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Register a new userSearch for heavy neutrinos with the T2K near detector ND280
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This paper reports on the search for heavy neutrinos with masses in the range $140 < M_N < 493$ MeV/c$^2$ using the off-axis near detector ND280 of the T2K experiment. These particles can be produced from kaon decays in the standard neutrino beam and then subsequently decay in ND280. The decay modes under consideration are $N to ell^{pm}_{alpha} pi^{mp}$ and $N to ell^+_{alpha} ell^-_{beta} u (bar u)$ ($alpha,beta=e,mu$). A search for such events has been made using the Time Projection Chambers of ND280, where the background has been reduced to less than two events in the current dataset in all channels. No excess has been observed in the signal region. A combined Bayesian statistical approach has been applied to extract upper limits on the mixing elements of heavy neutrinos to electron-, muon- and tau- flavoured currents ($U_e^2$, $U_{mu}^2$, $U_{tau}^2$) as a function of the heavy neutrino mass, e.g. $U_e^2 < 10^{-9}$ at $90%$ C.L. for a mass of $390$ MeV/c$^2$. These constraints are competitive with previous experiments.
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Neutrino neutral-current induced single photon production is a sub-leading order process for accelerator-based neutrino beam experiments including T2K. It is, however, an important process to understand because it is a background for electron (anti)neutrino appearance oscillation experiments. Here, we performed the first search of this process below 1 GeV using the fine-grained detector at the T2K ND280 off-axis near detector. By reconstructing single photon kinematics from electron-positron pairs, we achieved 95% pure gamma ray sample from 5.738$times 10^{20}$ protons-on-targets neutrino mode data. We do not find positive evidence of neutral current induced single photon production in this sample. We set the model-dependent upper limit on the cross-section for this process, at 0.114$times 10^{-38}$ cm$^2$ (90% C.L.) per nucleon, using the J-PARC off-axis neutrino beam with an average energy of $left<E_ uright>sim 0.6$ GeV. This is the first limit on this process below 1 GeV which is important for current and future oscillation experiments looking for electron neutrino appearance oscillation signals.
The T2K experiment studies oscillations of an off-axis muon neutrino beam between the J-PARC accelerator complex and the Super-Kamiokande detector. Special emphasis is placed on measuring the mixing angle theta_13 by observing electron neutrino appearance via the sub-dominant muon neutrino to electron neutrino oscillation, and searching for CP violation in the lepton sector. The experiment includes a sophisticated, off-axis, near detector, the ND280, situated 280 m downstream of the neutrino production target in order to measure the properties of the neutrino beam and to understand better neutrino interactions at the energy scale below a few GeV. The data collected with the ND280 are used to study charged- and neutral-current neutrino interaction rates and kinematics prior to oscillation, in order to reduce uncertainties in the oscillation measurements by the far detector. A key element of the near detector is the ND280 electromagnetic calorimeter (ECal), consisting of active scintillator bars sandwiched between lead sheets and read outwith multi-pixel photon counters (MPPCs). The ECal is vital to the reconstruction of neutral particles, and the identification of charged particle species. The ECal surrounds the Pi-0 detector (P0D) and the tracking region of the ND280, and is enclosed in the former UA1/NOMAD dipole magnet. This paper describes the design, construction and assembly of the ECal, as well as the materials from which it is composed. The electronic and data acquisition (DAQ) systems are discussed, and performance of the ECal modules, as deduced from measurements with particle beams, cosmic rays, the calibration system, and T2K data, is described.
The T2K experiment has performed a search for $ u_e$ disappearance due to sterile neutrinos using $5.9 times 10^{20}$ protons on target for a baseline of $280 m$ in a neutrino beam peaked at about $500 MeV$. A sample of u_e CC interactions in the off-axis near detector has been selected with a purity of 63% and an efficiency of 26%. The p-value for the null hypothesis is 0.085 and the excluded region at 95% CL is approximately $sin^2 2 theta_{ee} > 0.3$ for $Delta m^2_{eff} > 7 eV^2 / c^4$.
The Charged-Current Quasi-Elastic (CCQE) interaction, $ u_{l} + n rightarrow l^{-} + p$, is the dominant CC process at $E_ u sim 1$ GeV and contributes to the signal in accelerator-based long-baseline neutrino oscillation experiments operating at intermediate neutrino energies. This paper reports a measurement by the T2K experiment of the $ u_{mu}$ CCQE cross section on a carbon target with the off-axis detector based on the observed distribution of muon momentum ($p_mu$) and angle with respect to the incident neutrino beam ($theta_mu$). The flux-integrated CCQE cross section was measured to be $(0.83 pm 0.12) times 10^{-38}textrm{ cm}^{2}$ in good agreement with NEUT MC value of ${0.88 times 10^{-38}} textrm{ cm}^{2}$. The energy dependence of the CCQE cross section is also reported. The axial mass, $M_A^{QE}$, of the dipole axial form factor was extracted assuming the Smith-Moniz CCQE model with a relativistic Fermi gas nuclear model. Using the absolute (shape-only) $p_{mu}costheta_mu$ distribution, the effective $M_A^{QE}$ parameter was measured to be ${1.26^{+0.21}_{-0.18} textrm{ GeV}/c^{2}}$ (${1.43^{+0.28}_{-0.22} textrm{ GeV}/c^{2}}$).
The T2K experiment has reported the first observation of the appearance of electron neutrinos in a muon neutrino beam. The main and irreducible background to the appearance signal comes from the presence in the neutrino beam of a small intrinsic component of electron neutrinos originating from muon and kaon decays. In T2K, this component is expected to represent 1.2% of the total neutrino flux. A measurement of this component using the near detector (ND280), located 280 m from the target, is presented. The charged current interactions of electron neutrinos are selected by combining the particle identification capabilities of both the time projection chambers and electromagnetic calorimeters of ND280. The measured ratio between the observed electron neutrino beam component and the prediction is 1.01+-0.10 providing a direct confirmation of the neutrino fluxes and neutrino cross section modeling used for T2K neutrino oscillation analyses. Electron neutrinos coming from muons and kaons decay are also separately measured, resulting in a ratio with respect to the prediction of 0.68+-0.30 and 1.10+-0.14, respectively.
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