New results from T2K

Abstract

Abstract. The T2K experiment is a 295-km long-baseline neutrino experiment in Japan employing an off-axis muon neutrino beam with a ∼ 0.6 GeV peak energy. The beam, produced from 30-GeV protons at the J-PARC complex on the Pacific coast, is directed to the Super-Kamiokande detector. T2K released the first long-baseline measurement of a nonzero value for the θ 13 mixing parameter through the observation of electron neutrino appearance (ν µ → ν e ) and produced the most precise measurement of θ 23 through the observation of muon neutrino disappearance (ν µ → ν µ ). T2K data, in combination with reactor experiments, also excludes at 90% C.L. a significant region of the Dirac CP phase: δ CP < −3.02(−1.87) and δ CP > −0.49(−0.98) for the normal (inverted) hierarchy. A full joint appearance and disappearance fit including both neutrino (7×10 20 protons on target, PoT) and anti-neutrino (4 × 10 20 PoT) data and, for the first time, a constraint from water target data in the near detector, is presented yielding improved sensitivity on δ CP and improved precision on sin 2 2θ 23 and the atmospheric mass splitting. The T2K experimentT2K is a long-baseline neutrino oscillation experiment located in Japan, exploiting 30 GeV protons from the Japan Proton Accelerator Research Center (J-PARC, in Tōkai, Ibaraki Prefecture) to create a muon neutrino beam. The proton beam is directed onto a graphite target and the resulting pions and kaons focussed by a system of three magnetic horns into a 96 m-long decay tunnel filled with helium gas. Depending on the direction of the 250 kA pulsed current, horns can select either positively charged particles, for a beam composed mainly of ν µ (ν-mode), or negatively charged particles, for a beam composed mainly ofν µ (ν-mode).The neutrino beam is measured by two near detectors (INGRID, ND280) located 280 m from the target and a far detector, Super-Kamiokande (SK). The far detector and one of the near detectors (ND280) are placed 2.5 • off-axis with respect to the protons' direction, which results in a narrow neutrino energy spectrum with a peak at about 0.6 GeV. The baseline between neutrino production and the far detector, 295 km, is chosen to correspond to the first minimum in the ν µ survival probability at the peak energy.T2K can measure neutrino oscillation in two channels: ν µ disappearance (ν µ → ν µ ), which is dominated by the oscillation parameters sin 2 2θ 23 and ∆m 2 32 and ν e appearance (ν µ → ν e ), which is sensitive to sin 2 2θ 13 , the octant of θ 23 and through subleading terms to δ CP and the mass hierarchy 1 a e-mail: longhin@pd.infn.it 1 "Normal" mass hierarchy (NH) indicates the ∆m 2 32 , ∆m 2 31 > 0 case (state m 3 having the largest mass) while the other case is denoted as "inverted" mass hierarchy (IH).