Antineutrino Science in KamLAND

TL;DR

KamLAND's research provides crucial evidence for neutrino oscillations, measures neutrino mixing parameters, and pioneers the detection of Earth's geoneutrinos, significantly advancing understanding in neutrino physics and Earth's interior composition.

Contribution

This paper reports the first detection of geoneutrinos and precise measurements of neutrino oscillation parameters using KamLAND data, confirming the LMA solution to the solar neutrino problem.

Findings

Fewer reactor antineutrinos observed than expected, confirming oscillation effects.

Measured neutrino oscillation parameters with high precision.

Detected geoneutrinos from Earth's interior, informing Earth's composition models.

Abstract

The primary goal of KamLAND is a search for the oscillation of $\bar{\nu}_{\rm e}$$'$s emitted from distant power reactors. The long baseline, typically 180 km, enables KamLAND to address the oscillation solution of the solar neutrino problem with $\bar{\nu}_{\rm e}$ under laboratory conditions. KamLAND found fewer reactor $\bar{\nu}_{\rm e}$ events than expected from standard assumptions about $\bar{\nu}_{\rm e}$$'$s propagation at more than 9 {\sigma} confidence level (C.L.). The observed energy spectrum disagrees with the expected spectral shape at more than 5 {\sigma} C.L., and prefers the distortion from neutrino oscillation effects. A three-flavor oscillation analysis of the data from KamLAND and KamLAND + solar neutrino experiments with CPT invariance, yields ${\Delta}{\rm m}^{2}_{21} = [{7.54}^{+0.19}_{0.18},\ {7.53}^{+0.19}_{0.18}]$ $\times {10}^{-5} {\rm eV}^{2}$, $\tan^{2} {\theta}_{12} = [{0.481}^{+0.092}_{-0.080},\ {0.437}^{+0.029}_{-0.026}]$, and ${\sin}^{2} {\theta}_{13} = [{0.010}^{+0.033}_{-0.034},{0.023}^{+0.015}_{-0.015}]$. All solutions to the solar neutrino problem except for the large mixing angle (LMA) region are excluded. KamLAND also demonstrated almost two cycles of the periodic feature expected from neutrino oscillation effects. KamLAND performed the first experimental study of antineutrinos from the Earth$'$s interior so-called geoneutrinos (geo $\bar{\nu}_{\rm e}$$'$s), and succeeded in detecting geo $\bar{\nu}_{\rm e}$$`$s produced by the decays of $^{238}$U and $^{232}$Th within the Earth. Assuming a chondritic Th/U mass ratio, we obtain ${116}^{+28}_{-27}$ $\bar{\nu}_{\rm e}$ events from $^{238}$U and $^{232}$Th, corresponding a geo $\bar{\nu}_{\rm e}$ flux of ${3.4}^{+0.8}_{-0.8} \times {10}^{6} {\rm cm}^{-2}{\rm s}^{-1}$ at the KamLAND location. We evaluate various bulk silicate Earth composition models using the observed geoneutrino rate.