A hybrid setup for fundamental unknowns in neutrino oscillations using T2HK ($\nu$) and $\mu$-DAR ($\bar{\nu }$)

Abstract

Neutrino mass hierarchy, CP-violation, and octant of ${\theta }_{23}$ are the fundamental unknowns in neutrino oscillations. In order to address all these three unknowns, we study the physics reach of a setup, where we replace the antineutrino run of T2HK with antineutrinos from muon decay at rest ($\mu$-DAR). This approach has the advantages of having higher statistics in both neutrino and antineutrino modes, and lower beam-on backgrounds for antineutrino run with reduced systematics. We find that a hybrid setup consisting of T2HK ($\nu$) and $\mu$-DAR ($\bar{\nu }$) in conjunction with full exposure from T2K and NO$\nu$A can resolve the issue of mass hierarchy at greater than 3$\sigma$ C.L. irrespective of the choices of hierarchy, ${\delta }_{\mathrm{CP}}$, and ${\theta }_{23}$. This hybrid setup can also establish the CP-violation at 5$\sigma$ C.L. for $\sim$ 55% choices of ${\delta }_{\mathrm{CP}}$, whereas the same for conventional T2HK ($\nu +\bar{\nu }$) setup along with T2K and NO$\nu$A is around 30%. As far as the octant of ${\theta }_{23}$ is concerned, this hybrid setup can exclude the wrong octant at 5$\sigma$ C.L. if ${\theta }_{23}$ is at least $3^{\circ }$ away from maximal mixing for any ${\delta }_{\mathrm{CP}}$.

Comments: 18 pages, 28 pdf figures, 1 table. Minor changes in the text. A new footnote added. Accepted in JHEP