It is likely that the investigation of neutrinos, particularly through the phenomenon of neutrino oscillations, can provide vital clues to the future development of particle physics. However such investigations are limited by the precision with which neutrino interactions with nuclei are understood. One such interaction, neutrino induced coherent pion production, is of particular importance as a potential source of background to νe oscillation searches, and as part of the broader goal of understanding neutrino induced pion production in general.
A review of experimental data on coherent pion production was presented, noting in particular the reduced NC and lack of evidence for CC cross-sections recently measured below 3 GeV, in conflict with the Rein-Sehgal model which explained well data at higher energies. Investigation of the Rein-Sehgal model found that its predictions vary greatly depending on the choice of inputs, but also that its validity at low energy is questionable. One alternative low energy model, the Alvarez-Ruso model, was therefore implemented in the GENIE interaction simulation for the purposes of comparison. This implementation represents the first time a “microscopic” coherent model has been included in a full neutrino interaction simulation.
A search for coherent pion production was conducted using data gathered from the T2K experiment's near detector, ND280, and predictions from the GENIE interaction simulation. This analysis found a 3.0σ excess of events over the background expectation, constituting the first experimental evidence of νμ CC coherent pion production below 7 GeV. The analysis was repeated using alternative signal (Alvarez-Ruso) and background (NEUT) models, both of which supported this conclusion.
In order to put this result in context of those from other experiments, it was found that the size of the excess seen in data is compatible with a flux-averaged cross-section which is 92 % of that predicted by the Rein-Sehgal model in GENIE. This is compatible with other comparable data, in particular that from SciBooNE (Figure 7.1).
The primary goal for improving understanding of coherent pion production must be the identification of a model which can better explain the process at low energies, but the uncertainties in this analysis are not yet low enough to allow discrimination between the signal models used. However, the strong evidence found here should motivate continued investigation in T2K which, with improved statistics and reduced background systematic uncertainties, should be capable of constraining the available models. In combination with similar searches at other experiments, this goal should be attainable in the near future.