Nonmaximal neutrino mixing at NOνA from nonstandard interactions

Muon neutrino disappearance measurements at NO$\nu$A suggest that maximal $\theta_{23}$ is excluded at the 2.6$\sigma$ CL. This is in mild tension with T2K data which prefer maximal mixing. Considering that NO$\nu$A has a much longer baseline than T2K, we point out that the apparent departure from maximal mixing in NO$\nu$A may be a consequence of nonstandard neutrino propagation in matter.