D. Seipt

Family of equations of state based on lattice QCD: Impact on flow in ultrarelativistic heavy-ion collisions

We construct a family of equations of state within a quasiparticle model by relating pressure, energy density, baryon density, and susceptibilities adjusted to first-principles lattice QCD calculations. The relation between pressure and energy density from lattice QCD is surprisingly insensitive to details of the simulations. Effects from different lattice actions, quark masses, and lattice spacings used in the simulations show up mostly in the quark-hadron phase transition region, which we bridge over by a set of interpolations to a hadron resonance gas equation of state. Within our optimized quasiparticle model we then examine the equation of state along isentropic expansion trajectories at small net baryon densities, as relevant for experiments and hydrodynamic simulations at RHIC and LHC energies. We illustrate its impact on azimuthal flow anisotropies and on the transverse momentum spectra of various hadron species.

Lifting shell structures in the dynamically assisted Schwinger effect in periodic fields

Abstract The dynamically assisted pair creation (Schwinger effect) is considered for the superposition of two periodic electric fields acting in a finite time interval. We find a strong enhancement by orders of magnitude caused by a weak field with a frequency being a multitude of the strong-field frequency. The strong low-frequency field leads to shell structures which are lifted by the weaker high-frequency field. The resonance type amplification refers to a new, monotonously increasing mode, often hidden in some strong oscillatory transient background, which disappears during the smoothly switching off the background fields, thus leaving a pronounced residual shell structure in phase space.

Dynamical Schwinger process in a bifrequent electric field of finite duration: Survey on amplification

The electron-positron pair production due to the dynamical Schwinger process in a slowly oscillating strong electric field is enhanced by the superposition of a rapidly oscillating weaker electric field. A systematic account of the enhancement by the resulting bifrequent field is provided for the residual phase space distribution. The enhancement is explained by a severe reduction of the suppression in both the tunneling and multiphoton regimes.

Two-photon Compton process in pulsed intense laser fields

Based on strong-field QED in the Furry picture we use the Dirac-Volkov propagator to derive a compact expression for the differential emission probability of the two-photon Compton process in a pulsed intense laser field. The relation of real and virtual intermediate states is discussed, and the natural regularization of the on-shell contributions due to the finite laser pulse is highlighted. The inclusive two-photon spectrum is two orders of magnitude stronger than expected from a perturbative estimate.

Quasi-particle description of strongly interacting matter: Towards a foundation

We confront our quasi-particle model for the equation of state of strongly interacting matter with recent first-principle QCD calculations. In particular, we test its applicability at finite baryon densities by comparing with Taylor expansion coefficients of the pressure for two quark flavours. We outline a chain of approximations starting from the Φ-functional approach to QCD which motivates the quasi-particle picture.

Spectral caustics in laser assisted Breit–Wheeler process

Abstract Electron–positron pair production by the Breit–Wheeler process embedded in a strong laser pulse is analyzed. The transverse momentum spectrum displays prominent peaks which are interpreted as caustics, the positions of which are accessible by the stationary phases. Examples are given for the superposition of an XFEL beam with an optical high-intensity laser beam. Such a configuration is available, e.g., at LCLS at present and at European XFEL in near future. It requires a counter propagating probe photon beam with high energy which can be generated by synchronized inverse Compton backscattering.

Determination of the carrier envelope phase for short, circularly polarized laser pulses

We analyze the impact of the carrier envelope phase on the differential cross sections of the Breit-Wheeler and the generalized Compton scattering in the interaction of a charged electron (positron) with an intensive ultra-short electromagnetic (laser) pulse. The differential cross sections as a function of the azimuthal angle of the outgoing electron have a clear bump structure, where the bump position coincides with the value of the carrier phase. This effect can be used for the carrier envelope phase determination.

QCD matter within a quasi-particle model and the critical end point

We compare our quasi-particle model with recent lattice QCD results for the equation of state at finite temperature and baryo-chemical potential. The inclusion of the QCD critical end point into models is discussed. We propose a family of equations of state to be employed in hydrodynamical calculations of particle spectra at RHIC energies and compare with the differential azimuthal anisotropy of strange and charm hadrons.

Isentropic equation of state of two-flavour QCD in a quasi-particle model

We examine the isentropic QCD equation of state within a quasi-particle model being adjusted to first principle QCD calculations of two quark flavours. In particular, we compare with Taylor expansion coefficients of energy and entropy densities and with the isentropic trajectories describing the hydrodynamical expansion of a heavy-ion collision fireball.