The need for an early anti-neutrino run of NOvA

Abstract

The moderately large value of $\ty$, 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 $\anu$ runs, will be able to determine the mass hierarchy if one of the following two favourable combinations is true: normal hierarchy with $-180^\circ \leq \dcp \leq 0$ or inverted hierarchy with $0\leq \dcp \leq 180^\circ$. In this report, we study the hierarchy reach of the first 3 years of \nova data. Since $\sin^2 2 \tz$ is measured to be non-maximal, $\tz$ can be either in the lower or higher octant. Pure $\nu$ data is affected by $\ty$-hierarchy and octant-hierarchy degeneracies, which limit the hierarchy sensitivity of such data. A combination of $\nu$ and $\anu$ 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 $\anu$ modes give good hierarchy sensitivity for all the four combinations.