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B. Kaempfer

Publications and source records attributed to B. Kaempfer.

At least 19 recordsLinked to original sources
arXiv2018

Analytical properties of the gluon propagator from truncated Dyson-Schwinger equation in complex Euclidean space

We suggest a framework based on the rainbow approximation with effective parameters adjusted to lattice data. The analytic structure of the gluon and ghost propagators of QCD in Landau gauge is analyzed by means of numerical solutions of the coupled system of truncated Dyson-Schwinger equations. We find that the gluon and ghost dressing functions are singular in complex Euclidean space with singularities as isolated pairwise conjugated poles. These poles hamper solving numerically the Bethe-Salpeter equation for glueballs as bound states of two interacting dressed gluons. Nevertheless, we argue that, by knowing the position of the poles and their residues, a reliable algorithm for numerical solving the Bethe-Salpeter equation can be established.

hep-ph↗
arXiv2018

Centrality determination of Au+Au collisions at 1.23A GeV with HADES

The centrality determination for Au+Au collisions at 1.23A GeV, as measured with HADES at the GSI-SIS18, is described. In order to extract collision geometry related quantities, such as the average impact parameter or number of participating nucleons, a Glauber Monte Carlo approach is employed. For the application of this model to collisions at this relatively low centre-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 2.42$ GeV special investigations were performed. As a result a well defined procedure to determine centrality classes for ongoing analyses of heavy-ion data is established.

nucl-ex↗
arXiv2016

Field induced phase transition in the few photon regime

Some features of the field induced phase transition accompanied by the vacuum creation of an electron-positron plasma (EPP) in strong time-dependent electric fields have been discussed in the work [1] in the domain of the tunneling mechanism ($ω\ll m$, where $ω$ is the characteristic frequency of the external field and $m$ is the electron mass). In the present contribution the features of the this process will be considered in the few photon domain where $ω\sim m$. We observe a narrowing of the transient domain of the fast oscillations and, mainly, a considerable growth of the effectiveness of the EPP production. Under these circumstances, we see an increase of the effectiveness of the EPP creation in the particular case of a bifrequent excitation, where both mechanisms (tunneling and few photon) act simultaneously [2,3].

physics.plasm-ph↗
arXiv2016

Vacuum particle-antiparticle creation in strong fields as a field induced phase transition

The features of vacuum particle creation in an external classical field are studied for simplest external field models in $3 + 1$ dimensional QED. The investigation is based on a kinetic equation that is a nonperturbative consequence of the fundamental equations of motion of QED. The observed features of the evolution of the system apply on the qualitative level also for systems of other nature and therefore are rather general. Examples from cosmology and condensed matter physics illustrate this statement. The common basis for the description of these systems are kinetic equations for vacuum particle creation belonging to the class of integro-differential equations of non-Markovian type with fastly oscillating kernel. This allows to characterize processes of this type as belonging to the class of field induced phase transitions.

hep-ph↗
arXiv2015

Assisted dynamical Schwinger effect: pair production in a pulsed bifrequent field

Electron-positron pair production by the superposition of two laser pulses with different frequencies and amplitudes is analyzed as a particular realization of the assisted dynamic Schwinger effect. It is demonstrated that, within a non-perturbative kinetic equation framework, an amplification effect is conceivable for certain parameters. When both pulses have wavelengths longer than the Compton wavelength, the residual net density of produced pairs is determined by the resultant field strength. The number of pairs starts to grow rapidly if the wavelength of the high-frequency laser component gets close to the Compton wavelength.

quant-ph↗
arXiv2013

Searching a Dark Photon with HADES

We present a search for the e+e- decay of a hypothetical dark photon, also names U vector boson, in inclusive dielectron spectra measured by HADES in the p (3.5 GeV) + p, Nb reactions, as well as the Ar (1.756 GeV/u) + KCl reaction. An upper limit on the kinetic mixing parameter squared epsilon^{2} at 90% CL has been obtained for the mass range M(U) = 0.02 - 0.55 GeV/c2 and is compared with the present world data set. For masses 0.03 - 0.1 GeV/c^2, the limit has been lowered with respect to previous results, allowing now to exclude a large part of the parameter region favoured by the muon g-2 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3 * 10^{-6} has been set on the helicity-suppressed direct decay of the eta meson, eta-> e+e-, at 90% CL.

hep-ex↗
arXiv2013

Properties of the electron-positron plasma created from vacuum in a strong laser field. Quasiparticle excitations

Solutions of a kinetic equation are investigated which describe, on a nonperturbative basis, the vacuum creation of quasiparticle electron-positron pairs due to a strong laser field. The dependence of the quasiparticle electron (positron) distribution function and the particle number density is explored in a wide range of the laser radiation parameters, i.e., the wavelength λand amplitude of electric field strength E_0. Three domains are found: the domain of vacuum polarization effects where the density of the e-e+ pairs is small (the "calm valley"), and two accumulation domains in which the production rate of the e-e+ pairs is strongly increased and the e-e+ pair density can reach a significant value (the short wave length domain and the strong field one). In particular, the obtained results point to a complicated short-distance electromagnetic structure of the physical vacuum in the domain of short wavelengths λ\lessim λ_{acc} = π/m. For moderately strong fields E_0 \lesssim E_c=m^2/e, the accumulation regime can be realized where a plasma with a high density of e-e+ quasiparticles can be achieved. In this domain of the field strengths and in the whole investigated range of wavelengths, an observation of the dynamical Schwinger effect can be facilitated.

physics.plasm-ph↗
arXiv2013

Inclusive pion and eta production in p+Nb collisions at 3.5 GeV beam energy

Data on inclusive pion and eta production measured with the dielectron spectrometer HADES in the reaction p+93Nb at a kinetic beam energy of 3.5 GeV are presented. Our results, obtained with the photon conversion method, supplement the rather sparse information on neutral meson production in proton-nucleus reactions existing for this bombarding energy regime. The reconstructed e+e-e+e- transverse-momentum and rapidity distributions are confronted with transport model calculations, which account fairly well for both pi0 and eta production.

nucl-ex↗
arXiv2012

Photon cluster excitation mechanism in an e-e+ plasma created from vacuum by a strong electromagnetic field

In the low density approximation of nonperturbative kinetic theory we investigate the vacuum creation of an electron-positron plasma (EPP) under the action of an external monochromatic "laser" electric field with the focus on multiphoton processes. We analyse in some detail the role of the photon cluster mechanism of EPP excitation and show that this mechanism plays the dominating role in the case when a large photon number from the external field reservoir takes part in the breakdown of the energy gap. We obtain the distribution function in this approximation and discuss its properties.

physics.plasm-ph↗
arXiv2012

Influence of Laser Pulse Parameters on the Properties of e-e+ Plasmas Created from Vacuum

We use the low density approximation within the kinetic theory approach to vacuum creation of an e-e+ pair plasma (EPPP) in a strong laser field in order to investigate the dependence of the observed EPPP on the form and parameters of a single laser pulse. The EPPP distribution function is calculated for an arbitrary time dependence of the electric field in the multiphoton domain (adiabaticity parameter gamma >> 1). The dependence on the field strength, the form and spectrum of the field pulse is investigated on the basis both analytical and numerical methods. The obtained results can be useful for examining some observable secondary processes associated with the dynamical Schwinger effect.

physics.plasm-ph↗
arXiv2012

Momentum dependence of the phi-meson nuclear transparency

The production of phi mesons in proton collisions with C, Cu, Ag, and Au targets has been studied via the phi -> K+K- decay at an incident beam energy of 2.83 GeV using the ANKE detector system at COSY. For the first time, the momentum dependence of the nuclear transparency ratio, the in-medium phi width, and the differential cross section for phi meson production at forward angles have been determined for these targets over the momentum range of 0.6 - 1.6 GeV/c. There are indications of a significant momentum dependence in the value of the extracted phi width, which corresponds to an effective phi-N absorption cross section in the range of 14 - 21 mb.

nucl-ex↗
arXiv2011

Baryonic resonances close to the Kbar-N threshold: the case of Sigma(1385)^+ in pp collisions

We present results of an exclusive measurement of the first excited state of the Sigma hyperon, Sigma(1385)^+, produced in p+p -> Sigma^+ + K^+ + n at 3.5 GeV beam energy. The extracted data allow to study in detail the invariant mass distribution of the Sigma(1385)^+. The mass distribution is well described by a relativistic Breit-Wigner function with a maximum at m_0 = 1383.2 +- 0.9 MeV/c^2 and a width of 40.2 +- 2.1 MeV/c^2. The exclusive production cross-section comes out to be 22.27 +- 0.89 +- 1.56 +3.07 -2.10 mu b. Angular distributions of the Sigma(1385)^+ in different reference frames are found to be compatible with the hypothesis that 33 % of Sigma(1385)^+ result from the decay of an intermediate Delta^{++} resonance.

nucl-ex↗
arXiv2011

Hyperon production in Ar+KCl collisions at 1.76A GeV

We present transverse momentum spectra, rapidity distribution and multiplicity of Lambda-hyperons measured with the HADES spectrometer in the reaction Ar(1.76A GeV)+KCl. The yield of Xi- is calculated from our previously reported Xi-/(Lambda+Sigma0) ratio and compared to other strange particle multiplicities. Employing a strangeness balance equation the multiplicities of the yet unmeasured charged Sigma hyperons can be estimated. Finally a statistical hadronization model is used to fit the yields of pi-, K+, K0s, K-, phi, Lambda and Xi-. The resulting chemical freeze-out temperature of T=(76+-2) MeV is compared to the measured slope parameters obtained from fits to the transverse mass distributions of the particles.

nucl-ex↗
arXiv2010

A Combined Solution of the Schwinger-Dyson and Bethe-Salpeter Equations for Mesons as $q\bar q$ Bound States

The mass spectrum of heavy pseudoscalar mesons, described as quark-antiquark bound systems, is considered within the Bethe-Salpeter formalism with momentum dependent masses of the constituents. This dependence is found by solving the Schwinger-Dyson equation for quark propagators in rainbow-ladder approximation. Such an approximation is known to provide both a fast convergence of numerical methods and accurate results for lightest mesons. However, as the meson mass increases, the method becomes less stable and special attention must be devoted to details of numerical means of solving the corresponding equations. We focus on the pseudoscalar sector and show that our numerical scheme describes fairly accurately the $π$, $K$, $D$, $D_s$ and $η_c$ ground states. Excited states are considered as well. Our calculations are directly related to the future physics programme at FAIR.

nucl-th↗
arXiv2010

Heavy pseudoscalar mesons in a Schwinger-Dyson--Bethe-Salpeter approach

The mass spectrum of heavy pseudoscalar mesons, described as quark-antiquark bound systems, is considered within the Bethe-Salpeter formalism with momentum-dependent masses of the constituents. This dependence is found by solving the Schwinger-Dyson equation for quark propagators in rainbow-ladder approximation. Such an approximation is known to provide both a fast convergence of numerical methods and accurate results for lightest mesons. However, as the meson mass increases, the method becomes less stable and special attention must be devoted to details of numerical means of solving the corresponding equations. We focus on the pseudoscalar sector and show that our numerical scheme describes fairly accurately the $π$, $K$, $D$, $D_s$ and $η_c$ ground states. Excited states are considered as well. Our calculations are directly related to the future physics programme at FAIR.

nucl-th↗
arXiv2010

Measurement of the in-medium phi-meson width in proton-nucleus collisions

The production of phi mesons in the collisions of 2.83 GeV protons with C, Cu, Ag, and Au at forward angles has been measured via the phi -> K+K- decay using the COSY-ANKE magnetic spectrometer. The phi meson production cross section follows a target mass dependence of A^0.56+/-0.02 in the momentum region of 0.6-1.6 GeV/c. The comparison of the data with model calculations suggests that the in-medium phi width is about an order of magnitude larger than its free value.

nucl-ex↗
arXiv2009

A versatile method for simulating pp -> ppe+e- and dp -> pne+e-p_spec reactions

We have developed a versatile software package for the simulation of di-electron production in $pp$ and $dp$ collisions at SIS energies. Particular attention has been paid to incorporate different descriptions of the Dalitz decay $Δ\to N e^+e^-$ via a common interface. In addition, suitable parameterizations for the virtual bremsstrahlung process $NN \to NN e^+e^-$ based on one-boson exchange models have been implemented. Such simulation tools with high flexibility of the framework are important for the interpretation of the di-electron data taken with the HADES spectrometer and the design of forthcoming experiments.

nucl-ex↗