Precision Measurement of Neutrino Oscillation Parameters with KamLAND

TL;DR

The KamLAND experiment precisely measured neutrino oscillation parameters using increased reactor anti-neutrino exposure, providing strong constraints on $ heta_{12}$ and $ riangle m^2_{21}$, and combining results with solar neutrino data.

Contribution

This work provides the most precise measurement of $ riangle m^2_{21}$ and $ heta_{12}$ using reactor neutrinos, with improved data and analysis techniques.

Findings

Precise value of $ riangle m^2_{21}$ at approximately 7.58 x 10^{-5} eV^2.

Stringent constraints on mixing angle $ heta_{12}$ with $ an^2 heta_{12}$ around 0.56.

Rejection of an undistorted reactor neutrino spectrum at >5$\sigma$.

Abstract

The KamLAND experiment has determined a precise value for the neutrino oscillation parameter $\Delta m^{2}_{21}$ and stringent constraints on $\theta_{12}$. The exposure to nuclear reactor anti-neutrinos is increased almost fourfold over previous results to 2.44$\times10^{32}$ proton-yr due to longer livetime and an enlarged fiducial volume. An undistorted reactor $\bar{\nu}_{e}$ energy spectrum is now rejected at >5$\sigma$. Extending the analysis down to the inverse beta decay energy threshold, and incorporating geo-neutrinos, gives a best-fit at $\Delta m^{2}_{21}$= $7.58^{+0.14}_{-0.13}(stat)^{+0.15}_{-0.15}(syst)\times10^{-5}$ eV$^{2}$ and $\tan^2 \theta_{12}$=$0.56^{+0.10}_{0.07}(stat)^{+0.10}_{-0.06}(syst)$. Local $\Delta \chi^2$-minima at higher and lower $\Delta m^{2}_{21}$ are disfavored at >4$\sigma$. Combining with solar neutrino data, we obtain $\Delta m^{2}_{21}$= $7.59^{+0.21}_{-0.21}\times10^{-5}$ eV$^{2}$ and $\tan^2 \theta_{12}$=$0.47^{+0.06}_{-0.05}$.