The need for an early anti-neutrino run of NOvA

Abstract

The moderately large value of $\text{ty}$, measured recently by reactor experiments, is very welcome news for the future neutrino experiments. In particular, the \nova experiment, with 3 years each of $\nu$ and $\text{anu}$ runs, will be able to determine the mass hierarchy if one of the following two favourable combinations is true: normal hierarchy with $-180^{\circ }\le \text{dcp}\le 0$ or inverted hierarchy with $0\le \text{dcp}\le 180^{\circ }$. In this report, we study the hierarchy reach of the first 3 years of \nova data. Since ${\sin }^{2}2\text{tz}$ is measured to be non-maximal, $\text{tz}$ can be either in the lower or higher octant. Pure $\nu$ data is affected by $\text{ty}$-hierarchy and octant-hierarchy degeneracies, which limit the hierarchy sensitivity of such data. A combination of $\nu$ and $\text{anu}$ data is not subject to these degeneracies and hence has much better hierarchy discrimination capability. We find that, with a 3 year $\nu$ run, hierarchy determination is possible for only two of the four octant-hierarchy combinations. Equal 1.5 year runs in $\nu$ and $\text{anu}$ modes give good hierarchy sensitivity for all the four combinations.