First Results from KamLAND: Evidence for Reactor Antineutrino Disappearance

Abstract

KamLAND has measured the flux of ${\overline{\ensuremath{\nu}}}_{e}$'s from distant nuclear reactors. We find fewer ${\overline{\ensuremath{\nu}}}_{e}$ events than expected from standard assumptions about ${\overline{\ensuremath{\nu}}}_{e}$ propagation at the 99.95% C.L. In a $162\text{ }\mathrm{\text{ton}}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{y}\mathrm{r}$ exposure the ratio of the observed inverse $\ensuremath{\beta}$-decay events to the expected number without ${\overline{\ensuremath{\nu}}}_{e}$ disappearance is $0.611\ifmmode\pm\else\textpm\fi{}0.085\mathrm{(}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{)}\ifmmode\pm\else\textpm\fi{}0.041\mathrm{(}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{)}$ for ${\overline{\ensuremath{\nu}}}_{e}$ energies $>3.4\text{ }\text{ }\mathrm{M}\mathrm{e}\mathrm{V}$. In the context of two-flavor neutrino oscillations with CPT invariance, all solutions to the solar neutrino problem except for the ``large mixing angle'' region are excluded.