arXiv subjects

Hisashi Kikuchi

Publications and source records attributed to Hisashi Kikuchi.

12 recordsLinked to original sources

arXiv1998

Valley Views: Instantons, Large Order Behaviors, and Supersymmetry

The elucidation of the properties of the instantons in the topologically trivial sector has been a long-standing puzzle. Here we claim that the properties can be summarized in terms of the geometrical structure in the configuration space, the valley. The evidence for this claim is presented in various ways. The conventional perturbation theory and the non-perturbative calculation are unified, and the ambiguity of the Borel transform of the perturbation series is removed. A `proof' of Bogomolny's ``trick'' is presented, which enables us to go beyond the dilute-gas approximation. The prediction of the large order behavior of the perturbation theory is confirmed by explicit calculations, in some cases to the 478-th order. A new type of supersymmetry is found as a by-product, and our result is shown to be consistent with the non-renormalization theorem. The prediction of the energy levels is confirmed with numerical solutions of the Schrödinger equation.

hep-th↗

arXiv1997

Valleys in Quantum Mechanics

Conventionally, perturbative and non-perturbative calculations are performed independently. In this paper, valleys in the configuration space in quantum mechanics are investigated as a way to treat them in a unified manner. All the known results of the interplay of them are reproduced naturally. The prescription for separating the non-perturbative contribution from the perturbative is given in terms of the analytic continuation of the valley parameter. Our method is illustrated on a new series of examples with the asymmetric double-well potential. We obtain the non-perturbative part explicitly, which leads to the prediction of the large order behavior of the perturbative series. We calculate the first 200 perturbative coefficients for a wide range of parameters and confirm the agreement with the prediction of the valley method.

quant-ph↗

arXiv1997

Recent Developments in the Theory of Tunneling

Path-integral approach in imaginary and complex time has been proven successful in treating the tunneling phenomena in quantum mechanics and quantum field theories. Latest developments in this field, the proper valley method in imaginary time, its application to various quantum systems, complex time formalism, asympton theory for the large order analysis of the perturbation theory, are reviewed in a self-contained manner.

hep-th↗

arXiv1996

Fake Instability in the Euclidean Formalism

We study the path-integral formalism in the imaginary-time to show its validity in a case with a metastable ground state. The well-known method based on the bounce solution leads to the imaginary part of the energy even for a state that is only metastable and has a simple oscillating behavior instead of decaying. Although this has been argued to be the failure of the Euclidean formalism, we show that proper account of the global structure of the path-space leads to a valid expression for the energy spectrum, without the imaginary part. For this purpose we use the proper valley method to find a new type of instanton-like configuration, the ``valley instantons''. Although valley instantons are not the solutions of equation of motion, they have dominant contribution to the functional integration. A dilute-gas approximation for the valley instantons is shown to lead to the energy formula. This method extends the well-known imaginary-time formalism so that it can take into account the global behavior of the theory.

hep-th↗

arXiv1995

Enhancement of the Transition Magnetic Moments of a Neutrino by Degenerate Electrons

The one-loop induced magnetic dipole moments of a neutrino are examined in a background of degenerate electrons in the standard model. For the nonrelativistic neutrino, they are enhanced by a factor $ (8\pF/3\mnu) $, where \pF\ is the electron Fermi momentum and \mnu\ the neutrino mass. For the relativistic neutrino, they enhance the flavor-changing but helicity-conserving process because of the absence of the GIM cancellation.

hep-ph↗

arXiv1995

Flavor-Changing Magnetic Dipole Moment and Oscillation of a Neutrino in a Degenerate Electron Plasma

The standard model prediction for a magnetic dipole moment of a neutrino is proportional to the neutrino mass and extremely small. It also generates a flavor-changing process, but the GIM mechanism reduces the corresponding amplitude. These properties of a neutrino magnetic moment change drastically in a degenerate electron plasma. We have shown that an electron-hole excitation gives a contribution proportional to the electrons' Fermi momentum. Since this effect is absent in $μ$ and $τ$ sector, the GIM cancellation does not work. The magnetic moment induces a neutrino oscillation if a strong enough magnetic field exists in the plasma. The required magnitude of the field strength that affects the \nue\ burst from a supernova is estimated to be the order of $10^8 $ Gauss.

hep-ph↗

arXiv1995

Decay of Z into Two Light Higgs Bosons

If the standard electroweak gauge model is extended to include two or more Higgs doublets, there may be a neutral Higgs boson $h$ which is light (with a mass of say 10 GeV) but the $hZZ$ coupling is suppressed so that it has so far escaped experimental detection. However, the effective $hhZZ$ coupling is generally unsuppressed, hence the decay of Z into two light Higgs bosons plus a fermion-antifermion pair may have an observable branching fraction, especially if $h$ decays invisibly as for example in the recently proposed doublet Majoron model.

hep-ph↗

arXiv1994

Heavy Tau Neutrino as the Late Decaying Particle in the Cold Dark Matter Scenario

The tau neutrino with a mass of about 10 MeV can be the ``late decaying particle'' in the cold dark matter scenario for the formation of structure in the Universe. We show how this may be realized specifically in the recently proposed doublet Majoron model.

hep-ph↗

arXiv1994

Neutrino Decay in the Doublet Majoron Model

A new Majoron model is presented within the framework of the seesaw mechanism. Its Higgs sector consists of only doublet representations and the lepton-number violation takes place at the same scale of the electroweak symmetry breaking. This model is different from the singlet- or triplet-Majoron model in several respects: it is free from the $ρ$-parameter constraint and it provides moderately fast neutrino decay, but the constraint from the stellar cooling of red giants is satisfied only with an imposed approximate symmetry. A $τ$ neutrino as heavy as 10 MeV is possible in this model despite various cosmological and astrophysical constraints.

hep-ph↗

arXiv1993

Chiral phase dependence of fermion partition function in two dimension

The chiral phase dependence of fermion partition function in spherically symmetric U(1) gauge field background is analyzed in two dimensional space-time. A well-defined method to calculated the path integral which apply to the continuous fermion spectrum is described. The one-to-one correspondence between the nonzero energy continuous spectra of two pertinent hamiltonians, which are defined by the Dirac operator to make the path integral well-defined, is shown to be exact. The asymptotic expansion for the chiral phase dependence in $1/|m|^2$ ($m$ is mass of the fermion.) is proposed and the coefficients in the expansion are evaluated up to the next-to-next leading term. Up to this order, the chiral phase dependence is given only by the winding number of background field and the corrections vanish.

hep-th↗

arXiv1993

CP Violation and Leptogenesis

Recently a model of chaotic inflation was proposed, where the right handed sneutrinos drive the baryogenesis. We study some of the details of the model, particularly the aspect of CP violation, and determine the number of right handed sneutrinos required for the viability of such models.

hep-ph↗

arXiv1993

Poincare invariance in temporal gauge canonical quantization and $θ$-vacua

The Poincare invariance in the temporal gauge canonical quantization of QCD is shown manifestly by verifying the energy-momentum-vector and angular-momentum-tensor satisfy the Poincare algebra in the physical Hilbert space. Two different values of $θ$ for the $θ$-term in QCD lagrangian lead to different representations of the Poincare group, which are, however, connected by an unitary transformation. Thus the parameter $θ$ becomes physically irrelevant unless we can further restrict the physical Hilbert space.

hep-th↗