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T2K Collaboration(Abe, K. et al), Cervera-Villanueva, A., Escudero, L., Gomez-Cadenas, J. J., Izmaylov, A., Monfregola, L., et al. (2013). Evidence of electron neutrino appearance in a muon neutrino beam. Phys. Rev. D, 88(3), 032002–41pp.
Abstract: The T2K Collaboration reports evidence for electron neutrino appearance at the atmospheric mass splitting, vertical bar Delta m(32)(2)vertical bar approximate to 2.4 X 10(-3) eV(2). An excess of electron neutrino interactions over background is observed from a muon neutrino beam with a peak energy of 0.6 GeV at the Super-Kamiokande (SK) detector 295 km from the beam's origin. Signal and background predictions are constrained by data from near detectors located 280 m from the neutrino production target. We observe 11 electron neutrino candidate events at the SK detector when a background of 3.3 +/- 0.4(syst) events is expected. The background-only hypothesis is rejected with a p value of 0.0009 (3.1 sigma), and a fit assuming nu(mu) -> nu(e) oscillations with sin (2)2 theta(23) = 1, delta(CP) = 0 and vertical bar Delta m(32)(2)vertical bar = 2.4 X 10(-3) eV(2) yields sin (2)2 theta(13) = 0.088(-0.039)(+0.049)(stat + syst).
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T2K Collaboration(Abe, K. et al), Cervera-Villanueva, A., Escudero, L., Gomez-Cadenas, J. J., Izmaylov, A., Monfregola, L., et al. (2013). Measurement of Neutrino Oscillation Parameters from Muon Neutrino Disappearance with an Off-Axis Beam. Phys. Rev. Lett., 111(21), 211803–7pp.
Abstract: The T2K Collaboration reports a precision measurement of muon neutrino disappearance with an off-axis neutrino beam with a peak energy of 0.6 GeV. Near detector measurements are used to constrain the neutrino flux and cross section parameters. The Super-Kamiokande far detector, which is 295 km downstream of the neutrino production target, collected data corresponding to 3.01 x 10(20) protons on target. In the absence of neutrino oscillations, 205 +/- 17 (syst) events are expected to be detected while only 58 muon neutrino event candidates are observed. A fit to the neutrino rate and energy spectrum, assuming three neutrino flavors and normal mass hierarchy yields a best-fit mixing angle sin(2) (theta(23)) = 0.514 +/- 0.082 and mass splitting vertical bar Delta m(32)(2)vertical bar = 2.44(-0.15)(+0.17) x 10(-3) eV(2)/c(4). Our result corresponds to the maximal oscillation disappearance probability.
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Gomez-Cadenas, J. J., Martin-Albo, J., Muñoz Vidal, J., & Pena-Garay, C. (2013). Discovery potential of xenon-based neutrinoless double beta decay experiments in light of small angular scale CMB observations. J. Cosmol. Astropart. Phys., 03(3), 043–17pp.
Abstract: The South Pole Telescope (SPT) has probed an expanded angular range of the CMB temperature power spectrum. Their recent analysis of the latest cosmological data prefers nonzero neutrino masses, with Sigma m(nu) = (0.32 +/- 0.11) eV. This result, if con firmed by the upcoming Planck data, has deep implications on the discovery of the nature of neutrinos. In particular, the values of the effective neutrino mass m(beta beta) involved in neutrinoless double beta decay (beta beta 0 nu) are severely constrained for both the direct and inverse hierarchy, making a discovery much more likely. In this paper, we focus in xenon-based beta beta 0 nu experiments, on the double grounds of their good performance and the suitability of the technology to large-mass scaling. We show that the current generation, with effective masses in the range of 100 kg and conceivable exposures in the range of 500 kg.year, could already have a sizeable opportunity to observe beta beta 0 nu events, and their combined discovery potential is quite large. The next generation, with an exposure in the range of 10 ton.year, would have a much more enhanced sensitivity, in particular due to the very low specific background that all the xenon technologies (liquid xenon, high-pressure xenon and xenon dissolved in liquid scintillator) can achieve. In addition, a high-pressure xenon gas TPC also features superb energy resolution. We show that such detector can fully explore the range of allowed effective Majorana masses, thus making a discovery very likely.
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Gomez-Cadenas, J. J., Martin-Albo, J., Sorel, M., Ferrario, P., Monrabal, F., Muñoz, J., et al. (2011). Sense and sensitivity of double beta decay experiments. J. Cosmol. Astropart. Phys., 06(6), 007–30pp.
Abstract: The search for neutrinoless double beta decay is a very active field in which the number of proposals for next-generation experiments has proliferated. In this paper we attempt to address both the sense and the sensitivity of such proposals. Sensitivity comes first, by means of proposing a simple and unambiguous statistical recipe to derive the sensitivity to a putative Majorana neutrino mass, m(beta beta). In order to make sense of how the different experimental approaches compare, we apply this recipe to a selection of proposals, comparing the resulting sensitivities. We also propose a “physics-motivated range” (PMR) of the nuclear matrix elements as a unifying criterium between the different nuclear models. The expected performance of the proposals is parametrized in terms of only four numbers: energy resolution, background rate (per unit time, isotope mass and energy), detection efficiency, and beta beta isotope mass. For each proposal, both a reference and an optimistic scenario for the experimental performance are studied. In the reference scenario we find that all the proposals will be able to partially explore the degenerate spectrum, without fully covering it, although four of them (KamLAND-Zen, CUORE, NEXT and EXO) will approach the 50 meV boundary. In the optimistic scenario, we find that CUORE and the xenon-based proposals (KamLAND-Zen, EXO and NEXT) will explore a significant fraction of the inverse hierarchy, with NEXT covering it almost fully. For the long term future, we argue that Xe-136-based experiments may provide the best case for a 1-ton scale experiment, given the potentially very low backgrounds achievable and the expected scalability to large isotope masses.
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Freitas, E. D. C., Monteiro, C. M. B., Ball, M., Gomez-Cadenas, J. J., Lopes, J. A. M., Lux, T., et al. (2010). Secondary scintillation yield in high-pressure xenon gas for neutrinoless double beta decay (0 nu beta beta) search. Phys. Lett. B, 684(4-5), 205–210.
Abstract: The search for neutrinoless double beta decay (0 nu beta beta) is an important topic in contemporary physics with many active experiments. New projects are planning to use high-pressure xenon gas as both source and detection medium. The secondary scintillation processes available in noble gases permit large amplification with negligible statistical fluctuations, offering the prospect of energy resolution approaching the Fano factor limit. This Letter reports results for xenon secondary scintillation yield, at room temperature, as a function of electric field in the gas scintillation gap for pressures ranging from 2 to 10 bar. A Large Area Avalanche Photodiode (LAAPD) collected the VUV secondary scintillation produced in the gas. X-rays directly absorbed in the LAAPD are used as a reference for determining the number of charge carriers produced by the scintillation pulse and, hence, the number of photons impinging the LAAPD. The number of photons produced per drifting electron and per kilovolt, the so-called scintillation amplification parameter, displays a small increase with pressure, ranging from 141 +/- 6 at 2 bar to 170 +/- 10 at 8 bar. In our setup, this Parameter does not increase above 8 bar due to nonnegligible electron attachment. The results are in good agreement with those presented in the literature in the 1 to 3 bar range. The increase of the scintillation amplification parameter with pressure for high gas densities has been also observed in former work at cryogenic temperatures.
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