Measurement of Muon Antineutrino Oscillations with an Accelerator-Produced Off-Axis Beam

Abstract

T2K reports its first measurements of the parameters governing the disappearance of $\bar{\hat{I}}{½}_{\hat{I}}¼$ in an off-axis beam due to flavor change induced by neutrino oscillations. The quasimonochromatic $\bar{\hat{I}}{½}_{\hat{I}}¼$ beam, produced with a peak energy of 0.6 GeV at J-PARC, is observed at the far detector Super-Kamiokande, 295 km away, where the $\bar{\hat{I}}{½}_{\hat{I}}¼$ survival probability is expected to be minimal. Using a dataset corresponding to $4.01\times 10^{20}$ protons on target, $34$ fully contained $\hat{I}¼$-like events were observed. The best-fit oscillation parameters are ${\sin }^2(\bar{\hat{I}}{¸}_{23})=0.45$ and $|\hat{I}”{\bar{m}}_{32}^2|=2.51\times 10^{-3}$ eV${}^2$ with 68% confidence intervals of 0.38 - 0.64 and 2.26 - 2.80 $\times 10^{-3}$ eV${}^2$ respectively. These results are in agreement with existing antineutrino parameter measurements and also with the $\hat{I}{½}_{\hat{I}}¼$ disappearance parameters measured by T2K.

8 pages, 4 figures, revision made following to referee comments, the link to the data release page was added