First measurement of high-energy reactor antineutrinos at Daya Bay

TL;DR

This paper reports the first direct measurement of high-energy reactor antineutrinos at Daya Bay, observing a significant flux and spectrum deficit, and providing a new data-based antineutrino spectrum for the 8-12 MeV range.

Contribution

It presents the first measurement of high-energy reactor antineutrinos, including spectrum unfolding and flux deficit analysis, advancing understanding of reactor antineutrino production.

Findings

Detected nearly 9000 inverse beta decay events in 8-12 MeV range

Rejected the no-antineutrino hypothesis above 10 MeV with 6.2 sigma significance

Observed a 29% flux deficit compared to recent models

Abstract

This Letter reports the first measurement of high-energy reactor antineutrinos at Daya Bay, with nearly 9000 inverse beta decay candidates in the prompt energy region of 8-12~MeV observed over 1958 days of data collection. A multivariate analysis is used to separate 2500 signal events from background statistically. The hypothesis of no reactor antineutrinos with neutrino energy above 10~MeV is rejected with a significance of 6.2 standard deviations. A 29\% antineutrino flux deficit in the prompt energy region of 8-11~MeV is observed compared to a recent model prediction. We provide the unfolded antineutrino spectrum above 7 MeV as a data-based reference for other experiments. This result provides the first direct observation of the production of antineutrinos from several high-$Q_{\beta}$ isotopes in commercial reactors.