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Mayumi Aoki

Publications and source records attributed to Mayumi Aoki.

At least 73 records · Page 4Linked to original sources
arXiv2002

Measuring the CP-violating phase by a long base-line neutrino experiment with Hyper-Kamiokande

We study the sensitivity of a long-base-line (LBL) experiment with neutrino beams from the High Intensity Proton Accelerator (HIPA), that delivers 10^{21} POT per year, and a proposed 1Mt water-Cherenkov detector, Hyper-Kamiokande (HK) 295km away from the HIPA, to the CP phase (delta_{M N S}) of the three-flavor lepton mixing matrix. We examine a combination of the nu_mu narrow-band beam (NBB) at two different energies, vev{p_pi}=2, 3GeV, and the bar{nu}_mu NBB at vev{p_pi}=2GeV. By allocating one year each for the two nu_mu beams and four years for the bar{nu}_mu beam, we can efficiently measure the nu_mu to nu_e and bar{nu}_mu to bar{nu}_e transition probabilities, as well as the nu_mu and bar{nu}_mu survival probabilities. CP violation in the lepton sector can be established at 4sigma (3sigma) level if the MSW large-mixing-angle scenario of the solar-neutrino deficit is realized, |\dmns| or |delta{M N S}-180^{circ}| > 30^{circ}, and if 4|U_{e3}|^2 (1-|U_{e3}|^2) equiv sin^2 2 theta_{CHOOZ} > 0.03 (0.01). The phase delta_{M N S} is more difficult to constrain by this experiment if there is little CP violation, delta_{M N S} sim 0^{circ} or 180^{circ}, which can be distinguished at 1sigma level if sin^2 2 theta_{CHOOZ} >~ 0.01.

hep-ph↗
arXiv2002

Physics potential of future long baseline neutrino oscillation experiments with KEK-JAERI HIPA

We study physics potential of Very Long Base-Line (VLBL) Neutrino-Oscillation Experiments with the High Intensity Proton Accelerator, which will be completed by the year 2007 in Tokai-village, Japan. As a target, a 100 kton-level water-$\check {\rm C}$erenkov detector is considered at 2,100 km away. Assuming the pulsed narrow-band $ν_μ$ beams, we study sensitivity of such experiments to the neutrino mass hierarchy, the mass-squared differences, one CP phase and three angles of the lepton-flavor-mixing matrix. We find that experiments at a distance 2,100 km can determine the neutrino mass hierarchy if the mixing matrix element $|U_{e3}|$ is not too small. The CP phase and $|U_{e3}|$ can be constrained if the large-mixing-angle solution of the solar-neutrino deficit is realized.

hep-ph↗
arXiv2001

Large effects on $\BsBs$ mixing by vector-like quarks

We calculate the contributions of the vector-like quark model to $\BsBs$ mixing, taking into account the constraints from the decay $B\to X_sγ$. In this model the neutral bosons mediate flavor-changing interactions at the tree level. However, $\BsBs$ mixing is dominated by contributions from the box diagrams with the top quark and the extra up-type quark. In sizable ranges of the model parameters, the mixing parameter $x_s$ is much different from the standard model prediction.

hep-ph↗
arXiv2001

Contributions of Vector-Like Quarks to $B \to X_s γ$ and $B_s^0 \bar B_s^0$ Mixing

The radiative decay $\BSgamma$ and the mixing $\BsBs$ are discussed in the vector-like quark model which contains extra SU(2)-singlet quarks with the same electric charges as the up-type and down-type quarks. Constraints on the extended Cabibbo-Kobayashi-Maskawa matrix are obtained from the experimental results for the branching ratio of $\BSgamma$ . Within these experimental bounds, any value of the branching ratio can be accounted for by the contributions from the vector-like quarks. In sizable ranges of the model parameters, the mixing parameter for $\BsBs$ is much different from the prediction of the standard model.

hep-ph↗
arXiv2000

Decay rate asymmetry for $B \to X_s γ$ in SUSY model

We discuss a rate asymmetry for the radiative $B$-meson decay $B \to X_s γ$ within the framework of the supersymmetric standard model based on N=1 supergravity. This model contains new sources of CP violation. In spite of severe experimental constraints on the electric dipole moment of the neutron, a new CP-violating phase may not be suppressed, leading to a sizable enhancement of the decay rate asymmetry. The magnitude of the asymmetry is predicted to be larger than that by the standard model in wide parameter regions where the branching ratio is consistent with its experimental bounds. A possible maximal magnitude is about 0.1, which will be well accessible at B factories.

hep-ph↗
arXiv2000

New supersymmetric standard model with stable proton

We discuss a supersymmetric extension of the standard model with an extra U(1) gauge symmetry. In this model, the proton stability is guaranteed by the gauge symmetry without invoking R parity. The gauge symmetry breakdown automatically generates an effective $μ$ term and large Majorana masses for right-handed neutrinos. The supersymmetry-soft-breaking terms for scalar fields could be universal at a very high energy scale, and the electroweak symmetry is broken through radiative corrections.

hep-ph↗
arXiv2000

Contributions of vector-like quarks to radiative B meson decay

We study the decay B\to X_sγin a minimal extension of the standard model with extra up- and down-type quarks whose left- and right-handed components are both SU(2) singlets. Constraints on the extended Cabibbo-Kobayashi-Maskawa matrix are obtained from the experimental results for the branching ratio. Even if the extra quarks are too heavy to be detected in near-future colliders, the branching ratio could have a value which is non-trivially different from the prediction of the standard model.

hep-ph↗
arXiv2000

Supersymmetric Extension of the Standard Model with Naturally Stable Proton

A new supersymmetric standard model based on N=1 supergravity is constructed, aiming at natural explanation for the proton stability without invoking an ad hoc discrete symmetry through R parity. The proton is protected from decay by an extra U(1) gauge symmetry. Particle contents are necessarily increased to be free from anomalies, making it possible to incorporate the superfields for right-handed neutrinos and an SU(2)-singlet Higgs boson. The vacuum expectation value of this Higgs boson, which induces spontaneous breakdown of the U(1) symmetry, yields large Majorana masses for the right-handed neutrinos, leading to small masses for the ordinary neutrinos. The linear coupling of SU(2)-doublet Higgs superfields, which is indispensable to the superpotential of the minimal supersymmetric standard model, is replaced by a trilinear coupling of the Higgs superfields, so that there is no mass parameter in the superpotential. The energy dependencies of the model parameters are studied, showing that gauge symmetry breaking is induced by radiative corrections. Certain ranges of the parameter values compatible with phenomena at the electroweak energy scale can be derived from universal values of masses-squared and trilinear coupling constants for scalar fields at a very high energy scale.

hep-ph↗
arXiv1999

A Supersymmetric Model with an Extra U(1) Gauge Symmetry

In the standard model the proton is protected from decay naturally by gauge symmetries, whereas in the ordinary minimal supersymmetric standard model an ad hoc discrete symmetry is imposed for the proton stability. We present a new supersymmetric model in which the proton decay is forbidden by an extra U(1) gauge symmetry. Particle contents are necessarily increased to be free from anomalies, incorporating right-handed neutrinos. Both Dirac and Majorana masses are generated for neutrinos, yielding non-vanishing but small masses. The superpotential consists only of trilinear couplings and the mass parameter $μ$ of the minimal model is induced by spontaneous breaking of the U(1) symmetry.

hep-ph↗
arXiv1999

CP asymmetry for radiative B-meson decay in the supersymmetric standard model

We study a rate asymmetry in the inclusive decay $\Bsgamma$, assuming the supersymmetric standard model based on N=1 supergravity. A complex coefficient $A$ for scalar-trilinear couplings is the source of CP violation, which is contained in the mass-squared matrices for squarks. The model parameters are nontrivially constrained by experimental results for the branching ratio of the radiative decay and for the electric dipole moments of the neutron and the electron. The decay rate asymmetry is predicted to be much larger than that by the standard model in a wide region of the parameter space compatible with the experiments. Its magnitude can be maximally around 0.1, which will be well accessible at B factories.

hep-ph↗
arXiv1998

Perturbation Analysis of Deformed Q-Balls and Primordial Magnetic Field

We study the excited states of the Q-balls by performing stationary perturbation on the spherical Q-balls. We find the exact solution of the stationary perturbation of the global Q-ball with thin wall approximation. For local Q-balls we solve the equations of motion for the perturbative part approximately by using expansion about the coupling constant. Furthermore we comment on the magnetic field generated by the excited Q-balls during the phase transition precipitated by solitosyhthesis and give an implication into cosmology.

hep-ph↗
arXiv1998

Decay rate asymmetry in $B \to X_s γ$ as a signature of supersymmetry

In the supersymmetric standard model, the radiative $B$-meson decay $B \to X_s γ$ could receive a large contribution from a new source of flavor-changing neutral current, which also violates CP invariance. The CP-conjugate processes then may sizably differ in decay width. If a new CP-violating phase is not suppressed, this decay rate asymmetry can be significantly larger than the prediction by the standard model, possibly detectable at B factories. Such a large asymmetry may be implied by the measured branching ratio which is consistent with the standard model.

hep-ph↗
arXiv1998

Decay Rate Asymmetry of Top Squark

We discuss a decay rate asymmetry of the top squark, which is induced by a new source of CP violation intrinsic in the supersymmetric standard model. Although new sources of CP violation in this model are severely constrained from the electric dipole moment of the neutron, an unsuppressed CP-violating phase can still coexist with a top squark whose mass is accessible by near-future colliders. Then the dominant decay mode of the top squark has a width different from its CP conjugate process. The magnitude of this $CP$ asymmetry becomes of order $10^{-3}$.

hep-ph↗
arXiv1998

Decay Rate Asymmetry of the Top Quark through Light Squarks

A decay rate asymmetry of the top quark is discussed within the framework of the supersymmetric standard model. Although new sources of CP violation in this model are severely constrained from the electric dipole moments of the neutron and the electron, there is a possibility that a top squark has a mass of around 100 GeV and CP violation in its interaction with a top quark is unsuppressed. Then partial widths could be different between the decays $t\to bW^+$ and $\bar t\to \bar bW^-$. The magnitude of this CP asymmetry can be of order $10^{-3}$, which may be detectable in the near future.

hep-ph↗
arXiv1998

Baryon asymmetry and neutron EDM

Baryon asymmetry of the universe can be generated by new sources of $CP$ violation intrinsic in supersymmetric extension of the standard model. This scenario predicts the neutron electric dipole moment to be not much smaller than its present experimental upper bound.

hep-ph↗
arXiv1998

Baryogenesis from top squark transport

Baryon asymmetry of the universe is generated in the supersymmetric standard model (SSM) through the charge transport mechanism mediated by top squarks. Necessary CP violation originates from a complex phase contained in mass-squared matrices of squarks and sleptons. This complex phase also gives contributions to the electric dipole moments (EDMs) of the neutron and the electron. The amount of induced baryon asymmetry is consistent with its observed value within reasonable ranges of SSM parameters. The EDMs are predicted to be slightly smaller than their experimental upper limits.

hep-ph↗
arXiv1998

CP violation in the top-quark system through light squarks

We study a decay rate asymmetry of the top quark in the supersymmetric standard model, taking into account the constraints from the electric dipole moments of the neutron and the electron. One CP-violating phase contained in the mass-squared matrices of squarks and sleptons is allowed to have an unsuppressed value, even if their masses are of order 100 GeV. Sizable $CP$ violation could then occur in the production and decay of the top quark through interactions with top squarks. The asymmetry between the widths for the decays $t\to bW^+$ and $\bar t\to \bar bW^-$ can be of order $10^{-3}$, which will possibly be detectable in the near future.

hep-ph↗