Nonstandard neutrino interactions in reactor and superbeam experiments

Abstract

The formalism of nonstandard four-fermion interactions provides a convenient, model-independent way of parametrizing a wide class of ``new physics'' scenarios. In this article, we study the performance of reactor and superbeam neutrino experiments in the presence of such nonstandard interactions (NSI). Because of interference between the standard and nonstandard amplitudes, sizeable effects are to be expected if the NSI parameters are close to their current upper limits. We derive approximate formulas for the relevant oscillation probabilities including NSI, and show how the leading effects can be understood intuitively even without any calculations. We will present a classification of all possible NSI according to their impact on reactor and superbeam experiments, and it will turn out that these experiments are highly complementary in terms of their sensitivity to the nonstandard parameters. The second part of the paper is devoted to detailed numerical simulations, which will demonstrate how a standard oscillation fit of the mixing angle ${\ensuremath{\theta}}_{13}$ may fail if experimental data is affected by NSI. We find that for some nonstandard terms, reactor and superbeam experiments would yield seemingly conflicting results, while in other cases, they may agree well with each other, but the resulting value for ${\ensuremath{\theta}}_{13}$ could be far from the true value. This offset may be so large that the true ${\ensuremath{\theta}}_{13}$ is even ruled out erroneously. In the last section of the paper, we demonstrate that reactor and superbeam data can actually establish the presence of nonstandard interactions. Throughout our discussion, we pay special attention to the impact of the complex phases, and of the near detectors.