Neutrino Oscillations: Hierarchy Question

Abstract

The only experimentally observed phenomenon that lies outside the standard model of the electroweak interaction is neutrino oscillations. A way to try to unify the extensive neutrino oscillation data is to add a phenomenological mass term to the Lagrangian that is not diagonal in the flavor basis. The goal is then to understand the world's data in terms of the parameters of the mixing matrix and the differences between the squares of the masses of the neutrinos. An outstanding question is what is the correct ordering of the masses, the hierarchy question. We point out a broken symmetry relevant to this question, the symmetry of the simultaneous interchange of hierarchy and the sign of $\theta_{13}$. We first present the results of an analysis of data that well determine the phenomenological parameters but are not sensitive to the hierarchy. We find $\theta_{13} = 0.152\pm 0.014$, $\theta_{23} = 0.25^{+0.03}_{-0.05} \pi$ and $\Delta_{32} = 2.45\pm 0.14 \times 10^{-3}$ eV$^2$, results consistent with others. We then include data that are sensitive to the hierarchy and the sign of $\theta_{13}$. We find, unlike others, four isolated minimum in the $\chi^2$-space as predicted by the symmetry. Now that Daya Bay and RENO have determined $\theta_{13}$ to be surprisingly large, the Super-K atmospheric data produce meaningful symmetry breaking such that the inverse hierarchy is preferred at the 97.2 % level.