arXiv subjects

B. Kampfer

Publications and source records attributed to B. Kampfer.

At least 37 records · Page 2Linked to original sources

arXiv2012

Subthreshold pair production in short laser pulses

The $e^+e^-$ pair production by a probe photon traversing a linearly polarized laser pulse is treated as generalized nonlinear Breit-Wheeler process. For short laser pulses with very few oscillations of the electromagnetic field we find below the perturbative weak-field threshold $\sqrt{s} = 2m$ a similar enhancement of the pair production rate as for circular polarization. The strong subthreshold enhancement is traced back to the finite bandwidth of the laser pulse. A folding model is developed which accounts for the interplay of the frequency spectrum and the intensity distribution in the course of the pulse.

hep-ph↗

arXiv2012

Enhanced subthreshold electron-positron production in short laser pulses

The emission of electron-positron pairs off a probe photon propagating through a polarized short-pulsed electromagnetic (e.g.\ laser) wave field is analyzed. A significant increase of the total cross section of pair production in the subthreshold region is found for decreasing laser pulse duration even in case of moderate laser pulse intensities.

hep-ph↗

arXiv2012

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.

hep-ph↗

arXiv2011

Non-linear Compton scattering of ultrahigh-intensity laser pulses

We present results for the photon spectrum emitted in non-linear Compton scattering of pulsed ultra-strong laser fields off relativistic electrons for intensities up to $a_0\gtrsim 100$ and pulse lengths of a few laser cycles. At ultrahigh laser intensity, it is appropriate to average over the sub- structures of the differential photon spectrum. Supplementing this procedure with a stationary phase approximation one can evaluate the total emission probability. We find the photon yield in pulsed fields to be up to a factor of ten larger than results obtained from a monochromatic wave calculation.

hep-ph↗

arXiv2011

Transport Coefficients in Gluodynamics: From Weak Coupling towards the Deconfinement Transition

We study the ratio of bulk to shear viscosity in gluodynamics within a phenomenological quasiparticle model. We show that at large temperatures this ratio exhibits a quadratic dependence on the conformality measure as known from weak coupling perturbative QCD. In the region of the deconfinement transition, however, this dependence becomes linear as known from specific strongly coupled theories. The onset of the strong coupling behavior is located near the maximum of the scaled interaction measure. This qualitative behavior of the viscosity ratio is rather insensitive to details of the equation of state.

hep-ph↗

arXiv2011

Shear and bulk viscosities of the gluon plasma in a quasiparticle description

Shear and bulk viscosities of deconfined gluonic matter are investigated within an effective kinetic theory by describing the strongly interacting medium phenomenologically in terms of quasiparticle excitations with medium-dependent self-energies. We show that the resulting transport coefficients reproduce the parametric dependencies on temperature and coupling obtained in perturbative QCD at large temperatures and small running coupling. The extrapolation into the non-perturbative regime results in a decreasing specific shear viscosity with decreasing temperature, exhibiting a minimum in the vicinity of the deconfinement transition, while the specific bulk viscosity is sizeable in this region falling off rapidly with increasing temperature. The temperature dependence of specific shear and bulk viscosities found within this quasiparticle description of the pure gluon plasma is in agreement with available lattice QCD results.

hep-ph↗

arXiv2011

Exclusive $\barΞΞ$ and $\barΞ_cΞ_c$ production in $\bar pp$ collisions at $\sqrt{s}\leq 15$ GeV

Estimates of differential cross sections and longitudinal asymmetries are presented for the reactions $\bar pp\to \bar ΞΞ$ and $\bar pp\to \bar Ξ_cΞ_c$ at energies $\sqrt{s}\lesssim 15$ GeV. The $Ξ$ and $Ξ_c$ hyperons are assumed to be produced in two-step processes: first, intermediate $\barΛΛ$ and/or $\barΛ_cΛ_c$ states are created which are converted afterwards into final states $\bar ΞΞ$, $\bar Ξ_cΞ_c$ and $\bar Ξ_cΞ_c$. The full amplitudes are described by loop diagrams within a modified Regge model, based on the topological decomposition of planar quark diagrams. A strong sensitivity of the ratio of yields of $\bar Ξ_cΞ_c$ to $\bar ΞΞ$ and of $\bar Λ_cΛ_c$ to $\bar ΛΛ$ to the degree of SU(4) symmetry violation is found.

hep-ph↗

arXiv2010

Viscosities of the quasigluon plasma

We investigate bulk and shear viscosities of the gluon plasma within relaxation time approximation to an effective Boltzmann-Vlasov type kinetic theory by viewing the plasma as describable in terms of quasigluon excitations with temperature dependent self-energies. The found temperature dependence of the transport coefficients agrees fairly well with available lattice QCD results. The impact of some details in the quasigluon dispersion relation on the specific shear viscosity is discussed.

hep-ph↗

arXiv2010

Neutrino pair emission off electrons in a strong electromagnetic wave field

The emission of $ν\barν$ pairs off electrons in a polarized ultra-intense electromagnetic (e.g.\ laser) wave field is analyzed. We elaborate on the significance of non-linear electrodynamics effects (i.e., multi-photon processes) and the peculiarities of neutrino production. Special attention is devoted to the convergence of the reaction probabilities as a function of the number of absorbed photons. Expressions for large field intensities are provided. The asymmetry between the probabilities of electron and $μ+τ$ neutrino production depends on initial conditions such as energy of the wave field photons and the field intensity. These findings differ from the lowest order perturbative calculation of the reaction $γ+ e \to e' + ν\bar ν$.

hep-ph↗

arXiv2010

Non-Linear Compton Scattering of Ultrashort and Ultraintense Laser Pulses

The scattering of temporally shaped intense laser pulses off electrons is discussed by means of manifestly covariant quantum electrodynamics. We employ a framework based on Volkov states with a time dependent laser envelope in light-cone coordinates within the Furry picture. An expression for the cross section is constructed, which is independent of the considered pulse shape and pulse length. A broad distribution of scatted photons with a rich pattern of subpeaks like that obtained in Thomson scattering is found. These broad peaks may overlap at sufficiently high laser intensity, rendering inappropriate the notion of individual harmonics. The limit of monochromatic plane waves as well as the classical limit of Thomson scattering are discussed. As a main result, a scaling law is presented connecting the Thomson limit with the general result for arbitrary kinematics. In the overlapping regions of the spectral density, the classical and quantum calculations give different results, even in the Thomson limit. Thus, a phase space region is identified where the differential photon distribution is strongly modified by quantum effects.

hep-ph↗

arXiv2010

Instantaneous Shape Sampling - a model for the $γ$-absorption cross section of transitional nuclei

The influence of the quadrupole shape fluctuations on the dipole vibrations in transitional nuclei is investigated in the framework of the Instantaneous Shape Sampling Model, which combines the Interacting Boson Model for the slow collective quadrupole motion with the Random Phase Approximation for the rapid dipole vibrations. Coupling to the complex background configurations is taken into account by folding the results with a Lorentzian with an energy dependent width. The low-energy energy portion of the $γ$- absorption cross section, which is important for photo-nuclear processes, is studied for the isotopic series of Kr, Xe, Ba, and Sm. The experimental cross sections are well reproduced. The low-energy cross section is determined by the Landau fragmentation of the dipole strength and its redistribution caused by the shape fluctuations. Collisional damping only wipes out fluctuations of the absorption cross section, generating the smooth energy dependence observed in experiment. In the case of semi-magic nuclei, shallow pygmy resonances are found in agreement with experiment.

nucl-th↗

arXiv2010

Relativistic Expansion of Electron-Positron-Photon Plasma Droplets and Photon Emission

The expansion dynamics of hot electron-positron-photon plasma droplets is dealt with within relativistic hydrodynamics. Such droplets, envisaged to be created in future experiments by irradiating thin foils with counter-propagating ultra-intense laser beams, are sources of flashes of gamma radiation. Warm electron-positron plasma droplets may be identified and characterized by a broadened 511 keV line.

physics.plasm-ph↗

arXiv2010

Chiral symmetry, di-electrons and charm

We survey some prospects of identifying further in-medium modifications of hadrons in a strongly interacting medium with respect to ongoing experiment series of the HADES Collaboration and planned experiments of the CBM Collaboration at FAIR. Di-electrons, strange and charm mesons are considered and their potential for signaling imprints of chiral restoration is highlighted.

nucl-th↗

arXiv2010

QCD sum rules for D mesons in dense and hot nuclear matter

Open charm mesons (pseudo-scalar and scalar as well as axial-vector and vector) propagating or resting in nuclear matter display an enhanced sensitivity to the chiral condensate. This offers new prospects to seek for signals of chiral restoration, in particular in p-A and p-bar-A reactions as envisaged in first-round experiments by the CBM and PANDA collaborations at FAIR. Weinberg type sum rules for charming chiral partners are presented, and the distinct in-medium modifications of open-charm mesons are discussed. We also address the gluon condensates near Tc and their impact on QCD sum rules.

nucl-th↗

arXiv2009

Cold quark stars from hot lattice QCD

Based on a quasiparticle model for βstable and electrically neutral deconfined matter we address the mass-radius relation of pure quark stars. The model is adjusted to recent hot lattice QCD results for 2 + 1 flavors with almost physical quark masses. We find rather small radii and masses of equilibrium configurations composed of cold deconfined matter, well distinguished from neutron or hybrid stars.

nucl-th↗

arXiv2009

Beam-Shape Effects in Nonlinear Compton and Thomson Scattering

We discuss intensity effects in collisions between beams of optical photons from a high-power laser and relativistic electrons. Our main focus are the modifications of the emission spectra due to realistic finite-beam geometries. By carefully analyzing the classical limit we precisely quantify the distinction between strong-field QED Compton scattering and classical Thomson scattering. A purely classical, but fully covariant, calculation of the bremsstrahlung emitted by an electron in a plane wave laser field yields radiation into harmonics, as expected. This result is generalized to pulses of finite duration and explains the appearance of line broadening and harmonic substructure as an interference phenomenon. The ensuing numerical treatment confirms that strong focussing of the laser leads to a broad continuum while higher harmonics become visible only at moderate focussing, hence lower intensity. We present a scaling law for the backscattered photon spectral density which facilitates averaging over electron beam phase space. Finally, we propose a set of realistic parameters such that the observation of intensity induced spectral red-shift, higher harmonics, and their substructure, becomes feasible.

hep-ph↗

arXiv2009

Role of phi decays for K- yields in relativistic heavy-ion collisions

The production of strange mesons in collisions of Ar+KCl at a kinetic beam energy of 1.756 AGeV is studied within a transport model of Boltzmann-Ühling-Uhlenbeck (BUU) type. In particular, $ϕ, K^+$ and $K^-$ yields and spectra are compared to the data mesured recently by the HADES collaboration and the $ϕ$ yield measured previously by the FOPI collaboration. Our results are in agreement with these data thus presenting an interpretation of the subleading role of $ϕ$ decays into $K^-$'s and confirming the importance of the strangeness-exchange channels for $K^-$ production.

nucl-th↗

arXiv2009

Omega and Eta (Eta-prime) mesons from NN and ND collisions at intermediate energies

The production of pseudo scalar, Eeta, Eta-prime, and vector, Omega, Rho, Phi, mesons in NN collisions at threshold-near energies is analyzed within a covariant effective meson-nucleon theory. It is shown that a good description of cross sections and angular distributions, for vector meson production, can be accomplished by considering meson and nucleon currents only, while for pseudo scalar production an inclusion of nucleon resonances is needed. The di-electron production from subsequent Dalitz decay of the produced mesons, $η'\to γγ^* \toγe^+e^-$ and $ω\to πγ^*\to πe^+e^-$ is also considered and numerical results are presented for intermediate energies and kinematics of possible experiments with HADES, CLAS and KEK-PS. We argue that the transition form factor $ω\to γ^*π$ as well as $η'\to γ^*γ$ can be defined in a fairly model independent way and the feasibility of an experimental access to transition form factors is discussed.

hep-ph↗