arXiv subjects

H. Stöcker

Publications and source records attributed to H. Stöcker.

At least 19 recordsLinked to original sources

arXiv2024

Pauli-type coupling between spinors and curved spacetime

In this study we prove that the Pauli interaction -- which is associated with a length parameter -- emerges when the minimal coupling recipe is applied to the non-degenerate version of the Dirac Lagrangian. The conventional Dirac Lagrangian is rendered non-degenerate if supplemented by a particular term quadratic in the derivatives of the spinors. For dimensional reasons, this non-degenerate Dirac Lagrangian is associated with a length parameter $\ell$. It yields the standard free Dirac equation in Minkowski space. However, if the Dirac spinor is minimally coupled to gauge fields, then the length parameter~$\ell$ becomes a physical coupling constant yielding novel interactions. For the U($1$) symmetry the Pauli coupling of fermions to electromagnetic fields arises, modifying the fermion's magnetic moment. In a second step we then investigate how analogous "Pauli-type" couplings of gravity and matter arise if fermions are embedded in curved spacetime. Minimal coupling of the Dirac field to the gauge field of gravity, the spin connection, leads to an anomalous spin-torsion interaction and a curvature-dependent mass correction. The relation of the latter to Mach's Principle is discussed. Moreover, it is found for a totally anti-symmetric torsion that an upper limit for the "strength" of the torsion exists in order for a solution to remain causal, while causality for a vector torsion requires a lower limit for its amplitude. We calculate the mass correction in the De~Sitter geometry of vacuum with the cosmological constant $Λ$. Possible implications for the existence of effective non-zero rest masses of neutrinos are addressed. Finally, an outlook on the impact of mass correction on the physics of "Big Bang" cosmology, black holes, and of neutron stars is provided.

gr-qc↗

arXiv2020

Radiation dominated implosion with flat target

Inertial Confinement Fusion is a promising option to provide massive, clean, and affordable energy for humanity in the future. The present status of research and development is hindered by hydrodynamic instabilities occurring at the intense compression of the target fuel by energetic laser beams. A recent proposal Csernai et al. (2018) combines advances in two fields: detonations in relativistic fluid dynamics and radiative energy deposition by plasmonic nano-shells. The initial compression of the target pellet can be eliminated or decreased, not to reach instabilities. A final and more energetic laser pulse can achieve rapid volume ignition, which should be as short as the penetration time of the light across the target. In the present study, we discuss a flat fuel target irradiated from both sides simultaneously. Here we propose an ignition energy with smaller compression, largely increased entropy and temperature increase, and instead of external indirect heating and huge energy loss, a maximized internal heating in the target with the help of recent advances in nano-technology. The reflectivity of the target can be made negligible, and the absorptivity can be increased by one or two orders of magnitude by plasmonic nano-shells embedded in the target fuel. Thus, higher ignition temperature and radiation dominated dynamics can be achieved. Here most of the interior will reach the ignition temperature simultaneously based on the results of relativistic fluid dynamics. This makes the development of any kind of instability impossible, which up to now prevented the complete ignition of the target.

physics.plasm-ph↗

arXiv2017

Conserved charge fluctuations are not conserved during the hadronic phase

We study the correlation between the distributions of the net-charge, net-kaon, net-baryon and net-proton number at hadronization and after the final hadronic decoupling by simulating ultra relativistic heavy ion collisions with the hybrid version of the ultrarelativistic quantum molecular dynamics (UrQMD) model. We find that due to the hadronic rescattering these distributions are not strongly correlated. The calculated change of the correlation, during the hadronic expansion stage, does not support the recent paradigm, namely that the measured final moments of the experimentally observed distributions do give directly the values of those distributions at earlier times, when the system had been closer to the QCD crossover.

nucl-th↗

arXiv2017

Influence of the hadronic phase on observables in ultrarelativistic heavy ion collisions

The hadronic phase in ultrarelativistic nuclear collisions has a large influence on final state observables like multiplicity, flow and $p_t$ spectra, as studied in the UrQMD approach. In this model one assumes that a non-equilibrium decoupling phase follows a fluid dynamical description of the high density phase. Hadrons are produced assuming local thermal equilibrium and dynamically decouple during the hadronic rescattering until the particles are registered in the detectors. This rescattering of hadrons modifies every hadronic bulk observable. The apparent multiplicity of resonances is suppressed as compared to a chemical equilibrium freeze-out model, because the decay products rescatter. Therefore the resonances, which decay in the early hadronic phase, cannot be identified anymore by the invariant mass method. Stable and unstable particles change their momentum distribution by more than 30$\%$ through rescattering and their multiplicity is modified by resonance production and annihilation on a similar magnitude. These findings show that it is all but trivial to conclude from the final state observables on the properties of the system at an earlier time where it may have been in local equilibrium.

nucl-th↗

arXiv2016

$Λ$ polarization in peripheral collisions at moderate relativistic energies

The polarization of $Λ$ hyperons from relativistic flow vorticity is studied in peripheral heavy ion reactions at FAIR and NICA energies, just above the threshold of the transition to the Quark-Gluon Plasma. Previous calculations at higher energies with larger initial angular momentum, predicted significant $Λ$ polarization based on the classical vorticity term in the polarization, while relativistic modifications decreased the polarization and changed its structure in the momentum space. At the lower energies studied here, we see the same effect namely that the relativistic modifications decrease the polarization arising from the initial shear flow vorticity.

nucl-th↗

arXiv2016

Artificial piezoelectricity in centrosymmetric SrTiO3

Defect engineering is an effective and powerful tool to control existing material properties and create completely new ones, which are symmetry-forbidden in a defect-free crystal. This letter reports on the creation of piezoelectrically active near-surface layer of centrosymmetric SrTiO3, modified by the electric field-induced migration of oxygen vacancies. We provide the unequivocal proof of piezoelectricity through the stroboscopic time-resolved X-ray diffraction under alternating electric field. The magnitude of the discovered piezoelectric effect is comparable with the bulk piezoelectric effect in commercial ferroelectric materials. Such artificially formed defect-mediated piezoelectricity can be important as an alternative road for smart materials design.

cond-mat.mtrl-sci↗

arXiv2014

Strangeness in Quark Matter 2013: Opening Talk

We discuss several new developments in the field of strange and heavy flavor physics in high energy heavy ion collisions. As shown by many recent theoretical works, heavy flavored particles give us a unique opportunity to study the properties of systems created in these collisions. Two in particular important aspects, the production of (multi) strange hypernuclei and the properties of heavy flavor mesons, are at the core of several future facilities and will be discussed in detail.

nucl-th↗

arXiv2014

Directed flow, a signal for the phase transition in Relativistic Nuclear Collisions?

The sign change of the slope of the directed flow of baryons has been predicted as a signal for a first order phase transition within fluid dynamical calculations. Recently, the directed flow of identified particles has been measured by the STAR collaboration in the beam energy scan (BES) program. In this article, we examine the collision energy dependence of directed flow $v_1$ in fluid dynamical model descriptions of heavy ion collisions for $\sqrt{s_{NN}}=3-20$ GeV. The first step is to reproduce the existing predictions within pure fluid dynamical calculations. As a second step we investigate the influence of the order of the phase transition on the anisotropic flow within a state-of-the-art hybrid approach that describes other global observables reasonably well. We find that, in the hybrid approach, there seems to be no sensitivity of the directed flow on the equation of state and in particular on the existence of a first order phase transition. In addition, we explore more subtle sensitivities like e.g. the Cooper-Frye transition criterion and discuss how momentum conservation and the definition of the event plane affects the results. At this point, none of our calculations matches qualitatively the behavior of the STAR data, the values of the slopes are always larger than in the data.

nucl-th↗

arXiv2013

Chiral Hadronic Mean Field Model including Quark Degrees of Freedom

In an approach inspired by Polyakov loop extended NJL models, we present a nonlinear hadronic SU(3) sigma-omega mean field model augmented by quark degrees of freedom. By introducing the effective Polyakov loop related scalar field Φand an associated effective potential, the model includes all known hadronic degrees of freedom at low temperatures and densities as well as a quark phase at high temperatures and densities. Hadrons in the model exhibit a finite volume in order to suppress baryons at high T and μ. This ensures that the right asymptotic degrees of freedom are attained for the description of strongly interacting matter and allows to study the QCD phase diagram in a wide range of temperatures and chemical potentials. Therefore, with this model it is possible to study the phase transition of chiral restoration and deconfinement. In this paper, the impact of quarks on the resulting phase diagram is shown. The results from the chiral model are compared to recent data from lattice QCD.

hep-ph↗

arXiv2012

Hypernuclei, dibaryon and antinuclei production in high energy heavy ion collisions: Thermal production vs. Coalescence

We study the production of (hyper-)nuclei and di-baryons in most central heavy Ion collisions at energies of $E_{lab}=1-160 A$ GeV. In particular we are interested in clusters produced from the hot and dense fireball. The formation rate of strange and non-strange clusters is estimated by assuming thermal production from the intermediate phase of the UrQMD-hydro hybrid model and alternatively by the coalescence mechanism from a hadronic cascade model. Both model types are compared in detail. For most energies we find that both approaches agree in their predictions for the yields of the clusters. Only for very low beam energies, and for di-baryons including $Ξ$'s, we observe considerable differences. We also study the production of anti-matter clusters up to top RHIC energies and show that the observation of anti-$^4He$ and even anti-$^4_ΛHe$ is feasible. We have found a considerable qualitative difference in the energy dependence of the strangeness population factor $R_H$ when comparing the thermal production with the coalescence results.

nucl-th↗

arXiv2012

Resonance States in an Effective Chiral Hadronic Model

With an effective chiral flavour SU(3) model we show the effect of hadronic resonances on the QCD phase diagram. We state that varying the resonance couplings to the scalar and vector fields affects the order and location of the phase transition, the possible existence of a critical end point (CEP), and the thermodynamic properties. We present (strange) quark number susceptibilities at zero baryochemical potential and at three different points at the phase transition. Comparing results to lattice QCD, we state that reasonable large vector couplings limit the phase transition to a smooth crossover ruling out a CEP.

hep-ph↗

arXiv2011

From FAIR to RHIC, hyper clusters and an effective strange EoS for QCD

Two major aspects of strange particle physics at the upcoming FAIR and NICA facilities and the RHIC low energy scan will be discussed. A new distinct production mechanism for hypernuclei will be presented, namely the production abundances for hypernuclei from $Λ$'s absorbed in the spectator matter in peripheral heavy ion collisions. As strangeness is not uniformly distributed in the fireball of a heavy ion collision, the properties of the equation of state therefore depend on the local strangeness fraction. The same, inside neutron stars strangeness is not conserved and lattice studies on the properties of finite density QCD usually rely on an expansion of thermodynamic quantities at zero strange chemical potential, hence at non-zero strange-densities. We will therefore discuss recent investigations on the EoS of strange-QCD and present results from an effective EoS of QCD that includes the correct asymptotic degrees of freedom and a deconfinement and chiral phase transition.

hep-ph↗

arXiv2011

The hadronic SU(3) Parity Doublet Model for Dense Matter, its extension to quarks and the strange equation of state

A chiral model is introduced that is based on the parity doublet formulation of chiral symmetry including hyperonic degrees of freedom. The phase structure of the model is determined. Depending on the masses of the chiral partners the transition to the chirally restored phase shows a first-order line with critical endpoints as function of chemical potential and temperature in additional to the standard liquid-gas phase transition of self-bound nuclear matter. We extend the parity doublet model to describe the deconfinement phase transition which is in quantitative agreement with lattice data at $μ_B=0$. The phase diagram of the model is presented which shows a decoupling of chiral symmetry restoration and deconfinement. Loosening the constraint of strangeness conservation we also investigate the phase diagram at net strangeness density. We calculate the strangeness per baryon fraction and the baryon strangeness correlation factor, two quantities that are sensitive on deconfinement and that can be used to interpret lattice calculations.

hep-ph↗

arXiv2011

Fluid Dynamical Prediction of Changed v1-flow at LHC

Substantial collective flow is observed in collisions between Lead nuclei at LHC as evidenced by the azimuthal correlations in the transverse momentum distributions of the produced particles. Our calculations indicate that the Global v1-flow, which at RHIC peaked at negative rapidities (named as 3rd flow component or anti-flow), now at LHC is going to turn toward forward rapidities (to the same side and direction as the projectile residue). Potentially this can provide a sensitive barometer to estimate the pressure and transport properties of the Quark-Gluon Plasma. Our calculations also take into account the initial state Center of Mass rapidity fluctuations, and demonstrate that these are crucial for v1 simulations. In order to better study the transverse momentum flow dependence we suggest a new "symmetrized" v1S flow component; and we also propose a new method to disentangle Global v1 flow from the contribution generated by the random fluctuations in the initial state. This will enhance the possibilities of studying the collective Global v1 flow both at the STAR Beam Energy Scan program and at LHC.

nucl-th↗

arXiv2011

An effective chiral Hadron-Quark Equation of State

We construct an effective model for the QCD equation of state, taking into account chiral symmetry restoration as well as the deconfinement phase transition. The correct asymptotic degrees of freedom at the high and low temperature limits are included (quarks $\leftrightarrow$ hadrons). The model shows a rapid crossover for both order parameters, as is expected from lattice calculations. We then compare the thermodynamic properties of the model at $μ_B=0$ which turn out to be in qualitative agreement with lattice data, while apparent quantitative differences can be attributed to hadronic contributions and excluded volume corrections. Furthermore we discuss the effects of a repulsive vector type quark interaction at finite baryon number densities on the resulting phase diagram of the model. Our current model is able to reproduce a first-order liquid gas phase transition as expected, but does not show any signs of a first order deconfinement or chiral phase transition. Both transitions rather appear as a very wide crossover in which heavily medium modified hadron coexist with free quarks.

hep-ph↗

arXiv2010

Structure and giant magnetoresistance of granular Co-Cu nanolayers prepared by cross-beam PLD

A series of Co_xCu_{100-x} (x = 0, 40...75, 100) layers with thicknesses in-between 13 nm and 55 nm were prepared on silicon substrates using cross-beam pulsed laser deposition. Wide-angle X-ray diffraction (WAXRD), transmission electron microscopy (TEM) and electrical transport measurements revealed a structure consisting of decomposed cobalt and copper grains with grain sizes of about 10 nm. The influence of cobalt content and layer thickness on the grain size is discussed. Electron diffraction (ED) indicates the presence of an intermetallic Co-Cu phase of Cu3Au structure-type. Thermal treatment at temperatures between 525 K and 750 K results in the progressive decomposition of Co and Cu, with an increase of the grain sizes up to about 100 nm. This is tunable by controlling the temperature and duration of the anneal, and is directly observable in WAXRD patterns and TEM images. A careful analysis of grain size and the coherence length of the radiation used allows for an accurate interpretation of the X-ray diffraction patterns, by taking into account coherent and non-coherent scattering. The alloy films show a giant magnetoresistance of 1...2.3 % with the maximum obtained after annealing at around 725 K.

cond-mat.mtrl-sci↗

arXiv2009

An effective chiral Hadron-Quark Equation of State Part I: Zero baryochemical potential

hep-ph↗

arXiv2006

Mini Black Holes at the LHC: Discovery Through Di-Jet Suppression, Mono-Jet Emission and a Supersonic Boom in the Quark-Gluon Plasma in ALICE, ATLAS and CMS

We examine experimental signatures of TeV-mass black hole formation in heavy ion collisions at the LHC. We find that the black hole production results in a complete disappearance of all very high $p_T$ ({$> 500$} GeV) back-to-back correlated di-jets of total mass {$M > M_f \sim 1$}TeV. We show that the subsequent Hawking-decay produces multiple hard mono-jets and discuss their detection. We study the possibility of cold black hole remnant (BHR) formation of mass $\sim M_f$ and the experimental distinguishability of scenarios with BHRs and those with complete black hole decay. Finally we point out that a Heckler-Kapusta-Hawking plasma may form from the emitted mono-jets. In this context we present new simulation data of Mach shocks and of the evolution of initial conditions until the freeze-out.

hep-ph↗