_\mu$, coming from SUSY contributions, hardly depends on the lepton flavor mixing and, thus, there is no direct relation between $\delta a_\mu$ and the LFV processes. On the contrary, if the first two generations' sleptons are much heavier than the 3rd one, i.e., in the effective SUSY scenario, the two quantities are closely related. In the latter scenario, the SUSY parameter space to account for the experimental $\delta a_\mu$ is quite different from the case of no lepton flavor mixing. Especially, the Higgsino mass parameter $\mu$ can be either positive or negative.Comment: 22 pages, 9 figures; Some discussions are modifie

μ→eγ\mu \to e \gamma and μ→3e\mu \to 3e processes with polarized muons and supersymmetric grand unified theories

$\mu^{+} \to e^{+} \gamma$ and $\mu^{+} \to e^{+}e^{+}e^{-}$ processes are analyzed in detail with polarized muons in supersymmetric grand unified theories. We first present Dalitz plot distribution for $\mu^{+} \to e^{+}e^{+}e^{-}$ decay based on effective Lagrangian with general lepton-flavor-violating couplings and define various P- and T-odd asymmetries. We calculate branching ratios and asymmetries in supersymmetric SU(5) and SO(10) models taking into account complex soft supersymmetry breaking terms. Imposing constraints from experimental bounds on the electron, neutron and atomic electric dipole moments, we find that the T-odd asymmetry for $\mu^{+} \to e^{+}e^{+}e^{-}$ can be 15% in the SU(5) case. P-odd asymmetry with respect to muon polarization for $\mu^{+} \to e^{+} \gamma$ varies from -20% to -100% for the SO(10) model while it is $+100$ in the SU(5) case. We also show that the P-odd asymmetries in $\mu^{+} \to e^{+}e^{+}e^{-}$ and the ratio of $\mu^{+} \to e^{+}e^{+}e^{-}$ and $\mu^{+} \to e^{+} \gamma$ branching fractions are useful to distinguish different models.Comment: 52 pages, 15 figure