A measurement of electron neutrino appearance in the MINOS experiment after four years of data

This work attempts to measure or set a limit on sin 2 (2[straight theta] 13 ), the parameter which describes ν μ [arrow right]ν e oscillations. The MINOS detectors at Fermilab are used to perform a search for the oscillations utilizing a beam of ν μ neutrinos created in the NuMI beamline by the collisions of 120 GeV protons with a carbon target. These collisions create π ± and K ± which are focused with magnetic horns, are allowed to decay, and result in a beam of ν μ in the energy range of 1 to 30 GeV. Two functionally identical steel-scintillator calorimetric detectors are utilized to measure the interactions of the generated neutrinos. A detector close to the NuMI beam. located 104 m underground and 1040 m from the target, is used to measure the properties of the neutrino beam, including the flux, composition, and energy spectrum. This information is used in part to generate a predicted spectrum of neutrinos in absence of ν μ [arrow right]ν e oscillations in the detector located far from the target, at a distance of 705 in underground and 735.5 km from the target. An excess of predicted ν e charged current events in this far detector will be interpreted as ν μ [arrow right]ν e oscillations, and a measurement of sin 2 (2[straight theta] 13 ) will be made using a Feldman-Cousins analysis. The measurement of ν μ [arrow right]ν e requires the separation of ν e candidates from background events. New reconstruction software was developed with a focus on identifying ν e candidate events in order to reduce systematic errors. The event parameters measured by this software were used as an input to an artificial neutral network event discriminator. The details of this reconstruction software and the other steps of the analysis necessary to making the measurement will be discussed. This work builds on a previous measurement made with this experiment. After two years of running with 3.14×10 20 protons-on-target (POT), a limit was set at δ CP =0 for the normal (inverted) hierarchy of sin 2 (2[straight theta] 13 )<0.29 (0.42) at 90% C.L. This study finds after four years of data with an accumulated intensity of 7×10 20 POT that sin 2 (2[straight theta] 13 )<0.12 (0.20) with δ CP =0 at 90% C.L. for the normal (inverted) hierarchy.