Two modes of searching for new neutrino interactions at MINOS

The Super-Kamiokande atmospheric neutrino measurements leave substantial room for nonstandard interactions (NSI) of neutrinos with matter in the ${\ensuremath{\nu}}_{e}\mathrm{\text{\ensuremath{-}}}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ sector. Large values of the NSI couplings are accommodated if the vacuum oscillation parameters are changed from their standard values. Short and medium baseline neutrino beams can break this degeneracy by measuring the true vacuum oscillation parameters with the ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ disappearance mode, for which the matter effects are negligible or subdominant. These experiments can also search for the ${\ensuremath{\nu}}_{e}\mathrm{\text{\ensuremath{-}}}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ flavor-changing effects directly, by looking for ${\ensuremath{\nu}}_{\ensuremath{\mu}}\mathrm{\text{\ensuremath{-}}}{\ensuremath{\nu}}_{e}$ conversion caused by the intervening matter. We discuss both of these methods for the case of MINOS. We find that, while the present MINOS data on ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ disappearance induce only minor changes on the constraints on the NSI parameters, the situation will improve markedly with the planned increase of the statistics by an order of magnitude. In that case, the precision will be enough to distinguish certain presently allowed NSI scenarios from the no-NSI case. NSI per quark of about 10% the size of the standard weak interaction could give a ${\ensuremath{\nu}}_{\ensuremath{\mu}}\mathrm{\text{\ensuremath{-}}}{\ensuremath{\nu}}_{e}$ conversion probability of the order $\ensuremath{\sim}{10}^{\ensuremath{-}2}$, measurable by MINOS in the same high statistics scenario. In this ${\ensuremath{\nu}}_{\ensuremath{\mu}}\mathrm{\text{\ensuremath{-}}}{\ensuremath{\nu}}_{e}$ channel, the small effects of NSI could be comparable or larger than the vacuum contribution of the small angle ${\ensuremath{\theta}}_{13}$. The expected ${\ensuremath{\theta}}_{13}$ bound at MINOS should be more properly interpreted as a bound in the ${\ensuremath{\theta}}_{13}$-NSI parameter space.