arXiv subjects

Burkhard Kämpfer

Publications and source records attributed to Burkhard Kämpfer.

15 recordsLinked to original sources

arXiv2026

Two-Fluid Schwarzschild Solution

We consider the interior two-fluid Schwarzschild solution. That is a model of compact (neutron) stars with admixed dark matter. The two constant energy densities generalize the interior Schwarschild solution to the case of two fluids (representing standard model matter and dark matter, for instance), interacting mutually solely by the common gravitational field. In general, the two-fluid core is surrounded by a one-fluid corona (envelope). Only for special parameters (the model depends on energy densities $e_1,e_2$ and central pressures $p_{\mathrm{c}1},p_{\mathrm{c}2}$ of the fluids), both fluids occupy the same region. Despite the sum of two energy densities in that region, the compactness bound of 8/9 is not exceeded. A conceivable object is one with a stark leakage of one fluid (e.g. dark matter) beyond the two-fluid core.

gr-qc↗

arXiv2025

Core-Corona Decomposition of Very Compact (Neutron) Stars: Accounting for Current Data of XTE J1814-338

A core-corona decomposition of compact (neutron) star models was compared with the current mass-radius data of the outlier XTE~J1814-338. The corona (which may also be dubbed the envelope, halo or outer crust) is assumed to be of Standard Model matter, with an equation of state that is supposed to be faithfully known and accommodates nearly all other neutron star data. The core, solely parameterized by its mass, radius and transition pressure, presents a challenge regarding its composition. We derived a range of core parameters needed to describe the current data of XTE J1814-338.

astro-ph.HE↗

arXiv2025

Note on Klein-Nishina effect in strong-field QED: the case of nonlinear Compton scattering

Suitably normalized differential probabilities of one-photon emission in external electromagnetic fields are compared to quantify the transit of nonlinear Compton scattering to linear Compton scattering, described by the Klein-Nishina formula, and to constant crossed field treatment. The known Klein-Nishina suppression at large energies is further enforced by increasing field intensity. In view of the Ritus-Narozhny conjecture, we demonstrate that different paths in the field intensity vs. energy plane towards large values of the quantum non-linearity parameter $χ$ facilitate significantly different asymptotic dependencies, both in the Klein-Nishina regime and the constant crossed field regime and in between.

hep-ph↗

arXiv2024

Core-corona decomposition of compact (neutron) stars compared to NICER data including XTE J1814-338

A core-corona decomposition of compact (neutron) star models is compared to recent NICER data of masses and radii. It is in particular interesting to capture the outlier XTE~J1814-338. Instead of integrating the TOV equations from the center to surface, we follow here another pathway by accommodating all uncertainties of the equation(s) of state (EoS) at supra-nuclear density or/and an unknown dark matter admixture in a parameterization of the core by its radius $r_x$, the included mass $m_x$ and the pressure $p_x$ at $r_x$. The corona, which may be dubbed also envelope or halo or outer crust, is assumed to be of standard-model matter where the EoS is supposed to be faithfully known.

astro-ph.HE↗

arXiv2024

Towards a warm holographic equation of state by an Einstein-Maxwell-dilaton model

The holographic Einstein-Maxwell-dilaton model is employed to map state-of-the-art lattice QCD thermodynamics data from the temperature ($T$) axis towards the baryon-chemical potential ($μ_B$) axis aimed at gaining a warm equation of state (EoS) of deconfined QCD matter which can be supplemented with a cool and confined part suitable for subsequent compact (neutron) star (merger) investigations. The model exhibits a critical end point (CEP) at $T_\mathrm{CEP} = \mathcal{O}(100)$ MeV and $μ_{B \, \mathrm{CEP}} = 500 \ldots 700$ MeV with emerging first-order phase transition (FOPT) curve which extends to large values of $μ_B$ without approaching the $μ_B$ axis. We consider the impact and peculiarities of the related phase structure on the EoS for the employed dilaton potential and dynamical coupling parameterizations. These seem to prevent to design an overall trustable EoS without recourse to hybrid constructions.

hep-th↗

arXiv2023

Strong-field QED in Furry-picture momentum-space formulation: Ward identities and Feynman diagrams

The impact of a strong electromagnetic background field on otherwise perturbative QED processes is studied in the momentum-space formulation. The univariate background field is assumed to have finite support in time, thus being suitable to provide a model for a strong laser pulse in plane-wave approximation. The usually employed Furry picture in position space must be equipped with some non-obvious terms to ensure the Ward identity. In contrast, the momentum space formulation allows for an easy and systematic account of these terms, both globally and order-by-order in the weak-field expansion. In the limit of an infinitely long-acting (monochromatic) background field, these terms become gradually suppressed, and the standard perturbative QED Feynman diagrams are recovered in the leading-order weak-field limit. A few examples of three- and four-point amplitudes are considered to demonstrate the application of our Feynman rules which employ free Dirac spinors, the free photon propagator, and the free Fermion propagator, while the external field impact is solely encoded in the Fermion-Fermion-photon vertex function. The appearance of on-/off-shell contributions, singular structures, and Oleinik resonances is pointed out.

hep-ph↗

arXiv2022

Exotic Cores with and without Dark-Matter Admixtures in Compact Stars

We parameterize the core of compact spherical star configurations by a mass ($m_x$) and a radius ($r_x$) and study the resulting admissible areas in the total-mass vs. total-radius plane. The employed fiducial equation-of-state models of the corona at radii $r \ge r_x$ and pressures $p \le p_x = p(r = r_x)$ are that (i) of constant sound velocity and (ii) a proxy of DY$Δ$ DD-ME2 provided by Buchdahl's exactly solvable ansatz. The core ($r < r_x$) may contain any type of material, e.g. Standard-Model matter with unspecified equation of state or/and an unspecified Dark-Matter admixture. Employing a toy model for the cool equation of state with first-order phase transition we discuss also the mass-radius relation of compact stars with an admixture of Dark Matter in a Mirror-World scenario.

gr-qc↗

arXiv2021

Holographic bottomonium formation in a cooling strong-interaction medium at finite baryon density

The shrinking of the bottomonium spectral function towards narrow quasi-particle states in a cooling strong-interaction medium at finite baryon density is followed within a holographic bottom-up model. The 5-dimensional Einstein-dilaton-Maxwell background is adjusted to lattice-QCD results of sound velocity and susceptibilities. The zero-temperature bottomonium spectral function is adjusted to experimental $Υ$ ground-state mass and first radial excitations. At baryo-chemical potential $μ_B = 0$, these two pillars let emerge the narrow quasi-particle state of the $Υ$ ground state at a temperature of about 150 MeV. Excited states are consecutively formed at lower temperatures by about 10 (20) MeV for the $2S$ ($3S$) vector states. The baryon density, i.e. $μ_B > 0$, pulls that formation pattern to lower temperatures. At $μ_B =$ 200 MeV, we find a shift by about 15 MeV.

hep-th↗

arXiv2021

Rise and fall of laser-intensity effects in spectrally resolved Compton process

The spectrally resolved differential cross section of Compton scattering, $d σ/ d ω' \vert_{ω' = const}$, rises from small towards larger laser intensity parameter $ξ$, reaches a maximum, and falls towards the asymptotic strong-field region. Expressed by invariant quantities: $d σ/du \vert_{u = const}$ rises from small towards larger values of $ξ$, reaches a maximum at $ξ_{max} = \frac49 {\cal K} u m^2 / k \cdot p$, ${\cal K} = {\cal O} (1)$, and falls at $ξ> ξ_{max}$ like $\propto ξ^{-3/2} \exp \left (- \frac{2 u m^2}{3 ξ\, k \cdot p} \right )$ at $u \ge 1$. [The quantity $u$ is the Ritus variable related to the light-front momentum-fraction $s = (1 + u)/u = k \cdot k' / k \cdot p$ of the emitted photon (four-momentum $k'$, frequency $ω'$), and $k \cdot p/m^2$ quantifies the invariant energy in the entrance channel of electron (four-momentum $p$, mass $m$) and laser (four-wave vector $k$).] Such a behavior of a differential observable is to be contrasted with the laser intensity dependence of the total probability, $\lim_{χ= ξk \cdot p/m^2, ξ\to \infty} \mathbb{P} \propto αχ^{2/3} m^2 / k \cdot p$, which is governed by the soft spectral part. We combine the hard-photon yield from Compton with the seeded Breit-Wheeler pair production in a folding model and obtain a rapidly increasing $e^+ e^-$ pair number at $ξ\lesssim 4$. Laser bandwidth effects are quantified in the weak-field limit of the related trident pair production.

hep-ph↗

arXiv2020

Non-perturbative signatures of non-linear Compton scattering

The probabilities of various elementary laser - photon - electron/positron interactions display in selected phase space and parameter regions typical non-perturbative dependencies such as $\propto {\cal P} \exp\{- a E_{crit} /E\}$, where ${\cal P}$ is a pre-exponential factor, $E_{crit}$ denotes the critical Sauter-Schwinger field strength, and $E$ characterizes the (laser) field strength. While the Schwinger process with $a = a_S \equiv π$ and the non-linear Breit-Wheeler process in the tunneling regime with $a = a_{n \ell BW} \equiv 4 m / 3 ω'$ (with $ω'$ the probe photon energy and $m$ the electron/positron mass) are famous results, we point out here that also the non-linear Compton scattering exhibits a similar behavior when focusing on high harmonics. Using a suitable cut-off $c > 0$, the factor $a$ becomes $a = a_{n \ell C} \equiv \frac23 c m /(p_0 + \sqrt{p_0^2 -m^2)}$. This opens the avenue towards a new signature of the boiling point of the vacuum even for field strengths $E$ below $E_{crit}$ by employing a high electron beam-energy $p_0$ to counter balance the large ratio $E_{crit} / E$ by a small factor $a$ to achieve $E / a \to E_{crit}$. In the weak-field regime, the cut-off facilitates a threshold leading to multi-photon signatures showing up in the total cross section at sub-threshold energies.

hep-ph↗

arXiv2019

Laser pulse-length effects in trident pair production

Laser pulses facilitate multiphoton contributions to the trident pair production $e_L^- \to e_L^- + e_L^- + e_L^+$ , where the label $L$ indicates a laser field dressed electron ($e^-$) or positron ($e^+$). We isolate the impact of the pulse envelope in the trident S matrix element, formulated within the Furry picture, in leading order of a series expansion in the classical non-linearity parameter $a_0$. Generally, the Fourier transform of the envelope carries the information on the pulse length, which becomes an easily tractable function in the case of a $\cos^2$ pulse envelope. The transition to a monochromatic laser wave can be handled in a transparent manner, as also the onset of bandwidth effects for short pulses can be factorized out and studied separately.

hep-ph↗

arXiv2018

Assisted Vacuum Decay by Time Dependent Electric Fields

We consider the vacuum decay by electron-positron pair production in spatially homogeneous, time dependent electric fields by means of quantum kinetic equations. Our focus is on the impact of various pulse shapes as envelopes of oscillating fields and the assistance effects in multi-scale fields, which are also seen in photons accompanying the creation and motion of pairs.

hep-ph↗

arXiv2017

Afterglow of the dynamical Schwinger process: soft photons amass

We consider the conversion of an electric field into photons as a secondary probe of the dynamical Schwinger process. In spatially homogeneous electric fields, quantum fluctuations of electron-positron ($e^+e^-$) pairs are lifted on the mass shell leaving asymptotically a small finite pair density. The $e^+e^-$ dynamics in turn couples to the quantized photon field and drives its on-shell mode occupation. The spectral properties of the emerging asymptotic photons accompanying the Schwinger process are calculated in lowest-order perturbation theory. Soft photons in the optical range are produced amass in the sub critical region, thus providing a promising discovery avenue, e.g. for laser parameters of the Extreme Light Initiative (ELI-NP) to be put in operation soon.

hep-ph↗

arXiv2017

Holographic vector mesons in a dilaton background

Within a holographic framework, we consider vector mesons riding on a gravity-dilaton background. The latter one is determined directly from a Schrödinger equivalent potential which delivers a proper $ρ$ meson Regge trajectory. The mapping on the dilaton potential yields a thermodynamic phase structure with a first-order transition.

hep-th↗

arXiv2015

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.

hep-ph↗