A Search for Neutrino Induced Coherent NC(π0) Production in the MINOS Near Detector

The production of single, highly forward π0 mesons by NC coherent neutrino-nucleus interactions (νμ + N → νμ + N + π0) is a process which probes fundamental aspects of the weak interaction. This reaction may also pose as a limiting background for long baseline searches for νμ → νe oscillations if the neutrino mixing angle θ13 is very small. The high-statistics sample of neutrino interactions recorded by the MINOS Near Detector provides an opportunity to measure the cross section of this coherent reaction on a relatively large-A nucleus at an average Eν = 4.9 GeV. A major challenge for this measurement is the isolation of forward-going electromagnetic (EM) showers produced by the relatively rare coherent NC(π0) process amidst an abundant rate of incoherently produced EM showers. The backgrounds arise from single π0 dominated NC events and also from quasi-elastic-like CC scattering of electron neutrinos. In this Thesis the theory of coherent interactions is summarized, and previous measurements of the coherent NC(π0) cross section are reviewed. Then, methods for selecting a sample of coherent NC(π0) like events, extracting the coherent NC(π0) event rate from that sample, estimating the analysis uncertainties, and calculating a cross section, are presented. A signal for neutrino-induced NC(π0) production is observed in the relevant kinematic regime as an excess of events of three standard deviations above background. The reaction cross sections, averaged over an energy window of 2.5 ≤ Eν ≤ 9.0 GeV is determined to be (31.6±10.5) x 10-40 cm2/nucleus. The result is the first evidence obtained for neutrino-nucleus coherent NC(π0) scattering on iron, and is the first measurement on an average nuclear target above A = 30. The cross section measurement is in agreement with NEUGEN3 implementation of the model by Rein and Sehgal which is motived by the PCAC hypothesis.