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Hisakazu Minakata

Publications and source records attributed to Hisakazu Minakata.

At least 73 records · Page 4Linked to original sources
arXiv2006

Resolving theta_{23} Degeneracy by Accelerator and Reactor Neutrino Oscillation Experiments

If the lepton mixing angle theta_{23} is not maximal, there arises a problem of ambiguity in determining theta_{23} due to the existence of two degenerate solutions, one in the first and the other in the second octant. We discuss an experimental strategy for resolving the theta_{23} octant degeneracy by combining reactor measurement of theta_{13} with accelerator nu_{mu} disappearance and nu_{e} appearance experiments. The robustness of the theta_{23} degeneracy and the difficulty in lifting it only by accelerator experiments with conventional nu_{mu} (and nu_{mu}-bar) beam are demonstrated by analytical and numerical treatments. Our method offers a way to overcome the difficulty and can resolve the degeneracy between solutions sin^2 theta_{23} = 0.4 and sin^2 theta_{23} = 0.6 if sin^2 (2 theta_{13}) \gsim 0.05 at 95% CL by assuming the T2K phase II experiment and a reactor measurement with an exposure of 10 GW.kt.yr. The dependence of the resolving power of the octant degeneracy on the systematic errors of reactor experiments is also examined.

hep-ph↗
arXiv2006

Do Neutrinos Violate CP?

In trying to answer the question in the title of my talk, I have argued on ground of naturalness that leptonic CP violation is very likely to exist both in the form of the Kobayashi-Maskawa type and the Majorana type phases. The latter part of the argument has to be backed up by a general argument by Yanagida which states that neutrinos must be Majorana particles because our universe is asymmetric with respect to baryon number. The argument is reviewed. Since the naturalness argument raises the possibility of naturally small theta_{13} and theta_{23} - pi/4, I discuss possible experimental methods for probing into these two small quantities. They include recent proposal of the resonant nu_e-bar absorption reaction enhanced by the M"ossbauer effect which may allow extremely high sensitivity for not only theta_{13} but also Delta m^2_{31}. The issue of how to resolve the theta_{23} octant degeneracy is briefly discussed with emphasis on the atmospheric neutrino observation and the reactor-accelerator combined methods.

hep-ph↗
arXiv2005

Testing CPT Symmetry with Supernova Neutrinos

Diagnosing core of supernova requires favor-dependent reconstruction of three species of neutrino spectra, ν_e, \barν_{e} and ν_x (a collective notation for ν_μ, \barν_μ, ν_τ, and \barν_τ). We point out that, assuming the information available, CPT symmetry can be tested with supernova neutrinos. We classify all possible level crossing patterns of neutrinos and antineutrinos into six cases and show that half of them contains only the CPT violating mass and mixing patterns. We discuss how additional informations from terrestrial experiments help identifying CPT violation by narrowing down the possible flux patterns. Although the method may not be good at precision test, it is particularly suited to uncover gross violation of CPT such as different mass patterns of neutrinos and antineutrinos. The power of the method is due to the nature of level crossing in supernova which results in the sensitivity to neutrino mass hierarchy and to the unique characteristics of in situ preparation of both νand \barν beams. Implications of our discussion to the conventional analyses with CPT conservation are also briefly mentioned.

hep-ph↗
arXiv2005

Resolving Neutrino Mass Hierarchy and CP Degeneracy by Two Identical Detectors with Different Baselines

We explore the possibility of simultaneous determination of neutrino mass hierarchy and the CP violating phase by using two identical detectors placed at different baseline distances. We focus on a possible experimental setup using neutrino beam from J-PARC facility in Japan with beam power of 4MW and megaton (Mton)-class water Cherenkov detectors, one placed in Kamioka and the other at somewhere in Korea. We demonstrate, under reasonable assumptions of systematic uncertainties, that the two-detector complex with each fiducial volume of 0.27 Mton has potential of resolving neutrino mass hierarchy up to sin^2 2theta_{13} > 0.03 (0.055) at 2σ(3σ) CL for any values of delta and at the same time has the sensitivity to CP violation by 4 + 4 years running of nu_e and nu_e-bar appearance measurement. The significantly enhanced sensitivity is due to clean detection of modulation of neutrino energy spectrum, which is enabled by cancellation of systematic uncertainties between two identical detectors which receive the neutrino beam with the same energy spectrum in the absence of oscillations.

hep-ph↗
arXiv2005

Quark-Lepton Complementarity; a Review

It has been recognized recently that there is a remarkable empirical relation between lepton and quark mixing angles, theta_{12} + theta_C \approx pi/4. If not accidental, it should testify for yet uncovered new relationship between the fundamental twin particles in nature which only differ in their ability to feel color. The nontrivial structure which is presumed to exist behind the empirical relation is named as ``quark-lepton complementarity''. In this talk, I review the idea at the kind request of the organizer. Starting from pedagogical discussions of bimaximal mixing, which likely to be involved in the whole picture, I try to give a flavor of the new field which is still in rapid development. Toward the more balanced knowledges of flavor mixing in lepton and quark sectors, I describe a promising way for precision measurement of theta_{12} which utilizes solar and reactor neutrinos.

hep-ph↗
arXiv2004

Determination of theta_{23} in Long-Baseline Neutrino Oscillation Experiments with Three-Flavor Mixing Effects

We examine accuracy of theta_{23} determination in future long-baseline (LBL) nu_{mu} disappearance experiments in the three-flavor mixing scheme of neutrinos. Despite that the error of sin^2{2theta_{23}} is indeed a few % level at around the maximal mixing, we show that the error of physics variable s^2_{23} is large, delta(s^2_{23})/s^2_{23} \simeq 10-20%, depending upon regions of theta_{23}. The errors are severely affected by the octant degeneracy of theta_{23}, and delta(s^2_{23}) is largely amplified by the Jacobian factor relating these two variables in a region near to the maximal mixing. The errors are also affected by the uncertainty due to unknown value of theta_{13}; delta(s^2_{23}) is doubled at off maximal in the second octant of theta_{23} where the effect is largest. To overcome this problem, we discuss combined analysis with nu_e appearance measurement in LBL experiments, or with reactor measurement of theta_{13}. For possible relevance of sub-leading effects even in the next-generation LBL experiments, we give a self-contained derivation of the survival probability to the next to leading order in s^2_{13} and Delta m^2_{21} / Delta m^2_{31}.

hep-ph↗
arXiv2004

Neutrino Mixing and Quark-Lepton Complementarity

As a result of identification of the solution to the solar neutrino problem, a rather precise relation theta_{sun} + theta_C = pi/4 between the leptonic 1-2 mixing angle theta_{sun} and the Cabibbo angle has emerged. It would mean that the lepton and the quark mixing angles add up to the maximal, suggesting a deep structure by which quarks and leptons are interrelated. We refer the relation ``quark-lepton complementarity'' (QLC) in this paper. We formulate general conditions under which the QLC relation is realized. We then present several scenarios which lead to the relation and elaborate on phenomenological consequences which can be tested by the future experiments. We also discuss implications of the QLC relation for the quark-lepton symmetry and the mechanism of neutrino mass generation.

hep-ph↗
arXiv2004

Unity of CP and T Violation in Neutrino Oscillations

In a previous work a simultaneous P- CP[P] and P- T[P] bi-probability plot was proposed as a useful tool for unified graphical description of CP and T violation in neutrino oscillation. The ``baseball diamond'' structure of the plot is understood as a consequence of the approximate CP-CP and the T-CP relations obeyed by the oscillation probabilities. In this paper, we make a step forward toward deeper understanding of the unified graphical representation by showing that these two relations are identical in its content, suggesting a truly unifying view of CP and T violation in neutrino oscillations. We suspect that the unity reflects the underlying CPT theorem. We also present calculation of corrections to the CP-CP and the T-CP relations to leading order in Delta m^2_{21} / Delta m^2_{31} and s^2_{13}.

hep-ph↗
arXiv2004

New Views on the Problem of CP Violation

After briefly recollecting basic features of the good-old way of detecting CP violation by comparing nu_{e} and nu_{e}-bar appearance measurement in long-baseline (LBL) neutrino oscillation experiments, I discuss two new ways of exploring leptonic CP violation. First, I discuss the reactor-LBL method in which reactor measurement of theta_{13} is combined with nu_{e} (no nu_{e}-bar) appearance measurement in LBL.. Assuming \sim 10^{3} GW_{th} ton year operation of a reactor experiment, CP sensitivity at 90 % CL is shown to exists in sin^2{2theta_{13}} > 0.03 (0.04) with 2 years running of Hyper-Kamiokande (10 years running of SK). Second, I review the method which I call the BNL strategy, in which one tries to explore CP violation through observing oscillatory pattern of neutrino oscillation. Motivated by the approval of the JPARC neutrino program, I also discuss a low-energy realization of the BNL strategy. It is meant to measure neutrino oscillation at the first and the second oscillation maxima by two HKs placed at Kamioka and somewhere in Korea. It is suggested by very rough argument that 8 years running in nu_{e} and nu_{e}-bar modes with one HK at Kamioka is equivalent to 2 years running in nu_{e} mode with two HK complex.

hep-ph↗
arXiv2003

Exploring Leptonic CP Violation by Reactor and Neutrino Superbeam Experiments

We point out the possibility that reactor measurement of theta_{13}, when combined with high-statistics nu_e appearance accelerator experiments, can detect leptonic CP violation. Our proposal is based on a careful statistical analysis under reasonable assumptions on systematic errors, assuming 2 years running of the neutrino mode J-PARC -> Hyper-Kamiokande experiment and a few years running of a reactor experiment with 100 ton detectors at the Kashiwazaki-Kariwa nuclear power plant. We show that the method can be arranged to be insensitive to the intrinsic parameter degeneracy but is affected by the one due to unknown sign of Delta m^2_{31}.

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arXiv2003

Long Baseline Neutrino Oscillations: Parameter Degeneracies and JHF/NuMI Complementarity

A summary of the parameter degeneracy issue for long baseline neutrino oscillations is presented and how a sequence of measurements can be used to resolve all degeneracies. Next, a comparison of the JHF and NuMI Off-Axis proposals is made with emphasis on how both experiments running neutrinos can distinguish between the normal and inverted hierarchies provided the E/L of NuMI is less than or equal to the E/L of JHF. Due to the space limitations of this proceedings only an executive style summary can be presented here, but the references and transparencies of the talk contain the detailed arguments.

hep-ph↗
arXiv2003

CP and T Violation in Neutrino Oscillations

In this short lecture, we discuss some basic phenomenological aspects of CP and T violation in neutrino oscillation. Using CP/T trajectory diagrams in the bi-probability space, we try to sketch out some essential features of the interplay between the effect of CP/T violating phase and that of the matter in neutrino oscillation.

hep-ph↗
arXiv2003

Double Beta Decay Constraints on Neutrino Masses and Mixing; Reanalysis with KamLAND Data

In the light of KamLAND data just released, we reanalyze and update the constraints on neutrino masses and mixing parameters, the most general ones that can be derived in three-flavor mixing scheme of neutrinos with use of the bounds imposed by neutrinoless double beta decay search and reactor experiments. We point out that with KamLAND data and assuming Majorana neutrinos one can derive, for the first time, an upper bound on neutrino contribution to the cosmological Ωparameter, Ω_ν \leq 0.070 h^{-2}, by using the current upper bound on mass parameters obtained by Heidelberg-Moscow group, < m >_{ββ} < 0.35 eV (90 % CL). If the bound is tighten to < m >_{ββ} < 0.1 eV by future experiments, it would lead to a far severer bound Ω_ν \lsim 0.01 h^{-2}.

hep-ph↗
arXiv2003

The Complementarity of Eastern and Western Hemisphere Long-Baseline Neutrino Oscillation Experiments

We present a general formalism for extracting information on the fundamental parameters associated with neutrino masses and mixings from two or more long baseline neutrino oscillation experiments. This formalism is then applied to the current most likely experiments using neutrino beams from the Japan Hadron Facility (JHF) and Fermilab's NuMI beamline. Different combinations of muon neutrino or muon anti-neutrino running are considered. To extract the type of neutrino mass hierarchy we make use of the matter effect. Contrary to naive expectation, we find that both beams using neutrinos is more suitable for determining the hierarchy provided that the neutrino energy divided by baseline ($E/L$) for NuMI is smaller than or equal to that of JHF. Whereas to determine the small mixing angle, $θ_{13}$, and the CP or T violating phase $δ$, one neutrino and the other anti-neutrino is most suitable. We make extensive use of bi-probability diagrams for both understanding and extracting the physics involved in such comparisons.

hep-ph↗
arXiv2002

Parameter Degeneracies in Neutrino Oscillation Measurement of Leptonic CP and T Violation

The measurement of the mixing angle θ_{13}, sign of Δm^2_{13} and the CP or T violating phase δis fraught with ambiguities in neutrino oscillation. In this paper we give an analytic treatment of the paramater degeneracies associated with measuring the ν_μ-> ν_e probability and its CP and/or T conjugates. For CP violation, we give explicit solutions to allow us to obtain the regions where there exist two-fold and four-fold degeneracies. We calculate the fractional differences, Δθ/ \barθ, between the allowed solutions which may be used to compare with the expected sensitivities of the experiments. For T violation we show that there is always a complete degeneracy between solutions with positive and negative Δm^2_{13} which arises due to a symmetry and cannot be removed by observing one neutrino oscillation probability and its T conjugate. Thus, there is always a four fold parameter degeneracy apart from exceptional points. Explicit solutions are also given and the fractional differences are computed. The bi-probability CP/T trajectory diagrams are extensively used to illuminate the nature of the degeneracies.

hep-ph↗
arXiv2002

CP and T Trajectory Diagrams for a Unified Graphical Representation of Neutrino Oscillations

Recently the CP trajectory diagram was introduced to demonstrate the difference between the intrinsic CP violating effects to those induced by matter for neutrino oscillation. In this paper we introduce the T trajectory diagram. In these diagrams the probability for a given oscillation process is plotted versus the probability for the CP- or the T-conjugate processes, which forms an ellipse as the CP or T violating phase is varied. Since the CP and the T conjugate processes are related by CPT symmetry, even in the presence of matter, these two trajectory diagrams are closely related with each other and form a unified description of neutrino oscillations in matter.

hep-ph↗
arXiv2002

Constraints on Neutrino Mixing Parameters By Observation of Neutrinoless Double Beta Decay

Assuming positive observation of neutrinoless double bata decay together with the CHOOZ reactor bound, we derive constraints imposed on neutrino mixing parameters, the solar mixing angle θ_{12} and the observable mass parameter < m >_β in single beta decay experiments. We show that 0.05 eV < < m >_β < 2 eV at the best fit parameters of the LMA MSW solar neutrino solution by requiring the range of the parameter < m >_{ββ} deduced from recently announced double beta decay events at 95 % CL with +/- 50 % uncertainty of nuclear matrix elements.

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