T2K Collaboration(Abe, K. et al), Cervera-Villanueva, A., Escudero, L., Izmaylov, A., Sorel, M., & Stamoulis, P. (2015). Neutrino oscillation physics potential of the T2K experiment. Prog. Theor. Exp. Phys., (4), 043C01–36pp.
Abstract: The observation of the recent electron neutrino appearance in a muon neutrino beam and the high-precision measurement of the mixing angle theta(13) have led to a re-evaluation of the physics potential of the T2K long-baseline neutrino oscillation experiment. Sensitivities are explored for CP violation in neutrinos, non-maximal sin(2) 2 theta(23), the octant of theta(23), and the mass hierarchy, in addition to the measurements of delta CP, sin(2) theta(23), and Delta m(32)(2), for various combinations of nu-mode and (nu) over bar -mode data-taking. With an exposure of 7.8 x 10(21) protons-on-target, T2K can achieve 1 sigma resolution of 0.050 (0.054) on sin(2) theta(23) and 0.040 (0.045) x 10(-3) eV(2) on Delta m(32)(2) for 100% (50%) neutrino beam mode running assuming sin(2) theta(23) = 0.5 and Delta m(32)(2) = 2.4 x 10(-3) eV(2). T2K will have sensitivity to the CP-violating phase delta(CP) at 90% C.L. or better over a significant range. For example, if sin(2) 2 theta(23) is maximal (i.e.theta(23) = 45 degrees) the range is -115 degrees < delta(CP) < -60 degrees for normal hierarchy and +50 degrees < delta(CP) < + 130 degrees for inverted hierarchy. When T2K data is combined with data from the NO nu A experiment, the region of oscillation parameter space where there is sensitivity to observe a non-zero delta CP is substantially increased compared to if each experiment is analyzed alone.
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T2K Collaboration(Abe, K. et al), Cervera-Villanueva, A., Escudero, L., Izmaylov, A., Sorel, M., & Stamoulis, P. (2015). Measurement of the nu(mu) charged current quasielastic cross section on carbon with the T2K on-axis neutrino beam. Phys. Rev. D, 91(11), 112002–17pp.
Abstract: We report a measurement of the nu(mu) charged current quasielastic cross-sections on carbon in the T2K on-axis neutrino beam. The measured charged current quasielastic cross-sections on carbon at mean neutrino energies of 1.94 GeV and 0.93 GeV are (11.95 +/- 0.19(stat)(-1.47)(+1.82)(syst)) x 10(-39) cm(2)/neutron, and (10.64 +/- 0.37(stat)(-1.65)(+2.03)(syst)) x 10(-39) cm(2)/neutron, respectively. These results agree well with the predictions of neutrino interaction models. In addition, we investigated the effects of the nuclear model and the multi-nucleon interaction.
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NEXT Collaboration(Renner, J. et al), Alvarez, V., Carcel, S., Cervera-Villanueva, A., Diaz, J., Ferrario, P., et al. (2015). Ionization and scintillation of nuclear recoils in gaseous xenon. Nucl. Instrum. Methods Phys. Res. A, 793, 62–74.
Abstract: Ionization and scintillation produced by nuclear recoils in gaseous xenon at approximately 14 bar have been simultaneously observed in an electroluminescent time projection chamber. Neutrons from radioisotope a-Be neutron sources were used to induce xenon nuclear recoils, and the observed recoil spectra were compared to a detailed Monte Carlo employing estimated ionization and scintillation yields for nuclear recoils. The ability to discriminate between electronic and nuclear recoils using the ratio of ionization to primary scintillation is demonstrated. These results encourage further investigation on the use of xenon in the gas phase as a detector medium in dark matter direct detection experiments.
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T2K Collaboration(Abe, K. et al), Cervera-Villanueva, A., Escudero, L., Izmaylov, A., Sorel, M., & Stamoulis, P. (2015). Measurement of the electron neutrino charged-current interaction rate on water with the T2K ND280 pi(0) detector. Phys. Rev. D, 91(11), 112010–11pp.
Abstract: This paper presents a measurement of the charged current interaction rate of the electron neutrino beam component of the beam above 1.5 GeV using the large fiducial mass of the T2K pi(0) detector. The predominant portion of the v(e) flux (similar to 85%) at these energies comes from kaon decays. The measured ratio of the observed beam interaction rate to the predicted rate in the detector with water targets filled is 0.89 +/- 0.08(stat) +/- 0.11(sys), and with the water targets emptied is 0.90 +/- 0.09(stat) +/- 0.13 (sys). The ratio obtained for the interactions on water only from an event subtraction method is 0.87 +/- 0.33(stat) +/- 0.21(sys). This is the first measurement of the interaction rate of electron neutrinos on water, which is particularly of interest to experiments with water Cherenkov detectors.
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NEXT Collaboration(Serra, L. et al), Sorel, M., Alvarez, V., Carcel, S., Cervera-Villanueva, A., Diaz, J., et al. (2015). An improved measurement of electron-ion recombination in high-pressure xenon gas. J. Instrum., 10, P03025–21pp.
Abstract: We report on results obtained with the NEXT-DEMO prototype of the NEXT-100 high-pressure xenon gas time projection chamber (TPC), filled with pure xenon gas at 10 bar pressure and exposed to an alpha decay calibration source. Compared to our previous measurements with alpha particles, an upgraded detector and improved analysis techniques have been used. We measure event-by-event correlated fluctuations between ionization and scintillation due to electronion recombination in the gas, with correlation coefficients between -0.80 and -0.56 depending on the drift field conditions. By combining the two signals, we obtain a 2.8% FWHM energy resolution for 5.49 MeV alpha particles and a measurement of the optical gain of the electroluminescent TPC. The improved energy resolution also allows us to measure the specific activity of the radon in the gas due to natural impurities. Finally, we measure the average ratio of excited to ionized atoms produced in the xenon gas by alpha particles to be 0.561 +/- 0.045, translating into an average energy to produce a primary scintillation photon of W-ex = (39.2 +/- 3.2) eV.
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NEXT Collaboration(Cebrian, S. et al), Alvarez, V., Carcel, S., Cervera-Villanueva, A., Diaz, J., Ferrario, P., et al. (2015). Radiopurity assessment of the tracking readout for the NEXT double beta decay experiment. J. Instrum., 10, P05006–16pp.
Abstract: The “Neutrino Experiment with a Xenon Time-Projection Chamber” (NEXT) is intended to investigate the neutrinoless double beta decay of Xe-136, which requires a severe suppression of potential backgrounds; therefore, an extensive screening and selection process is underway to control the radiopurity levels of the materials to be used in the experimental set-up of NEXT. The detector design combines the measurement of the topological signature of the event for background discrimination with the energy resolution optimization. Separate energy and tracking readout planes are based on different sensors: photomultiplier tubes for calorimetry and silicon multi-pixel photon counters for tracking. The design of a radiopure tracking plane, in direct contact with the gas detector medium, was specially challenging since the needed components like printed circuit boards, connectors, sensors or capacitors have typically, according to available information in databases and in the literature, activities too large for experiments requiring ultra-low background conditions. Here, the radiopurity assessment of tracking readout components based on gamma-ray spectroscopy using ultra-low background germanium detectors at the Laboratorio Subterraneo de Canfranc (Spain) is described. According to the obtained results, radiopure enough printed circuit boards made of kapton and copper, silicon photomultipliers and other required components, fulfilling the requirement of an overall background level in the region of interest of at most 8 x 10(-4) counts keV(-1) kg(-1) y(-1), have been identified.
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NEXT Collaboration(Cebrian, S. et al), Alvarez, V., Carcel, S., Cervera-Villanueva, A., Diaz, J., Ferrario, P., et al. (2015). Accurate gamma and MeV-electron track reconstruction with an ultra-low diffusion Xenon/TMA TPC at 10 atm. Nucl. Instrum. Methods Phys. Res. A, 804, 8–24.
Abstract: We report the performance of a 10 atm Xenon/trimethylamine time projection chamber (TPC) for the detection of X-rays (30 keV) and gamma-rays (0.511-1.275 MeV) in conjunction with the accurate tracking of the associated electrons. When operated at such a high pressure and in similar to 1%-admixtures, trimethylamine (TMA) endows Xenon with an extremely low electron diffusion (1.3 +/- 0.13 mm-sigma (longitudinal), 0.95 +/- 0.20 mm-sigma (transverse) along 1 m drift) besides forming a convenient Penning-Fluorescent' mixture. The TPC, that houses 1.1 kg of gas in its fiducial volume, operated continuously for 100 live-days in charge amplification mode. The readout was performed through the recently introduced microbulk Micromegas technology and the AFTER chip, providing a 3D voxelization of 8 mm x 8 mm x 1.2 mm for approximately 10 cm/MeV-long electron tracks. Resolution in energy (epsilon) at full width half maximum (R) inside the fiducial volume ranged from R = 14.6% (30 keV) to R = 4.6% (1.275 MeV). This work was developed as part of the R&D program of the NEXT collaboration for future detector upgrades in the search of the neutrino-less double beta decay (beta beta 0 nu) in Xe-136, specifically those based on novel gas mixtures. Therefore we ultimately focus on the calorimetric and topological properties of the reconstructed MeV-electron tracks. In particular, the obtained energy resolution has been decomposed in its various contributions and improvements towards achieving the R =1.4%root MeV/epsilon levels obtained in small sensors are discussed.
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T2K Collaboration(Abe, K. et al), Cervera-Villanueva, A., Escudero, L., Izmaylov, A., Sorel, M., & Stamoulis, P. (2015). Measurement of the nu(mu) charged-current quasielastic cross section on carbon with the ND280 detector at T2K. Phys. Rev. D, 92(11), 112003–14pp.
Abstract: This paper reports a measurement by the T2K experiment of the nu(mu) charged current quasielastic (CCQE) cross section on a carbon target with the off-axis detector based on the observed distribution of muon momentum (rho(mu)) and angle with respect to the incident neutrino beam (theta(mu)). The flux-integrated CCQE cross section was measured to be <sigma > = (0.83 +/- 0.12) x 10(-38) 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) rho(mu)-cos theta(mu) distribution, the effective M-A(QE) parameter was measured to be 1.26(-0.18)(+0.21) GeV/c(2) (1.43(-0.22)(+0.28) GeV/c(2)).
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T2K Collaboration(Abe, K. et al), Cervera-Villanueva, A., Escudero, L., Izmaylov, A., Sorel, M., & Stamoulis, P. (2016). Upper bound on neutrino mass based on T2K neutrino timing measurements. Phys. Rev. D, 93(1), 012006–15pp.
Abstract: The Tokai to Kamioka (T2K) long-baseline neutrino experiment consists of a muon neutrino beam, produced at the J-PARC accelerator, a near detector complex and a large 295-km-distant far detector. The present work utilizes the T2K event timing measurements at the near and far detectors to study neutrino time of flight as a function of derived neutrino energy. Under the assumption of a relativistic relation between energy and time of flight, constraints on the neutrino rest mass can be derived. The sub-GeV neutrino beam in conjunction with timing precision of order tens of ns provide sensitivity to neutrino mass in the few MeV/c(2) range. We study the distribution of relative arrival times of muon and electron neutrino candidate events at the T2K far detector as a function of neutrino energy. The 90% C.L. upper limit on the mixture of neutrino mass eigenstates represented in the data sample is found to be m(v)(2) < 5.6 MeV2/c(4).
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NEXT Collaboration(Ferrario, P. et al), Laing, A., Lopez-March, N., Gomez-Cadenas, J. J., Alvarez, V., Carcel, S., et al. (2016). First proof of topological signature in the high pressure xenon gas TPC with electroluminescence amplification for the NEXT experiment. J. High Energy Phys., 01(1), 104–18pp.
Abstract: The NEXT experiment aims to observe the neutrinoless double beta decay of Xe-136 in a high-pressure xenon gas TPC using electroluminescence (EL) to amplify the signal from ionization. One of the main advantages of this technology is the possibility to reconstruct the topology of events with energies close to Q(beta beta). This paper presents the first demonstration that the topology provides extra handles to reject background events using data obtained with the NEXT-DEMO prototype. Single electrons resulting from the interactions of Na-22 1275 keV gammas and electron-positron pairs produced by conversions of gammas from the Th-228 decay chain were used to represent the background and the signal in a double beta decay. These data were used to develop algorithms for the reconstruction of tracks and the identification of the energy deposited at the end-points, providing an extra background rejection factor of 24.3 +/- 1.4 (stat.)%, while maintaining an efficiency of 66.7 +/- 1.% for signal events.
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