Giles Barr

Abstract:

T2K is a second generation long-baseline experiment in Japan, designed to perform precision studies of neutrino oscillations. A neutrino beam originating at the J-PARC facility is sampled by two near detectors, approximately 280m distant from the source and directed towards the Kamioka underground facility, 295km distant, where the Super-Kamiokande water Cerenkov detector is situated. Both the on-axis (INGRID) and off-axis (ND280) near detectors employ the MIDAS framework for their data acquisition systems. Events are selected via a flexible hardware trigger, which allows mixed operating modes incorporating beam spill, cosmic and calibration triggers. Data are acquired from the front-end electronics over optical gigabit Ethernet links by custom MIDAS front-end applications distributed over a cluster of Linux computers. Event fragments from all sub-detectors are merged and logged on a central backend processor and then copied to remote storage for longterm archival. The system is also designed to allow flexible partitioning of sub-detectors for standalone diagnostics and calibration. We present the design of the DAQ system and report on experience gathered during early operation of the near detectors. © 2010 IEEE.

Abstract:

We searched for a sidereal modulation in the MINOS far detector neutrino rate. Such a signal would be a consequence of Lorentz and CPT violation as described by the standard-model extension framework. It also would be the first detection of a perturbative effect to conventional neutrino mass oscillations. We found no evidence for this sidereal signature, and the upper limits placed on the magnitudes of the Lorentz and CPT violating coefficients describing the theory are an improvement by factors of 20-510 over the current best limits found by using the MINOS near detector. © 2010 The American Physical Society.

Abstract:

This paper reports results from a search for νμ→ν e transitions by the MINOS experiment based on a 7×1020 protons-on-target exposure. Our observation of 54 candidate νe events in the far detector with a background of 49.1±7.0(stat)±2. 7(syst) events predicted by the measurements in the near detector requires 2 sin2(2θ13)sin2θ23<0. 12(0.20) at the 90% C.L. for the normal (inverted) mass hierarchy at δCP=0. The experiment sets the tightest limits to date on the value of θ13 for nearly all values of δCP for the normal neutrino mass hierarchy and maximal sin2(2θ 23). © 2010 The American Physical Society.

Abstract:

A search for depletion of the combined flux of active neutrino species over a 735 km baseline is reported using neutral-current interaction data recorded by the MINOS detectors in the NuMI neutrino beam. Such a depletion is not expected according to conventional interpretations of neutrino oscillation data involving the three known neutrino flavors. A depletion would be a signature of oscillations or decay to postulated noninteracting sterile neutrinos, scenarios not ruled out by existing data. From an exposure of 3.18×1020 protons on target in which neutrinos of energies between ∼500MeV and 120 GeV are produced predominantly as νμ, the visible energy spectrum of candidate neutral-current reactions in the MINOS far detector is reconstructed. Comparison of this spectrum to that inferred from a similarly selected near-detector sample shows that of the portion of the νμ flux observed to disappear in charged-current interaction data, the fraction that could be converting to a sterile state is less than 52% at 90% confidence level (C.L.). The hypothesis that active neutrinos mix with a single sterile neutrino via oscillations is tested by fitting the data to various models. In the particular four-neutrino models considered, the mixing angles θ24 and θ34 are constrained to be less than 11° and 56° at 90% C.L., respectively. The possibility that active neutrinos may decay to sterile neutrinos is also investigated. Pure neutrino decay without oscillations is ruled out at 5.4 standard deviations. For the scenario in which active neutrinos decay into sterile states concurrently with neutrino oscillations, a lower limit is established for the neutrino decay lifetime τ3/m3>2.1×10-12s/eV at 90% C.L. © 2010 The American Physical Society.

Abstract:

The temperature of the upper atmosphere affects the height of primary cosmic ray interactions and the production of high-energy cosmic ray muons which can be detected deep underground. The MINOS far detector at Soudan, MN, has collected over 67×106 cosmic ray induced muons. The underground muon rate measured over a period of five years exhibits a 4% peak-to-peak seasonal variation which is highly correlated with the temperature in the upper atmosphere. The coefficient, αT, relating changes in the muon rate to changes in atmospheric temperature was found to be αT=0.873±0. 009(stat)±0.010(syst). Pions and kaons in the primary hadronic interactions of cosmic rays in the atmosphere contribute differently to αT due to the different masses and lifetimes. This allows the measured value of αT to be interpreted as a measurement of the K/π ratio for Ep 7TeV of 0.12-0.05+0.07, consistent with the expectation from collider experiments. © 2010 The American Physical Society.