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M. Salle

Publications and source records attributed to M. Salle.

9 recordsLinked to original sources
arXiv2001

Initial conditions for simulated `tachyonic preheating' and the Hartree ensemble approximation

In numerical simulations studying preheating in the classical approximation there is the problem how to derive the classical initial conditions from the quantum vacuum fluctuations. In past treatments, the initial conditions often put an energy density into the classical field of order of the cutoff, leading to a divergent temperature after thermalization. We suggest a solution to the problem which follows naturally from a Hartree ensemble approximation, introduced recently as an improvement over the standard Hartree approximation. We study the effects on particle numbers of the various treatments, within the context of `tachyonic preheating' in 1+1 dimensional $ϕ^4$ theory.

hep-ph↗
arXiv2001

Staying Thermal with Hartree Ensemble Approximations

We study thermal behavior of a recently introduced Hartree ensemble approximation, which allows for non-perturbative inhomogeneous field configurations as well as for approximate thermalization, in the $ϕ^4$ model in 1+1 dimensions. Using ensembles with a free field thermal distribution as out-of-equilibrium initial conditions we determine thermalization time scales. The time scale for which the system stays in approximate quantum thermal equilibrium is an indication of the time scales for which the approximation method stays reasonable. This time scale turns out to be two orders of magnitude larger than the time scale for thermalization, in the range of couplings and temperatures studied. We also discuss simplifications of our method which are numerically more efficient and make a comparison with classical dynamics.

hep-ph↗
arXiv2001

Thermalization in a Hartree Ensemble Approximation to Quantum Field dynamics

For homogeneous initial conditions, Hartree (gaussian) dynamical approximations are known to have problems with thermalization, because of insufficient scattering. We attempt to improve on this by writing an arbitrary density matrix as a superposition of gaussian pure states and applying the Hartree approximation to each member of such an ensemble. Particles can then scatter via their back-reaction on the typically inhomogeneous mean fields. Starting from initial states which are far from equilibrium we numerically compute the time evolution of particle distribution functions and observe that they indeed display approximate thermalization on intermediate time scales by approaching a Bose-Einstein form. However, for very large times the distributions drift towards classical-like equipartition.

hep-ph↗
arXiv2000

Thermalisation of inhomogeneous quantum scalar fields in 1+1D

Using an improved version of the Hartree approximation, allowing for ensembles of inhomogeneous configurations, we show in a $λϕ^4$ theory, that initially the system thermalises with a Bose-Einstein distribution. For later times and larger couplings we see deviations.

hep-ph↗
arXiv2000

Finite Temperature Simulations from Quantum Field Dynamics?

We describe a Hartree ensemble method to approximately solve the Heisenberg equations for the ϕ^4 model in 1+1 dimensions. We compute the energies and number densities of the quantum particles described by the ϕfield and find that the particles initially thermalize with a Bose-Einstein distribution for the particle density. Gradually, however, the distribution changes towards classical equipartition. Using suitable initial conditions quantum thermalization is achieved much faster than the onset of this undesirable equipartition. We also show how the numerical efficiency of our method can be significantly improved.

hep-lat↗
arXiv2000

Damping and the Hartree Ensemble Approximation

We study a Hartree ensemble approximation for real-time dynamics in the toy model of 1+1 dimensional scalar field theory. Damping behavior seen in numerical simulations is compared with analytical predictions based on perturbation theory in the original (non-Hartree-approximated) model.

hep-lat↗
arXiv2000

Twin Peaks

The on-shell imaginary part of the retarded selfenergy of massive $\vr^4$ theory in 1+1 dimensions is logarithmically infrared divergent. This leads to a zero in the spectral function, separating its usual bump into two. The twin peaks interfere in time-dependent correlation functions, which causes oscillating modulations on top of exponential-like decay, while the usual formulas for the decay rate fail. We see similar modulations in our numerical results for a mean field correlator, using a Hartree ensemble approximation.

hep-ph↗
arXiv2000

Scalar Field Dynamics: Classical, Quantum and in Between

Using a Hartree ensemble approximation, we investigate the dynamics of the \f^4 model in 1+1 dimensions. We find that the fields initially thermalize with a Bose-Einstein distribution for the fields. Gradually, however, the distribution changes towards classical equipartition. Using suitable initial conditions quantum thermalization is achieved much faster than the onset of this undesirable equipartition. We also show how the numerical efficiency of our method can be significantly improved.

hep-ph↗