arXiv subjects

Junji Hisano

Publications and source records attributed to Junji Hisano.

103 records · Page 6Linked to original sources

arXiv2002

Scenery from the Top: Study of the Third Generation Squarks at CERN LHC

In the minimal supersymmetric standard model (MSSM) properties of the third generation sfermions are important from the viewpoint of discriminating the SUSY breaking models and in the determination of the Higgs boson mass. If gluinos are copiously produced at CERN LHC, gluino decays into tb through stop and sbottom can be studied using hadronic decays of the top quark. The kinematical endpoint of the gluino decays can be evaluated using a W sideband method to estimate combinatorial backgrounds. This implies that fundamental parameters related to the third generation squarks can be reliably measured. The top-quark polarization dependence in the decay process may also be extracted by looking at the b jet distribution near the kinematical endpoint.

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arXiv2002

Slepton Oscillation at Large Hadron Collider

Measurement of Lepton-Flavor Violation (LFV) in the minimal SUSY Standard Model (MSSM) at Large Hadron Collider (LHC) is studied based on a realistic simulation. We consider the LFV decay of the second-lightest neutralino, $\tildeχ^0_2 \to \tilde{l} l' \to l l' \tildeχ^0_1$, in the case where the flavor mixing exists in the right-handed sleptons. We scan the parameter space of the minimal supergravity model (MSUGRA) and a more generic model in which we take the Higgsino mass $μ$ as a free parameter. We find that the possibility of observing LFV at LHC is higher if $μ$ is smaller than the MSUGRA prediction; the LFV search at LHC can cover the parameter range where the $μ\to e γ$ decay can be suppressed by the cancellation among the diagrams for this case.

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arXiv2002

The stau exchange contribution to muon g-2 in the decoupling solution

We study the possibility that the lepton-flavor changing process can induce the suitable magnitude of the muon anomalous magnetic moment (g_μ-2) in the decoupling solution to the flavor problem in the minimal supersymmetric standard model. Our analyses introduce the flavor mixings of left- and right-handed stau and smuon phenomenologically. It is found that if both the left- and right-handed sleptons have sizable flavor mixings, the correction to g_μ-2 from the lighter slepton can reach to 10^{-9} while the correction to the branching ratio of τ\to μγsatisfies the current experimental bound. On the other hand, when only the left-handed or right-handed sleptons have the large flavor mixing, the suitable magnitude of the correction to g_μ-2 is not realized owing to the experimental bound of τ\to μγ.

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arXiv2001

CP Violation in the Minimal Supersymmetric Seesaw Model

We study CP violation in the lepton sector of the supersymmetric extension of the Standard Model with three generations of massive singlet neutrinos with Yukawa couplings $Y_ν$ to lepton doublets, in a minimal seesaw model for light neutrino masses and mixing. This model contains six physical CP-violating parameters, namely the phase $δ$ observable in oscillations between light neutrino species, two Majorana phases $ϕ_{1,2}$ that affect $ββ_{0 ν}$ decays, and three independent phases appearing in ${Y_ν}{Y_ν}^\dagger$, that control the rate of leptogenesis. Renormalization of the soft supersymmetry-breaking parameters induces observable CP violation at low energies, including T-odd asymmetries in polarized $μ\to eee$ and $τ\to \ell \ell \ell$ decays, as well as lepton electric dipole moments. In the leading-logarithmic approximation in which the massive singlet neutrinos are treated as degenerate, these low-energy observables are sensitive via ${Y_ν}^\dagger{Y_ν}$ to just one combination of the leptogenesis and light-neutrino phases. We present numerical results for the T-odd asymmetry in polarized $μ\to eee$ decay, which may be accessible to experiment, but the lepton electric dipole moments are very small in this approximation. To the extent that the massive singlet neutrinos are not degenerate, low-energy observables become sensitive also to two other combinations of leptogenesis and light-neutrino phases, in this minimal supersymmetric seesaw model.

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arXiv2001

Lepton Electric Dipole Moments in Non-Degenerate Supersymmetric Seesaw Models

In the context of supersymmetric seesaw models of neutrino masses with non-degenerate heavy neutrinos, we show that Dirac Yukawa interactions N^c_i (Y_nu)_{ij} L_j H_2 induce large threshold corrections to the slepton soft masses via renormalization. While still yielding rates for lepton-flavour-violating processes below the experimental bounds, these contributions may increase the muon and electron electric dipole moments d_mu and d_e by several orders of magnitude. In the leading logarithmic approximation, this is due to three additional physical phases in Y_nu, one of which also contributes to leptogenesis. The naive relation d_mu/d_e\approx -m_mu/m_e is violated strongly in the case of successful phenomenological textures for Y_nu, and the values of d_mu and/or d_e may be within the range of interest for the future experiments.

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arXiv2001

Neutrino masses, muon g-2, and lepton-flavour violation in the supersymmetric see-saw model

In the light of the recent muon (g_mu-2) result by the E821 experiment at the Brookhaven National Laboratory, we study the event rates of the charged lepton-flavour-violating (LFV) processes in the supersymmetric standard model (SUSY SM) with the heavy right-handed neutrinos (SUSY see-saw model). Since the left-handed sleptons get the LFV masses via the neutrino Yukawa interaction in this model, the event rate of mu->e gamma and the SUSY-SM correction to (g_mu-2)/2 (δa_mu^{SUSY}) are strongly correlated. When the left-handed sleptons have a LFV mass between the first and second generations ((m^2_L)_{12}) in the mass matrix, it should be suppressed by \sim 10^{-3} ({10^-9}/δa_mu^{SUSY}) compared with the diagonal components (m_{SUSY}^2), from the current experimental bound on mu-> e gamma. The recent (g_mu-2) result indicates {δa_mu^{SUSY}}\sim 10^{-9}. The future charged LFV experiments could cover (m^2_L)_{12}/m_{SUSY}^2\gsim 10^{-(5-6)}. These experiments will give a significant impact on the flavour models and the SUSY-breaking models. In the SUSY see-saw model (m^2_L)_{12} is proportional to square of the tau-neutrino Yukawa-coupling constant. In the typical models where the neutrino-oscillation results are explained and the top-quark and tau-neutrino Yukawa couplings are unified at the GUT scale, a large LFV mass of (m^2_L)_{12}/m_{SUSY}^2\gsim 10^{-4} is generated, and the large LFV event rates are predicted. We impose a so-called no-scale condition for the SUSY-breaking parameters at the GUT scale, which suppresses the FCNC processes, and derive the conservative lower bound on mu-> egamma. The predicted Br(mu-> e gamma) could be covered at the future LFV experiments.

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arXiv2001

The fate of the B ball

The gauge-mediated SUSY-breaking (GMSB) model needs entropy production at a relatively low temperature in the thermal history of the Universe for the unwanted relics to be diluted. This requires a mechanism for the baryogenesis after the entropy production, and the Affleck and Dine (AD) mechanism is a promising candidate for it. The AD baryogenesis in the GMSB model predicts the existence of the baryonic Q ball, that is the B ball, and this may work as the dark matter in the Universe. In this article, we discuss the stability of the B ball in th presence of baryon-number violating interactions. We find that the evaporation rate increases monotonically with the B-ball charge because the large field value inside the B ball enhances the effect of the baryon-number-violating operators. While there are some difficulties to evaluate the evaporation rate of the B ball, we derive the evaporation time (lifetime) of the B ball for the mass-to-charge ratio $ω_0\gsim 100 \MEV$. The lifetime of the B ball and the distortion of the cosmic ray positron flux and the cosmic background radiation from the B ball evaporation give constraints on the baryon number of the B ball and the interaction, if the B ball is the dark matter. We also discuss some unresolved properties of the B ball.

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arXiv2000

Natural Effective Supersymmetry

Much heavier sfermions of the first-two generations than the other superparticles provide a natural explanation for the flavor and CP problems in the supersymmetric standard model (SUSY SM). However, the heavy sfermions may drive the mass squareds for the light third generation sfermions to be negative through two-loop renormalization group (RG) equations, breaking color and charge. Introducing extra matters to the SUSY SM, it is possible to construct models where the sfermion masses are RG invariant at the two-loop level in the limit of vanishing gaugino-mass and Yukawa-coupling contributions. We calculate the finite corrections to the light sfermion masses at the two-loop level in the models. We find that the finite corrections to the light-squark mass squareds are negative and can be less than (0.3-1)% of the heavy-squark mass squareds, depending on the number and the parameters of the extra matters. We also discuss whether such models realized by the U(1)_X gauge interaction at the GUT scale can satisfy the constraints from Δm_K and ε_K naturally. When both the left- and right-handed down-type squarks of the first-two generations have common U(1)_X charges, the supersymmetric contributions to Δm_K and ε_K are sufficiently suppressed without spoiling naturalness, even if the flavor-violating supergravity contributions to the sfermion mass matrices are included. When only the right-handed squarks of the first-two generations have a common U(1)_X charge, we can still satisfy the constraint from Δm_K naturally, but evading the bound from ε_K requires the CP phase smaller than 10^{-2}.

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arXiv1999

Bulk Standard Model in the Randall-Sundrum Background

We discuss issues in an attempt to put the Standard Model (SM) in five-dimensional anti-de Sitter spacetime compactified on $S^1/Z_2$. The recently-proposed approach to the gauge hierarchy problem by using this background geometry, with the SM confined on a boundary, is extended to a situation where (some of) the SM particles reside in the five dimensional bulk. In particular, we find a localization of zero modes of bulk fermions near the boundary with a negative tension. Unlike the compactification with the flat metric, these fermion zero modes couple to Kaluza-Klein (KK) excitations of the SM gauge bosons. Interestingly, only low-lying modes of such KK gauge bosons have non-negligible couplings. Current electroweak precision data give a constraint that the first KK mode be heavier than 9 TeV. We also argue that at least the Higgs field should be confined on the brane to utilize the Randall-Sundrum background as a solution to the gauge hierarchy.

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arXiv1999

On Effective Theory of Brane World with Small Tension

The five dimensional theory compactified on $S^1$ with two ``branes'' (two domain walls) embedded in it is constructed, based on the field-theoretic mechanism to generate the ``brane''. Some light states localized in the ``brane'' appear in the theory. One is the Nambu-Goldstone boson, which corresponds to the breaking of the translational invariance in the transverse direction of the ``brane''. In addition, if the tension of the ``brane'' is smaller than the fundamental scale of the original theory, it is found that there may exist not only massless states but also some massive states lighter than the fundamental scale in the ``brane''. We analyze the four dimensional effective theory by integrating out the freedom of the fifth dimension. We show that some effective couplings can be explicitly calculated. As one of our results, some effective couplings of the state localized in the ``brane'' to the higher Kaluza-Klein modes in the bulk are found to be suppressed by the width of the ``brane''. The resultant suppression factor can be quantitatively different from the one analyzed by Bando et al. using the Nambu-Goto action, while they are qualitatively the same.

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arXiv1998

Lepton-Flavor Violation in the Left-handed Slepton Production at Future Lepton Colliders

The Super-Kamiokande atmospheric neutrino data suggest existence of the large lepton-flavor violating (LFV) interaction in the higher energy scale. If the minimal supersymmetric standard model is extended to have right-handed neutrinos, the left-handed sleptons in the second and third generations are expected to have the LFV masses in the minimal supergravity scenario. In this article we study the LFV signals in the left-handed slepton production at $μ^+μ^-$ colliders and $\e^+\e^-$ linear colliders (LC's), $μ^+μ^-(\e^+\e^-)\toτμ+4jets + \E$ and $μ^+μ^-(\e^+\e^-)\toτμl+ 2jets+ \E$. The main background comes from decay of a tau lepton into a muon in the lepton-flavor conserving slepton pair production. They are significantly reduced by the energy and the impact parameter cuts for the muon. At $μ^+μ^-$ colliders (LC's) it may be possible to reach the mixing angle $\sin 2θ_{\tildeν} \gsim 0.1(0.5)$ and the mass difference $Δm_{\tildeν}\gsim 0.1(0.4)$ GeV for the sneutrinos in the second and third generations at the statistical significance of $3σ$. Such small mass difference and a mixing angle may be induced radiatively even if the Yukawa coupling constant for the tau neutrino is of the order of 0.1.

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arXiv1997

Upper Bound on R_b in Two Higgs Doublet Model from Lepton Universality

It has been known that $R_b$ can be enhanced in the two Higgs doublet model if $\tan β$ is large. We point out that a similar enhancement in $Γ(Z \to τ^+ τ^-)$ is large enough to place a constraint on such a possibility. We obtain a 95% CL upper bound $ΔR_b/R_{b} < 0.73%$ in this model for the $\overline{MS}$ mass $m_{b} (m_{Z}) = 3.0$ GeV. The 1996 world average is $ΔR_{b}/R_{b}$ = $0.97% \pm 0.51%$. We used the $m_{b}(m_{Z})$ to determine the bottom Yukawa coupling instead of $m_b(m_{b})$ unlike in previous analyses, and also an improved experimental test of the lepton universality in $Z$ decay, which made our results qualitatively different.

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arXiv1996

Hard gluon emission from colored scalar pairs\break in e^+ e^- annihilation

We study QCD correction to the pair production of colored scalar particles in electron-positron annihilation with an emphasis on gluon emission in the final state. We discuss the usefulness of working in a ``quasi-two-body'' frame and present the helicity amplitudes for the process. We compare the final state configuration with fermion pair production and find that the three-jet fraction for the scalars shows quantitative difference from that for fermions.

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