Neutrinoless double-beta decay and seesaw mechanism

From the standard seesaw mechanism of neutrino mass generation, which is based on the assumption that the lepton number is violated at a large (~10exp(+15) GeV) scale, follows that the neutrinoless double-beta decay is ruled by the Majorana neutrino mass mechanism. Within this notion, for the inverted neutrino-mass hierarchy we derive allowed ranges of half-lives of the neutrinoless double-beta decay for nuclei of experimental interest with different sets of nuclear matrix elements. The present-day results of the calculation of the neutrinoless double-beta decay nuclear matrix elements are briefly discussed. We argue that if neutrinoless double-beta decay will be observed in future experiments sensitive to the effective Majorana mass in the inverted mass hierarchy region, a comparison of the derived ranges with measured half-lives will allow us to probe the standard seesaw mechanism assuming that future cosmological data will establish the sum of neutrino masses to be about 0.2 eV.Comment: Some changes in sections I, II, IV, and V; two new figures; additional reference

Indication for the disappearance of reactor electron antineutrinos in the Double Chooz experiment

The Double Chooz Experiment presents an indication of reactor electron antineutrino disappearance consistent with neutrino oscillations. A ratio of 0.944 $\pm$ 0.016 (stat) $\pm$ 0.040 (syst) observed to predicted events was obtained in 101 days of running at the Chooz Nuclear Power Plant in France, with two 4.25 GW$_{th}$ reactors. The results were obtained from a single 10 m$^3$ fiducial volume detector located 1050 m from the two reactor cores. The reactor antineutrino flux prediction used the Bugey4 measurement as an anchor point. The deficit can be interpreted as an indication of a non-zero value of the still unmeasured neutrino mixing parameter \sang. Analyzing both the rate of the prompt positrons and their energy spectrum we find \sang = 0.086 $\pm$ 0.041 (stat) $\pm$ 0.030 (syst), or, at 90% CL, 0.015 $<$ \sang $\ <$ 0.16.Comment: 7 pages, 4 figures, (new version after PRL referee's comments

Observation of Geo-Neutrinos

Geo-neutrinos, electron anti-neutrinos produced in beta decays of naturally occurring radioactive isotopes in the Earth, are a unique direct probe of our planet's interior. We report the first observation at more than 3$\sigma$ C.L. of geo-neutrinos, performed with the Borexino detector at Laboratori Nazionali del Gran Sasso. Anti-neutrinos are detected through the neutron inverse beta decay reaction. With a 252.6 ton-yr fiducial exposure after all selection cuts, we detected 9.9^{+4.1}_{-3.4}(^{+14.6}_{-8.2}) geo-neutrino events, with errors corresponding to a 68.3%(99.73%) C.L. From the $\ln{\cal{L}}$ profile, the statistical significance of the Borexino geo-neutrino observation corresponds to a 99.997% C.L. Our measurement of the geo-neutrinos rate is 3.9^{+1.6}_{-1.3}(^{+5.8}_{-3.2}) events/(100ton-yr). This measurement rejects the hypothesis of an active geo-reactor in the Earth's core with a power above 3 TW at 95% C.L. The observed prompt positron spectrum above 2.6 MeV is compatible with that expected from european nuclear reactors (mean base line of approximately 1000 km). Our measurement of reactor anti-neutrinos excludes the non-oscillation hypothesis at 99.60% C.L.Comment: 8 pages, 4 figures, 3 table

A White Paper on keV Sterile Neutrino Dark Matter

We present a comprehensive review of keV-scale sterile neutrino Dark Matter,collecting views and insights from all disciplines involved - cosmology,astrophysics, nuclear, and particle physics - in each case viewed from boththeoretical and experimental/observational perspectives. After reviewing therole of active neutrinos in particle physics, astrophysics, and cosmology, wefocus on sterile neutrinos in the context of the Dark Matter puzzle. Here, wefirst review the physics motivation for sterile neutrino Dark Matter, based onchallenges and tensions in purely cold Dark Matter scenarios. We then round outthe discussion by critically summarizing all known constraints on sterileneutrino Dark Matter arising from astrophysical observations, laboratoryexperiments, and theoretical considerations. In this context, we provide abalanced discourse on the possibly positive signal from X-ray observations.Another focus of the paper concerns the construction of particle physicsmodels, aiming to explain how sterile neutrinos of keV-scale masses could arisein concrete settings beyond the Standard Model of elementary particle physics.The paper ends with an extensive review of current and future astrophysical andlaboratory searches, highlighting new ideas and their experimental challenges,as well as future perspectives for the discovery of sterile neutrinos

A White Paper on keV sterile neutrino Dark Matter

We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved—cosmology, astrophysics, nuclear, and particle physics—in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos

First measurement of θ<inf>13</inf> from delayed neutron capture on hydrogen in the Double Chooz experiment

The Double Chooz experiment has determined the value of the neutrino oscillation parameter θ13 from an analysis of inverse beta decay interactions with neutron capture on hydrogen. This analysis uses a three times larger fiducial volume than the standard Double Chooz assessment, which is restricted to a region doped with gadolinium (Gd), yielding an exposure of 113.1 GW-ton-years. The data sample used in this analysis is distinct from that of the Gd analysis, and the systematic uncertainties are also largely independent, with some exceptions, such as the reactor neutrino flux prediction. A combined rate- and energy-dependent fit finds sin22θ13=0.097±0.034 (stat.)±0.034 (syst.), excluding the no-oscillation hypothesis at 2.0. This result is consistent with previous measurements of sin22θ13

Letter of Intent for Double-CHOOZ: a Search for the Mixing Angle Theta13

Tremendous progress has been achieved in neutrino oscillation physics during the last few years. However, the smallness of the \t13 neutrino mixing angle still remains enigmatic. The current best constraint comes from the CHOOZ reactor neutrino experiment \s2t13 < 0.2 (at 90% C.L., for \adm2=2.0 10^{-3} \text{eV}^2). We propose a new experiment on the same site, Double-CHOOZ, to explore the range of \s2t13 from 0.2 to 0.03, within three years of data taking. The improvement of the CHOOZ result requires an increase in the statistics, a reduction of the systematic error below one percent, and a careful control of the cosmic ray induced background. Therefore, Double-CHOOZ will use two identical detectors, one at $\sim$150 m and another at 1.05 km distance from the nuclear cores. The plan is to start data taking with two detectors in 2008, and to reach a sensitivity of 0.05 in 2009, and 0.03 in 2011

First Test of Lorentz Violation with a Reactor-based Antineutrino Experiment

We present a search for Lorentz violation with 8249 candidate electron antineutrino events taken by the Double Chooz experiment in 227.9 live days of running. This analysis, featuring a search for a sidereal time dependence of the events, is the first test of Lorentz invariance using a reactor-based antineutrino source. No sidereal variation is present in the data and the disappearance results are consistent with sidereal time independent oscillations. Under the Standard-Model Extension (SME), we set the first limits on fourteen Lorentz violating coefficients associated with transitions between electron and tau flavor, and set two competitive limits associated with transitions between electron and muon flavor