arXiv subjects

Hisakazu Minakata

Publications and source records attributed to Hisakazu Minakata.

At least 37 records · Page 2Linked to original sources

arXiv2019

Perturbing neutrino oscillations around the solar resonance

Atmospheric neutrinos at low energies, $E \lsim 500$ MeV, is known to be a rich source of information of lepton mixing parameters. We formulate a simple perturbative framework to elucidate the characteristic features of neutrino oscillation at around the solar-scale enhancement due to the matter effect. The clearest message we could extract from our perturbation theory is that CP violation in the appearance oscillation probability is large, a factor of $\sim 10$ times larger than CP violation at around the atmospheric-scale oscillation maximum. Underlying mechanism for it is that one of the suppression factors on the CP phase dependent terms due to smallness of $Δm^2_{21} / Δm^2_{31}$ are dynamically lifted by the solar-scale enhancement. Our framework has a unique feature as a perturbation theory in which large $Δm^2_{31}$ term outside the key 1-2 sector for the solar-scale resonance does not yield sizeable corrections. On the contrary, the larger the $Δm^2_{31}$, the smaller the higher order corrections.

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arXiv2019

Non-unitary evolution of neutrinos in matter and the leptonic unitarity test

We present a comprehensive study of the three-active plus $N$ sterile neutrino model as a framework for constraining leptonic unitarity violation induced at energy scales much lower than the electroweak scale. We formulate a perturbation theory with expansion in small unitarity violating matrix element $W$ while keeping (non-$W$ suppressed) matter effect to all orders. We show that under the same condition of sterile state masses $0.1\, \text{eV}^2 \lesssim m^2_{J} \lesssim (1-10)\, \text{GeV}^2$ as in vacuum, assuming typical accelerator based long-baseline neutrino oscillation experiment, one can derive a very simple form of the oscillation probability which consists only of zeroth-order terms with the unique exception of probability leaking term $\mathcal{C}_{αβ}$ of $\mathcal{O} (W^4)$. We argue, based on our explicit computation to fourth-order in $W$, that all the other terms are negligibly small after taking into account the suppression due to the mass condition for sterile states, rendering the oscillation probability {\em sterile-sector model independent}. Then, we identify a limited energy region in which this suppression is evaded and the effects of order $W^2$ corrections may be observable. Its detection would provide another way, in addition to detecting $\mathcal{C}_{αβ}$, to distinguish between low-scale and high-scale unitarity violation. We also solve analytically the zeroth-order system in matter with uniform density to provide a basis for numerical evaluation of non-unitary neutrino evolution.

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arXiv2018

Compact Perturbative Expressions for Neutrino Oscillations in Matter: II

In this paper we rewrite the neutrino mixing angles and mass squared differences in matter given, in our original paper, in a notation that is more conventional for the reader. Replacing the usual neutrino mixing angles and mass squared differences in the expressions for the vacuum oscillation probabilities with these matter mixing angles and mass squared differences gives an excellent approximation to the oscillation probabilities in matter. Comparisons for T2K, NOvA, T2HKK and DUNE are also given for neutrinos and anti-neutrinos, disappearance and appearance channels, normal ordering and inverted ordering.

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arXiv2018

Analytic Neutrino Oscillation Probabilities in Matter: Revisited

We summarize our recent paper on neutrino oscillation probabilities in matter, explaining the importance, relevance and need for simple, highly accurate approximations to the neutrino oscillation probabilities in matter. Simple expressions for the neutrino mixing angles and mass squared differences in matter are given in an Appendix. Using these in the vacuum oscillation probabilities instead of the vacuum values, gives an excellent approximation to the oscillation probabilities in matter.

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arXiv2017

An Effective Two-Flavor Approximation for Neutrino Survival Probabilities in Matter

It is known in vacuum that the three-flavor neutrino survival probability can be approximated by the effective two-flavor form to first orders in $ε\equiv Δm^2_{21} / Δm^2_{31}$, with introduction of the effective $Δm^2_{αα}$ ($α= e, μ, τ$), in regions of neutrino energy $E$ and baseline $L$ such that $Δm^2_{31} L / 2E \sim π$. Here, we investigate the question of whether the similar effective two-flavor approximation can be formulated for the survival probability in matter. Using a perturbative framework with the expansion parameters $ε$ and $s_{13} \propto \sqrtε$, we give an affirmative answer to this question and the resultant two-flavor form of the probability is valid to order $ε$. However, we observe a contrived feature of the effective $Δm^2_{αα} (a)$ in matter. It ceases to be a combination of the fundamental parameters and has energy dependence, which may be legitimate because it comes from the matter potential. But, it turned out that $Δm^2_{μμ} (a)$ becomes $L$-dependent, though $Δm^2_{ee} (a)$ is not, which casts doubt on adequacy of the concept of effective $Δm^2$ in matter. We also find that the appearance probability in vacuum admits, to order $ε$, the similar effective two-flavor form with a slightly different effective $Δm^2_{βα}$ from the disappearance channel. A general result is derived to describe suppression of the matter effect in the oscillation probability.

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arXiv2017

A framework for testing leptonic unitarity by neutrino oscillation experiments

If leptonic unitarity is violated by new physics at an energy scale much lower than the electroweak scale, which we call low-scale unitarity violation, it has different characteristic features from those expected in unitarity violation at high-energy scales. They include maintaining flavor universality and absence of zero-distance flavor transition. We present a framework for testing such unitarity violation at low energies by neutrino oscillation experiments. Starting from the unitary 3 active plus $N$ (arbitrary integer) sterile neutrino model we show that by restricting the active-sterile and sterile-sterile neutrino mass squared differences to $\gtrsim$ 0.1 eV$^2$ the oscillation probability in the $(3+N)$ model becomes insensitive to details of the sterile sector, providing a nearly model-independent framework for testing low-scale unitarity violation. Yet, the presence of the sterile sector leaves trace as a constant probability leaking term, which distinguishes low-scale unitarity violation from the high-scale one. The non-unitary mixing matrix in the active neutrino subspace is common for the both cases. We analyze how severely the unitarity violation can be constrained in $ν_{e}$-row by taking a JUNO-like setting to simulate medium baseline reactor experiments. Possible modification of the features of the $(3+N)$ model due to matter effect is discussed to first order in the matter potential.

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arXiv2016

Compact Perturbative Expressions For Neutrino Oscillations in Matter

We further develop and extend a recent perturbative framework for neutrino oscillations in uniform matter density so that the resulting oscillation probabilities are accurate for the complete matter potential versus baseline divided by neutrino energy plane. This extension also gives the exact oscillation probabilities in vacuum for all values of baseline divided by neutrino energy. The expansion parameter used is related to the ratio of the solar to the atmospheric $Δm^2$ scales but with a unique choice of the atmospheric $Δm^2$ such that certain first-order effects are taken into account in the zeroth-order Hamiltonian. Using a mixing matrix formulation, this framework has the exceptional feature that the neutrino oscillation probability in matter has the same structure as in vacuum, to all orders in the expansion parameter. It also contains all orders in the matter potential and $\sinθ_{13}$. It facilitates immediate physical interpretation of the analytic results, and makes the expressions for the neutrino oscillation probabilities extremely compact and very accurate even at zeroth order in our perturbative expansion. The first and second order results are also given which improve the precision by approximately two or more orders of magnitude per perturbative order.

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arXiv2015

Simple and Compact Expressions for Neutrino Oscillation Probabilities in Matter

We reformulate perturbation theory for neutrino oscillations in matter with an expansion parameter related to the ratio of the solar to the atmospheric $Δm^2$ scales. Unlike previous works, we use a renormalized basis in which certain first-order effects are taken into account in the zeroth-order Hamiltonian. We show that the new framework has an exceptional feature that leads to the neutrino oscillation probability in matter with the same structure as in vacuum to first order in the expansion parameter. It facilitates immediate physical interpretation of the formulas, and makes the expressions for the neutrino oscillation probabilities extremely simple and compact. We find, for example, that the $ν_e$ disappearance probability at this order is of a simple two-flavor form with an appropriately identified mixing angle and $Δm^2$. More generally, all the oscillation probabilities can be written in the universal form with the channel-discrimination coefficient of $0,~\pm1$ or simple functions of $θ_{23}$. Despite their simple forms they include all order effects of $θ_{13}$ and all order effects of the matter potential, to first order in our expansion parameter.

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arXiv2015

Flavor-Universal Form of Neutrino Oscillation Probabilities in Matter

We construct a new perturbative framework to describe neutrino oscillation in matter with the unique expansion parameter ε, which is defined as Δm^2_{21} / Δm^2_{ren} with the renormalized atmospheric Δm^2_{ren} \equiv Δm^2_{31} - s^2_{12} Δm^2_{21}. It allows us to derive the maximally compact expressions of the oscillation probabilities in matter to order εin the form akin to those in vacuum. This feature allows immediate physical interpretation of the formulas, and facilitates understanding of physics of neutrino oscillations in matter. Moreover, quite recently, we have shown that our three-flavor oscillation probabilities P(ν_α\rightarrow ν_β) in all channels can be expressed in the form of universal functions of L/E. The ν_e disappearance oscillation probability P(ν_e \rightarrow ν_e) has a special property that it can be written as the two-flavor form which depends on the single frequency. This talk is based on the collaborating work with Stephen Parke [1].

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arXiv2015

Constraint on Neutrino Decay with Medium-Baseline Reactor Neutrino Oscillation Experiments

The experimental bound on lifetime of nu_3, the neutrino mass eigenstate with the smallest nu_e component, is much weaker than those of nu_1 and nu_2 by many orders of magnitude to which the astrophysical constraints apply. We argue that the future reactor neutrino oscillation experiments with medium-baseline (~ 50 km), such as JUNO or RENO-50, has the best chance of placing the most stringent constraint on nu_3 lifetime among all neutrino experiments which utilize the artificial source neutrinos. Assuming decay into invisible states, we show by a detailed chi^2 analysis that the nu_3 lifetime divided by its mass, tau_3/m_3, can be constrained to be tau_3/m_3 > 7.5 (5.5) x 10^{-11} s/eV at 95% (99%) C.L. by 100 kt.years exposure by JUNO. It may be further improved to the level comparable to the atmospheric neutrino bound by its longer run. We also discuss to what extent nu_3 decay affects mass-ordering determination and precision measurements of the mixing parameters.

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arXiv2015

Possible Interpretations of IceCube High-Energy Neutrino Events

We discuss possible interpretations of the 37 high energy neutrino events observed by the IceCube experiment in the South Pole. We examine the possibility to explain the observed neutrino spectrum exclusively by the decays of a heavy long-lived particle of mass in the PeV range. We compare this with the standard scenario, namely, a single power-law spectrum related to neutrinos produced by astrophysical sources and a viable hybrid situation where the spectrum is a product of two components: a power-law and the long-lived particle decays. We present a simple extension of the Standard Model that could account for the heavy particle decays that are needed in order to explain the data. We show that the current data equally supports all above scenarios and try to evaluate the exposure needed in order to falsify them in the future.

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arXiv2015

Constraining Majorana CP Phase in Precision Era of Cosmology and Double Beta Decay Experiment

We show that precision measurement of (1) sum of neutrino masses by cosmological observation and (2) lifetime of neutrinoless double beta decay in ton-scale experiments, with supplementary use of (3) effective mass measured in single beta decay experiment, would allow us to obtain information on the Majorana phase of neutrinos. To quantify the sensitivity to the phase we use the CP exclusion fraction, a fraction of the CP phase parameter space that can be excluded for a given set of assumed input parameters, a global measure for CP violation. We illustrate the sensitivity under varying assumptions, from modest to optimistic ones, on experimental errors and theoretical uncertainty of nuclear matrix elements. Assuming that the latter can be reduced to a factor of ~1.5 we find that one of the two Majorana phases (denoted as alpha_(21)) can be constrained by excluding ~10-40% of the phase space at 2sigma CL even with the modest choice of experimental error for the lowest neutrino mass of 0.1 eV. The characteristic features of the sensitivity to alpha_(21), such as dependences on the true values of alpha_(21), are addressed.

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arXiv2014

Interplay between Appearance and Disappearance Channels for Precision Measurements of $θ_{23}$ and $δ$

We discuss how the CP violating phase $δ$ and the mixing angle $θ_{23}$ can be measured precisely in an environment where there are strong correlations between them. This is achieved by paying special attention to the mutual roles and the interplay between the appearance and the disappearance channels in long-baseline neutrino oscillation experiments. We analyze and clarify the general structure of the $θ_{23} - θ_{13} - δ$ degeneracy for both the appearance and disappearance channels in a more complete fashion than what has previously been discussed in the literature. A full understanding of this degeneracy is of vital importance if $θ_{23}$ is close to maximal mixing. The relative importance between the appearance and disappearance channels depends upon the particular setup and how close to maximal mixing Nature has chosen the value for $θ_{23}$. For facilities that operate with a narrow band beam or a wide band beam centered on the first oscillation extremum, the contribution of the disappearance channel depends critically on the systematic uncertainties assumed for this channel. Whereas for facilities that operate at energies above the first oscillation extremum or at the second oscillation extremum the appearance channels dominate. On the other hand, for $δ$ we find that the disappearance channel usually improves the sensitivity, modestly for facilities around the first oscillation extremum and more significantly for facilities operating at an energy above the first oscillation extremum, especially near $δ\sim \pm π/2$.

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arXiv2014

Neutrino Physics Now and in the Near Future

The current status of neutrino physics is reviewed with some near future perspective. After recollecting the birth of modern neutrino physics with nonzero masses and flavor mixing, I summarize the present status of measurement of the mixing parameters in 2-3, 1-2, and 1-3 sectors of the MNS matrix. Then, I describe the attempts to uncover the regularities, if any, in the measured values of the mixing angles; mostly reviewing. Yet, a possible large deviation of θ_{23} to the second octant may trigger interests in the triangle relation of the lepton mixing angles. In the latter part of my lecture some perspective of determination of the mass hierarchy and measurement of lepton Kobayashi-Maskawa phase $δ$ are described. Finally, I discuss the prospects of the new, fast developing field of high-energy neutrino astrophysics, and the emerging new precision era of cosmology and particle physics. I conclude with optimistic speculations.

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arXiv2013

Correlated, Precision Measurements of θ_{23} and $δ$ using only the Electron Neutrino Appearance Experiments

Precision measurement of the leptonic CP violating phase $δ$ will suffer from the, then surviving, large uncertainty of sin^2 θ_{23} of 10-20% in the experimentally interesting region near maximal mixing of θ_{23}. We advocate a new method for determination of both θ_{23} and $δ$ at the same time using only the ν_e and \barν_e appearance channels, and show that sin^2 θ_{23} can be determined automatically with much higher accuracy, approximately a factor of six, than \sin δ. In this method, we identify a new degeneracy for the simultaneous determination of θ_{23} and δ, the θ_{23} intrinsic degeneracy, which must be resolved in order to achieve precision measurement of these two parameters. Spectral information around the vacuum oscillation maxima is shown to be the best way to resolve this degeneracy.

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arXiv2012

Solar Neutrino Observables Sensitive to Matter Effects

We discuss constraints on the coefficient A_{MSW} which is introduced to simulate the effect of weaker or stronger matter potential for electron neutrinos with the current and future solar neutrino data. The currently available solar neutrino data leads to a bound A_{MSW} = 1.47^{-0.42}_{+0.54} (^{-0.82}_{+1.88}) at 1σ(3σ) CL, which is consistent with the Standard Model prediction A_{MSW} = 1. For weaker matter potential (A_{MSW} < 1), the constraint which comes from the flat ^8B neutrino spectrum is already very tight, indicating the evidence for matter effects. Whereas for stronger matter potential (A_{MSW} > 1), the bound is milder and is dominated by the day-night asymmetry of ^8B neutrino flux recently observed by Super-Kamiokande. Among the list of observable of ongoing and future solar neutrino experiments, we find that (1) an improved precision of the day-night asymmetry of ^8B neutrinos, (2) precision measurements of the low energy quasi-monoenergetic neutrinos, and (3) the detection of the upturn of the ^8B neutrino spectrum at low energies, are the best choices to improve the bound on A_{MSW}.

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arXiv2012

Phenomenology of future neutrino experiments with large Theta(13)

The question "how small is the lepton mixing angle θ_{13}?" had a convincing answer in a surprisingly short time, θ_{13} \simeq 9^{\circ}, a large value comparable to the Chooz limit. It defines a new epoch in the program of determining the lepton mixing parameters, opening the door to search for lepton CP violation of the Kobayashi-Maskawa-type. I discuss influences of the large value of θ_{13} to search for CP violation and determination of the neutrino mass hierarchy, the remaining unknowns in the standard three-flavor mixing scheme of neutrinos. I emphasize the following two points: (1) Large θ_{13} makes determination of the mass hierarchy easier. It stimulates to invent new ideas and necessitates quantitative reexamination of practical ways to explore it. (2) However, large θ_{13} does not quite make CP measurement easier so that we do need a "guaranteeing machine" to measure CP phase δ.

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arXiv2011

If Theta(13) is large, then what?

As indicated by the recent data obtained by the T2K and the MINOS experiments θ_{13} can be large, even be comparable to the Chooz limit. Assuming that it will be confirmed by ongoing reactor and accelerator experiments I discuss its possible implications in the context of how to explore the remaining unknowns in the lepton flavor mixing. While it opens wide range of possibilities to explore CP and the mass hierarchy, I emphasize that the large θ_{13} allows us to take "all in one" (everything in a lunch box) approach.

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