Sergio Palomares-Ruiz

The next-generation liquid-scintillator neutrino observatory LENA

Abstract As part of the European LAGUNA design study on a next-generation neutrino detector, we propose the liquid-scintillator detector LENA (Low Energy Neutrino Astronomy) as a multipurpose neutrino observatory. The outstanding successes of the Borexino and KamLAND experiments demonstrate the large potential of liquid-scintillator detectors in low-energy neutrino physics. Low energy threshold, good energy resolution and efficient background discrimination are inherent to the liquid-scintillator technique. A target mass of 50xa0kt will offer a substantial increase in detection sensitivity. At low energies, the variety of detection channels available in liquid scintillator will allow for an energy – and flavor-resolved analysis of the neutrino burst emitted by a galactic Supernova. Due to target mass and background conditions, LENA will also be sensitive to the faint signal of the Diffuse Supernova Neutrino Background. Solar metallicity, time-variation in the solar neutrino flux and deviations from MSW–LMA survival probabilities can be investigated based on unprecedented statistics. Low background conditions allow to search for dark matter by observing rare annihilation neutrinos. The large number of events expected for geoneutrinos will give valuable information on the abundances of Uranium and Thorium and their relative ratio in the Earth’s crust and mantle. Reactor neutrinos enable a high-precision measurement of solar mixing parameters. A strong radioactive or pion decay-at-rest neutrino source can be placed close to the detector to investigate neutrino oscillations for short distances and sub-MeV to MeV energies. At high energies, LENA will provide a new lifetime limit for the SUSY-favored proton decay mode into kaon and antineutrino, surpassing current experimental limits by about one order of magnitude. Recent studies have demonstrated that a reconstruction of momentum and energy of GeV particles is well feasible in liquid scintillator. Monte Carlo studies on the reconstruction of the complex event topologies found for neutrino interactions at multi-GeV energies have shown promising results. If this is confirmed, LENA might serve as far detector in a long-baseline neutrino oscillation experiment currently investigated in LAGUNA-LBNO.

Low reheating temperature and the visible sterile neutrino.

We present here a scenario, based on a low reheating temperature T(R)<<100 MeV at the end of (the last episode of) inflation, in which the coupling of sterile neutrinos to active neutrinos can be as large as experimental bounds permit (thus making this neutrino visible in future experiments). In previous models this coupling was forced to be very small to prevent a cosmological overabundance of sterile neutrinos. Here the abundance depends on how low the reheating temperature is. For example, the sterile neutrino required by the Liquid Scintillator Neutrino Detector result may not have any cosmological problem within our scenario.

A novel way of constraining WIMPs annihilations in the Sun: MeV neutrinos

Annihilation of dark matter particles accumulated in the Sun would produce a flux of high-energy neutrinos whose prospects of detection in neutrino telescopes and detectors have been extensively discussed in the literature. However, for annihilations into Standard Model particles, there would also be a flux of neutrinos in the MeV range from the decays at rest of muons and positively charged pions. These low-energy neutrinos have never been considered before and they open the possibility to also constrain dark matter annihilation in the Sun into e + e , + or light quarks. Here we perform a detailed analysis using the recent Super-Kamiokande data in the few tens of MeV range to set limits on the WIMP-nucleon scattering cross section for dierent annihilation channels and computing the evaporation rate of WIMPs from the Sun for all values of the scattering cross section in a consistent way.

Sterile neutrinos in light of recent cosmological and oscillation data: a multi-flavor scheme approach

Light sterile neutrinos might mix with the active ones and be copiously produced in the early Universe. In the present paper, a detailed multi-flavor analysis of sterile neutrino production is performed. Making some justified approximations allows us to consider not only neutrino interactions with the primeval medium and neutrino coherence breaking effects, but also oscillation effects arising from the presence of three light (mostly-active) neutrino states mixed with two heavier (mostly-sterile) states. First, we emphasize the underlying physics via an analytical description of sterile neutrino abundances that is valid for cases with small mixing between active and sterile neutrinos. Then, we study in detail the phenomenology of (3+2) sterile neutrino models in light of short-baseline oscillation data, including the LSND and MiniBooNE results. Finally, by using the information provided by this analysis, we obtain the expected sterile neutrino cosmological abundances and then contrast them with the most recent available data from Cosmic Microwave Background and Large Scale Structure observations. We conclude that (3+2) models are significantly more disfavored by the internal inconsistencies between sterile neutrino interpretations of appearance and disappearance short-baseline data themselves, rather than by the used cosmological data.

Is it possible to explain neutrino masses with scalar dark matter

We present a scenario in which a remarkably simple relation linking dark matter properties and neutrino masses naturally emerges. This framework points towards a low energy theory where the neutrino mass originates from the existence of a light scalar dark matter particle in the MeV mass range. A very surprising aspect of this scenario is that the required MeV dark matter is one of the favoured candidates to explain the mysterious emission of 511 keV photons in the centre of our galaxy. A possible interpretation of these findings is that dark matter is the stepping stone of a theory beyond the standard model instead of being an embarrassing relic whose energy density must be accounted for in any successful model building.

Model-independent bound on the dark matter lifetime

Abstract If dark matter (DM) is unstable, in order to be present today, its lifetime needs to be longer than the age of the Universe, t U ≃ 4 × 10 17 s . It is usually assumed that if DM decays it would do it with some strength through a radiative mode. In this case, very constraining limits can be obtained from observations of the diffuse gamma ray background. However, although reasonable, this is a model-dependent assumption. Here our only assumption is that DM decays into, at least, one Standard Model (SM) particle. Among these, neutrinos are the least detectable ones. Hence, if we assume that the only SM decay daughters are neutrinos, a limit on their flux from DM decays in the Milky Way sets a conservative, but stringent and model-independent bound on its lifetime.

Three-neutrino oscillations of atmospheric neutrinos, θ13, neutrino mass hierarchy and iron magnetized detectors

Abstract We derive predictions for the Nadir angle ( θ n ) dependence of the ratio N μ − / N μ + of the rates of the μ − and μ + multi-GeV events, and for the μ − – μ + event rate asymmetry, A μ − μ + = [ N ( μ − ) − N ( μ + ) ] / [ N ( μ − ) + N ( μ + ) ] , in iron-magnetized calorimeter detectors (MINOS, INO, etc.) in the case of 3-neutrino oscillations of the atmospheric ν μ and ν ¯ μ , driven by one neutrino mass squared difference, | Δ m 31 2 | ∼ ( 2.0 – 3.0 ) × 10 −3 xa0eV 2 ≫ Δ m 21 2 . The asymmetry A μ − μ + (the ratio N μ − / N μ + ) is shown to be particularly sensitive to the Earth matter effects in the atmospheric neutrino oscillations, and thus to the values of sin 2 θ 13 and sin 2 θ 23 , θ 13 and θ 23 being the neutrino mixing angle limited by the CHOOZ and Palo Verde experiments and that responsible for the dominant atmospheric ν μ → ν τ ( ν ¯ μ → ν ¯ τ ) oscillations. It is also very sensitive to the type of neutrino mass spectrum which can be with normal ( Δ m 31 2 > 0 ) or with inverted ( Δ m 31 2 0 ) hierarchy. We find that for sin 2 θ 23 ≳ 0.50 , sin 2 2 θ 13 ≳ 0.06 and | Δ m 31 2 | = ( 2 – 3 ) × 10 −3 xa0eV 2 , the Earth matter effects produce a relative difference between the integrated asymmetries A ¯ μ − μ + and A ¯ μ − μ + 2 ν in the mantle ( cos θ n = 0.30 – 0.84 ) and core ( cos θ n = 0.84 – 1.0 ) bins, which is bigger in absolute value than approximately ∼15%, can reach the values of ( 30 – 35 ) % , and thus can be sufficiently large to be observable. The sign of the indicated asymmetry difference is anticorrelated with the sign of Δ m 31 2 . An observation of the Earth matter effects in the Nadir angle distribution of the asymmetry A μ − μ + (ratio N μ − / N μ + ) would clearly indicate that sin 2 2 θ 13 ≳ 0.06 and sin 2 θ 23 ≳ 0.50 , and would lead to the determination of the sign of Δ m 31 2 .

MeV sterile neutrinos in low reheating temperature cosmological scenarios

It is commonly assumed that the cosmological and astrophysical bounds on the mixings of sterile with active neutrinos are much more stringent than those obtained from laboratory measurements. We point out that in scenarios with a very low reheating temperature MeV at the end of (the last episode of) inflation or entropy creation, the abundance of sterile neutrinos becomes greatly suppressed with respect to that obtained within the standard framework. Thus, in this case cosmological bounds become much less stringent than usually assumed, allowing sterile neutrinos to be visible in future experiments. Here, we concentrate on massive (mostly sterile) neutrinos with masses ms>1 MeV for TRH≤ms.

Constraints on dark matter annihilation from CMB observations before Planck

We compute the bounds on the dark matter (DM) annihilation cross section using the most recent Cosmic Microwave Background measurements from WMAP9, SPT11 and ACT10. We consider DM with mass in the MeV–TeV range annihilating 100% into either an e+e− or a μ+μ− pair. We consider a realistic energy deposition model, which includes the dependence on the redshift, DM mass and annihilation channel. We exclude the canonical thermal relic abundance cross section (σv = 3 × 10−26cm3s−1) for DM masses below 30 GeV and 15 GeV for the e+e− and μ+μ− channels, respectively. A priori, DM annihilating in halos could also modify the reionization history of the Universe at late times. We implement a realistic halo model taken from results of state-of-the-art N-body simulations and consider a mixed reionization mechanism, consisting on reionization from DM as well as from first stars. We find that the constraints on DM annihilation remain unchanged, even when large uncertainties on the halo model parameters are considered.

Explaining LSND by a decaying sterile neutrino

We propose an explanation of the LSND evidence for electron antineutrino appearance based on neutrino decay. We introduce a heavy neutrino, which is produced in pion and muon decays because of a small mixing with muon neutrinos, and then decays into a scalar particle and a light neutrino, predominantly of the electron type. We require values of gm4 ∼ few eV, g being the neutrino–scalar coupling and m4 the heavy neutrino mass, e.g. m4 in the range from 1 keV to 1 MeV and g ∼ 10 −6 –10 −3 . Performing a fit to the LSND data as well as all relevant nullresult experiments, we show that all data can be explained within this decay scenario. In the minimal version of the decay model, we predict a signal in the upcoming MiniBooNE experiment corresponding to a transition probability of the same order as seen in LSND. In addition, we show that extending our model to two nearly degenerate heavy neutrinos it is possible to introduce CP violation in the decay, which can lead to a suppression of the signal in MiniBooNE running in the neutrino mode. We briefly discuss signals in future neutrino oscillation experiments, we show that our scenario is compatible with bounds from laboratory experiments, and we comment on implications in astrophysics and cosmology.