Model-independent search for T violation with T2HK and DUNE

TL;DR

This work develops a model-independent method to test time-reversal violation in neutrino oscillations by comparing appearance probabilities at two baselines for the same energy, using a near detector to control zero-distance effects. The core idea is the observable $X_T = P_{ u_1\to\nu_e}(L_2) - P_{ u_1\to\nu_e}(L_1) - \delta_0 P^{\rm ND}_{\nu_1\to\nu_e}$, which can reveal T violation if negative under plausible assumptions about oscillation frequencies and matter effects. The authors identify the T2HK and DUNE combination around $E_\nu\approx 0.86$ GeV as particularly suitable and perform GLoBES-based simulations to assess experimental requirements, including event statistics, energy resolution, and zero-distance constraints. They find that achieving $\sim$3σ sensitivity requires enhanced DUNE sub-GeV statistics (roughly a decade of neutrino running with improved resolution) and near-detector bounds at the $\sim$1% level, while antineutrino data offer limited additional sensitivity. The study highlights a promising, largely model-independent avenue to probe fundamental T violation in the lepton sector with upcoming experiments, and suggests exploring alternative baselines for potential gains.

Abstract

We consider the time reversal (T) transformation in neutrino oscillations in a model-independent way by comparing the observed transition probabilities at two different baselines at the same neutrino energy. We show that, under modest model assumptions, if the transition probability $P_{ν_μ\toν_e}$ around $E_ν\simeq 0.86$ GeV measured at DUNE is smaller than the one at T2HK the T symmetry has to be violated. Experimental requirements needed to achieve good sensitivity to this test for T violation are to obtain enough statistics at DUNE for $E_ν\lesssim 1$ GeV (around the 2nd oscillation maximum), good energy resolution (better than 10%), and near-detector measurements with a precision of order 1% or better.

Figures (6)

• Figure 1: Regions in energy and distance where the conditions \ref{['eq:cond1', 'eq:cond2']} are fulfilled. Blue and red regions correspond to \ref{['eq:cond1']} and \ref{['eq:cond2']}, respectively, and purple regions to both conditions simultaneously. In the upper panels we fix $L_1$ to $L_{\rm T2K}$ and $L_{\rm DUNE}$, respectively, vary $L_2$ on the vertical axis and show the neutrino energy on the horizontal axis. In the lower panels we show the two distances $L_{1,2}$ on the axes for four fixed energies $E_\nu = 0.85, 1.0, 1.3, 1.6$ GeV in each triangle section of the panels, respectively. We assume neutrinos and normal mass ordering.
• Figure 2: Number of $\nu_\mu\to\nu_e$ (left) and $\bar{\nu}_\mu\to\bar{\nu}_e$ (right) signal events per 0.125 GeV reconstructed neutrino energy bins. For T2HK we assume an exposure of 607.75 (1823.25) kt MW yr for neutrino (antineutrino) running. For DUNE we show spectra for a nominal exposure of 168 kt MW yr by green curves, as well as exposures increased by a factor 5 (10) for neutrinos (antineutrinos) as red curves. Dashed curves indicate events due to the wrong-sign beam component, i.e., $\bar{\nu}_\mu\to\bar{\nu}_e$ for the left panel (hardly visible) and $\nu_\mu\to\nu_e$ for the right panel. The vertical bar indicates the energy bin sensitive to the T-violation test. The insets show a zoom into the relevant energy range. We assume standard oscillations with the parameters given in \ref{['tab:osc-params']} and $\delta_{\rm CP} = 90^\circ$ (left panel) and $\delta_{\rm CP} = 270^\circ$ (right panel).
• Figure 3: T2HK + DUNE sensitivity to T violation as a function of true CP phase $\delta_{\rm CP}^{\rm true}$. The top-right panel corresponds to the predicted sensitive energy window [0.80-0.92] GeV, \ref{['eq:bin']}, top-left and bottom-left show its lower and upper neighboring bins; in the bottom-right panel we show the combination of the three bins. Green curves correspond to the default energy resolution according to \ref{['eq:resolution']}; for blue and red curves we re-scale the width of the resolutions globally by a factor 0.5 and 0.2, respectively. We assume exposures of 608 (840) kt MW yr for T2HK (DUNE) in the neutrino mode.
• Figure 4: $\Delta\chi^2_T$ as a function of the DUNE neutrino exposure for true $\delta_{\rm CP} = 90^\circ$ summing the three relevant energy bins. The T2HK exposure is kept fixed at 608 kt MW yr. Different curves correspond to different assumptions on the neutrino energy resolutions. The green curve represents our default resolution according to \ref{['eq:resolution']}, for the red (blue) curve the resolutions for both, T2HK and DUNE have been re-scaled by a factor of 0.2 (0.5), while for the magenta curve only the DUNE resolution has been re-scaled by a factor 2.
• Figure 5: Sensitivity to T violation for $\delta_{\rm CP}^{\rm true} = 90^\circ$ as a function of the prior on the effective mass-squared difference in matter, $\Delta m^2_{31,\rm eff}$, for different assumptions on the near detector constraint on the zero-distance effect, $\sigma_\epsilon = 0.1\%, 1\%, 5\%$ for the green, blue, red curves, respectively. For the solid curves we assume the default energy resolution from \ref{['eq:resolution']}, for the dashed curves the energy resolution is re-scaled by a factor 0.5 for both experiments. Exposures have been set to our default assumptions.