The second order hydrodynamic transport coefficient κ for the gluon plasma from the lattice

The quark gluon plasma produced in heavy ion collisions behaves like an almost ideal fluid described by viscous hydrodynamics with a number of transport coefficients. The second order coefficient κ is related to a Euclidean correlator of the energy-momentum tensor at vanishing frequency and low momentum. This allows for a lattice determination without maximum entropy methods or modelling, but the required lattice sizes represent a formidable challenge. We calculate κ in leading order lattice perturbation theory and simulations on 120 3  × 6, 8 lattices with a  < 0.1 fm. In the temperature range 2 T c  − 10 T c we find κ  = 0.36(15) T 2 . The error covers both a suitably rescaled AdS/CFT prediction as well as, remarkably, the result of leading order perturbation theory. This suggests that appropriate noise reduction methods on the lattice and NLO perturbative calculations could provide an accurate QCD prediction in the near future

Heavy dense QCD and nuclear matter from an effective lattice theory

A three-dimensional effective lattice theory of Polyakov loops is derived from QCD by expansions in the fundamental character of the gauge action, u , and the hopping parameter, κ , whose action is correct to κ n u m with n + m = 4. At finite baryon density, the effective theory has a sign problem which meets all criteria to be simulated by complex Langevin as well as by Monte Carlo on small volumes. The theory is valid for the thermodynamics of heavy quarks, where its predictions agree with simulations of full QCD at zero and imaginary chemical potential. In its region of convergence, it is moreover amenable to perturbative calculations in the small effective couplings. In this work we study the challenging cold and dense regime. We find unambiguous evidence for the nuclear liquid gas transition once the baryon chemical potential approaches the baryon mass, and calculate the nuclear equation of state in the limit of heavy baryons. In particular, we find a negative binding energy per nucleon causing the condensation, whose absolute value decreases exponentially as mesons get heavier. For decreasing meson mass, we observe a first order liquid gas transition with an endpoint at some finite temperature, as well as a gap between the onset of isospin and baryon condensation

Effective lattice Polyakov loop theory vs. full SU(3) Yang-Mills at finite temperature

A three-dimensional effective theory of Polyakov loops has recently been derived from Wilson’s Yang-Mills lattice action by means of a strong coupling expansion. It is valid in the confined phase up to the deconfinement phase transition, for which it predicts the correct order and gives quantitative estimates for the critical coupling. In this work we study its predictive power for further observables like correlation functions and the equation of state. We find that the effective theory correctly reproduces qualitative features and symmetries of the full theory as the continuum is approached. Regarding quantitative predictions, we identify two classes of observables by numerical comparison as well as analytic calculations: correlation functions and their associated mass scales cannot be described accurately from a truncated effective theory, due to its inherently non-local nature involving long-range couplings. On the other hand, phase transitions and bulk thermodynamic quantities are accurately reproduced by the leading local part of the effective theory. In particular, the effective theory description is numerically superior when computing the equation of state at low temperatures or the properties of the phase transition

Formation of hypermatter and hypernuclei within transport models in relativistic ion collisions

Within a combined approach we investigate the main features of the production of hyper-fragments in relativistic heavy-ion collisions. The formation of hyperons is modeled within the UrQMD and HSD transport codes. To describe the hyperon capture by nucleons and nuclear residues a coalescence of baryons (CB) model was developed. We demonstrate that the origin of hypernuclei of various masses can be explained by typical baryon interactions, and that it is similar to processes leading to the production of conventional nuclei. At high beam energies we predict a saturation of the yields of all hyper-fragments, therefore, this kind of reactions can be studied with high yields even at the accelerators of moderate relativistic energies

Conserved charge fluctuations in a chiral hadronic model including hadrons and quarks

In this work the baryon number and strange susceptibility of second and fourth order are presented. The results at zero baryon-chemical potential are obtained using a well tested chiral effective model including all known hadron degrees of freedom and additionally implementing quarks and gluons in a PNJL-like approach. Quark and baryon number susceptibilities are sensitive to the fundamental degrees of freedom in the model and signal the shift from massive hadrons to light quarks at the deconfinement transition by a sharp rise at the critical temperature. Furthermore, all susceptibilities are found to be largely suppressed by repulsive vector field interactions of the particles. In the hadronic sector vector repulsion of baryon resonances restrains fluctuations to a large amount and in the quark sector above <math altimg="si1.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mrow><mi>T</mi></mrow><mrow><mi>c</mi></mrow></msub></math> even small vector field interactions of quarks quench all fluctuations unreasonably strong. For this reason, vector field interactions for quarks have to vanish in the deconfinement limit

Direct determination of ground-state transition widths of low-lying dipole states in 140 Ce with the self-absorption technique

The technique of self absorption has been applied for the first time to study the decay pattern of low-lying dipole states of 140 Ce. In particular, ground-state transition widths Γ0 and branching ratios Γ0Γ to the ground state have been investigated in the energy domain of the pygmy dipole resonance. Relative self-absorption measurements allow for a model-independent determination of Γ0 . Without the need to perform a full spectroscopy of all decay channels, also the branching ratio to the ground state can be determined. The experiment on 140 Ce was conducted at the bremsstrahlung facility of the superconducting Darmstadt electron linear accelerator S-DALINAC. In total, the self-absorption and, thus, Γ0 were determined for 104 excited states of 140 Ce. The obtained results are presented and discussed with respect to simulations of γ cascades using the DICEBOX code

Super-accelerating bouncing cosmology in asymptotically free non-local gravity

Recently, evidence has been collected that a class of gravitational theories with certain non-local operators is renormalizable. We consider one such model which, at the linear perturbative level, reproduces the effective non-local action for the light modes of bosonic closed string-field theory. Using the property of asymptotic freedom in the ultraviolet and fixing the classical behavior of the scale factor at late times, an algorithm is proposed to find general homogeneous cosmological solutions valid both at early and late times. Imposing a power-law classical limit, these solutions (including anisotropic ones) display a bounce, instead of a big-bang singularity, and super-accelerate near the bounce even in the absence of an inflaton or phantom field

Baryon resonance production and dielectron decays in proton-proton collisions at 3.5 GeV

We report on baryon resonance production and decay in proton-proton collisions at a kinetic energy of $3.5$ GeV based on data measured with HADES. The exclusive channels $pp \rightarrow np\pi^{+}$ and $pp \rightarrow pp\pi^{0}$ as well as $pp \rightarrow ppe^{+}e^{-}$ are studied simultaneously for the first time. The invariant masses and angular distributions of the pion-nucleon systems were studied and compared to simulations based on a resonance model ansatz assuming saturation of the pion production by an incoherent sum of baryonic resonances (R) with masses $<2~$ GeV/$c^2$. A very good description of the one-pion production is achieved allowing for an estimate of individual baryon-resonance production-cross-sections which are used as input to calculate the dielectron yields from $R\rightarrow pe^+e^-$ decays. Two models of the resonance decays into dielectrons are examined assuming a point-like $RN \gamma^*$ coupling and the dominance of the $\rho$ meson. The results of model calculations are compared to data from the exclusive $ppe^{+}e^{-}$ channel by means of the dielectron and $pe^+e^-$ invariant mass distributions

Searching a dark photon with HADES

We present a search for the <math altimg="si1.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup></math> decay of a hypothetical dark photon, also named U vector boson, in inclusive dielectron spectra measured by HADES in the p(3.5 GeV) + p, Nb reactions, as well as the Ar ( <math altimg="si2.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mn>1.756</mn><mtext> GeV</mtext><mo stretchy="false">/</mo><mtext>u</mtext></math> ) + KCl reaction. An upper limit on the kinetic mixing parameter squared <math altimg="si3.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>ϵ</mi></mrow><mrow><mn>2</mn></mrow></msup></math> at 90% CL has been obtained for the mass range <math altimg="si4.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mrow><mi>M</mi></mrow><mrow><mi>U</mi></mrow></msub><mo>=</mo><mn>0.02</mn><mtext>–</mtext><mn>0.55</mn><mtext> GeV</mtext><mo stretchy="false">/</mo><msup><mrow><mi>c</mi></mrow><mrow><mn>2</mn></mrow></msup></math> and is compared with the present world data set. For masses <math altimg="si5.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mn>0.03</mn><mtext>–</mtext><mn>0.1</mn><mtext> GeV</mtext><mo stretchy="false">/</mo><msup><mrow><mi>c</mi></mrow><mrow><mn>2</mn></mrow></msup></math> , the limit has been lowered with respect to previous results, allowing now to exclude a large part of the parameter region favored by the muon <math altimg="si6.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mi>g</mi><mo>−</mo><mn>2</mn></math> anomaly. Furthermore, an improved upper limit on the branching ratio of <math altimg="si7.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mn>2.3</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>6</mn></mrow></msup></math> has been set on the helicity-suppressed direct decay of the eta meson, <math altimg="si8.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mi>η</mi><mo stretchy="false">→</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup></math> , at 90% CL