Helmholtz International Center for FAIR

We study ground states and excitations of light octet and decuplet baryons within the framework of Dyson-Schwinger and Faddeev equations. We improve upon similar approaches by explicitly taking into account the momentum-dependent dynamics of the quark-gluon interaction that leads to dynamical chiral symmetry breaking. We perform calculations in both the three-body Faddeev framework and the quark-diquark approximation in order to assess the impact of the latter on the spectrum. Our results indicate that both approaches agree well with each other. The resulting spectra furthermore agree one-to-one with experiment, provided well-known deficiencies of the rainbow-ladder approximation are compensated for. We also discuss the mass evolution of the Roper and the excited $\mathrm{\ensuremath{\Delta}}$ with varying pion mass and analyze the internal structure in terms of their partial wave decompositions.

We complement studies of the neutral pion transition form factor ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{(*)}{\ensuremath{\gamma}}^{(*)}$ with calculations for the electromagnetic decay widths of the processes ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$, ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\gamma}$ and ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}{e}^{+}{e}^{\ensuremath{-}}$. Their common feature is that the singly or doubly virtual transition form factor serves as a vital input that is tested in the nonperturbative low-momentum region of QCD. We determine this form factor from a well-established and symmetry-preserving truncation of the Dyson-Schwinger equations. Our results for the three- and four-body decays match results of previous theoretical calculations and experimental measurements. For the rare decay we employ a numerical method to calculate the process directly by deforming integration contours, which in principle can be generalized to arbitrary integrals as long as the analytic structure of the integrands are known. Our result for the rare decay is in agreement with dispersive calculations but still leaves a $2\ensuremath{\sigma}$ discrepancy between theory and experiment.

We study the transition of non-perturbative to perturbative QCD in situations with possible violations of scaling limits. To this end we consider the singly- and doubly-virtual pion transition form factor π0→γγ at all momentum scales of symmetric and asymmetric photon momenta within the Dyson–Schwinger/Bethe–Salpeter approach. For the doubly virtual form factor we find good agreement with perturbative asymptotic scaling laws. For the singly-virtual form factor our results agree with the Belle data. At very large off-shell photon momenta we identify a mechanism that introduces quantitative modifications to Efremov–Radyushkin–Brodsky–Lepage scaling.

Compact stars made of quark matter, rather than confined hadronic matter, are expected to form a color superconductor. This superconductor ought to be threaded with rotational-vortex lines, within which the star's interior magnetic field is at least partially confined. The vortices (and thus magnetic flux) would be expelled from the star during stellar spin-down, leading to magnetic reconnection at the surface of the star and the prolific production of thermal energy. In this paper, we show that this energy release can reheat quark stars to exceptionally high temperatures, such as observed for soft gamma repeaters, anomalous x-ray pulsars, and x-ray dim isolated neutron stars. Moreover, our numerical investigations of the temperature evolution, spin-down rate, and magnetic field behavior of such superconducting quark stars suggest that soft gamma repeaters, anomalous x-ray pulsars, and x-ray dim isolated neutron stars may be linked ancestrally. Finally, we discuss the possibility of a time delay before the star enters the color-superconducting phase, which can be used to estimate the density at which quarks deconfine. From observations, we find this density to be of the order of 5 times that of nuclear saturation.

Abstract Measuring density in coral skeletons with high precision is challenging and forms the dominant source of experimental uncertainty in related studies. To reduce this uncertainty, we developed a precise and easy‐to‐handle γ‐densitometer. This instrument illuminates coral samples with a close‐to‐monochromatic γ‐ray beam and measures the attenuation in the material by means of single photon counting. Knowing the thickness of the sample, the density can be extracted from the attenuation. After calibration, we obtained a precision of 3.7% for absolute and 0.6% for relative density measurements. The spatial resolution is 0.5 mm. The detector system has been tested with the genus Porites , typically used in paleoclimate studies in the Indo‐Pacific, and with the genus Orbicella (formerly Montastraea ), which has been commonly used in the Atlantic. A record ( a . d . 1965–1999) from an Orbicella faveolata from the Belize Barrier Reef exhibits a decrease in skeletal density and in calcification rate; extension rates increase over time. A record from an O. faveolata from the offshore Glovers Reef Atoll, Belize, exhibits a density increase during a . d . 1900–2005 while skeletal extension rate decreased; calcification rate shows a decline over time. Skeletal density in a Porites lutea from the Maldives decreases from a . d . 1917–2007 whereas skeletal extension rate and calcification rate exhibit increasing trends. These first results along with those from other studies suggest that coral density data from numerous corals in a region are needed to establish robust trends in coral calcification over time, and, that susceptibility to ocean acidification apparently might vary among coral taxa.

We report on a new experimental setup for ion energy loss measurements in dense moderately coupled plasma which has recently been developed and tested at GSI Darmstadt. A partially ionized, moderately coupled carbon plasma (ne ≤ 0.8• 1022 cm-3, Te = 15 eV, z = 2.5, Γ = 0.5) is generated by volumetrical heating of two thin carbon foils with soft X-rays. This plasma is then probed by a bunched heavy ion beam. For that purpose, a special double gold hohlraum target of sub-millimeter size has been developed which efficiently converts intense laser light into thermal radiation and guarantees a gold-free interaction path for the ion beam traversing the carbon plasma. This setup allows to do precise energy loss measurements in non-ideal plasma at the level of 10 percent solid-state density.

Simulations have been performed to study the energy loss of carbon ions in a hot, laser-generated plasma in the velocity region of the stopping-power maximum. In this parameter range, discrepancies of up to 30% exist between the various stopping theories and hardly any experimental data are available. The considered plasma, created by irradiating a thin carbon foil with two high-energy laser beams, is fully-ionized with a temperature of nearly 200 eV. To study the interaction at the maximum stopping power, Monte-Carlo calculations of the ion charge state in the plasma are carried out at a projectile energy of 0.5 MeV per nucleon. The predictions of various stopping-power theories are compared and experimental campaigns are planned for a first-time theory benchmarking in this low-velocity range.

We present recent results from lattice simulations of 2+1 flavors of improved staggered fermions at zero baryon number density near the high temperature crossover. Included are new results from simulations of asqtad fermions at N = 12 and a nearly physical Goldstone pion mass and from simulations of HISQ fermions at N = 6 and 8. We focus on observables sensitive to chiral symmetry and confinement. A companion HotQCD talk discusses the effects of staggered-fermion taste-symmetry breaking on thermodynamic quantities.

We summarise recent results on the spectrum of ground-state and excited baryons and their form factors in the framework of functional methods. As an improvement upon similar approaches we explicitly take into account the underlying momentum-dependent dynamics of the quark-gluon interaction that leads to dynamical chiral symmetry breaking. For light octet and decuplet baryons we find a spectrum in very good agreement with experiment, including the level ordering between the positive- and negative-parity nucleon states. Comparing the three-body framework with the quark-diquark approximation, we do not find significant differences in the spectrum for those states that have been calculated in both frameworks. This situation is different in the electromagnetic form factor of the Δ, which may serve to distinguish both pictures by comparison with experiment and lattice QCD.