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T2K Collaboration(Abe, K. et al), Cervera-Villanueva, A., Escudero, L., Gomez-Cadenas, J. J., Hansen, C., Monfregola, L., et al. (2012). First muon-neutrino disappearance study with an off-axis beam. Phys. Rev. D, 85(3), 031103–8pp.
Abstract: We report a measurement of muon-neutrino disappearance in the T2K experiment. The 295-km muon-neutrino beam from Tokai to Kamioka is the first implementation of the off-axis technique in a long-baseline neutrino oscillation experiment. With data corresponding to 1.43 x 10(20) protons on target, we observe 31 fully-contained single mu-like ring events in Super-Kamiokande, compared with an expectation of 104 +/- 14 (syst) events without neutrino oscillations. The best-fit point for two-flavor nu(mu) -> nu(tau) oscillations is sin(2)(2 theta(23)) = 0.98 and vertical bar Delta m(32)(2)vertical bar = 2.65 x 10(-3) eV(2). The boundary of the 90% confidence region includes the points sin(2)(2 theta(23)), vertical bar Delta m(32)(2)vertical bar = (1.0, 3.1 x 10(-3) eV(2)), (0.84, 2.65 x 10(-3) eV(2)) and (1.0, 2.2 x 10(-3) eV(2)).
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Gomez-Cadenas, J. J., Martin-Albo, J., Mezzetto, M., Monrabal, F., & Sorel, M. (2012). The search for neutrinoless double beta decay. Riv. Nuovo Cimento, 35(2), 29–98.
Abstract: In the last two decades the search for neutrinoless double beta decay has evolved into one of the highest priorities for understanding neutrinos and the origin of mass. The main reason for this paradigm shift has been the discovery of neutrino oscillations, which clearly established the existence of massive neutrinos. An additional motivation for conducting such searches comes from the existence of an unconfirmed, but not refuted, claim of evidence for neutrinoless double decay in Ge-76. As a consequence, a new generation of experiments, employing different detection techniques and beta beta isotopes, is being actively promoted by experimental groups across the world. In addition, nuclear theorists are making remarkable progress in the calculation of the neutrinoless double beta. decay nuclear matrix elements, thus eliminating a substantial part of the theoretical uncertainties affecting the particle physics interpretation of this process. In this report, we review the main aspects of the double beta decay process and some of the most relevant experiments. The picture that emerges is one where searching for neutrinoless double beta decay is recognized to have both far-reaching theoretical implications and promising prospects for experimental observation in the near future.
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Gomez-Cadenas, J. J., Guinea, F., Fogler, M. M., Katsnelson, M. I., Martin-Albo, J., Monrabal, F., et al. (2012). GraXe, graphene and xenon for neutrinoless double beta decay searches. J. Cosmol. Astropart. Phys., 02(2), 037–17pp.
Abstract: We propose a new detector concept, GraXe (to be pronounced as grace), to search for neutrinoless double beta decay in Xe-136. GraXe combines a popular detection medium in rare-event searches, liquid xenon, with a new, background-free material, grapheme. In our baseline design of GraXe, a sphere made of graphene-coated titanium mesh and filled with liquid xenon (LXe) enriched in the Xe-136 isotope is immersed in a large volume of natural LXe instrumented with photodetectors. Liquid xenon is an excellent scintillator, reasonably transparent to its own light. Graphene is transparent over a large frequency range, and impermeable to the xenon. Event position could be deduced from the light pattern detected in the photosensors. External backgrounds would be shielded by the buffer of natural LXe, leaving the ultra-radiopure internal volume virtually free of background. Industrial graphene can be manufactured at a competitive cost to produce the sphere. Enriching xenon in the isotope Xe-136 is easy and relatively cheap, and there is already near one ton of enriched xenon available in the world (currently being used by the EXO, KamLAND-Zen and NEXT experiments). All the cryogenic know-how is readily available from the numerous experiments using liquid xenon. An experiment using the GraXe concept appears realistic and affordable in a short time scale, and its physics potential is enormous.
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NEXT Collaboration(Alvarez, V. et al), Agramunt, J., Ball, M., Bayarri, J., Carcel, S., Cervera-Villanueva, A., et al. (2012). SiPMs coated with TPB: coating protocol and characterization for NEXT. J. Instrum., 7, P02010.
Abstract: Silicon photomultipliers (SiPM) are the photon detectors chosen for the tracking read-out in NEXT, a neutrinoless beta beta decay experiment which uses a high pressure gaseous xenon time projection chamber (TPC). The reconstruction of event track and topology in this gaseous detector is a key handle for background rejection. Among the commercially available sensors that can be used for tracking, SiPMs offer important advantages, mainly high gain, ruggedness, cost-effectiveness and radio-purity. Their main drawback, however, is their non sensitivity in the emission spectrum of the xenon scintillation (peak at 175 nm). This is overcome by coating these sensors with the organic wavelength shifter tetraphenyl butadiene (TPB). In this paper we describe the protocol developed for coating the SiPMs with TPB and the measurements performed for characterizing the coatings as well as the performance of the coated sensors in the UV-VUV range.
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NEXT Collaboration(Alvarez, V. et al), Carcel, S., Cervera-Villanueva, A., Diaz, J., Ferrario, P., Gil, A., et al. (2012). NEXT-100 Technical Design Report (TDR). Executive summary. J. Instrum., 7, T06001–34pp.
Abstract: In this Technical Design Report (TDR) we describe the NEXT-100 detector that will search for neutrinoless double beta decay (beta beta 0v) in Xe-136 at the Laboratorio Subterraneo de Canfranc (LSC), in Spain. The document formalizes the design presented in our Conceptual Design Report (CDR): an electroluminescence time projection chamber, with separate readout planes for calorimetry and tracking, located, respectively, behind cathode and anode. The detector is designed to hold a maximum of about 150 kg of xenon at 15 bar, or 100 kg at 10 bar. This option builds in the capability to increase the total isotope mass by 50% while keeping the operating pressure at a manageable level. The readout plane performing the energy measurement is composed of Hamamatsu R11410-10 photomultipliers, specially designed for operation in low-background, xenon-based detectors. Each individual PMT will be isolated from the gas by an individual, pressure resistant enclosure and will be coupled to the sensitive volume through a sapphire window. The tracking plane consists in an array of Hamamatsu S10362-11-050P MPPCs used as tracking pixels. They will be arranged in square boards holding 64 sensors (8 x 8) with a 1-cm pitch. The inner walls of the TPC, the sapphire windows and the boards holding the MPPCs will be coated with tetraphenyl butadiene (TPB), a wavelength shifter, to improve the light collection.
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