Nonstandard neutrino interactions at DUNE, T2HK and T2HKK

We study the matter effect caused by nonstandard neutrino interactions (NSI) in the next generation long-baseline neutrino experiments, DUNE, T2HK and T2HKK. If multiple NSI parameters are nonzero, the potential of these experiments to detect CP violation, determine the mass hierarchy and constrain NSI is severely impaired by degeneracies between the NSI parameters and by the generalized mass hierarchy degeneracy. In particular, a cancellation between leading order terms in the appearance channels when $\epsilon_{e\tau} = \cot\theta_{23} \epsilon_{e\mu}$, strongly affects the sensitivities to these two NSI parameters at T2HK and T2HKK. We also study the dependence of the sensitivities on the true CP phase $\delta$ and the true mass hierarchy, and find that overall DUNE has the best sensitivity to the magnitude of the NSI parameters, while T2HKK has the best sensitivity to CP violation whether or not there are NSI. Furthermore, for T2HKK a smaller off-axis angle for the Korean detector is better overall. We find that due to the structure of the leading order terms in the appearance channel probabilities, the NSI sensitivities in a given experiment are similar for both mass hierarchies, modulo the phase change $\delta \to \delta + 180^\circ$.