arXiv subjects

Kari Enqvist

Publications and source records attributed to Kari Enqvist.

At least 109 records · Page 6Linked to original sources

arXiv1998

Damping rates in the MSSM and electroweak baryogenesis

We present an analysis of the thermalization rate of Higgsinos and winos based on the imaginary part of the two-point Green function in the {\it unbroken} phase of the MSSM. We use improved propagators including resummation of hard thermal loops and the thermalization rate is computed at the one-loop level in the high temperature approximation. We find that the damping is typically dominated by scattering with gauge bosons, resulting in a damping rate of about $γ_{\Ht}\simeq 0.025T$, $γ_{\Wt}\simeq 0.065T$. The contribution from scattering with scalars is relatively small. Implications for baryogenesis are also discussed.

hep-ph↗

arXiv1998

Dark Matter from Unstable B-balls

The spectrum of MSSM admits solitons carrying baryonic charge, or B-balls. In an inflationary universe they can be produced in significant numbers by a break-up of a scalar condensate along the flat directions. It is shown that if SUSY breaking is mediated to the observable sector by gravity, B-balls are unstable but decay to baryons and LSPs typically well below the electroweak phase transition. It is argued that B-balls could be the source of most baryons and cold dark matter in the universe, with their number densities related by $n_{LSP}\simeq 3n_{B}$. For B-balls to survive thermalization, the reheating temperature after inflation should be less than about $10^{3}$ GeV.

hep-ph↗

arXiv1998

Strongly first order electroweak phase transition induced by primordial hypermagnetic fields

We consider the effect of the presence of a hypermagnetic field at the electroweak phase transition. Screening of the Z-component inside a bubble of the broken phase delays the phase transition and makes it stronger first order. We show that the sphaleron constraint can be evaded for m_H up to 100 GeV if a B_Y\gsim 0.3 T^2 exists at the time of the EW phase transition, thus resurrecting the possibility for baryogenesis within the minimal standard model (provided enough CP violation can be obtained). We estimate that for m_H\gsim 100 GeV the Higgs condendsate behaves like a type II superconductor with Z-vortices penetrating the bubble. Also, for such high Higgs masses the minimum B_Y field required for a strong first order phase transition is large enough to render the W-field unstable towards forming a condensate which changes the simple picture of the symmetry breaking.

hep-ph↗

arXiv1998

Q-balls in the MSSM

In the MSSM with gravity mediated supersymmetry breaking, there may exist unstable but long-lived solitons carrying large baryonic charge, or B-balls. These decay well afterthe electroweak phase transition, giving rise to B-ball baryogenesis. Being made of squarks, B-ball decays produce also LSPs and hence can be the source for all cold dark matter.

hep-ph↗

arXiv1998

MSSM Dark Matter Constraints and Decaying B-balls

In the MSSM, LSP dark matter could arise from the decay of B-balls rather than from thermal relics, with a quite different dependence on the MSSM parameters and a natural correlation with the baryon asymmetry. We discuss the constraints imposed on the properties of B-balls and the MSSM spectrum by experimental constraints and show that B-balls cannot form from an Affleck-Dine condensate with 100% efficiency in the absence of LSP annihilations after they decay. For likely formation efficiencies the LSP and slepton masses are typically constrained to be light. The effects of LSP annihilations after the B-balls decay are discussed; for sufficiently small decay temperatures annihilations will play no role, opening up the possibility of experimentally testing the scenario.

hep-ph↗

arXiv1998

Testing neutrino magnetic moments with AGNs

We propose to test the magnetic transition moments of Majorana neutrinos by comparing the fluxes of different flavours of neutrinos coming from active galactic nuclei (AGN). We show that, with reasonable assumptions about the magnetic field of the AGN, it is possible to obtain limits on $ν_τν_{e}$ and $ν_τν_μ$ transition moments which are three to five orders of magnitude better than the laboratory limits. We also point out that with certain parameter values the ratio $ν_τ/ν_{e,μ}$, when measured from different sources, is expected to vary from zero to values somewhat higher than one, providing an unambigious signal of a magnetic transition within the AGN which cannot be explained by neutrino oscillations.

hep-ph↗

arXiv1998

Primordial Magnetic Fields

The explanation of the observed galactic magnetic fields may require the existence of a primordial magnetic field. Such a field may arise during the early cosmological phase transitions, or because of other particle physics related phenomena in the very early universe reviewed here. The turbulent evolution of the initial, randomly fluctuating microscopic field to a large-scale macroscopic field can be described in terms of a shell model, which provides an approximation to the complete magnetohydrodynamics. The results indicate that there is an inverse cascade of magnetic energy whereby the coherence of the magnetic field is increased by many orders of magnitude. Cosmological seed fields roughly of the order of $10^{-20}$ G at the scale of protogalaxy, as required by the dynamo explanation of galactic magnetic fields, thus seem plausible.

astro-ph↗

arXiv1998

Q-Balls and Baryogenesis in the MSSM

We show that Q-balls naturally exist in the Minimal Supersymmetric Standard Model (MSSM) with soft SUSY breaking terms of the minimal N=1 SUGRA type. These are associated with the F- and D-flat directions of the scalar potential once radiative corrections are taken into account. We consider two distinct cases, corresponding to the "H_u L" (slepton) direction with L-balls and the "udd" and "uude" (squark) directions with B-balls. The L-ball always has a small charge, typically of the order of 1000, whilst the B-ball can have an arbitrarily large charge, which, when created cosmologically by the collapse of an unstable Affleck-Dine condensate, is likely to be greater that 10^14. The B-balls typically decay at temperatures less than that of the electroweak phase transition, leading to a novel version of Affleck-Dine baryogenesis, in which the B asymmetry comes from Q-ball decay rather than condensate decay. This mechanism can work even in the presence of additional L violating interactions or B-L conservation, which would rule out conventional Affleck-Dine baryogenesis.

hep-ph↗

arXiv1997

Baryogenesis and the Thermalization Rate of Stop

We take the first steps towards the complete computation of the thermalization rate of the supersymmetric particles involved in electroweak baryogenesis by computing the thermalization rate of the right-handed stop from the imainary part of the two-point Green function. We use improved propagators including resummation of hard thermal loops. The thermalization rate is computed at the one-loop level in the high temperature approximation as a function of $M_{\tilde t_R}(T)$. We also give an estimate for the magnitude of the two-loop contributions which dominate the rate for small $M_{\tilde t_R}(T)$. If the stop is non-relativistic with $M_{\tilde t_R}(T)\gg T$, thermalization takes place by decay and is very fast.

hep-ph↗

arXiv1997

Magnetic Fields in the Early Universe

The observed galactic magnetic fields may have a primordial origin. I briefly review the observations, their interpretation in terms of the dynamo theory, and the current limits on cosmological magnetic fields. Several possible mechanisms for generating a primordial magnetic field are then discussed. Turbulence and the evolution of the microscopic fields to macroscopic fields is described in terms of a shell model, which provides an approximation to the full magnetohydrodynamics and indicates the existence of an inverse cascade of magnetic energy. Cosmological seed fields roughly of the order of $10^{-20}$ G at the scale of protogalaxy, as required by the dynamo explanation of galactic magnetic fields, seem rather plausible.

astro-ph↗

arXiv1997

Neutrinos with Magnetic Moment: Depolarization Rate in Plasma

Neutrinos with a magnetic moment $μ$ change their helicity when interacting with an electromagnetic field. Various aspects of this effect have been described as spin precession, spin-flip scattering, and magnetic Cherenkov radiation. These perspectives are unified in an expression for the $ν_L\toν_R$ transition rate which involves the correlators of the electromagnetic field distribution. Our general formula corrects a previous result and generalizes it to the case where the fields cannot be viewed as classical and where the momentum transfers need not be small. We evaluate our result explicitly for a relativistic QED plasma and determine the depolarization rate to leading order in the fine structure constant. Assuming that big-bang nucleosynthesis constraints do not allow a right-handed neutrino in equilibrium we derive the limit $μ<6.2\EE{-11}μ_B$ on the neutrino magnetic moment. Bounds on $μ$ from a possible large scale magnetic fields are found to be more stringent even for very weak fields

hep-ph↗

arXiv1997

Can Cosmic Ray Catalysed Vacuum Decay Dominate Over Tunnelling?

We consider the question of whether cosmic ray catalysed false vacuum decay can be phenomenologically more important than spontaneous decay via quantum tunnelling. We extend the zero bubble wall width Landau-WKB analysis of catalysed false vacuum decay to include the leading order effects of finite wall width and derive an expression for the thin-wall bubble action. Using this we calculate the exponential suppression factor for the catalysed decay rate at the critical bubble energy, corresponding to the largest probability of catalysed decay. We show that, in general, cosmic ray catalysed decay is likely to be more important than spontaneous decay for sufficiently thin-walled bubbles (wall thickness less than about 30 % of the initial bubble radius), but that spontaneous decay will dominate for the case of thick-walled bubbles. Since any perturbative model with a cosmologically significant false vacuum decay rate will almost certainly produce thick-walled bubbles, we can conclude that cosmic ray catalysed false vacuum decay will never dominate over tunnelling in imposing phenomenological constraints on perturbative particle physics models.

hep-ph↗

arXiv1997

Magnetic field generation in first order phase transition bubble collisions

We consider the formation of a ring-like magnetic field in collisions of bubbles of broken phase in an abelian Higgs model. Particular attention is paid on multiple collisions. The small collision velocity limit, appropriate to the electroweak phase transition, is discussed. We argue that after the completion of the electroweak phase transition, when averaged over nucleation center distances, there exists a mean magnetic field $B\simeq 2.0\times 10^{20}$ G with a coherence length $9.1 \times 10^3 GeV^{-1}$ (for m_H=68 GeV). Because of the ring-like nature of B, the volume average behaves as $B \sim 1/L$. Taking into account the turbulent enhancement of the field by inverse cascade, we estimate that colliding electroweak bubbles would give rise to a mean field $B_{rms}\simeq 10^{-21}$ G at 10 Mpc comoving scale today.

hep-ph↗

arXiv1996

Cosmological abundances of right-handed Dirac neutrinos

The equilibration of the right-helicity states $ν_+$ of light Dirac-neutrinos is discussed. I point out that the $ν_+$ production rate is enhanced by weak gauge boson pole effects so that the right-helicity component of $ν_τ$ is brought into equilibrium at $T\simeq 10$ GeV independently of the initial abundance, provided $m_{ν_τ}\gsim 10$ keV. Neutrino spin flip in primordial magnetic fields and the resulting bound on $μ_ν$ is also discussed.

hep-ph↗

arXiv1996

The effect of Silk damping on primordial magnetic fields

We study the effects of plasma viscosity on the dynamics of primordial magnetic fields by simulating magnetohydrodynamics in the early universe by appropriate non-linear cascade models. We find numerically that even in the presence of large kinetic viscosity, magnetic energy is transferred to large length scales. There are indications, however, that the inverse cascade stops at a given time which depends on the magnitude of viscosity. For realistic viscosities we do not find equipartition between magnetic and kinetic energies.

hep-ph↗

arXiv1996

Cosmological abundances of right-handed neutrinos

We study the equilibration of the right-helicity states of light Dirac neutrinos in the early universe by solving the momentum dependent Boltzmann equations numerically. We show that the main effect is due to electroweak gauge boson poles, which enhance thermalization rates by some three orders of magnitude. The right-helicity states of tau neutrinos will be brought in equilibrium independently of their initial distribution at a temperature above the poles if the tau neutrino mass is larger than about 10 keV.

hep-ph↗

arXiv1996

Electrical conductivity in the early universe

We solve numerically the Boltzmann equation in the early universe in the presence of a constant electric field and find the electrical conductivity $σ$ in the range $1\MeV\lsim T\lsim M_W$. The main contribution to $σ$ is shown to be due to leptonic interactions. For $T\lsim 100\MeV$ we find $σ\simeq 0.76T$ while at $T\simeq M_W$ we obtain $σ\simeq 6.7T$

hep-ph↗

arXiv1996

Large-scale magnetic fields from hydromagnetic turbulence in the very early universe

We investigate hydromagnetic turbulence of primordial magnetic fields using magnetohydrodynamics (MHD) in an expanding universe. We present the basic, covariant MHD equations, find solutions for MHD waves in the early universe, and investigate the equations numerically for random magnetic fields in two spatial dimensions. We find the formation of magnetic structures at larger and larger scales as time goes on. In three dimensions we use a cascade (shell) model, that has been rather successful in the study of certain aspects of hydrodynamic turbulence. Using such a model we find that after ${\cal O}(10^9)$ times the initial time the scale of the magnetic field fluctuation (in the comoving frame) has increased by 4-5 orders of magnitude as a consequence of an inverse cascade effect (i.e. transfer of energy from smaller to larger scales). Thus {\it at large scales} primordial magnetic fields are considerably stronger than expected from considerations which do not take into account the effects of MHD turbulence.

astro-ph↗