arXiv subjects

Hitoshi Murayama

Publications and source records attributed to Hitoshi Murayama.

At least 289 records · Page 16Linked to original sources

arXiv1995

$(S_3)^3$ flavor symmetry and $p\to K^0 e^+$

We show how to incorporate the lepton sector in a supersymmetric theory of flavor based on the discrete flavor group $(S_3)^3$. Assuming that all possible nonrenormalizable operators are generated at the Planck scale, we show that the transformation properties of the leptons and of the flavor-symmetry breaking fields are uniquely determined. We then demonstrate that the model has a viable phenomenology and makes one very striking prediction: the nucleon decays predominantly to $K l$ where $l$ is a {\em first} generation lepton. We show that the modes $n \rightarrow K^0 \barν_e$, $p\rightarrow K^+ \barν_e$, and $p\rightarrow K^0 e^+$ occur at comparable rates, and could well be discovered simultaneously at the SuperKamiokande experiment.

hep-ph↗

arXiv1995

Viable $t$-$b$-$τ$ Yukawa Unification in SUSY SO(10)

The supersymmetric SO(10) GUT with $t$--$b$--$τ$ Yukawa coupling unification has problems with correct electroweak symmetry breaking, experimental constraints (especially $b\rightarrow sγ$) and neutralino abundance, if the scalar masses are universal at the GUT scale. We point out that non-universality of the scalar masses at the GUT scale generated both by (1) renormalization group running from the Planck scale to the GUT scale and (2) $D$--term contribution induced by the reduction of the rank of the gauge group, has a desirable pattern to make the model phemenologically viable (in fact the only one which is consistent with experimental and cosmological constraints). At the same time the top quark mass has to be either close to its quasi IR--fixed point value or below $\sim$170 GeV. We also briefly discuss the spectrum of superpartners which is then obtained.

hep-ph↗

arXiv1995

Phenomenology of Minimal SU(5) Unification with Dynamical Supersymmetry Breaking

We consider the constraints from proton decay and $b$-$τ$ unification in the minimal supersymmetric SU(5) grand unified theory with a `visible' dynamical supersymmetry breaking sector. We show how the presence of vector-like messenger fields and the constrained superparticle mass spectrum affect the phenomenology of the model. We include the messenger fields in our renormalization group analysis between the messenger scale ($\sim 100$ TeV) and the GUT scale. We show that the simplest model of this type, a minimal SU(5) GUT with an additional {\bf 5}+\bfbar{5} of messenger fields is excluded by the constraints from proton decay and $b$-$τ$ unification.

hep-ph↗

arXiv1995

A Geometry of the Generations

We propose a geometric theory of flavor based on the discrete group $(S_3)^3$, in the context of the minimal supersymmetric standard model. The group treats three objects symmetrically, while making fundamental distinctions between the generations. The top quark is the only heavy quark in the symmetry limit, and the first and second generation squarks are degenerate. The hierarchical nature of Yukawa matrices is a consequence of a sequential breaking of $(S_3)^3$.

hep-ph↗

arXiv1995

Light U(1) Gauge Boson Coupled to Baryon Number

We discuss the phenomenology of a light U(1) gauge boson, $γ_B$, that couples only to baryon number. Gauging baryon number at high energies can prevent dangerous baryon-number violating operators that may be generated by Planck scale physics. However, we assume at low energies that the new U(1) gauge symmetry is spontaneously broken and that the $γ_B$ mass $m_B$ is smaller than $m_Z$. We show for $m_Υ<m_B<m_Z$ that the $γ_B$ coupling $α_B$ can be as large as $\sim 0.1$ without conflicting with the current experimental constraints. We argue that $α_B\sim 0.1$ is large enough to produce visible collider signatures and that evidence for the $γ_B$ could be hidden in existing LEP data. We show that there are realistic models in which mixing between the $γ_B$ and the electroweak gauge bosons occurs only as a radiative effect and does not lead to conflict with precision electroweak measurements. Such mixing may nevertheless provide a leptonic signal for models of this type at an upgraded Tevatron.

hep-ph↗

arXiv1995

Studying Non-calculable Models of Dynamical Supersymmetry Breaking

There are supersymmetric gauge theories which do not possess any parameters nor flat directions, and hence cannot be studied anywhere in the field space using holomorphy (``non-calculable''). Some of them are believed to break supersymmetry dynamically. We propose a simple technique to analyze these models. Introducing a vector-like field into the model, one finds flat directions where one can study the dynamics. We unambiguously show that the supersymmetry is broken when the mass of the vector-like field is small but finite, and hence Witten index vanishes. If we increase the mass of the vector-like field, it eventually decouples from the dynamics and the models reduce to the original non-calculable models. Assuming the continuity of the Witten index in the parameter space, one can establish the dynamical supersymmetry breaking in the non-calculable models.

hep-th↗

arXiv1995

Third family flavor physics in an SU(3)^3 \times SU(2)_L \times U(1)_Y model

We consider a model in which each family transforms under a different SU(3) color group. The low-energy effective theory is an extension of the Standard Model, with additional color octet gauge bosons $G_H$ with mass $M$ that couple preferentially to the third family quarks. We show that there are two distinct regions of the model's parameter space in which we can simultaneously evade all the current experimental constraints, one with $M \approx 250$ GeV and the other with $M \gtrsim 600$ GeV. Within each allowed region, we can obtain a correction to the $Zb\overline{b}$ vertex that is consistent with the slightly high value of $R_b$ observed at LEP. We show that there are $ΔB = 1$ operators in our model that can suppress the $B$-meson semileptonic branching ratio $B_{SL}$ and the charm multiplicity per decay $n_c$ by enough to reconcile the spectator parton model predictions with the experimental data. In the non-supersymmetric version of our model, we can only obtain the desired corrections to $R_b$, $B_{SL}$ and $n_c$ in different regions of the allowed parameter space, while in the supersymmetric version, we can obtain all three corrections simultaneously. We also discuss a strong-coupling limit of our model in which the third-family quarks become composite.

hep-ph↗

arXiv1995

Preserving Flat Directions During Inflation

Supersymmetry is generally broken by the non-vanishing vacuum energy density present during inflation. In supergravity models, such a source of supersymmetry breaking typically makes a contribution to scalar masses of order ${\tilde m}^2 \sim H^2$, where $H^2 \sim V/M_P^2$ is the Hubble parameter during inflation. We show that in supergravity models which possess a Heisenberg symmetry, supersymmetry breaking makes no contribution to scalar masses, leaving supersymmetric flat directions flat at tree-level. One-loop corrections in general lift the flat directions, but naturally give small negative squared masses $\sim - g^2 H^2/(4π)^2$ for all flat directions that do not involve the stop. No-scale supergravity of the SU(N,1) type and the untwisted sectors from orbifold compactifications are special cases of this general set of models. We point out the importance of the preservation of flat directions for baryogenesis.

hep-ph↗

arXiv1995

NON-UNIVERSAL SUSY BREAKING, HIERARCHY AND SQUARK DEGENERACY

I discuss non-trivial effects in the soft SUSY breaking terms which appear when one integrates out heavy fields. The effects exist only when the SUSY breaking terms are non-universal. They may spoil (1) the hierarchy between the weak and high-energy scales, or (2) degeneracy among the squark masses even in the presense of a horizontal symmetry. I argue, in the end, that such new effects may be useful in probing physics at high-energy scales from TeV-scale experiments.

hep-ph↗

arXiv1995

New Symmetries of Supersymmetric Effective Lagrangians

We consider the structure of effective lagrangians describing the low-energy dynamics of supersymmetric theories in which a global symmetry $G$ is spontaneously broken to a subgroup $H$ while supersymmetry is unbroken. In accordance with the supersymmetric Goldstone theorem, these lagrangians contain Nambu--Goldstone superfields associated with a coset space $G^c / \hat{H}$, where $G^c$ is the complexification of $G$ and $\hat{H}$ is the largest subgroup of $G^c$ that leaves the order parameter invariant. The lagrangian may also contain additional light matter fields. To analyze the effective lagrangian for the matter fields, we first consider the case where the effective lagrangian is obtained by integrating out heavy modes at weak coupling (but including non-perturbative effects such as instantons). We show that the superpotential of the matter fields is $\hat{H}$ invariant, which can give rise to non-trivial relations among independent $H$-invariants in the superpotential. We also show that the Kahler potential of the matter fields can be restricted by a remnant of $\hat{H}$ symmetry. These results are non-perturbative and have a simple group-theoretic interpretation. When we relax the weak-coupling constraint, there appear to be additional possibilities for the action of $\hat{H}$ on the matter fields, hinting that the constraints imposed by $\hat{H}$ may be even richer in strongly coupled theories.

hep-ph↗

arXiv1995

REALISTIC MODELS WITH A LIGHT U(1) GAUGE BOSON COUPLED TO BARYON NUMBER

We recently showed that a new gauge boson $γ_B$ coupling only to baryon number is phenomenologically allowed, even if $m_B<m_Z$ and $α_B \approx 0.2$. In our previous work we assumed that kinetic mixing between the baryon number and hypercharge gauge bosons (via an $F^{μν}_B F_{μν}^Y$ term) was small enough to evade constraints from precision electroweak measurements. In this paper we propose a class of models in which this term is naturally absent above the electroweak scale. We show that the generation of a mixing term through radiative corrections in the low-energy effective theory does not lead to conflict with precision electroweak measurements and may provide a leptonic signal for models of this type at an upgraded Tevatron.

hep-ph↗

arXiv1994

Possible Light U(1) Gauge Boson Coupled to Baryon Number

We discuss the phenomenology of a light U(1) gauge boson, $γ_B$, that couples only to baryon number. We assume that the new U(1) gauge symmetry is spontaneously broken and that the $γ_B$ mass is smaller than $m_Z$. Nevertheless, we show that the model survives the current experimental constraints. In addition, we argue that evidence for the existence of such a particle could be hidden in existing LEP and Tevatron data. We determine the allowed regions of the $m_B$-$α_B$ plane, where $m_B$ is the $γ_B$ mass, and where $4 πα_B$ is the squared gauge coupling. We point out that in some parts of the allowed parameter space our model can account for rapidity gap events in proton-antiproton scattering seen at the Fermilab Tevatron.

hep-ph↗

arXiv1994

Supersymmetry

I review phenomenologically interesting aspects of supersymmetry. First I point out that the discovery of the positron can be regarded as a historic analogue to the would-be discovery of supersymmetry. Second I review the recent topics on the unification of the gauge coupling constants, $m_b$--$m_τ$ relation, proton decay, and baryogenesis. I also briefly discuss the recent proposals to solve the problem of flavor changing neutral currents. Finally I argue that the measurements of supersymmetry parameters may probe the physics at the Planck scale.

hep-ph↗

arXiv1994

Family Symmetries and Proton Decay

The proton decay modes $p\to K^0 e^+$ and $p\to K^0 μ^+$ may be visible in certain supersymmetric theories, and if seen would provide evidence for new flavor physics at extremely short distances. These decay modes can arise from the dimension five operator $(Q_1Q_1Q_2L_{1,2})$, where $Q_i$ and $L_i$ are ${i^{th}}$ generation quark and lepton superfields respectively. Such an operator is not generated at observable levels due to gauge or Higgs boson exchange in a minimal GUT. However in theories that explain the fermion mass hierarchy, it may be generated at the Planck scale with a strength such that the decays $p\to K^0 \ell^+$ are both compatible with the proton lifetime and visible at Super-Kamiokande. Observable proton decay can even occur in theories without unification.

hep-ph↗

arXiv1994

Upper bound on Hot Dark Matter Density from $SO(10)$ Yukawa Unification

We study low-energy consequences of supersymmetric $SO(10)$ models with Yukawa unification $h_t = h_N$ and $h_b = h_τ$. We find that it is difficult to reproduce the observed $m_b/m_τ$ ratio when the third-generation right-handed neutrino is at an intermediate scale, especially for small $\tan β$. We obtain a conservative lower bound on the mass of the right-handed neutrino $M_N > 6 \times 10^{13}$~GeV for $\tan β< 10$. This bound translates into an upper bound on the $τ$-neutrino mass, and therefore on its contribution to the hot dark matter density of the present universe, $Ω_νh^2 < 0.004$. Our analysis is based on the full two-loop renormalization group equations with one-loop threshold effects. However, we also point out that physics above the GUT-scale could modify the Yukawa unification condition $h_b = h_τ$ for $\tan β\lsim 10$. This might affect the prediction of $m_b/m_τ$ and the constraint on $M_N$.

hep-ph↗

arXiv1994

Low-Energy Effective Lagrangian in Unified Theories with Non-Universal Supersymmetry Breaking Terms

Supersymmetric grand unified theories with non-universal soft supersymmetry breaking terms are studied. By integrating out the superheavy fields at an unification scale, we compute their low-energy effective Lagrangian. We find new contributions to the scalar potential specific to the non-universal supersymmetry breaking. $D$-term contribution to the scalar masses is one example. The gauge hierarchy achieved by a fine-tuning in the superpotential would be violated in general due to the non-universal SUSY breaking terms. We show, however, it is preserved for a certain class of the soft terms derived from a {\em hidden} ansatz. We also discuss some phenomenological implications of the non-universal supersymmetry breaking, including predictions of the radiative electroweak symmetry breaking scenario and of no-scale type models.

hep-ph↗

arXiv1994

On Preserving a B+L Asymmetry Produced in the Early Universe

One of the most efficient mechanisms for producing the baryon asymmetry of the Universe is the decay of scalar condensates in a SUSY GUT as was first suggested by Affleck and Dine. We show that given a large enough asymmetry, the baryon number will be preserved down to low temperatures even if B - L = 0, because the baryon number carrying scalars form bose condensates that give the W a mass. We derive the conditions on the condensate needed to suppress electroweak sphaleron interactions which would otherwise drive the baryon asymmetry to zero when B - L = 0.

hep-ph↗

arXiv1994

Looking up at the GUT/Planck World

The importance of the mass spectroscopy of the superparticles is emphasized. It will be shown that the gauge coupling constants give us information on the GUT-scale mass spectrum once the superparticle masses are known. The gaugino masses will provide us a model-independent test of the grand unification irrespective of the symmetry breaking pattern. The sfermion masses carry the information on the intermediate symmetries, if present. Combining the mass spectrum, we will be able to distinguish among the GUT models, and hopefully supergravity models also.

hep-ph↗