arXiv subjects

Hitoshi Murayama

Publications and source records attributed to Hitoshi Murayama.

At least 235 records · Page 13Linked to original sources

arXiv2000

LSND, SN1987A, and CPT Violation

We point out that neutrino events observed at Kamiokande and IMB from SN1987A disfavor the neutrino oscillation parameters preferred by the LSND experiment. For $Δm^2 > 0$ (the light side), the electron neutrinos from the neutronization burst would be lost, while the first event at Kamiokande is quite likely to be due to an electron neutrino. For $Δm^2 < 0$ (the dark side), the average energy of the dominantly $\barν_e$ events is already lower than the theoretical expectations, which would get aggravated by a complete conversion from $\barν_μ$ to $\barν_e$. If taken seriously, the LSND data are disfavored independent of the existence of a sterile neutrino. A possible remedy is CPT violation, which allows different mass spectra for neutrinos and anti-neutrinos and hence can accommodate atmospheric, solar and LSND data without a sterile neutrino. If this is the case, Mini-BooNE must run in $\barν$ rather than the planned $ν$ mode to test the LSND signal. We speculate on a possible origin of CPT violation.

hep-ph↗

arXiv2000

Neutrino Masses at v^{3/2}

Theories in which neutrino masses are generated by a conventional see-saw mechanism generically yield masses which are O(v^2) in units where M_{Pl}=1, which is naively too small to explain the results from SuperKamiokande. In supersymmetric theories with gravity mediated supersymmetry breaking, the fundamental small parameter is not v/M_{Pl}, but m_I/M_{Pl}, where m_I is the scale of supersymmetry breaking in the hidden sector. We note that m_I^3/M_{Pl}^2 is only slightly too large to explain SuperKamiokande, and present two models that achieve neutrino masses at this order in m_I, one of which has an additional suppression lambda_tau^2, while the other has additional suppression arising from a loop factor. The latter model shares a great deal of phenomenology with a class of models previously explored, including the possibility of viable sneutrino dark matter.

hep-ph↗

arXiv2000

Dynamical (Super)Symmetry Breaking

Dynamical Symmetry Breaking (DSB) is a concept theorists rely on very often in the discussions of strong dynamics, model building, and hierarchy problems. In this talk, I will discuss why this is such a permeating concept among theorists and how they are used in understanding physics. I also briefly review recent progress in using dynamical symmetry breaking to construct models of supersymmetry breaking and fermion masses.

hep-ph↗

arXiv2000

Confinement and Flavor Symmetry Breaking via Monopole Condensation

We discuss dynamics of N=2 supersymmetric SU(n_c) gauge theories with n_f quark hypermultiplets. Upon N=1 perturbation of introducing a finite mass for the adjoint chiral multiplet, we show that the flavor U(n_f) symmetry is dynamically broken to U(r) \times U(n_f-r), where r\leq [n_f/2] is an integer. This flavor symmetry breaking occurs due to the condensates of magnetic degrees of freedom which acquire flavor quantum numbers due to the quark zero modes. We briefly comment on the USp(2n_c) gauge theories. This talk is based on works with Giuseppe Carlino and Ken Konishi, hep-th/0001036 and hep-th/0005076.

hep-th↗

arXiv2000

Anarchy and Hierarchy

We advocate a new approach to study models of fermion masses and mixings, namely anarchy proposed in hep-ph/9911341. In this approach, we scan the O(1) coefficients randomly. We argue that this is the correct approach when the fundamental theory is sufficiently complicated. Assuming there is no physical distinction among three generations of neutrinos, the probability distributions in MNS mixing angles can be predicted independent of the choice of the measure. This is because the mixing angles are distributed according to the Haar measure of the Lie groups whose elements diagonalize the mass matrices. The near-maximal mixings, as observed in the atmospheric neutrino data and as required in the LMA solution to the solar neutrino problem, are highly probable. A small hierarchy between the Delta m^2 for the atmospheric and the solar neutrinos is obtained very easily; the complex seesaw case gives a hierarchy of a factor of 20 as the most probable one, even though this conclusion is more measure-dependent. U_{e3} has to be just below the current limit from the CHOOZ experiment. The CP-violating parameter sin delta is preferred to be maximal. We present a simple SU(5)-like extension of anarchy to the charged-lepton and quark sectors which works well phenomenologically.

hep-ph↗

arXiv2000

A New Perspective on Cosmic Coincidence Problems

Cosmological data suggest that we live in an interesting period in the history of the universe when ρ_Λ\sim ρ_M \sim ρ_R. The occurence of any epoch with such a "triple coincidence" is puzzling, while the question of why we happen to live during this special epoch is the "Why now?" problem. We introduce a framework which makes the triple coincidence inevitable; furthermore, the ``Why now?'' problem is transformed and greatly ameliorated. The framework assumes that the only relevant mass scales are the electroweak scale, M_{EW}, and the Planck scale, M_{Pl}, and requires ρ_Λ^{1/4} \sim M_{EW}^2/M_{Pl} parametrically. Assuming that the true vacuum energy vanishes, we present a simple model where a false vacuum energy yields a cosmological constant of this form.

astro-ph↗

arXiv2000

Dynamical Symmetry Breaking in Supersymmetric SU(n_c) and USp(2n_c) Gauge Theories

We find the phase and flavor symmetry breaking pattern of each N=1 supersymmetric vacuum of SU(n_c) and USp(2 n_c) gauge theories, constructed from the exactly solvable N=2 theories by perturbing them with small adjoint and generic bare hypermultiplet (quark) masses. In SU(n_c) theories with n_f \leq n_c the vacua are labelled by an integer r, in which the flavor U(n_f) symmetry is dynamically broken to U(r) \times U(n_f-r) in the limit of vanishing bare hyperquark masses. In the r=1 vacua the dynamical symmetry breaking is caused by the condensation of magnetic monopoles in the n_f representation. For general r, however, the monopoles in the {}_{n_f}C_r representation, whose condensation could explain the flavor symmetry breaking but would produce too-many Nambu--Goldstone multiplets, actually "break up" into "magnetic quarks": the latter with nonabelian interactions condense and induce confinement and dynamical symmetry breaking. In USp(2n_c) theories with n_f \leq n_c + 1, the flavor SO(2n_f) symmetry is dynamically broken to U(n_f), but with no description in terms of a weakly coupled local field theory. In both SU(n_c) and USp(2 n_c) theories, with larger numbers of quark flavors, besides the vacua with these properties, there exist also vacua in free magnetic phase, with unbroken global symmetry.

hep-th↗

arXiv2000

Determining the Flavour Content of the Low-Energy Solar Neutrino Flux

We study the sensitivity of the HELLAZ and Borexino solar neutrino experiments on discriminating the neutrino species nu_e, anti-nu_e, nu_{mu,tau}, anti-nu_{mu,tau}, and nu_{sterile} using the difference in the recoil electron kinetic energy spectra in elastic neutrino-electron scattering. We find that one can observe a non-vanishing nu_{mu,tau} component in the solar neutrino flux, especially when the nu_e survival probability is low. Also, if the data turn out to be consistent with nu_e <-> nu_{mu,tau} oscillations, an anti-nu_e component can be excluded effectively.

hep-ph↗

arXiv2000

The Higgs Mass and New Physics Scales in the Minimal Standard Model

We study the theoretical correlation between the Higgs mass of the minimal standard model and the scale at which new physics is expected to occur. In addition to the classic constraints of unitarity, triviality and vacuum stability, we reexamine the constraints imposed by the precision electroweak data. We then pay particular attention to the constraint imposed by the absence of fine-tuning in the Higgs mass parameter (the Veltman condition). We find that the fine-tuning condition places a significant constraint on the new physics scale for the Higgs mass range 100 GeV < m_h < 200 GeV mostly unconstrained by the classic constraints.

hep-ph↗

arXiv2000

Supersymmetry Phenomenology

This is a very pedagogical review of supersymmetry phenomenology, given at ICTP Summer School in 1999, aimed mostly at students who had never studied supersymmetry before. It starts with an analogy that the reason why supersymmetry is needed is similar to the reason why the positron exists. It introduces the construction of supersymmetric Lagrangians in a practical way. The low-energy constraints, renormalization-group analyses, collider phenomenology, and frameworks of mediating supersymmetry breaking are briefly discussed.

hep-ph↗

arXiv2000

The Dark Side of the Solar Neutrino Parameter Space

Results of neutrino oscillation experiments have always been presented on the (sin^2 2theta, Delta m^2) parameter space for the case of two-flavor oscillations. We point out, however, that this parameterization misses the half of the parameter space pi/4 < theta <= pi/2 (``the dark side''), which is physically inequivalent to the region 0 <= theta <= pi/4 (``the light side'') in the presence of matter effects. The MSW solutions to the solar neutrino problem can extend to the dark side, especially if we take the conservative attitude to allow higher confidence levels, ignore some of the experimental results in the fits, or relax theoretical predictions. Furthermore even the so-called ``vacuum oscillation'' solution distinguishes the dark and the light sides. We urge experimental collaborations to present their results on the entire parameter space.

hep-ph↗

arXiv2000

CP violating Z t anti-t and gamma t anti-t Couplings at a Future e+ e- Collider

The effect of new operators that give rise to CP-violating couplings of the type $Z t \bar{t}$ and $γt \bar{t}$ are examined at future electron positron Linear Colliders (FLC). The impact of these CP-violating interactions over Standard Model predictions was studied for the process e+ e^- -> t anti-t with the subsequent decays t -> b l+ nu_l and anti-t -> anti-b l- anti-nu_l, called as dilepton mode, and t -> b l^+ nu_l and anti-t -> anti-b anti-q q' or t -> b q anti-q' and anti-t -> anti-b l- anti-nu_l, called as single lepton mode, where the final leptons are l = e or mu, and the final quarks are q(q')= u(d) or c(s). Polarized electron beam and CP observables and asymmetries are used to impose bounds on the anomalous couplings.

hep-ph↗

arXiv2000

Dynamics of Supersymmetric SU(n_c) and USp(2n_c) Gauge Theories

We study dynamical flavor symmetry breaking in the context of a class of N=1 supersymmetric SU(n_c) and USp(2 n_c) gauge theories, constructed from the exactly solvable N=2 theories by perturbing them with small adjoint and generic bare hypermultiplet (quark) masses. We find that the flavor U(n_{f}) symmetry in SU(n_{c}) theories is dynamically broken to $U(r)\times U(n_{f}-r)$ groups for $n_f \leq n_c$. In the r=1 case the dynamical symmetry breaking is caused by the condensation of monopoles in the $\underline{n_{f}}$ representation. For general r, however, the monopoles in the $\underline{{}_{n_{f}}C_{r}}$ representation, whose condensation could explain the flavor symmetry breaking but would produce too-many Nambu--Goldstone multiplets, actually ``break up'' into ``magnetic quarks'' which condense and induce confinement and the symmetry breaking. In USp(2n_c) theories with $n_f \leq n_c + 1$, the flavor SO(2n_f) symmetry is dynamically broken to U(n_f), but with no description in terms of a weakly coupled local field theory. In both SU(n_c) and USp(2 n_c) theories, with larger numbers of quark flavors, besides the vacua with these properties, there exist also vacua with no flavor symmetry breaking.

hep-th↗

arXiv1999

Magnetic Field Generation from Self-Consistent Collective Neutrino-Plasma Interactions

A new Lagrangian formalism for self-consistent collective neutrino-plasma interactions is presented in which each neutrino species is described as a classical ideal fluid. The neutrino-plasma fluid equations are derived from a covariant relativistic variational principle in which finite-temperature effects are retained. This new formalism is then used to investigate the generation of magnetic fields and the production of magnetic helicity as a result of collective neutrino-plasma interactions.

hep-ph↗

arXiv1999

Neutrino Mass Anarchy

What is the form of the neutrino mass matrix which governs the oscillations of the atmospheric and solar neutrinos? Features of the data have led to a dominant viewpoint where the mass matrix has an ordered, regulated pattern, perhaps dictated by a flavor symmetry. We challenge this viewpoint, and demonstrate that the data are well accounted for by a neutrino mass matrix which appears to have random entries.

hep-ph↗

arXiv1999

Earth Matter Effect in 7Be Solar Neutrino Experiments

We determine the sensitivity of the KamLAND and Borexino experiments to the neutrino regeneration effect in the Earth as a function of Delta m^2 and theta, using realistic numbers for the signal and background rates. We compare the results obtained with the chi^2 method with those obtained from the conventional day-night asymmetry analysis. We also investigate how well one should be able to measure the neutrino oscillation parameters if a large day-night asymmetry is observed, taking the LOW solution as an example. We present an enlarged parameter space, which contains mixing angles greater than pi/4 where the heavy mass eigenstate is predominantly nu_e, and determine the electron neutrino survival probability for this traditionally neglected scenario. We emphasize that this portion of the parameter space yields different physics results when dealing with the MSW solutions to the solar neutrino puzzle and should not be neglected.

hep-ph↗

arXiv1999

CP Violation in Hyperon Decays from Supersymmetry

It was pointed out recently that supersymmetry can generate flavor-changing gluonic dipole operators with sufficiently large coefficients to dominate the observed value of epsilon'/epsilon. We point out that the same operators contribute to direct CP violation in hyperon decay and can generate a CP violating asymmetry A(Λ^0_-) in the range probed by the current E871 experiment. Interestingly, models that naturally reproduce the relation lambda = sqrt{m_d/m_s} do not generate epsilon'/epsilon but could lead to an A(Λ^0_-) of O(10^{-3}).

hep-ph↗

arXiv1999

Can epsilon'/epsilon be supersymmetric?

I first motivate why we may want to look at possible new physics contributions to epsilon' given relatively clear experimental but unclear theoretical situations. I reexamine the supersymmetric contribution to epsilon' and find an important one generally missed in the literature. Based on rather model-independent arguments based on flavor symmetries, an estimate of the possible supersymmetric epsilon' is given, which interestingly come around the reported values without fine-tuning. If the observed values are dominated by supersymmetry, it is likely to give interesting consequences on hyperon CP violation, mu --> e+gamma, and neutron and electron electric dipole moments.

hep-ph↗