Hisakazu Minakata

Exploring neutrino mixing with low-energy superbeams

Department of Physics Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397

Resolving the neutrino mass hierarchy and CP degeneracy by two identical detectors with different baselines

We explore the possibility of the 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 a neutrino beam from the J-PARC facility in Japan with a beam power of 4 MW and megaton (Mton)-class water Cherenkov detectors, one placed in Kamioka and the other 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 a potential of resolving the neutrino mass hierarchy up to sin{sup 2}2{theta}{sub 13}>0.03 (0.055) at 2{sigma} (3{sigma}) C.L. for any values of {delta} and at the same time has the sensitivity to CP violation by 4+4 years running of {nu}{sub e} and {nu}{sub e} appearance measurement. The significantly enhanced sensitivity is due to clean detection of the modulation of the neutrino energy spectrum, which is enabled by the cancellation of systematic uncertainties between two identical detectors which receive the neutrino beam with the same energy spectrum in the absence of oscillations.

How to measure CP violation in neutrino oscillation experiments

Abstract We propose a new method for measuring CP violation in neutrino oscillation experiments. The idea is to isolate the term due to the CP-violating phase out of the oscillation probability by taking difference between yields of two (or three) detectors at path-lengths L = 250( E 1.35 GeV )( Δm 2 10 −2 eV 2 ) −1 km and at L 3 (and also at 2L 3 in the case of three detectors). We use possible hierarchies in neutrino masses suggested by the astrophysical and the cosmological observations to motivate the idea and to examine how the method works.

Measuring leptonic CP violation by low-energy neutrino oscillation experiments

Abstract We uncover an interesting phenomenon that neutrino flavor transformation in slowly varying matter density imitates almost exactly that of vacuum neutrino oscillation under suitably chosen experimental parameters. It allows us to have relatively large CP violating measure ΔP≡P(ν μ →ν e )−P( ν μ → ν e ) which is essentially free from matter effect contamination. We utilize this phenomenon to design a low-energy long-baseline neutrino oscillation experiment to measure the leptonic CP violating phase.

Inverted hierarchy of neutrino masses disfavored by supernova 1987A

Abstract We discuss the flavor conversion of supernova neutrinos in the three-flavor mixing scheme of neutrinos. We point out that by neutrino observation from supernova one can discriminate the inverted hierarchy of neutrino masses from the normal one if s132≳a few×10−4, irrespective of which oscillation solution to the solar neutrino problem is realized in nature. We perform an analysis of data of SN1987A and obtain a strong indication that the inverted mass hierarchy is disfavored unless s132≲a few×10−4.

Probing non-standard neutrino interactions with neutrino factories

We discuss the sensitivity reach of a neutrino factory measurement to non-standard neutrino interactions (NSI), which may exist as a low-energy manifestation of physics beyond the Standard Model. We use the muon appearance modes νe→νμ/e→μ and consider two detectors, one at L = 3000 km and the other at L = 7000 km; The latter is nearly at the magic baseline which is known to have a great sensitivity to matter density determination. Assuming the effects of NSI at the production and the detection are negligible, we discuss the sensitivities to NSI and the simultaneous determination of θ13 and δ by examining the effects in the neutrino propagation of various systems in which two NSI parameters eαβ are switched on. The sensitivities to off-diagonal es are found to be excellent up to small values of θ13. At sin 22θ13 = 10−4, for example, |eeτ| a few × 10−3 at 3σ CL for 2 degrees of freedom, whereas the ones for the diagonal es are also acceptable, |eee|(|eττ|) 0.1(0.2) at the same CL. We demonstrate that the two-detector setting is powerful enough to resolve the θ13-NSI confusion problem, a notorious one which is thought to be an obstacle in determining θ13 and δ. We believe that the results obtained in this paper open the door to the possibility of using neutrino factory as a discovery machine for NSI while keeping its primary function of performing precision measurements of the lepton mixing parameters.

CP violation versus matter effect in long baseline neutrino oscillation experiments

We investigate, within the framework of three generations of neutrinos, the effects of

Parameter degeneracies in neutrino oscillation measurement of leptonic CP and T violation

\mathrm{CP}

Resolving theta(23) degeneracy by accelerator and reactor neutrino oscillation experiments

violation in long-baseline neutrino oscillation experiments. We aim at illuminating the global feature of the interplay between a genuine effect due to the

Complementarity of eastern and western hemisphere long-baseline neutrino oscillation experiments

\mathrm{CP}