arXiv subjects

Tomislav Prokopec

Publications and source records attributed to Tomislav Prokopec.

At least 145 records · Page 8Linked to original sources

arXiv2000

Large-Scale Primordial Magnetic Fields from Inflation and Preheating

We consider models in which the (transverse) photon mass is non-zero during inflation and drops to zero non-adiabatically at the end of inflation. Through this process, vacuum fluctuations of the photon field are converted into physical, long-wavelength modes with high amplitude. The resulting spectrum of the field strength is approximately B_l ~ 1/l, where l is the relevant coherence scale. With a reasonable model of field evolution we obtain, on comoving galactic scales, a magnetic field strong enough to seed a dynamo mechanism and generate the observed galactic magnetic fields.

astro-ph↗

arXiv2000

Electrogenesis in a scalar field dominated epoch

In this talk I discuss models in which a homogeneous scalar field is used to modify standard cosmology above the nucleosynthesis scale to provide an explanation for the observed matter-antimatter asymmetry of the Universe.

hep-ph↗

arXiv2000

Primordial Spectrum of Gauge Fields from Inflation

We show that conformal invariance of gauge fields is naturally broken in inflation, having as a consequence amplification of gauge fields. The resulting spectrum of the field strength is approximately B_L ~ L^(-1), where L is the relevant coherence scale. One realisation of our scenario is scalar electrodynamics with a scalar whose mass is large enough to evade observational constraints - the obvious candidates being supersymmetric partners of the standard-model fermions. Our mechanism also leads naturally to amplification of the standard-model Z-boson field due to its coupling to the electroweak Higgs field. At preheating, the spectrum of the Z field is transferred to the hypercharge field, which remains frozen in the plasma and is converted into a magnetic field at the electroweak phase transition. With a reasonable model of field evolution one obtains a magnetic field strength of the order of $10^{-29}$ Gauss on a scale of 100 pc, the size of the largest turbulent eddy in a virialised galaxy. Resonant amplification in preheating can lead to primordial fields as large as $10^{-24}$ Gauss, consistent with the seed field required for the galactic dynamo mechanism.

astro-ph↗

arXiv2000

Fermion propagator in a nontrivial background field

We study the fermion propagator in a spatially varying classical background field, and show that, contrary to common wisdom, it may get nontrivial gradient corrections already at the first order in derivative expansion. This occurs whenever the fermion self-energy acquires a spatially (or temporally) varying pseudoscalar term, a simple example of which is given by a complex mass term m(x)= m_R + i gamma_5 m_I. Such effective mass terms arise for example in extensions of the Standard Model during the electroweak transition, and they are crucial in providing the CP-violation necessary for electroweak baryogenesis.

hep-ph↗

arXiv2000

A comment on baryogenesis at the electroweak scale in alternative cosmologies

In this study we examine post-inflationary cosmologies dominated by a scalar field with the equation of state: p_phi=w_phi rho_phi (0 leq w_phi leq 1) in order to facilitate baryogenesis at the electroweak scale. We take a more conventional approach from one in Ref. 1 [Joyce, Prokopec] and assume that the Universe reheats by the scalar field decay before the nucleosynthesis epoch, and find a larger expansion rate at the electroweak scale than the one obtained in Ref. 1. The decaying field models suffer however from an entropy release that dilutes the baryon number produced at the electroweak scale. This dilution is minimized when the kinetic scalar field mode dominates (w_phi = 1), singling it out as the preferred cosmology with regard to baryogenesis. We study both cases, the electroweak transition with an expansion driven departure from equilibrium, and a first order phase transition. We show that in the former case with some tuning one can produce the amount of matter consistent with observation. In the latter case the expansion rate at the electroweak scale may be almost as large as the symmetric phase sphaleron rate, so that even the electroweak models with a relatively weak first order phase transition can be viable for baryogenesis.

hep-ph↗

arXiv2000

The Quantum Boltzmann Equation in a Nontrivial Background

This talk is a status report on our study of quantum transport equations relevant for baryogenesis computations. Our main finding is that, as a consequence of localization in space, the quasiparticle picture of the plasma dynamics breaks down at first non-trivial order in gradient expansion. While in this talk we focus on bosons, we expect that a similar picture holds for fermions. We then argue that the quasiparticle picture is recovered in the adiabatic limit of frequent scattering.

hep-ph↗

arXiv1999

The semiclassical propagator in field theory

We consider scalar field theory in a changing background field. As an example we study the simple case of a spatially varying mass for which we construct the semiclassical approximation to the propagator. The semiclassical dispersion relation is obtained by consideration of spectral integrals and agrees with the WKB result. Further we find that, as a consequence of localization, the semiclassical approximation necessarily contains quantum correlations in momentum space.

hep-ph↗

arXiv1999

Quantum Transport Equations for a Scalar Field

We derive quantum Boltzmann equations from Schwinger-Dyson equations in gradient expansion for a weakly coupled scalar field theory with a spatially varying mass. We find that at higher order in gradients a full description of the system requires specifying not only an on shell distribution function but also a finite number of its derivatives, or equivalently its higher moments. These derivatives describe quantum coherence arising as a consequence of localization in position space. We then show that in the limit of frequent scatterings coherent quantum effects are suppressed, and the transport equations reduce to the single Boltzmann equation for particle density, in which particles flow along modified semiclassical trajectories in phase space.

hep-ph↗

arXiv1999

Quantum Boltzmann equations for mixing scalar fields

We report on a work in progress, whose goal is a systematic field theoretical derivation of the quantum transport equations for baryon production in the electroweak plasma at a first order phase transition in the limit of slowly varying background fields (thick wall limit). We start with the Schwinger-Dyson equations for the two point Green function written in the closed time contour (CTC) formalism. The quantum Boltzmann equations for the density matrix arise when the SD-equations are expanded to the first order in the gradients in the on-shell limit. In this paper we consider only scalar fields, but the formalism can easily be extended to fermions.

hep-ph↗

arXiv1997

Turning Around the Sphaleron Bound: Electroweak Baryogenesis in an Alternative Post-inflationary Cosmology

The usual sphaleron bound and the statement of the impossibility of baryon production at a second order phase transition or analytic cross-over are reformulated in the first part of the paper as requirements of the expansion rate of the Universe at the electroweak scale. With an (exact or effective) additional contribution to the energy density scaling as 1/a^6, which dominates until just before nucleosynthesis, the observed baryon asymmetry may be produced at the electroweak scale in simple extensions of the Minimal Standard Model, even in the case that the phase transition is not first order. We focus our attention on one such cosmology, in which the Universe goes through a period termed `kination' in which its energy is dominated by the kinetic energy of a scalar field. The required kinetic energy dominated modes can occur either as a field rolls down an exponential (or steeper) potential, or in the oscillation of a field about the minimum of a steep power-law potential. We implement in detail the former case with a single exponential field first driving inflation, and then rolling into a kinetic energy dominated mode. Reheating is achieved using an alternative to the usual mechanism due to Spokoiny, in which the Universe is `reheated' by particle creation in the expanding background. Density perturbations of the magnitude required for structure formation may also be generated. We show that the analogous model for the power-law potential cannot be consistently implemented. In models with inflation driven by a second field and the usual mechanism of reheating (by decay of the inflaton) the required kinetic energy dominated cosmology is viable in both types of potential.

hep-ph↗

arXiv1997

Lattice study of classical inflaton decay

We study numerically the decay of the inflaton by solving the full non-linear equations of motion on the lattice. We confirm that parametric resonance is effective in transferring energy from the inflaton to a scalar field as long as the self-interactions of the second field are very small. However, in the very broad resonance case (q>>1) the decay rate is limited by scatterings, which significantly slows down the decay. We also find that the inflaton cannot decay via parametric resonance into a scalar field with moderate self-interactions. This means that the preheating stage may be completely absent in many natural inflationary models.

hep-ph↗

arXiv1997

Negative coupling instability and grandunified baryogenesis

I review my recent work with Brian Greene and Thomas Roos. First I discuss the effect of a negative cross-coupling on the inflaton decay in two scalar field theories. Our main finding is a new effect, the negative coupling instability, which leads to explosive particle production and very fast inflaton decay. Then I discuss the consequences of this instability for grandunified baryogenesis models. The novel aspect of this review is an intuitive explanation of the negative coupling instability, using field trajectories in the configuration space.

hep-ph↗

arXiv1997

Inflaton Decay and Heavy Particle Production with Negative Coupling

We study the decay of the inflaton in a renormalizable two scalar theory. Since the dynamics of the system is dominated by states with large occupation numbers which admit a semiclassical description, the decay can be studied by solving the classical equations of motion on the lattice. Of particular interest is the case when the cross-coupling between the inflaton and the second scalar field is negative, which is naturally allowed in many realistic models. While the inflaton decays via parametric resonance in the positive coupling case we find that for negative coupling there is a new mechanism of particle production which we call negative coupling instability. Due to this new mechanism the variances of the fields grow significantly larger before the production is shut off by the backreaction of the created particles. We also find that heavy particles are produced much more efficiently with negative coupling, which is of prime importance for GUT baryogenesis. Using a simple toy model for baryogenesis and the results of our lattice simulations we show that for natural values of the cross-coupling enough 10^{14}GeV bosons are created to produce a baryon to entropy ratio consistent with observation. For positive coupling the value of the cross-coupling required to produce such massive particles is unnaturally large. In addition to our numerical results we obtain analytical estimates for the maximum variances of the fields in an expanding universe for all cases of interest in our model.

hep-ph↗

arXiv1996

Some aspects of thermal inflation: the finite temperature potential and topological defects

Currently favoured extensions of the Standard Model typically contain `flaton fields' defined as fields with large vacuum expectation values (vevs) and almost flat potentials. If a flaton field is trapped at the origin in the early universe, one expects `thermal inflation' to take place before it rolls away to the true vacuum, because the finite-temperature correction to the potential will hold it at the origin until the temperature falls below $1\TeV$ or so. In the first part of the paper, that expectation is confirmed by an estimate of the finite temperature corrections and of the tunneling rate to the true vacuum, paying careful attention to the validity of the approximations that are used. The second part of the paper considers topological defects which may be produced at the end of an era of thermal inflation. If the flaton fields associated with the era are GUT higgs fields, then its end corresponds to the GUT phase transition. In that case monopoles (as well as GUT higgs particles) will have to be diluted by a second era of thermal inflation. Such an era will not affect the cosmology of GUT strings, for which the crucial parameter is the string mass per unit length. Because of the flat Higgs potential, the GUT symmetry breaking scale required for the strings to be a candidate for the origin of large scale structure and the cmb anisotropy is about three times bigger than usual, but given the uncertainties it is still compatible with the one required by the unification of the Standard Model gauge couplings. The cosmology of textures and of global monopoles is unaffected by the flatness of the potential.

hep-ph↗

arXiv1996

The Impossibility of Baryogenesis at a Second Order Electroweak Phase Transition

We investigate whether baryogenesis is possible at a second order electroweak phase transition. We find that under rather general conditions, the departure from thermal equilibrium is suppressed by the expansion rate of the Universe, and hence baryon production is also suppressed by the expansion rate. We conclude that if no additional sources of departure from thermal equilibrium such as topological defects are present, then electroweak baryogenesis is ruled out if the phase transition is second order or crossover. However, a non-vanishing net baryon to entropy ratio {\it is} generated, and we provide both upper and lower bounds for the result. Our technique is also applicable to other baryogenesis mechanisms taking place during and immediately after a second order phase transition. We estimate the lowest value of the transition energy scale for which the resulting baryon to entropy ratio might be large enough to explain observations.

hep-ph↗

arXiv1995

Dynamical Breaking of CPT Symmetry in Defect Networks and Baryogenesis

Based on a study of {\it charge,} (C), {\it parity} (P) and {\it time reversal} (T) symmetries we show how a CP violating network of defects in the early Universe may bias baryon number production. A static network, even though it violates CP, respects CPT and hence does not bias baryon number production. On the other hand, the {\it ordering dynamics\/} of defects in a network, governed by the interplay of string tension, friction, inertia and the expansion of the Universe, results in the dynamical breakdown of CPT symmetry and may lead to a net baryon number production.

hep-ph↗

arXiv1995

Non-local Electroweak Baryogenesis Part I : Thin Wall Regime

We investigate `non-local' schemes for baryogenesis at a first order electroweak phase transition, in which the effects of a $CP$ violating condensate on the bubble wall propagate into the unbroken phase where the sphaleron rate is unsupressed. Such a condensate exists in multi-Higgs extensions of the standard model, and may exist due to an instability in the minimal standard model. In this paper we first discuss the general problem of determining the $CP$ violating perturbations, distinguishing two regimes (quantum and classical). We then give an analytic treatment of quantum mechanical reflection in the thin wall regime, in which interactions with the plasma can be neglected as a particle propagates across the wall. We focus on leptons because of their much weaker coupling to the plasma. We argue that they are likely to be accurately described by this calculation, but quarks are not. The relative magnitude of the baryon asymmetry produced for different fermions depends on their relative Yukawa couplings ({\it not} their zero temperature masses), their transport properties and their interactions. We calculate the baryon asymmetry for various parameter ranges and conclude that asymmetries comparable with observations can be generated.

hep-ph↗

arXiv1995

Non-local Electroweak Baryogenesis Part II : The Classical Regime

We investigate baryogenesis at a first order electroweak phase transition in the presence of a CP violating condensate on the bubble walls, in the regime in which the bubble walls are `thick', in the sense that fermions interact with the plasma many times as the bubble wall passes. Such a condensate is present in multi-Higgs extensions of the standard model and may be formed via an instability in the minimal standard model. We concentrate on particles with typical thermal energies in the plasma, whose interactions with the wall are accurately described by the WKB approximation, in which a classical chiral force is evident. The deviations from thermal equilibrium produced by motion of the wall are then treated using a classical Boltzmann equation which we solve in a fluid approximation. From the resulting equations we find two effects important for baryogenesis: (i) a classical chiral force term due to the $CP$ violating background, and (ii) a term arising from hypercharge violating interactions which are pushed out of equilibrium by the background{field. Provided the wall propagates slower than the speed of sound, both terms lead to the diffusion of a chiral asymmetry in front of the wall. This can produce a baryon asymmetry of the observed magnitude for typical wall velocities and thicknesses.

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