arXiv subjects
Publications and source records attributed to S. H. Hansen.
The low second acoustic peak in the recent Boomerang data may indicate a cosmological baryon density which is larger than allowed by standard big bang nucleosynthesis. We show that the decay of the tau-neutrino: nu_tau -> nu_e + phi, where nu_e is the electron neutrino and phi is a scalar, essentially can assure agreement between BBN calculations and light element observations for a large baryon density.
Constraints on the lifetime of the heavy sterile neutrino, that was proposed as a possible interpretation of the KARMEN anomaly, are derived from primordial nucleosynthesis and SN 1987A. Together with the recent experimental bounds on the nu_s lifetime, SN 1987A completely excludes this interpretation. Nucleosynthesis arguments permit a narrow window for the lifetime in the interval 0.1-0.2 sec. If nu_s possesses an anomalous interaction with nucleons, the SN bounds may not apply, while the nucleosynthesis ones would remain valid.
The lepton asymmetry that could be generated in the early universe through oscillations of active to sterile neutrinos is calculated (almost) analytically for small mixing angles, sin 2θ< 10^{-2}. It is shown that for a mass squared difference, δm^2=-1 eV^2 it may rise at most by 6 orders of magnitude from the initial ``normal'' value of 10^{-10}, since the back-reaction from the refraction index terminates this rise while the asymmetry is still small. Only for very large mass differences, δm^2 about 10^9 eV^2, the lepton asymmetry could reach a significant magnitude exceeding 0.1.
The influence of a massive Majorana tau-neutrino on primordial nucleosynthesis is rigorously calculated. The system of three integro-differential kinetic equations is solved numerically for the tau-neutrino mass in the interval from 0 to 20 MeV. It is found that the usual assumption of kinetic equilibrium is strongly violated and non-equilibrium corrections considerably amplify the effect. Even a very weak restriction from nucleosynthesis, allowing for one extra massless neutrino species, permits to conclude that m_ν_τ< 1 MeV. For a stricter bound, e.g. for dN < 0.3, the limit is m_ν_τ< 0.35 MeV.
The impact of unstable Majorana tau neutrinos on primordial nucleosynthesis is considered. The mass and lifetime of nu_tau are taken in the intervals 0.1-20 MeV and 0.001-400 sec respectively. The studied decay modes are nu_tau -> nu_mu + phi and nu_tau -> nu_e + phi, where phi is a massless (or light) scalar. Integro-differential kinetic equations are solved numerically without any simplifying assumptions. Our results deviate rather strongly from earlier calculations. Depending on mass, lifetime, and decay channels of the nu_tau, the number of effective neutrino species (found from He4), in addition to the 3 standard ones, varies from -2 to +2.5. The abundances of H2 and Li7 are also calculated.
In the one-loop approximation we derive the equation of motion for a classical scalar field ϕ_c (t) with the back reaction of particle production included. Renormalization of mass and couplings of ϕ_c is done explicitly. The equation is non-local in time, but can easily be treated perturbatively or numerically. For the weak trilinear coupling of the external field to the produced particles, the new equation gives the same solution as the familiar one with the Γϕ_c term. For a stronger coupling and other types of couplings the results are significantly different. The equation can be applied to the universe heating by the inflaton decay and to spontaneous baryogenesis.
We repeat our previous calculation of the spectrum distortion of massless neutrinos in the early universe with a considerably better accuracy and corrected for a missing numerical factor in one of the two ways of calculations presented in hep-ph/9703315. Now both ways of calculations are in perfect agreement and we essentially reproduce our old results presented in the abstract of the paper and used in the calculations of light element abundances. We disagree with the criticism of our calculations presented in astro-ph/9712199.
Distortion of the equilibrium spectra of cosmic neutrinos due to interaction with hotter electrons and positrons in the primeval cosmic plasma is considered. The set of integro-differential kinetic equations for neutrinos is accurately numerically solved. The relative corrections to neutrino energy densities are approximately 0.9% for $ν_e$ and 0.4% for $ν_μ$ and $ν_τ$. This effect results in $1.4 \cdot 10^{-4}$ increase in the primordial $^4 He$ abundance.