The JHF-Kamioka neutrino project

TL;DR

The paper outlines the JHF-Kamioka neutrino project, a long-baseline accelerator experiment designed to study neutrino masses and mixing beyond the Standard Model. It describes a high-intensity, narrow-band beam from a 50 GeV proton source directed at Super-Kamiokande over 295 km, with a two-phase plan culminating in Hyper-Kamiokande for increased sensitivity. The approach combines quasi-elastic energy reconstruction in a water Cherenkov detector, precision measurements of $\Delta m^2_{23}$ and $\sin^22\theta_{23}$ via $\nu_\mu$ disappearance, and $\nu_e$ appearance searches to probe $\theta_{13}$ and CP violation, while employing near detectors and beam monitors to constrain systematics. The first stage emphasizes high-precision oscillation measurements and $\nu_e$ appearance sensitivity, with neutral-current channels testing $\nu_μ\rightarrow\nu_τ$ and sterile-neutrino scenarios, laying groundwork for a potential proton-decay search and CP-violation studies in the subsequent phase.

Abstract

The JHF-Kamioka neutrino project is a second generation long base line neutrino oscillation experiment that probes physics beyond the Standard Model by high precision measurements of the neutrino masses and mixing. A high intensity narrow band neutrino beam is produced by secondary pions created by a high intensity proton synchrotron at JHF (JAERI). The neutrino energy is tuned to the oscillation maximum at ~1 GeV for a baseline length of 295 km towards the world largest water Cerenkov detector, Super-Kamiokande. Its excellent energy resolution and particle identification enable the reconstruction of the initial neutrino energy, which is compared with the narrow band neutrino energy, through the quasi-elastic interaction. The physics goal of the first phase is an order of magnitude better precision in the nu_mu to nu_tau oscillation measurement (delta(Delta m_23^2)=10^-4 eV^2 and delta(sin^22theta_23)=0.01), a factor of 20 more sensitive search in the nu_mu to nu_e appearance (sin^22theta_{mu e} ~ 0.5sin^22theta_{13}>0.003), and a confirmation of the nu_mu to nu_tau oscillation or discovery of sterile neutrinos by detecting the neutral current events. In the second phase, an upgrade of the accelerator from 0.75 MW to 4 MW in beam power and the construction of 1 Mt Hyper-Kamiokande detector at Kamioka site are envisaged. Another order of magnitude improvement in the nu_mu to nu_e oscillation sensitivity, a sensitive search of the CP violation in the lepton sector (CP phase "delta" down to 10-20 degrees), and an order of magnitude improvement in the proton decay sensitivity is also expected.

Figures (15)

• Figure 1: Baseline of the JHF-Kamioka neutrino project
• Figure 2: (left) The scatter plots of the reconstructed neutrino energy versus the true one for $\nu_\mu$ events. The method of the energy reconstruction is expressed in Equation \ref{['jhf:eq:qe']}. (right) The energy resolution of $\nu_\mu$ events for 2 degree off-axis beam. The shaded (red) histogram is for the true QE events.
• Figure 3: Layout of JHF.
• Figure 4: Neutrino energy spectra of charged current interactions. Thick solid, dashed and dotted histograms in (a) are LE1.5$\pi$, LE2$\pi$ and LE3$\pi$, and those in (b) are OA1$^\circ$, OA2$^\circ$ and OA3$^\circ$, respectively. WBB is drawn by thin solid histogram in both (a) and (b).
• Figure 5: Comparison of $\nu_e$ and $\nu_\mu$ spectra for (a) LE2$\pi$ and (b) OA2$^\circ$. Solid (black) histogram is $\nu_\mu$ and dashed (red) one is $\nu_e$. Hatched area is contribution from K decay.