Cristina Volpe

Large underground, liquid based detectors for astro-particle physics in Europe: Scientific case and prospects

This document reports on a series of experimental and theoretical studies conducted to assess the astro-particle physics potential of three future large scale particle detectors proposed in Europe as next generation underground observatories. The proposed apparatuses employ three different and, to some extent, complementary detection techniques: GLACIER (liquid argon TPC), LENA (liquid scintillator) and MEMPHYS (water Cherenkov), based on the use of large mass of liquids as active detection media. The results of these studies are presented along with a critical discussion of the performance attainable by the three proposed approaches coupled to existing or planned underground laboratories, in relation to open and outstanding physics issues such as the search for matter instability, the detection of astrophysical neutrinos and geo-neutrinos and to the possible use of these detectors in future high intensity neutrino beams.

Dynamical Collective Calculation of Supernova Neutrino Signals

We present the first calculations with three flavors of collective and shock wave effects for neutrino propagation in core-collapse supernovae using hydrodynamical density profiles and the S matrix formalism. We explore the interplay between the neutrino-neutrino interaction and the effects of multiple resonances upon the time signal of positrons in supernova observatories. A specific signature is found for the inverted hierarchy and a large third neutrino mixing angle and we predict, in this case, a dearth of lower energy positrons in Cherenkov detectors midway through the neutrino signal and the simultaneous revelation of valuable information about the original fluxes. We show that this feature is also observable with current generation neutrino detectors at the level of several sigmas.

Collective neutrino oscillations in matter and CP violation

We explore CP violation effects on the neutrino propagation in dense environments, such as in core-collapse supernovae, where the neutrino self-interaction induces nonlinear evolution equations. We demonstrate that the electron (anti)neutrino fluxes are not sensitive to the CP violating phase if the muon and tau neutrinos interact similarly with matter. On the other hand, we numerically show that new features arise, because of the nonlinearity and the flux dependence of the evolution equations, when the muon and tau neutrinos have different fluxes at the neutrinosphere (due to loop corrections or physics beyond the standard model). In particular, the electron (anti)neutrino probabilities and fluxes depend upon the CP violating phase. We also discuss the CP effects induced by radiative corrections to the neutrino refractive index.

Neutrino-antineutrino correlations in dense anisotropic media

We derive the most general evolution equations describing in-medium (anti)neutrino propagation in the mean-field approximation. In particular, we consider various types of neutrino-antineutrino mixing, for both Dirac and Majorana fields, resulting either from nontrivial pair correlations or from helicity coherence due to the nonvanishing neutrino masses. We show that, unless the medium is spatially homogeneous and isotropic, these correlations are sourced by the usual neutrino and antineutrino densities. This may be of importance in astrophysical environments such as core-collapse supernovae.

The neutrino signal at HALO: learning about the primary supernova neutrino fluxes and neutrino properties

Core-collapse supernova neutrinos undergo a variety of phenomena when they travel from the high neutrino density region and large matter densities to the Earth. We perform analytical calculations of the supernova neutrino fluxes including collective effects due to the neutrino-neutrino interactions, the Mikheev-Smirnov-Wolfenstein (MSW) effect due to the neutrino interactions with the background matter and decoherence of the wave packets as they propagate in space. We predict the numbers of one- and two-neutron charged and neutral-current electron-neutrino scattering on lead events. We show that, due to the energy thresholds, the ratios of one- to two-neutron events are sensitive to the pinching parameters of neutrino fluxes at the neutrinosphere, almost independently of the presently unknown neutrino properties. Besides, such events have an interesting sensitivity to the spectral split features that depend upon the presence/absence of energy equipartition among neutrino flavors. Our calculations show that a lead-based observatory like the Helium And Lead Observatory (HALO) has the potential to pin down important characteristics of the neutrino fluxes at the neutrinosphere, and provide us with information on the neutrino transport in the supernova core.

Neutrino–nucleus interactions as a probe to constrain double-beta decay predictions

We propose to use charged-current neutrino–nucleus interactions as a probe of the many virtual transitions involved in neutrinoless double-beta decay. By performing ν and interaction studies on the initial and final nucleus, respectively, one can get information on the two branches involved in the double-beta decay process. The measurement of such reactions could help to further constrain double-beta decay predictions on the half-lives. We discuss that such studies can be performed either with conventional beams or with low-energy beta-beams. The search for neutrinoless double-beta decay is a crucial issue for learning about neutrino properties, in particular about the Dirac or Majorana nature of neutrinos, important for various domains of physics.

Neutrino spectral split in core-collapse supernovae: A magnetic resonance phenomenon

A variety of neutrino flavour conversion phenomena occur in core-collapse supernova, due to the large neutrino density close to the neutrinosphere, and the importance of the neutrino-neutrino interaction. Three different regimes have been identified so far, usually called the synchronization, the bipolar oscillations and the spectral split. Using the formalism of polarization vectors, within two-flavours, we focus on the spectral split phenomenon and we show for the first time that the physical mechanism underlying the neutrino spectral split is a magnetic resonance phenomenon. In particular, we show that the precession frequencies fulfill the magnetic resonance conditions. Our numerical calculations show that the neutrino energies and the location at which the resonance takes place in the supernova coincide well with the neutrino energies at which a spectral swap occurs. The corresponding adiabaticity parameters present spikes at the resonance location.

Turbulence effects on supernova neutrinos

Multidimensional core-collapse supernova simulations exhibit turbulence of large amplitude and over large scales. As neutrinos pass through the supernova mantle the turbulence is expected to modify their evolution compared to the case where the explosion is free of turbulence. In this paper we study this turbulence effect upon the neutrinos modeling the turbulence expected from multidimensional simulations by adding matter density fluctuations to density profiles taken from one-dimensional hydrodynamical simulations. We investigate the impact upon the supernova neutrino transition probabilities as a function of the neutrino mixing angle θ 13 and turbulence amplitude. In the high resonant channel and with large θ 13 values we find that turbulence is effectively two flavor for fluctuation amplitudes ≲ 1% and have identified a new effect due to the combination of turbulence and multiple high resonances that leads to a sensitivity to fluctuations amplitudes as small as ~0.001%. At small values of θ 13 , beyond the range achievable in Earth based experiments, we find that turbulence leads to new flavor transient effects in the channel where the Mikheev-Smirnov-Wolfenstein high resonance occurs. Finally, we investigate large amplitude fluctuations which lead to three-flavor effects due to broken high-low factorization and significant nonresonant transitions and identify two nonresonant turbulence effects, one depending on the θ 13 , and the other independent of this angle and due to the low Mikheev-Smimov-Wolfenstein resonance.

Shockwaves in Supernovae: New Implications on the Diffuse Supernova Neutrino Background

We investigate shock wave effects upon the diffuse supernova neutrino background using dynamic profiles taken from hydrodynamical simulations and calculating the neutrino evolution in three flavors with the S-matrix formalism. We show that the shock wave impact is significant and introduces modifications of the relic fluxes by about 20% and of the associated event rates at the level of 10%-20%. Such an effect is important since it is of the same order as the rate variation introduced when different oscillation scenarios (i.e., hierarchy or θ 13 ) are considered. In addition, due to the shock wave, the rates become less sensitive to collective effects, in the inverted hierarchy and when sin 2 2θ 13 is between the Chooz limit and 10 -5 . We propose a simplified model to account for shock wave effects in future predictions.

Linearizing neutrino evolution equations includingνν¯pairing correlations

We linearize the neutrino mean-field evolution equations describing the neutrino propagation in a background of matter and of neutrinos, using techniques from many-body microscopic approaches. The procedure leads to an eigenvalue equation that allows to identify instabilities in the evolution, associated with a change of the curvature of the neutrino energy-density surface. Our result includes all contributions from the neutrino Hamiltonian and is generalizable to linearize the equations of motion at an arbitrary point of the evolution. We then consider the extended equations that comprise the normal mean field as well as the abnormal mean field that is associated with neutrino-antineutrino pairing correlations. We first re-derive the extended neutrino Hamiltonian and show that such a Hamiltonian can be diagonalized by introducing a generalized Bogoliubov-Valatin transformation with quasi-particle operators that mix neutrinos and antineutrinos. We give the eigenvalue equations that determine the energies of the quasi-particles eigenstates. Finally we derive the eigenvalue equation of the extended equations of motion, valid in the small amplitude approximation. Our results apply to an arbitrary number of neutrino families.