Confusing nonstandard neutrino interactions with oscillations at a neutrino factory

Abstract

Most neutrino mass theories contain nonstandard interactions (NSI) of neutrinos which can be either nonuniversal (NU) or flavor changing (FC). We study the impact of such interactions on the determination of neutrino mixing parameters at a neutrino factory using the so-called ``golden channels'' ${\stackrel{\mathrm{(-)}}{\ensuremath{\nu}}}_{e}\ensuremath{\rightarrow}{\stackrel{\mathrm{(-)}}{\ensuremath{\nu}}}_{\ensuremath{\mu}}$ for the measurement of ${\ensuremath{\theta}}_{13}.$ We show that a certain combination of FC interactions in neutrino source and earth matter can give exactly the same signal as oscillations arising due to ${\ensuremath{\theta}}_{13}.$ This implies that information about ${\ensuremath{\theta}}_{13}$ can only be obtained if bounds on NSI are available. Taking into account the existing bounds on FC interactions, this leads to a drastic loss in sensitivity in ${\ensuremath{\theta}}_{13},$ at least two orders of magnitude. A near detector at a neutrino factory offers the possibility to obtain stringent bounds on some NSI parameters. Such a near site detector constitutes an essential ingredient of a neutrino factory and a necessary step towards the determination of ${\ensuremath{\theta}}_{13}$ and subsequent study of leptonic $\mathrm{CP}$ violation.