Implications of $\delta_{CP}=-90^\circ$ towards determining hierarchy and octant at T2K and T2K-II

TL;DR

This paper explores how the T2K experiment's hint of a CP phase of -90° impacts the determination of neutrino mass hierarchy and octant, emphasizing the importance of neutrino and antineutrino data in resolving degeneracies.

Contribution

It analyzes the implications of a -90° CP phase on hierarchy and octant determination, highlighting the role of antineutrino data in resolving degeneracies and improving sensitivity.

Findings

Normal hierarchy is favored if δ_CP = -90° is true.

Antineutrino data helps resolve octant degeneracies.

Combined data still leaves octant undetermined, but hierarchy is identified as normal.

Abstract

The T2K experiment has provided the first hint for the best-fit value for the leptonic CP phase $\delta_{CP} \sim -90^\circ$ from neutrino data. This is now corroborated by the NO$\nu$A neutrino runs. We study the implications for neutrino mass hierarchy and octant of $\theta_{23}$ in the context of this data assuming that the true value of $\delta_{CP}$ in nature is $-90^\circ$. Based on simple arguments on degeneracies in the probabilities we show that a clear signal of $\delta_{CP}=-90^\circ$ coming from T2K neutrino (antineutrino) data is only possible if the true hierarchy is normal and the true octant is higher (lower). Thus if the T2K neutrino and antineutrino data are fitted separately and both give the true value of $\delta_{CP}=-90^\circ$, this will imply that nature has chosen the true hierarchy to be normal and $\theta_{23} \approx 45^\circ$. However we find that the combined fit of neutrino and antineutrino data will still point to true hierarchy as normal but the octant of $\theta_{23}$ will remain undetermined. We do our analysis for both, the current projected exposure ($7.8 \times 10^{21}$ pot) and planned extended exposure ($20 \times 10^{21}$ pot). We also present the CP discovery potential of T2K emphasizing on the role of antineutrinos. We find that one of the main contribution of the antineutrino data is to remove the degenerate solutions with the wrong octant. Thus the antineutrino run plays a more significant role for those hierarchy-octant combinations for which this degeneracy is present. If this degeneracy is absent, then only neutrino run gives a better result for fixed $\theta_{13}$. However if we marginalize over $\theta_{13}$ then, sensitivity corresponding to mixed run can be better than pure neutrino run.