arXiv subjects

Gianpiero Mangano

Publications and source records attributed to Gianpiero Mangano.

At least 37 records · Page 2Linked to original sources

arXiv2014

Damping the neutrino flavor pendulum by breaking homogeneity

The most general case of self-induced neutrino flavor evolution is described by a set of kinetic equations for a dense neutrino gas evolving both in space and time. Solutions of these equations have been typically worked out assuming that either the time (in the core-collapse supernova environment) or space (in the early universe) homogeneity in the initial conditions is preserved through the evolution. In these cases one can gauge away the homogeneous variable and reduce the dimensionality of the problem. In this paper we investigate if small deviations from an initial postulated homogeneity can be amplified by the interacting neutrino gas, leading to a new flavor instability. To this end, we consider a simple two flavor isotropic neutrino gas evolving in time, and initially composed by only $ν_e$ and $\barν_e$ with equal densities. In the homogeneous case, this system shows a bimodal instability in the inverted mass hierarchy scheme, leading to the well studied flavor pendulum behavior. This would lead to periodic pair conversions $ν_e \barν_e \leftrightarrow ν_x \barν_x$. To break space homogeneity, we introduce small amplitude space-dependent perturbations in the matter potential. By Fourier transforming the equations of motion with respect to the space coordinate, we then numerically solve a set of coupled equations for the different Fourier modes. We find that even for arbitrarily tiny inhomogeneities, the system evolution runs away from the stable pendulum behavior: the different modes are excited and the space-averaged ensemble evolves towards flavor equilibrium. We finally comment on the role of a time decaying neutrino background density in weakening these results.

hep-ph↗

arXiv2013

The strongest bounds on active-sterile neutrino mixing after Planck data

Light sterile neutrinos can be excited by oscillations with active neutrinos in the early universe. Their properties can be constrained by their contribution as extra-radiation, parameterized in terms of the effective number of neutrino species N_ eff, and to the universe energy density today Ω_νh^2. Both these parameters have been measured to quite a good precision by the Planck satellite experiment. We use this result to update the bounds on the parameter space of (3+1) sterile neutrino scenarios, with an active-sterile neutrino mass squared splitting in the range (10^{-5} - 10^2 ) eV^2. We consider both normal and inverted mass orderings for the active and sterile states. For the first time we take into account the possibility of two non-vanishing active-sterile mixing angles. We find that the bounds are more stringent than those obtained in laboratory experiments. This leads to a strong tension with the short-baseline hints of light sterile neutrinos. In order to relieve this disagreement, modifications of the standard cosmological scenario, e.g. large primordial neutrino asymmetries, are required.

astro-ph.CO↗

arXiv2013

Multi-momentum and multi-flavour active-sterile neutrino oscillations in the early universe: role of neutrino asymmetries and effects on nucleosynthesis

We perform a study of the flavour evolution in the early universe of a multi-flavour active-sterile neutrino system with parameters inspired by the short-baseline neutrino anomalies. In a neutrino-symmetric bath a "thermal" population of the sterile state would quickly grow, but in the presence of primordial neutrino asymmetries a self-suppression as well as a resonant sterile neutrino production can take place, depending on temperature and chosen parameters. In order to characterize these effects, we go beyond the usual average momentum and single mixing approximations and consider a multi-momentum and multi-flavour treatment of the kinetic equations. We find that the enhancement obtained in this case with respect to the average momentum approximation is significant, up to \sim 20 % of a degree of freedom. Such detailed and computationally demanding treatment further raises the asymmetry values required to significantly suppress the sterile neutrino production, up to large and preferentially net asymmetries |L_ν| > O(10^{-2}). For such asymmetries, however, the active-sterile flavour conversions happen so late that significant distortions are produced in the electron (anti)neutrino spectra. The larger |L_ν|, the more the impact of these distortions takes over as dominant cosmological effect, notably increasing the 4 He abundance in primordial nucleosynthesis (BBN). The standard expression of the primordial yields in terms of the effective number of neutrinos and asymmetries is also greatly altered. We numerically estimate the magnitude of such effects for a few representative cases and comment on possible implications for forthcoming cosmological measurements.

astro-ph.CO↗

arXiv2012

Cosmological lepton asymmetry with a nonzero mixing angle θ_{13}

While the baryon asymmetry of the Universe is nowadays well measured by cosmological observations, the bounds on the lepton asymmetry in the form of neutrinos are still significantly weaker. We place limits on the relic neutrino asymmetries using some of the latest cosmological data, taking into account the effect of flavor oscillations. We present our results for two different values of the neutrino mixing angle θ_{13}, and show that for large θ_{13} the limits on the total neutrino asymmetry become more stringent, diluting even large initial flavor asymmetries. In particular, we find that the present bounds are still dominated by the limits coming from Big Bang Nucleosynthesis, while the limits on the total neutrino mass from cosmological data are essentially independent of θ_{13}. Finally, we perform a forecast for COrE, taken as an example of a future CMB experiment, and find that it could improve the limits on the total lepton asymmetry approximately by up to a factor 6.6.

astro-ph.CO↗

arXiv2012

Future constraints on neutrino isocurvature perturbations in the curvaton scenario

In the curvaton scenario, residual isocurvature perturbations can be imprinted in the cosmic neutrino component after the decay of the curvaton field, implying in turn a non-zero chemical potential in the neutrino distribution. We study the constraints that future experiments like Planck, SPIDER or CMBPol will be able to put on the amplitude of isocurvature perturbations in the neutrino component. We express our results in terms of the square root γof the non-adiabaticity parameter αand of the extra relativistic degrees of freedom ΔN_eff. Assuming a fiducial model with purely adiabatic fluctuations, we find that Planck (SPIDER) will be able to put the following upper limits at the 1sigma level: γ< 5.3x10^-3 (1.2x10^-2) and ΔN_eff < 0.16 (0.40) . CMBPol will further improve these constraints to γ< 1.5x10^-3 and ΔN_eff < 0.043. Finally, we recast these bounds in terms of the background neutrino degeneracy parameter ξ and the corresponding perturbation amplitude σ_ξ, and compare with the bounds on ξ that can be derived from Big Bang Nucleosynthesis.

astro-ph.CO↗

arXiv2011

Updated BBN bounds on the cosmological lepton asymmetry for non-zero theta13

We discuss the bounds on the cosmological lepton number from Big Bang Nucleosynthesis (BBN), in light of recent evidences for a large value of the neutrino mixing angle theta13. The largest asymmetries for electron and muon or tau neutrinos compatible with 4He and 2H primordial yields are computed versus the neutrino mass hierarchy and mixing angles. The flavour oscillation dynamics is traced till the beginning of BBN and neutrino distributions after decoupling are numerically computed. The latter contains in general, non thermal distortion due to the onset of flavour oscillations driven by solar squared mass difference in the temperature range where neutrino scatterings become inefficient to enforce thermodynamical equilibrium. Depending on the value of theta13, this translates into a larger value for the effective number of neutrinos, N_eff. Upper bounds on this parameter are discussed for both neutrino mass hierarchies. Values for N_eff which are large enough to be detectable by the Planck experiment are found only for the (presently disfavoured) range sin^2(theta13)<0.01.

hep-ph↗

arXiv2011

Coarse-Grained Cosmological Perturbation Theory

Semi-analytical methods, based on Eulerian perturbation theory, are a promising tool to follow the time evolution of cosmological perturbations at small redshifts and at mildly nonlinear scales. All these schemes are based on two approximations: the existence of a smoothing scale and the single-stream approximation, where velocity dispersion of the dark matter fluid, as well as higher moments of the particle distributions, are neglected. Despite being widely recognized, these two assumptions are, in principle, incompatible, since any finite smoothing scale gives rise to velocity dispersion and higher moments at larger scales. We describe a new approach to perturbation theory, where the Vlasov and fluid equations are derived in presence of a finite coarse-graining scale: this allows a clear separation between long and short distance modes and leads to a hybrid approach where the former are treated perturbatively and the effect of the latter is encoded in external source terms for velocity, velocity dispersion, and all the higher order moments, which can be computed from N-body simulations. We apply the coarse-grained perturbation theory to the computation of the power spectrum and the cross-spectrum between density and velocity dispersion, and compare the results with N-body simulations, finding good agreement.

astro-ph.CO↗

arXiv2011

A robust upper limit on N_eff from BBN, circa 2011

We derive here a robust bound on the effective number of neutrinos from constraints on primordial nucleosynthesis yields of deuterium and helium. In particular, our results are based on very weak assumptions on the astrophysical determination of the helium abundance, namely that the minimum effect of stellar processing is to keep constant (rather than increase, as expected) the helium content of a low-metallicity gas. Using the results of a recent analysis of extragalactic HII regions as upper limit, we find that Delta Neff<= 1 at 95 % C.L., quite independently of measurements on the baryon density from cosmic microwave background anisotropy data and of the neutron lifetime input. In our approach, we also find that primordial nucleosynthesis alone has no significant preference for an effective number of neutrinos larger than the standard value. The ~2 sigma hint sometimes reported in the literature is thus driven by CMB data alone and/or is the result of a questionable regression protocol to infer a measurement of primordial helium abundance.

astro-ph.CO↗

arXiv2010

Shadows of trans-planckian physics on cosmology and the role of the zero-point energy density

We consider the role of the zero-point energy of a quantum field in cosmology and show that the flow of trans-planckian momenta due to redshift acts as a source for this energy, regularized with a cut-off Lambda in physical momenta. In order to fulfill Bianchi identity, we generalize Einstein equations, and discuss the corresponding Friedmann homogeneous and isotropic models. In case of a de Sitter phase, such as during inflation, the solution shows a logarithmic behaviour of the Hubble parameter, and a primordial spectrum of scalar perturbations characterized by the spectral index ns= 1- Lambda2/(3 pi mP2) with mP the Planck mass. We also discuss possible implications of the scenario on late accelerating stage of the Universe at small redshifts, and the emergence of a fluid characterized by an equation of state w=P/rho= -1+ Lambda2/(9 pi mP2). Primordial perturbation spectrum and dark energy parameter w are thus, predicted to be connected by the simple relation w=-(2+ns)/3.

astro-ph.CO↗

arXiv2010

Varying couplings in the early universe: correlated variations of $α$ and $G$

The Cosmic Microwave Background anisotropies provide a unique opportunity to constrain simultaneous variations of the fine-structure constant $α$ and Newton's gravitational constant $G$. Those correlated variations are possible in a wide class of theoretical models. In this brief paper we show that the current data, assuming that particle masses are constant, gives no clear indication for such variations, but already prefers that any relative variations in $α$ should be of the same sign of those of $G$ for variations of $\sim 1 %$. We also show that a cosmic complementarity is present with Big Bang Nucleosynthesis and that a combination of current CMB and BBN data strongly constraints simultaneous variations in $α$ and $G$. We finally discuss the future bounds achievable by the Planck satellite mission.

astro-ph.CO↗

arXiv2009

Low Energy Antineutrino Detection Using Neutrino Capture on EC Decaying Nuclei

In this paper we present a study of the interaction of low energy electron antineutrino on nuclei that undergo electron capture. We show that the two corresponding crossed reactions have a sizeable cross section and are both suitable for detection of low energy antineutrino. However, only in case very specific conditions on the Q-value of the decay are met or significant improvements on the performances of ion storage rings are achieved, these reactions could be exploited in the future to address the long standing problem of a direct detection of Cosmological Neutrino Background.

hep-ph↗

arXiv2008

Using BBN in cosmological parameter extraction from CMB: a forecast for Planck

Data from future high-precision Cosmic Microwave Background (CMB) measurements will be sensitive to the primordial Helium abundance $Y_p$. At the same time, this parameter can be predicted from Big Bang Nucleosynthesis (BBN) as a function of the baryon and radiation densities, as well as a neutrino chemical potential. We suggest to use this information to impose a self-consistent BBN prior on $Y_p$ and determine its impact on parameter inference from simulated Planck data. We find that this approach can significantly improve bounds on cosmological parameters compared to an analysis which treats $Y_p$ as a free parameter, if the neutrino chemical potential is taken to vanish. We demonstrate that fixing the Helium fraction to an arbitrary value can seriously bias parameter estimates. Under the assumption of degenerate BBN (i.e., letting the neutrino chemical potential $ξ$ vary), the BBN prior's constraining power is somewhat weakened, but nevertheless allows us to constrain $ξ$ with an accuracy that rivals bounds inferred from present data on light element abundances.

astro-ph↗

arXiv2007

Capturing Relic Neutrinos with beta-decaying nuclei

We summarize a novel approach which has been recently proposed for direct detection of low energy neutrino backgrounds such as the cosmological relic neutrinos, exploiting neutrino/antineutrino capture on nuclei that spontaneously undergo beta decay.

astro-ph↗

arXiv2007

Probing Low Energy Neutrino Backgrounds with Neutrino Capture on Beta Decaying Nuclei

We study the interaction of low energy neutrinos on nuclei that spontaneously undergo beta decay showing that the product of the cross section times neutrino velocity takes values as high as 10^{-42} cm^2 c for some specific nuclei that decay via allowed transitions. The absence of energy threshold and the value of the cross section single out these processes as a promising though very demanding approach for future experiments aimed at a direct detection of low energy neutrino backgrounds such as the cosmological relic neutrinos.

hep-ph↗

arXiv2007

Present bounds on the relativistic energy density in the Universe from cosmological observables

We discuss the present bounds on the relativistic energy density in the Universe parameterized in terms of the effective number of neutrinos N using the most recent cosmological data on Cosmic Microwave Background (CMB) temperature anisotropies and polarization, Large Scale galaxy clustering from the Sloan Digital Sky Survey (SDSS) and 2dF, luminosity distances of type Ia Supernovae, Lyman-alpha absorption clouds (Ly-alpha), the Baryonic Acoustic Oscillations (BAO) detected in the Luminous Red Galaxies of the SDSS and finally, Big Bang Nucleosynthesis (BBN) predictions for 4He and Deuterium abundances. We find N= 5.2+2.7-2.2 from CMB and Large Scale Structure data, while adding Ly-alpha and BAO we obtain N= 4.6+1.6-1.5 at 95 % c.l.. These results show some tension with the standard value N=3.046 as well as with the BBN range N= 3.1+1.4-1.2 at 95 % c.l., though the discrepancy is slightly below the 2-sigma level. In general, considering a smaller set of data weakens the constraints on N. We emphasize the impact of an improved upper limit (or measurement) of the primordial value of 3He abundance in clarifying the issue of whether the value of N at early (BBN) and more recent epochs coincide.

astro-ph↗

arXiv2006

Cosmological constraints on Neutrino - Dark Matter interactions

I summarize the results of a recent analysis where the cosmological effects of interactions of neutrinos with cold Dark Matter (DM) is investigated. This interaction produces diffusion-damped oscillations in the matter power spectrum, analogous to the acoustic oscillations in the baryon-photon fluid. I discuss the bounds from the Sloan Digital Sky Survey on the corresponding opacity defined as the ratio of neutrino-DM scattering cross section over DM mass, and compare with the constraint from observation of neutrinos from supernova 1987A.

astro-ph↗

arXiv2006

Relaxing Nucleosynthesis Constraints on Brans-Dicke Theories

We reconsider constraints on Brans-Dicke theories arising from the requirement of successful Big Bang Nucleosynthesis. Such constraints typically arise by imposing that the universe be radiation-dominated at early times, and therefore restricting the contribution that a Brans-Dicke scalar could make to the energy budget of the universe. However, in this paper we show how the dynamics of the Brans-Dicke scalar itself can mimic a portion of the radiation contribution, thereby allowing successful nucleosynthesis with a sizable contribution to the total cosmic energy density. This possibility significantly relaxes the existing bounds on Brans-Dicke fields, and opens the door to new possibilities for early universe cosmology. The necessary fine tunings required by such an arrangement are identified and discussed.

astro-ph↗

arXiv2006

Effects of non-standard neutrino-electron interactions on relic neutrino decoupling

We consider the decoupling of neutrinos in the early Universe in presence of non-standard neutral current neutrino-electron interactions (NSI). We first discuss a semi-analytical approach to solve the relevant kinetic equations and then present the results of fully numerical and momentum-dependent calculations, including flavor neutrino oscillations. We present our results in terms of both the effective number of neutrino species (N_eff) and the impact on the abundance of He-4 produced during Big Bang Nucleosynthesis. We find that, for NSI parameters within the ranges allowed by present laboratory data, non-standard neutrino-electron interactions do not essentially modify the density of relic neutrinos nor the bounds on neutrino properties from cosmological observables, such as their mass. Nonetheless, the presence of neutrino-electron NSI may enhance the entropy transfer from electron-positron pairs into neutrinos instead of photons, up to a value of N_eff=3.12. This is almost three times the correction to N_eff=3 that appears for standard weak interactions.

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