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

## TL;DR

This paper proposes a hybrid neutrino experiment setup combining T2HK with muon decay at rest antineutrinos, significantly improving the ability to determine neutrino mass hierarchy, CP-violation, and octant of theta_23.

## Contribution

It introduces a novel hybrid experimental configuration that enhances sensitivity to fundamental neutrino oscillation parameters over traditional setups.

## Key findings

- Resolves mass hierarchy at >3σ with full T2K and NOνA data.
- Establishes CP-violation at 5σ for ~55% of δ_CP values.
- Excludes wrong octant if θ_23 is at least 3° away from maximal mixing.

## 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 $\delta_{\mathrm{CP}}$.