# Extraction of the $^{235}$U and $^{239}$Pu Antineutrino Spectra at Daya Bay

## TL;DR

This paper presents the first extraction of individual antineutrino spectra from $^{235}$U and $^{239}$Pu at Daya Bay, revealing significant discrepancies with existing models in the 4-6 MeV energy range.

## Contribution

It introduces a novel method to extract isotope-specific antineutrino spectra using fission fraction evolution, improving understanding of reactor antineutrino emissions.

## Key findings

- Observed 7-9% excess in spectra compared to models.
- Discrepancy with Huber-Mueller model at 4.0σ for $^{235}$U.
- Disagreement in the prompt energy spectrum at 5.3σ.

## Abstract

This Letter reports the first extraction of individual antineutrino spectra from $^{235}$U and $^{239}$Pu fission and an improved measurement of the prompt energy spectrum of reactor antineutrinos at Daya Bay. The analysis uses $3.5\times 10^6$ inverse beta-decay candidates in four near antineutrino detectors in 1958 days. The individual antineutrino spectra of the two dominant isotopes, $^{235}$U and $^{239}$Pu, are extracted using the evolution of the prompt spectrum as a function of the isotope fission fractions. In the energy window of 4--6~MeV, a 7\% (9\%) excess of events is observed for the $^{235}$U ($^{239}$Pu) spectrum compared with the normalized Huber-Mueller model prediction. The significance of discrepancy is $4.0\sigma$ for $^{235}$U spectral shape compared with the Huber-Mueller model prediction. The shape of the measured inverse beta-decay prompt energy spectrum disagrees with the prediction of the Huber-Mueller model at $5.3\sigma$. In the energy range of 4--6~MeV, a maximal local discrepancy of $6.3\sigma$ is observed.