arXiv subjects

Sho Sugama

Publications and source records attributed to Sho Sugama.

6 recordsLinked to original sources

arXiv2026

Impact of matter effects on the unitarity test of lepton mixing

Testing the unitarity of the lepton mixing matrix, in a manner analogous to the unitarity tests of the CKM matrix in the quark sector, is an important step toward probing physics beyond the standard three-generation framework. In long baseline neutrino oscillation experiments, the formula of the oscillation probabilities can be written as a sum of terms with various combinations of the mixing-matrix elements, and their coefficients depend differently on energy. By observing the spectral information of long baseline experiments such as T2HK and a future neutrino factory at J-PARC with a $\nu_e$ beam, the elements of the mixing matrix can be extracted without assuming a specific parametrization of the mixing matrix. We investigate how such an extraction method can be applied to neutrino oscillations by taking into account matter effects, and discuss how one can test unitarity of the mixing matrix in future long baseline experiments. As a concrete example, we examine the unitarity test by using a four-generation model, where we look at a quantity which should be vanishing in a unitary model. Among possible combinations of measurements, the most powerful test can be provided from the energy spectra of the CP-conjugate appearance channels $\nu_\mu \to \nu_e$ and $\bar{\nu}_\mu \to \bar{\nu}_e$ at T2HK, as well as from the T-conjugate pair $\nu_\mu \to \nu_e$ and $\nu_e \to \nu_\mu$ available at neutrino factories.

hep-ph↗

arXiv2026

Solar Neutrino Flux Fluctuations Caused by Solar Gravity Modes

We have evaluated fluctuations in neutrino fluxes caused by solar gravity (g) modes based on the analysis of linear adiabatic oscillation of a spherically symmetric star. We find that the first-order fluctuation is zero due to geometrical cancellation. We still find that the second-order fluctuation is non-zero, which consists of time-varying and non-time-varying components. The amplitude of the time-varying component is small (${\sim} 10^{-9}$ in relative difference, in the case of $\mathrm{^{8}B}$ neutrino) and well below the detection limits of the current neutrino detectors, when we assume the g-mode amplitude parameter $A_{n \ell}$ to be $10^{-5}$, which corresponds to the assumed maximum relative temperature perturbation inside the Sun. Thus, it is at the moment fair to say that detecting individual solar g-modes via the solar neutrino flux measurement is almost impossible. However, the net increase in the mean neutrino flux that originates from the non-time-varying component could be non-negligible. In particular, since $A_{n \ell}$ may be related to convection amplitude, which could change in accordance with the solar magnetic activity, the total net increase in the neutrino flux, which is proportional to $A_{n \ell}^2$, should also change with the solar activity cycle. Such a long-period variation~(${\sim} 11$~years) in the neutrino flux could thus be interpreted as evidence for a bunch of solar g-modes. Comparison of the theoretical prediction with the solar neutrino measurements by, e.g., Super-Kamiokande, may have a potential to put constraints on the theory of the excitation mechanism of solar g-modes.

astro-ph.SR↗

arXiv2025

Unitarity test of lepton mixing via energy dependence of neutrino oscillation

We study the method to test the unitarity of the lepton mixing matrix by using only the long baseline neutrino oscillation experiments, such as the combination of the T2HK experiment and the one with the $\nu_e$ beam from a future neutrino factory at J-PARC. Without a specific parametrization, one can directly extract the elements of the lepton mixing matrix by observing the energy dependence of the oscillation probabilities. A non-trivial test of the unitarity under the three-generation assumption can thus be made possible by examining the orthogonality in a similar manner to the unitarity triangle in the quark sector. As the first trial, we perform the analysis based on the simplified situation where the matter effects in the neutrino oscillation can be neglected. Under this simplified analysis, we demonstrate the observation of the unitarity violation in the $3\times3$ part of the lepton mixing matrix for a parameter set in the four-generation model. The statistically most significant measurement can be provided by the energy dependences of the combination of the CP conjugate modes, $\nu_\mu \to \nu_e$ and $\bar \nu_\mu \to \bar \nu_e$, at T2HK and, independently, by the T conjugate modes, $\nu_\mu \to \nu_e$ and $\nu_e \to \nu_\mu$, with the latter measured at the neutrino factory experiments.

hep-ph↗

arXiv2024

T violation at a future neutrino factory (Contribution to the 25th International Workshop on Neutrinos from Accelerators)

We study the possibility of measuring T (time reversal) violation in a future long baseline neutrino oscillation experiment. By assuming a neutrino factory as a staging scenario of a muon collider at the J-PARC site, we find that the $\nu_e \to \nu_\mu$ oscillation probabilities can be measured with good accuracy at the Hyper-Kamiokande detector. By comparing with the probability of the time-reversal process, $\nu_\mu \to \nu_e$, measured at the T2K/T2HK, one can determine the CP phase $\delta$ in the neutrino mixing matrix if $|\sin(\delta)|$ is large enough. The determination of $\delta$ can be made with poor knowledge of the matter density of the earth as T violation is almost insensitive to the matter effects. The comparison of CP and T-violation measurements, {\it \`a la} the CPT theorem, provides us with a non-trivial check of the three neutrino paradigm based on the quantum field theory. This proceeding is based on JHEP 12 (2024), 014 [arXiv:hep-ph/2407.05807].

hep-ph↗

arXiv2024

T violation at a future neutrino factory

We study the possibility of measuring T (time reversal) violation in a future long baseline neutrino oscillation experiment. By assuming a neutrino factory as a staging scenario of a muon collider at the J-PARC site, we find that the ${\nu}_e \to {\nu}_{\mu}$ oscillation probabilities can be measured with a good accuracy at the Hyper-Kamiokande detector. By comparing with the probability of the time-reversal process, ${\nu}_{\mu} \to {\nu}_e$, measured at the T2K/T2HK experiments, one can determine the CP phase $\delta$ in the neutrino mixing matrix if $| \sin(\delta)|$ is large enough. The determination of $\delta$ can be made with poor knowledge of the matter density of the earth as T violation is almost insensitive to the matter effects. The comparison of CP and T-violation measurements, ${\it \`a\ la}$ the CPT theorem, provides us with a non-trivial check of the three neutrino paradigm based on the quantum field theory.

hep-ph↗

arXiv2023

Octant Degeneracy and Plots of Parameter Degeneracy in Neutrino Oscillations Revisited

The three kinds of parameter degeneracy in neutrino oscillation, the intrinsic, sign and octant degeneracy, form an eight-fold degeneracy. The nature of this eight-fold degeneracy can be visualized on the ($\sin^22\theta_{13}$, $1/\sin^2\theta_{23}$)-plane, through quadratic curves defined by $P(\nu_\mu\to\nu_e)=$ const. and $P(\bar{\nu}_\mu\to\bar{\nu}_e)=$ const., along with a straight line $P(\nu_\mu\to\nu_\mu)=$ const. After $\theta_{13}$ was determined by reactor neutrino experiments, the intrinsic degeneracy in $\theta_{13}$ transforms into an alternative octant degeneracy in $\theta_{23}$, which can potentially be resolved by incorporating the value of $P(\nu_\mu\to\nu_\mu)$. In this paper, we analytically discuss whether this octant parameter degeneracy is resolved or persists in the future long baseline accelerator neutrino experiments, such as T2HK, DUNE, T2HKK and ESS$\nu$SB. It is found that the energy spectra near the first oscillation maximum are effective in resolving the octant degeneracy, whereas those near the second oscillation maximum are not.

hep-ph↗