arXiv subjects

Kari Enqvist

Publications and source records attributed to Kari Enqvist.

119 records · Page 7Linked 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

R-parity breaking phase transition in the susy singlet majoron model

We consider thermal properties of the susy singlet majoron model. We compute the critical temperature $T_c$ and the subsequent reheating temperature $T_{\rm RH}$ for $R$-parity breaking. Succesful baryogenesis constrains the parameter space via the requirements that $T_c$ and $T_{\rm RH}$ are lower than the electroweak phase transition temperature. A further constraint is provided by requiring that the gauge singlet should be kinematically allowed to decay, in order not to have a matter dominated universe at the time of nucleosynthesis. We have made a detailed study of the parameter space and find an upper limit for the susy breaking scalar mass $\mm\lsim 750~(900)$ GeV if $\gluino=100~(1000)$ GeV, which is valid except for certain special values of the singlet sector parameters.

hep-ph↗

arXiv1995

Neutrino conversions in hot plasma

We discuss the excitation of sterile neutrinos in the early universe using general quantum kinetic equations, which also incorporate a possible primordial magnetic field $B$. We find a new contribution to the excitation propability, which has its origin in the shrinkage of the spin vector. In the absence of $B$ nucleosynthesis implies the constraint $\mid Δm^2\mid\sin^22θ_0\lsim 1.6\times 10^{-6}~{\rm eV}^2$ which is more restrictive than previous estimates. We also present examples of possible stringent limits for $B\ne 0$.

hep-ph↗

arXiv1994

Dirac Neutrinos and Primordial Magnetic Fields

We consider random primordial magnetic fields and discuss their dissipation, coherence length $L_0$, scaling behaviour and constraints implied by the primoridal nucleosynthesis. Such magnetic fields could excite the right-helicity states of Dirac neutrinos, with adverse consequences for nucleosynthesis. We present solutions to the spin kinetic equation of a Dirac neutrino traversing a random magnetic field in the cases of large and small $L_0$, taking also into account elastic collisions. Depending on the scaling behaviour and on the magnetic coherence length, the lower limit on the neutrino magnetic moment thus obtained could be as severe as $10^{-20}μ_B$.

hep-ph↗

arXiv1994

On Primordial Magnetic Fields

A primordial magnetic field could be responsible for the observed magnetic fields of the galaxies. One possibility is that such a primordial field is generated at the electroweak phase transition because of the fluctuations in the Higgs field gradients. I describe a statistical averaging procedure which gives rise to a field of a correct magnitude. Another possibility, where the Yang-Mills vacuum itself is ferrromagnetic, is also discussed.

hep-ph↗

arXiv1994

On the Evolution of Magnetic Fields in the Early Universe

We address the question of the existence of magnetic domains in a recent scenario for the creation of primordial magnetic fields. We write down the magnetohydrodynamic equation in an expanding universe in the infinite conductivity limit and discuss its solution. No domains are found.

hep-ph↗

arXiv1994

Ferromagnetic Vacuum and Galactic Magnetic Fields

Non-abelian gauge theories may have a ferromagnet-like vacuum with a non-zero magnetic field, which also exists at finite temperature. We argue that the formation of the ferromagnet-like vacuum at GUT scales gives rise to a Maxwell magnetic field imprinted on the comoving plasma, and that it is energetically favourable to align the field in different correlation volumes. This results in a coherent magnetic field pertaining the whole universe, with a magnitude $B_{now}\simeq 10^{-14}\G$, which is of the correct size to serve as the seed field for the galactic dynamo.

hep-ph↗

arXiv1993

Aspects of Neutrino Cosmology

Primordial nucleosynthesis constrains the properties of light, stable neutrinos. Apart from the well--known limit on the number of neutrino species, there are also bounds on neutrino masses and magnetic moments. I discuss also sterile neutrinos and neutrino propagation in a primordial magnetic field, such as could be the origin of the observed galactic magnetic fields.

hep-ph↗

arXiv1993

Thermalization of the Higgs Field at the Electroweak Phase Transition

The thermalization rate for long wavelength fluctuations in the Higgs field is calculated from the imaginary part of the finite temperature effective action in the unbroken phase of the Standard Model. We use improved propagators including a resummation of hard thermal loops. The thermalization rate is computed at one loop level, but an estimate of the two--loop contribution appears to give an indication that they are comparable to the one--loop result for small thermal higgs mass. We show also that the Higgs field fluctuations are likely to thermalize very fast compared with the electroweak phase transition time.

hep-ph↗

arXiv1993

Chemical equilibrium in QCD gas in the early universe

We compute the thermally averaged $\qq$--annihilation rate into two and three gluons in the early universe. We show that at very high temperatures $\qq\to ggg$ represents only a 3\%\ correction to $\qq\to gg$. Comparing the annihilation rate to the Hubble rate, corrected for particle interactions in the Standard Model gas, we find that quarks and gluons are not in chemical equilibrium when $T\gsim 3\times 10^{14}$ GeV.

hep-ph↗

arXiv1993

Finite temperature effective action and thermalization of perturbations

The effective action is computed for the \lphi--theory at finite temperature for small perturbations about a constant background field, using a generalized tadpole method. We find the complete effective action, including the real and imaginary parts, to all orders in derivatives and to order ${\cal O}(λ^2)$. We demonstrate that the high $T$ approximation, where only the zero Matsubara frequency is included, is incorrect for the imaginary part even though it is UV-finite. The solutions of the dispersion relations show that initial perturbations do not necessarily thermalize fast enough to be absent at the onset of phase transition.

hep-ph↗