arXiv subjects

Dario Grasso

Publications and source records attributed to Dario Grasso.

59 records · Page 4Linked to original sources

arXiv1997

On the Nature of the Magnetic Fields Generated During the Electroweak Phase Transition

In this Letter we reanalyse the question of the origin of magnetic fields during the electroweak phase transition. We show that their formation is intimately connected to some semiclassical configurations of the gauge fields, such as electroweak Z-strings and W-condensates. We describe the formation of these semiclassical configurations during a first order phase transition and argue that they might be generated also in the case of a second order phase transition. We suggest that the instability of electroweak strings does not imply the disappereance of the embedded magnetic field.

hep-ph↗

arXiv1996

Distortion of the acoustic peaks in the CMBR due to a primordial magnetic field

In this paper we study the effect of a magnetic field on the fluctuation spectrum of the cosmic microwave background. We find that upcoming measurements might give interesting bounds on large scale magnetic fields in the early Universe. If the effects are seen, it might be possible to establish the presence of different fields in different patches of the sky. Absence of any effect, will provide by one order of magnitude a better limit for a primordial field, now given by nucleosynthesis.

astro-ph↗

arXiv1996

Neutrino Dispersion in Magnetized Media and Spin Oscillations in the Early Universe

We derive general expressions for the neutrino dispersion relation in a magnetized plasma with a wide range of temperatures, chemical potentials, and magnetic field strengths. If the electron and proton chemical potentials vanish, as in the early Universe, there is no magnetization contribution to the neutrino refractive index to leading order in the Fermi coupling constant, contrary to claims in the recent literature. Therefore, as long as the magnetic field satisfies $B\simleq T^2$, the neutrino refractive index in the early Universe is dominated by the standard ``non-local term''. If neutrinos are Dirac particles with magnetic moment $μ$, then their right-handed components are thermally populated before the nucleosynthesis epoch by magnetically induced spin oscillations if $μB_0 \agt 10^{-6}μ_{\rm B}\,{\rm gauss}$, where $μ_{\rm B}=e/2m_e$ is the Bohr magneton and $B_0$ is a large-scale primordial magnetic field at $T_0\approx 1\,\rm MeV$. For a typically expected random field distribution, even smaller values for $μB_0$ would suffice to thermalize the right-handed Dirac components.

hep-ph↗

arXiv1996

Cosmological Bounds to the Magnetic Moment of Heavy Tau Neutrinos

The magnetic moment of tau neutrinos in the MeV mass range may be large enough to modify the cosmological freeze-out calculation and determine the tau-neutrino relic density. In this paper we revisit such a possibility. We calculate the evolution and freeze-out of the tau neutrino number density as a function of its mass and magnetic moment. We then determine its relic density, then calculate its effect upon primordial nucleosynthesis including previously neglected effects.

astro-ph↗

arXiv1995

The Polarization of a QED Plasma in a Strong Magnetic Field

In this paper we study the polarization tensor of photons in a QED plasma in the presence of a magnetic field. We do it both at vanishing and at finite temperature. We use two different methods to compute the polarization tensor components $Π^{μν}$ within the one-loop approximation. The first starts from the effective Lagrangian of the system and relates $Π^{μν}$ to the thermodynamic quantities. The second makes use of the electron propagator in an external magnetic field. In this second approach we use an imaginary time formalism. These methods give consistent results in the first non-vanishing order in the photon 4-momentum. Beyond this limit the first method is not applicable and the introduction of the second can not be avoided. We give some physical interpretations of our results.

hep-ph↗