Ultralight dark matter in long-baseline accelerator neutrino oscillations

We present a systematic study of the ultralight dark matter (ULDM) effects on neutrino oscillations using the latest long-baseline data from the T2K and NO$\nu$A experiments. Our analysis covers both flavor-universal and flavor-general scalar interactions, as well as vector interactions associated with the $L_e - L_\mu$ and $L_\mu - L_\tau$ gauge symmetries. Importantly, we explicitly consider the coherence properties of the ULDM by incorporating the resulting stochastic fluctuations into our statistical analysis. We find that in the low-mass regime, $m_\phi \lesssim 10^{-17}$ eV, where stochastic effects are maximal, constraints on the ULDM couplings are relaxed by roughly an order of magnitude compared to those in the high-mass regime, $m_\phi \gtrsim 10^{-15}$ eV, where such fluctuations are effectively averaged out. While the combined T2K and NO$\nu$A datasets impose nontrivial exclusion limits on the ULDM interactions, we find no statistically significant evidence that these effects alleviate the current tension in determining the CP-violating phase $\delta_{CP}$ between the two experiments. It will be, therefore, essential for future high-precision facilities to further probe the ULDM scenarios and achieve a definite measurement of $\delta_{CP}$.