Global Fit of KamLAND Data and the Daya Bay Antineutrino Energy Spectrum

TL;DR

This study develops a global analysis framework combining KamLAND and Daya Bay data to assess reactor neutrino oscillation parameters, highlighting the impact of antineutrino spectrum deviations on experimental results.

Contribution

The paper introduces a weakly flux-dependent global analysis method that incorporates Daya Bay's measured spectra to refine neutrino oscillation parameter estimates.

Findings

Replacing the flux model with Daya Bay spectra slightly decreases Δm^2_{21}

Results align better with JUNO measurements

Spectrum deviations may cause experimental tensions

Abstract

Recently, the JUNO experiment published its measurement of the solar neutrino oscillation parameters $\Delta m^2_{21}$ and $\sin^2\theta_{12}$ based on 59 days of data, with central values differing by $0.2\sigma$ from those released by the KamLAND experiment in 2013. Meanwhile, short-baseline reactor neutrino oscillation experiments such as Daya Bay, RENO, and Double Chooz have observed significant deviations between the measured antineutrino spectrum and the Huber-M\"{u}ller model prediction around 5~MeV. To further investigate the impact of these deviations on the measurement of reactor neutrino oscillation parameters, we construct a global analysis framework that is weakly dependent on the reactor antineutrino flux model. This framework is based on the independently measured $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ fission antineutrino spectra from the Daya Bay experiment, combined with the public data from KamLAND. First, using the Huber-M\"{u}ller model, we successfully reproduce the KamLAND 2013 results to within $0.1\sigma$. Then, replacing the Huber-M\"{u}ller model with the Daya Bay measured antineutrino spectra in a combined analysis, we find that the best-fit value of the mass-squared difference $\Delta m^2_{21}$ decreases from $7.53^{+0.17}_{-0.16}\times10^{-5}\,\mathrm{eV^2}$ to $7.50^{+0.19}_{-0.18}\times10^{-5}\,\mathrm{eV^2}$, while the best-fit value of the mixing angle $\tan^2\theta_{12}$ also shows a decreasing trend. This result is in better agreement with the latest JUNO measurement, suggesting that differences in the predicted reactor antineutrino spectra may be an important cause of the tension between the two experiments.