T2K Replica Target Hadron Production Measurements in NA61/SHINE and T2K Neutrino Flux Predictions

Accelerator based neutrino experiments generate their neutrino beams by impinging high energy protons on thick targets. The neutrino beam predictions are thus based on modeling the interactions of the beam protons inside the targets. Different hadronic models can be used with different accuracies depending on the energy range of the incident protons and on the target material. Nevertheless, none of the models can be seen as perfectly describing all different interactions. In order to reach high precision neutrino flux predictions, it is thus mandatory to be able to test and constrain the models with hadron production measurements. The T2K experiment in Japan uses the ancillary NA61/SHINE facility at CERN to constrain the production of hadrons resulting from the interactions of proton beam particles impinging on a 90cm long graphite target. Data taken by NA61/SHINE with a 30 GeV proton beam on a thin (4% interaction length) graphite target have been recorded in 2007 and 2009. They have been analysed and extensively used by T2K. These data allow to constrain up to 60% of the neutrino flux at the beam peak energy. The remaining 40% are generated by re-interactions of secondary particles within the target or with the material surrounding the target. These re-interactions cannot be directly constrained with the p+C at 30 GeV thin target measurements. By measuring the production of hadrons off a replica of the T2K target, up to 90% of the neutrino flux can be constrained. In 2007, the NA61/SHINE took a first pilot run with a replica of the T2K target. These data were analyzed and demonstrated the feasibility of such measurements. This first pilot run did not give a large enough statistical dataset to allow for a reduction in the uncertainty regarding the neutrino flux prediction. In 2009 and 2010, two larger data sets were recorded. In this thesis, the analysis of the 2009 data set is presented. Pion multiplicities at the surface of the target are extracted and systematic uncertainties are computed. The results are given in bins of momentum p, polar angle θ and longitudinal position z at the surface of the target. The uncertainties on these results are typically of the order of 7% for the statistical component and between 4% and 10% for the systematic component. A procedure based on some of the work developed within T2K is presented in order to compare the thin target and T2K replica target results of NA61/SHINE. This procedure consists of re-weighting each modeled interactions in the 90cm long target with the NA61/SHINE thin target measurements. The re-weighted pion multiplicities at the surface of the target can then be directly compared to the T2K replica target analysis results. Finally, the neutrino flux predictions are constrained by using the 2009 T2K replica target data and the uncertainties on this data set are propagated to the neutrino flux predictions. The 2010 data set will allow to reach ultimate precision with respect to hadron production off the T2K replica target. As a result of higher recorded statistical datasets, kaon multiplicities at the surface of the target could also be extracted. All future projects proposing long baseline neutrino experiments mention the need for high precision hadron production measurements in order to reach their physics goals. This opens new perspectives for NA61/SHINE. Its neutrino physics program will certainly remain of interest for the neutrino physics community