Matter versus vacuum oscillations at long-baseline accelerator neutrino experiments.

The neutrino oscillation probabilities at the long-baseline accelerator neutrino experiments are expected to be modified by matter effects. We search for evidence of such modification in the data of T2K and NO ν A, by fitting the data to the hypothesis of (a) matter modified oscillations and (b) vacuum oscillations. We find that vacuum oscillations provide as good a fit to the data as matter modified oscillations. Even extended runs of T2K and NO ν A, with five years in neutrino mode (5 ν) and five years in anti-neutrino mode (5 ν ̄) , cannot make a 3 σ distinction between vacuum and matter modified oscillations. The future experiment DUNE, with neutrino and anti-neutrino runs of five years each (5 ν + 5 ν ̄) , can rule out vacuum oscillations by itself at 5 σ if the hierarchy is normal. If the hierarchy is inverted, a 5 σ discrimination against vacuum oscillations requires the combination of (5 ν + 5 ν ̄) runs of T2K, NO ν A and DUNE. [ABSTRACT FROM AUTHOR]