Search for Electron Antineutrino Appearance in a Long-baseline Muon Antineutrino Beam
K. Abe, R. Akutsu, A. Ali, C. Alt, C. Andreopoulos, L. Anthony, M., Antonova, S. Aoki, A. Ariga, Y. Asada, Y. Ashida, E. T. Atkin, Y. Awataguchi,, S. Ban, M. Barbi, G. J. Barker, G. Barr, D. Barrow, C. Barry, M., Batkiewicz-Kwasniak, A. Beloshapkin, F. Bench, V. Berardi

TL;DR
The T2K experiment searched for electron antineutrino appearance in a long-baseline beam, observing a slight excess of events that marginally disfavors no appearance, consistent with PMNS matrix predictions.
Contribution
This study provides the first measurement of electron antineutrino appearance in a long-baseline experiment with constraints on the PMNS matrix parameters.
Findings
15 candidate events observed with 9.3 expected background
Disfavoring no electron antineutrino appearance at 2.40σ significance
Data consistent with PMNS matrix predictions
Abstract
Electron antineutrino appearance is measured by the T2K experiment in an accelerator-produced antineutrino beam, using additional neutrino beam operation to constrain parameters of the PMNS mixing matrix. T2K observes 15 candidate electron antineutrino events with a background expectation of 9.3 events. Including information from the kinematic distribution of observed events, the hypothesis of no electron antineutrino appearance is disfavored with a significance of 2.40{\sigma} and no discrepancy between data and PMNS predictions is found. A complementary analysis that introduces an additional free parameter which allows non-PMNS values of electron neutrino and antineutrino appearance also finds no discrepancy between data and PMNS predictions.
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