Exploring Earth's Matter Effect in High-Precision Long-Baseline Experiments

TL;DR

This paper investigates how combining data from DUNE and T2HKK neutrino experiments can significantly improve the measurement of Earth's matter effect and related oscillation parameters, overcoming degeneracies and uncertainties in future high-precision experiments.

Contribution

It demonstrates that combined data from DUNE and T2HKK can establish Earth's matter effect at over 6σ confidence level and measure average matter density with high precision, despite parameter degeneracies.

Findings

Combined experiments can detect Earth's matter effect at >6σ confidence level.

The relative precision of measuring Earth's average matter density can reach around 10%.

Degeneracies among oscillation parameters can be effectively reduced through combined experimental data.

Abstract

The Earth's matter effect is going to play a crucial role in measuring the unknown three-flavor neutrino oscillation parameters at high confidence level in future high-precision long-baseline experiments. We observe that owing to the new degeneracies among the most uncertain oscillation parameters ($\delta_{CP}, \theta_{23}$) and the average Earth's matter density ($\rho_{avg}$) for the 1300 km baseline, the sensitivity of the upcoming Deep Underground Neutrino Experiment (DUNE) to establish Earth's matter effect reaches only about 2$\sigma$ C.L. for all possible choices of oscillation parameters. We notice that the current uncertainty in $\delta_{CP}$ degrades the measurement of $\rho_{avg}$ more as compared to $\theta_{23}$. To lift these degeneracies, we explore the possible complementarity between DUNE and Tokai to Hyper-Kamiokande (T2HK/JD) facility with a second detector in Korea, popularly known as T2HKK or JD+KD setup. While DUNE uses wide-band beam with on-axis detector, T2HKK setup plans to use narrow-band beam with two off-axis detectors: one in Japan and other in Korea. We exhibit how the high-precision measurement of $\delta_{CP}$ in JD+KD setup and the information on $\rho_{avg}$ coming from DUNE can reduce the impact of these degeneracies in both ($\rho_{avg}-\delta_{CP}$) and ($\rho_{avg}-\theta_{23}$) planes. We show that the combined data from DUNE and JD+KD setups can establish Earth's matter effect at more than 6$\sigma$ C.L. irrespective of both the choices of mass hierarchy: normal (NH) and inverted (IH), $\delta_{CP}$, and $\theta_{23}$. With the help of this combined data set, we can measure the average matter density ($\rho_{avg}$) with a relative 1$\sigma$ precision of around 11.2% (9.4%) assuming true NH (IH) and $\delta_{CP} = -90^{\circ}/90^{\circ}$.