arXiv subjects

Georg Raffelt

Publications and source records attributed to Georg Raffelt.

76 records · Page 5Linked to original sources

arXiv1995

The paradox of axions surviving primordial magnetic fields

In the presence of primordial magnetic fields the oscillating cosmic axion field drives an oscillating electric field. The ensuing dissipation of axions is found to be inversely proportional to the conductivity of the primordial plasma. This counterintuitive result is essentially equivalent to ``Zeno's paradox'' or the ``watched-pot effect'' of quantum mechanics. It implies that the standard predictions of the cosmic axion density remain unaltered even if primordial magnetic fields are strong.

hep-ph↗

arXiv1995

AXIONS IN ASTROPHYSICS AND COSMOLOGY

If axions exist they are efficiently produced in the hot and dense interior of stars, providing a novel energy-loss mechanism. In order to avoid a conflict with the observed properties of stars, one can derive a lower limit on the Peccei-Quinn scale (an upper limit on the axion mass). In the early universe, axions are produced by the ``misalignment mechanism'' and the emission from global strings as well as the relaxation of the string-domain wall system formed at the QCD phase transition. In order to avoid an ``overclosed universe'' the Peccei-Quinn scale must obey an upper limit (a lower limit on the axion mass). The current values of these bounds are reviewed. There remains a ``window of opportunity'' $10^{-5}\,\eV\lapprox m_a\lapprox 10^{-2}\,\eV$, with large uncertainties on either side, where axions could still exist. [Contribution to the XVth Moriond workshop, 21-28 Jan. 1995, Villars-sur-Ollon, Switzerland.]

hep-ph↗

arXiv1994

Reduced Neutrino Opacities and the SN 1987A Signal

Axial-vector interactions of neutrinos and axions with a medium are based on structure functions which cannot be calculated in simple perturbative terms for nuclear densities. We use the SN~1987A neutrino signal duration to estimate the range of allowed neutrino opacities of a supernova (SN) core. We perform numerical simulations of the evolution from the hot, lepton-rich post-collapse stage to the cold, deleptonized neutron star and compare the predicted neutrino signal characteristics with the SN~1987A data. Besides the neutrino opacities we vary the mass, temperature profile, and equation of state of the initial model. Within plausible limits of these quantities the neutrino opacity cannot be much smaller than its ``standard'' value. This limits the width of the spin-density structure function to much smaller values than indicated by a perturbative calculation, implying that bremsstrahlung processes such as axion emission probably saturate at around 10\% nuclear density. A substantial suppression at yet larger densities that might have been expected does not seem to occur.

hep-ph↗

arXiv1993

A Self-Consistent Approach to Neutral-Current Processes in Supernova Cores

The problem of neutral-current processes (neutrino scattering, pair emission, pair absorption, axion emission, \etc) in a nuclear medium can be separated into an expression representing the phase space of the weakly interacting probe, and a set of dynamic structure functions of the medium. For a non-relativistic medium we reduce the description to two structure functions $S_A(ø)$ and $S_V(ø)$ of the energy transfer, representing the axial-vector and vector interactions. $S_V$ is well determined by the single-nucleon approximation while $S_A$ may be dominated by multiply interacting nucleons. Unless the shape of $S_A(ø)$ changes dramatically at high densities, scattering processes always dominate over pair processes for neutrino transport or the emission of right-handed states. Because the emission of right-handed neutrinos and axions is controlled by the same medium response functions, a consistent constraint on their properties from consideration of supernova cooling should use the same structure functions for both neutrino transport and exotic cooling mechanisms.

astro-ph↗