Theoretical study of neutrino-induced coherent pion production off nuclei at T2K and MiniBooNE energies

Abstract

We have developed a model for neutrino-induced coherent pion production off nuclei in the energy regime of interest for present and forthcoming neutrino oscillation experiments. It is based on a microscopic model for pion production off the nucleon that, besides the dominant $\ensuremath{\Delta}$ pole contribution, takes into account the effect of background terms required by chiral symmetry. Moreover, the model uses a reduced nucleon-to-$\ensuremath{\Delta}$ resonance axial coupling, which leads to coherent pion production cross sections around a factor of 2 smaller than most of the previous theoretical estimates. In the coherent production, the main nuclear effects, namely, medium corrections on the $\ensuremath{\Delta}$ propagator and the final pion distortion, are included. We have improved on previous similar models by taking into account the nucleon motion and employing a more sophisticated optical potential. As found in previous calculations the modification of the $\ensuremath{\Delta}$ self-energy inside the nuclear medium strongly reduces the cross section, while the final pion distortion mainly shifts the peak position to lower pion energies. The angular distribution profiles are not much affected by nuclear effects. Nucleon motion increases the cross section by $\ensuremath{\sim}15%$ at neutrino energies of 650 MeV, while Coulomb effects on charged pions are estimated to be small. Finally, we discuss at length the deficiencies of the Rein-Sehgal pion coherent production model for neutrino energies below 2 GeV, and, in particular, for the MiniBooNE and T2K experiments. We also predict flux-averaged cross sections for these two latter experiments and K2K.