Neutrino masses and mixing: 2008 status

We present an analysis of atmospheric neutrino data from a 33.0 kiloton-year (535-day) exposure of the Super-Kamiokande detector. The data exhibit a zenith angle dependent deficit of muon neutrinos which is inconsistent with expectations based on calculations of the atmospheric neutrino flux. Experimental biases and uncertainties in the prediction of neutrino fluxes and cross sections are unable to explain our observation. The data are consistent, however, with two-flavor nu_mu nu_tau oscillations with sin^2(2theta)>0.82 and 5x10^-4 < delta m^2 < 6x10^-3 eV^2 at 90% confidence level.

Nailing down the unknown neutrino mixing angle theta_13 is one of the most important goals in current lepton physics. In this context, we perform a global analysis of neutrino oscillation data, focusing on theta_13, and including recent results [Neutrino 2008, Proceedings of the XXIII International Conference on Neutrino Physics and Astrophysics, Christchurch, New Zealand, 2008 (unpublished)]. We discuss two converging hints of theta_13>0, each at the level of ~1sigma: an older one coming from atmospheric neutrino data, and a newer one coming from the combination of solar and long-baseline reactor neutrino data. Their combination provides the global estimate sin^2(theta_13) = 0.016 +- 0.010 (1sigma), implying a preference for \theta_13>0 with non-negligible statistical significance (~90% C.L.). We discuss possible refinements of the experimental data analyses, which might sharpen such intriguing indication.

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 \(\tan^2 \theta_{12}\)=\(0.47^{+0.06}_{-0.05}\).