Neutrino Energy Loss at Matter-Radiation Decoupling Phase

Physical Review D, 1995

A calculation of neutrino decoupling in the early Universe, including full Fermi-Dirac statistics and electron mass dependence in the weak reaction rates, is presented. We nd that after decoupling, the electron neutrinos contribute 0.83% more to the relativistic energy density than in the standard scenario, where neutrinos are assumed not to share the heating from e 6 annihilation. The corresponding number for muon and tau neutrinos is 0.41%. This has the consequence of modifying the primordial 4 He abundance by 1Y = +1:0 2 10 04 , and the cosmological mass limit on light neutrinos by 0.2{0.5 eV. 95.30.Cq, 13.15.+g, 14.60.Pq, 98.80.Ft PhysRevD (in press)

Nuclear Physics B, 2006

We consider the decoupling of neutrinos in the early Universe in presence of nonstandard 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 4 He produced during Big Bang Nucleosynthesis. We find that 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 for NSI parameters within the ranges allowed by present laboratory data, which is almost three times the effect that appears for standard weak interactions. Thus 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.

1999

I discuss the implications of the latest data on solar and atmospheric neutrinos which strongly indicate the need for physics beyond the Standard Model. I review the theoretical options for reconciling these data in terms of three-neutrino oscillations. Even though not implied by the data, bi-maximal models of neutrino mixing emerge as an attractive possibility. Supersymmetry with broken R-parity provides a predictive way to incorporate it, opening the possibility of testing neutrino anomalies at high-energy collider experiments such as the LHC or at the upcoming long-baseline or neutrino factory experiments. Reconciling, in addition, the hint provided by the LSND experiment requires a fourth, light sterile neutrino. The simplest theoretical scenarios are the most symmetric ones, in which two of the four neutrinos are maximally-mixed and lie at the LSND scale, while the others are at the solar mass scale. The lightness of the sterile neutrino, the nearly maximal atmospheric neutrino mixing, and the generation of ∆m 2 ⊙ & ∆m 2 atm all follow naturally from the assumed lepton-number symmetry and its breaking. These two basic schemes can be distinguished at neutral-current-sensitive solar & atmospheric neutrino experiments such as the Sudbury Neutrino Observatory. However, underground experiments have not yet proven neutrino masses, since there is a variety of alternative mechanisms. For example, flavour changing interactions can play an important rôle in the explanation of solar and of contained atmospheric data and could be tested through effects such as µ → e + γ, µ − e conversion in nuclei, unaccompanied by neutrino-less double beta decay. Conversely, the room is still open for heavy unstable neutrinos. A short-lived ν µ might play a rôle in the explanation of the atmospheric data. Finally, in the presence of a sterile neutrino ν s , a long-lived ν τ in the MeV range could delay the time at which the matter and radiation contributions to the energy density of the Universe become equal, reducing the density fluctuations on the smaller scales, and rescuing the standard cold dark matter scenario for structure formation. In this case the light ν e , ν µ and ν s would account for the solar & atmospheric data.

Physical Review D, 2016

We calculate the evolution of the early universe through the epochs of weak decoupling, weak freeze-out and big bang nucleosynthesis (BBN) by simultaneously coupling a full strong, electromagnetic, and weak nuclear reaction network with a multi-energy group Boltzmann neutrino energy transport scheme. The modular structure of our code provides the ability to dissect the relative contributions of each process responsible for evolving the dynamics of the early universe in the absence of neutrino flavor oscillations. Such an approach allows a detailed accounting of the evolution of the νe,νe, νµ,νµ, ντ ,ντ energy distribution functions alongside and self-consistently with the nuclear reactions and entropy/heat generation and flow between the neutrino and photon/electron/positron/baryon plasma components. This calculation reveals nonlinear feedback in the time evolution of neutrino distribution functions and plasma thermodynamic conditions (e.g., electron-positron pair densities), with implications for: the phasing between scale factor and plasma temperature; the neutron-to-proton ratio; light-element abundance histories; and the cosmological parameter N eff. We find that our approach of following the time development of neutrino spectral distortions and concomitant entropy production and extraction from the plasma results in changes in the computed value of the BBN deuterium yield. For example, for particular implementations of quantum corrections in plasma thermodynamics, our calculations show a 0.4% increase in deuterium. These changes are potentially significant in the context of anticipated improvements in obversational and nuclear physics uncertainties.

Physics of Atomic Nuclei, 2000

I discuss the implications of the latest data on solar and atmospheric neutrinos which strongly indicate the need for physics beyond the Standard Model. I review the theoretical options for reconciling these data in terms of three-neutrino oscillations. Even though not implied by the data, bi-maximal models of neutrino mixing emerge as an attractive possibility. Supersymmetry with broken R-parity provides a predictive way to incorporate it, opening the possibility of testing neutrino anomalies at high-energy collider experiments such as the LHC or at the upcoming long-baseline or neutrino factory experiments. Reconciling, in addition, the hint provided by the LSND experiment requires a fourth, light sterile neutrino. The simplest theoretical scenarios are the most symmetric ones, in which two of the four neutrinos are maximally-mixed and lie at the LSND scale, while the others are at the solar mass scale. The lightness of the sterile neutrino, the nearly maximal atmospheric neutrino mixing, and the generation of ∆m 2 ⊙ & ∆m 2 atm all follow naturally from the assumed lepton-number symmetry and its breaking. These two basic schemes can be distinguished at neutral-current-sensitive solar & atmospheric neutrino experiments such as the Sudbury Neutrino Observatory. However, underground experiments have not yet proven neutrino masses, since there is a variety of alternative mechanisms. For example, flavour changing interactions can play an important rôle in the explanation of solar and of contained atmospheric data and could be tested through effects such as µ → e + γ, µ − e conversion in nuclei, unaccompanied by neutrino-less double beta decay. Conversely, the room is still open for heavy unstable neutrinos. A short-lived ν µ might play a rôle in the explanation of the atmospheric data. Finally, in the presence of a sterile neutrino ν s , a long-lived ν τ in the MeV range could delay the time at which the matter and radiation contributions to the energy density of the Universe become equal, reducing the density fluctuations on the smaller scales, and rescuing the standard cold dark matter scenario for structure formation. In this case the light ν e , ν µ and ν s would account for the solar & atmospheric data.

Modern Physics Letters A, 2013

We clarify in a quantitative way the impact that distinct chemical Tc and kinetic T k freeze-out temperatures have on the reduction of the neutrino fugacity Υν below equilibrium, i.e. Υν < 1, and the increase of the neutrino temperature Tν via partial reheating. We establish the connection between Υν and T k via the modified reheating relation Tν(Υν)/Tγ , where Tγ is the temperature of the background radiation. Our results demonstrate that one must introduce the chemical nonequilibrium parameter, i.e., the fugacity, Υν, as an additional standard cosmological model parameter in the evaluation of CMB fluctuations as its value allows measurement of T k .

A new equation of state is proposed in order to describe the thermal behavior of relic neutrinos. It is based on extensions of the MIT bag model to deal with the gravitational interaction and takes in account the fermionic character of neutrinos. The results for the temperature and entropy of relic neutrinos are compared with those of the cosmic background radiation, treated as a gas of photons at the temperature of 2.726 K. In particular, it is found that the temperature of the relic neutrinos is ¾ of that of the photon gas. The ratio between the two entropies is also estimate.

Science & Education, 2014

A historical case study concerning the serious doubts that arose in early 1930s about the validity of the law of energy conservation in nuclear disintegrations, and the hypothesis of neutrino, will be closely analyzed with the goal of promoting understanding of the nature of science. This work is based upon primary archival and printed sources, with a particular focus on the proceedings of the first International Conference of Nuclear Physics which was held in Rome on October 1931.

2009

Properties of the neutrino in the early universe have been investigated incorporating a small inhomogeneity in the mass density of the early universe. Dependence on this factor is found in studying mean free path and mass bound of neutrinos. The mass bound of neutrino is found to be in the range between 14 MeV and 76 MeV corresponding to the radiation and matter dominated eras respectively. The oscillation length for neutrinos have been estimated to be 86 km for beam energy 2.754 MeV and 220 km for beam energy 7 MeV.

Nuclear Physics B, 2005

In the early universe, neutrinos are slightly coupled when electron-positron pairs annihilate transferring their entropy to photons. This process originates non-thermal distortions on the neutrino spectra which depend on neutrino flavour, larger for ν e than for ν µ or ν τ. We study the effect of three-neutrino flavour oscillations on the process of neutrino decoupling by solving the momentum-dependent kinetic equations for the neutrino spectra. We find that oscillations do not essentially modify the total change in the neutrino energy density, giving N eff = 3.046 in terms of the effective number of neutrinos, while the small effect over the production of primordial 4 He is increased by O(20%), up to 2.1×10 −4. These results are stable within the presently favoured region of neutrino mixing parameters.