Searcharxiv⌕ Search

arXiv subjects

S. Zschocke

Publications and source records attributed to S. Zschocke.

17 recordsLinked to original sources
arXiv2016

Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes

Gaia Data Release 1 (Gaia DR1) contains astrometric results for more than 1 billion stars brighter than magnitude 20.7 based on observations collected by the Gaia satellite during the first 14 months of its operational phase. We give a brief overview of the astrometric content of the data release and of the model assumptions, data processing, and validation of the results. For stars in common with the Hipparcos and Tycho-2 catalogues, complete astrometric single-star solutions are obtained by incorporating positional information from the earlier catalogues. For other stars only their positions are obtained by neglecting their proper motions and parallaxes. The results are validated by an analysis of the residuals, through special validation runs, and by comparison with external data. Results. For about two million of the brighter stars (down to magnitude ~11.5) we obtain positions, parallaxes, and proper motions to Hipparcos-type precision or better. For these stars, systematic errors depending e.g. on position and colour are at a level of 0.3 milliarcsecond (mas). For the remaining stars we obtain positions at epoch J2015.0 accurate to ~10 mas. Positions and proper motions are given in a reference frame that is aligned with the International Celestial Reference Frame (ICRF) to better than 0.1 mas at epoch J2015.0, and non-rotating with respect to ICRF to within 0.03 mas/yr. The Hipparcos reference frame is found to rotate with respect to the Gaia DR1 frame at a rate of 0.24 mas/yr. Based on less than a quarter of the nominal mission length and on very provisional and incomplete calibrations, the quality and completeness of the astrometric data in Gaia DR1 are far from what is expected for the final mission products. The results nevertheless represent a huge improvement in the available fundamental stellar data and practical definition of the optical reference frame.

astro-ph.GA↗
arXiv2013

Towards sub-microarcsecond models for relativistic astrometry

Astrometric space missions like Gaia have stimulated a rapid advance in the field of relativistic astrometry. Present investigations in that field aim at accuracies significantly less than a microarcsecond. We review the present status of relativistic astrometry. As far as the problem of light propagation is concerned we face two problems: the form of the BCRS metric and solutions to the light-ray equation. Finally, work in progress in that field is briefly mentioned.

gr-qc↗
arXiv2010

Numerical versus analytical accuracy of the formulas for light propagation

Numerical integration of the differential equations of light propagation in the Schwarzschild metric shows that in some situations relevant for practical observations the well-known post-Newtonian solution for light propagation has an error up to 16 microarcsecond. The aim of this work is to demonstrate this fact, identify the reason for this error and to derive an analytical formula accurate at the level of 1 microarcsecond as needed for high-accuracy astrometric projects (e.g., Gaia). An analytical post-post-Newtonian solution for the light propagation for both Cauchy and boundary problems is given for the Schwarzschild metric augmented by the PPN and post-linear parameters $β$, $γ$ and $ε$. Using analytical upper estimates of each term we investigate which post-post-Newtonian terms may play a role for an observer in the solar system at the level of 1 microarcsecond and conclude that only one post-post-Newtonian term remains important for this numerical accuracy. In this way, an analytical solution for the boundary problem for light propagation is derived. That solution contains terms of both post-Newtonian and post-post-Newtonian order, but is valid for the given numerical level of 1 microarcsecond. The derived analytical solution has been verified using the results of a high-accuracy numerical integration of differential equations of light propagation and found to be correct at the level well below 1 microarcsecond for arbitrary observer situated within the solar system. Furthermore, the origin of the post-post-Newtonian terms relevant for the microarcsecond accuracy is elucidated. We demonstrate that these terms result from an inadequate choice of the impact parameter in the standard post-Newtonian formulas.

astro-ph.CO↗
arXiv2006

Impact of Four-Quark Condensates on In-Medium Effects of Hadrons

Spectral properties of hadrons in nuclear matter are treated in the framework of QCD sum rules. The influence of the ambient strongly interacting medium is encoded in various condensates. Especially, the structure of different four-quark condensates and their density dependencies in light quark systems are exemplified for the omega meson and the nucleon.

hep-ph↗
arXiv2006

Hunting Medium Modifications of the Chiral Condensate

With QCD sum rule evaluations, spectral changes of hadrons inside nuclear matter are considered, which shed light on QCD condensates and thus on the non-perturbative structure of the QCD ground state. For some light quark configurations, omega meson and nucleon, the relevance of four-quark condensates is compared; the D meson as a representative of heavy-light quark systems is also briefly discussed.

hep-ph↗
arXiv2005

Impact of Nucleon Mass Shift on the Freeze Out Process

The freeze out of a massive nucleon gas through a finite layer with time-like normal is studied. The impact of in-medium nucleon mass shift on the freeze out process is investigated. A considerable modification of the thermodynamical variables temperature, flow-velocity, energy density and particle density has been found. Due to the nucleon mass shift the freeze out particle distribution functions are changed noticeably in comparison with evaluations, which use vacuum nucleon mass.

hep-ph↗
arXiv2005

Evidence for In-Medium Changes of Four-Quark Condensates

Utilizing the QCD sum rule approach to the behavior of the omega meson in nuclear matter we derive evidence for in-medium changes of particular four-quark condensates from the recent CB-TAPS experiment for the reaction gamma + A -> A' + omega (-> pi0 gamma) with A = Nb and LH2.

hep-ph↗
arXiv2005

Electromagnetic Probes of Strongly Interacting Matter: Probes of Chiral Symmetry Restoration?

The QCD sum rule approach to in-medium modifications of the omega meson in nuclear matter is reviewed with emphasis of its relation to 4-quark condensates and chiral symmetry restoration. Possible implications of the CB-TAPS experiment for the reaction gamma A -> A' omega (-> pi0 gamma) are sketched and the particularly important role of di-electron probes, accessible with HADES, is highlighted. A brief update of a parametrization of the previous dilepton and photon probes from CERES and WA98 of heavy-ion collisions at CERN-SPS energies is presented.

hep-ph↗
arXiv2004

Rho - Omega Splitting and Mixing in Nuclear Matter

We investigate the splitting and mixing of $ρ$ and $ω$ mesons in nuclear matter. The calculations were performed on the basis of QCD sum rules and include all operators up to mass dimension-6 twist-4 and up to first order in the coupling constants. Special attention is devoted to the impact of the scalar 4-quark condensates on both effects. In nuclear matter the Landau damping governs the $ρ- ω$ mass splitting while the scalar 4-quark condensates govern the strenght of individual mass shifts. A strong in-medium mass splitting causes the disappearance of the $ρ- ω$ mixing.

hep-ph↗
arXiv2004

Dileptons and Open Charm: Probes of Chiral Restoration

We summarize the status of electromagnetic probes of strongly interacting matter produced in relativistic heavy-ion collisions at CERN-SPS with respect to indications of chiral symmetry restoration. Explorative results for studying the open charm dynamics at BNL-RHIC are presented.

nucl-th↗
arXiv2003

Impact of the QCD four-quark condensate on in-medium spectral changes of light vector mesons

Within the Borel QCD sum rule approach at finite baryon density we study the role of the four-quark condensates for the modifications of the vector mesons $ρ$, $ω$ and $ϕ$ in nuclear matter. We find that in-medium modifications of the $ρ$ and $ω$ mesons are essentially dominated by the dependence of the 4-quark condensate on the nucleon density. In particular, the numerical value of a parameter ($κ_N$), which describes the strength of the density dependence of the 4-quark condensate beyond the mean-field approximation, governs the decrease of the $ρ$ mass as a function of the density. For the $ω$ meson the sign of the in-medium mass shift is changed by variations of $κ_N$. To study consistently the in-medium broadening of the light vector mesons we employ $ρN$ and $ωN$ scattering amplitudes derived recently from a covariant unitary coupled channel approach adjusted to pion- and photo-induced reactions. In contrast to the $ρ$ and $ω$ mesons, the in-medium mass of the $ϕ$ meson is directly related to the chiral (strange) quark condensate. Measurements of the vector meson spectral change in heavy-ion collisions with HADES can shed light on the yet unknown density dependence of the 4-quark condensate.

hep-ph↗
arXiv2002

In-medium spectral change of omega mesons as a probe of QCD four-quark condensate

Within QCD sum rules at finite baryon density we show the crucial role of four-quark condensates for the in-medium modification of the omega meson spectral function. In particular, such a global property as the sign of the in-medium omega meson mass shift is found to be governed by a parameter which describes the strength of the density dependence of the four-quark condensate beyond mean-field approximation. To study self-consistently the broadening of the omega meson resonance we employ a hadron spectral function based on the omega meson propagator delivered by an effective chiral Lagrangian. Measurements of the omega meson spectral change in heavy-ion collisions with the HADES detector can reveal the yet unknown density dependence of the four-quark condensate.

hep-ph↗
arXiv2002

Probing the strange quark condensate by di-electrons from phi meson decays in heavy-ion collisions at SIS energies

QCD sum rules predict that the change of the strange quark condensate $<\bar s s>$ in hadron matter at finite baryon density causes a shift of the peak position of the di-electron spectra from $ϕ$ meson decays. Due to the expansion of hadron matter in heavy-ion collisions, the $ϕ$ peak suffers a smearing governed by the interval of density in the expanding fireball, which appears as effective broadening of the di-electron spectrum in the $ϕ$ region. The emerging broadening is sensitive to the in-medium change of $<\bar s s>$. This allows to probe directly in-medium modifications of $<\bar s s>$ via di-electron spectra in heavy-ion collisions at SIS energies with HADES.

nucl-th↗
arXiv2002

Evaluation of QCD sum rules for light vector mesons at finite density and temperature

QCD sum rules are evaluated at finite nucleon densities and temperatures to determine the change of mass parameters for the lightest vector mesons $ρ$, $ω$ and $ϕ$ in a strongly interacting medium. For conditions relevant for the starting experiments at HADES we find that the in-medium mass shifts of the $ρ$ and $ω$ mesons are governed, within the Borel QCD sum rule approach, by the density and temperature dependence of the four-quark condensate. In particular, the variation of the strength of the density dependence of the four-quark condensate reflects directly the decreasing mass of the $ρ$ meson and can lead to a change of the sign of the $ω$ meson mass shift as a function of the density. In contrast, the in-medium mass of the $ϕ$ meson is directly related to the chiral strange quark condensate which seems correspondingly accessible.

nucl-th↗
arXiv2002

Evidence for the absence of regularization corrections to the partial-wave renormalization procedure in one-loop self energy calculations in external fields

The equivalence of the covariant renormalization and the partial-wave renormaliz ation (PWR) approach is proven explicitly for the one-loop self-energy correction (SE) of a bound electron state in the presence of external perturbation potentials. No spurious correctio n terms to the noncovariant PWR scheme are generated for Coulomb-type screening potentia ls and for external magnetic fields. It is shown that in numerical calculations of the SE with Coulombic perturbation potential spurious terms result from an improper treatment of the unphysical high-energy contribution. A method for performing the PWR utilizing the relativistic B-spline approach for the construction of the Dirac spectrum in external magnetic fields is proposed. This method is applied for calculating QED corrections to the bound-electron $g$-factor in H-like ions. Within the level of accuracy of about 0.1% no spurious terms are generated in numerical calculations of the SE in magnetic fields.

hep-ph↗
arXiv2002

Evaluation of QCD sum rules for HADES

QCD sum rules are evaluated at finite nucleon densities and temperatures to determine the change of pole mass parameters for the lightest vector mesons $ρ$, $ω$ and $ϕ$ in a strongly interacting medium at conditions relevant for the starting experiments at HADES. The role of the four-quark condensate is highlighted. A few estimates (within a fire ball model and BUU calculations) of dilepton spectra in heavy-ion collisions at 1 AGeV are presented.

nucl-th↗