Measuring the Mass Hierarchy with Muon and Hadron Events in Atmospheric Neutrino Experiments

TL;DR

This paper explores how combining muon and hadron event data in atmospheric neutrino experiments enhances the measurement of the neutrino mass hierarchy, emphasizing the importance of detector resolution and data analysis techniques.

Contribution

It introduces a novel analysis method that includes hadron events with muon data, improving the sensitivity to the neutrino mass hierarchy in atmospheric neutrino experiments.

Findings

Hadron data increases mass hierarchy sensitivity.

Optimal sensitivity achieved by combining muon and hadron data.

Detector resolution impacts earth matter effect observation.

Abstract

Neutrino mass hierarchy can be measured in atmospheric neutrino experiments through the observation of earth matter effects. Magnetized iron calorimeters have been shown to be good in this regard due to their charge identification capabilities. The charged current interaction of $\nu_\mu$ in this detector, produces a muon track and a hadron shower. The direction of the muon track can be measured very accurately. We show the improvement expected in the reach of this class of experiments to the neutrino mass hierarchy, as we improve the muon energy resolution and the muon reconstruction efficiency. We next propose to include the hadron events in the analysis, by tagging them with the zenith angle of the corresponding muon and binning the hadron data first in energy and then in zenith angle. To the best of our knowledge this way of performing the analysis of the atmospheric neutrino data has not be considered before. We show that the hadron events increase the mass hierarchy sensitivity of the experiment. Finally, we show the expected mass hierarchy sensitivity in terms of the reconstructed neutrino energy and zenith angle. We show how the detector resolutions spoil the earth matter effects in the neutrino channel and argue why the sensitivity obtained from the neutrino analysis cannot be significantly better than that obtained from the analysis using muon data alone. As a result, the best mass hierarchy sensitivity is obtained when we add the contribution of the muon and the hadron data. For $\sin^22\theta_{13}=0.1$, $\sin^2\theta_{23}=0.5$, a muon energy resolution of 2\%, reconstruction efficiency of 80\% and exposure of $50\times 10$ kton-year, we could get up to $4.5\sigma$ signal for the mass hierarchy from combining the muon and hadron data.