Thermal electromagnetic radiation is emitted by the quark-gluon plasma (QGP) throughout its space-time evolution, with production rates that depend characteristically on the temperature. We study this temperature using thermal photons and dileptons using the heavy ion code, which is constrained by Bayesian analysis. In addition we present the elliptic flow of both the thermal photons and thermal dileptons including systematic uncertainties corresponding to the model parameter uncertainty. We give a comprehensive overview of the resulting effective temperatures $$T_{\textrm{eff}},$$ obtained from thermal photon transverse momentum and thermal dilepton invariant mass distributions, as well as the dependence of $$T_{\textrm{eff}}$$ on various selection criteria of these probes. We conclude that the $$T_{\textrm{eff}}$$ obtained from thermal photons is mostly insensitive to the temperature of the QGP with a value of $$T_{\textrm{eff}} \sim 250$$ –300 MeV depending on their transverse momentum, almost independent of collision centrality. Thermal dileptons are much better probes of the QGP temperature as they do not suffer from a blue shift as their invariant mass is used, allowing for a more precise constraint of the QGP temperature during different stages of the evolution of the system. By applying selection criteria on the dilepton transverse momentum and the invariant mass we are able to extract fluid temperatures on average times ranging from late emission $$(\langle \tau \rangle = 5.6~\text {fm}/c)$$ to very early emissions $$(\langle \tau \rangle < 1.0~\text {fm}/c).$$ Furthermore, we show how these selection criteria can be used to map the elliptic flow of the system all throughout its evolution.

Netherlands

Olaf Massen

Switzerland

Govert Nijs

Mike Sas

Wilke Schee

Raimond Snellings

Effective temperatures of the QGP from thermal photon and dilepton production

We present an estimate of the yield of hard probes expected for collisions of the isobars <math><msubsup><mrow></mrow><mrow><mn>44</mn></mrow><mrow><mn>96</mn></mrow></msubsup></math>Ru and <math><msubsup><mrow></mrow><mrow><mn>40</mn></mrow><mrow><mn>96</mn></mrow></msubsup></math>Zr at collision energies reachable at RHIC and the LHC. These yields are proportional to the number of binary nucleon-nucleon interactions, which is characteristically different due to the presence of the large neutron skin in <math><msubsup><mrow></mrow><mrow><mn>40</mn></mrow><mrow><mn>96</mn></mrow></msubsup></math>Zr. This provides an independent opportunity to measure the difference between the neutron skin of <math><msubsup><mrow></mrow><mrow><mn>44</mn></mrow><mrow><mn>96</mn></mrow></msubsup></math>Ru and <math><msubsup><mrow></mrow><mrow><mn>40</mn></mrow><mrow><mn>96</mn></mrow></msubsup></math>Zr, which can provide an important constraint on the Equation of State of cold neutron-rich matter.

CERN

Wilke van der Schee

United States

Yen-Jie Lee

Yi Chen

Hard probes in isobar collisions as a probe of the neutron skin

We study the generalized gluon distribution that governs the dynamics of quarkonium inside a non-Abelian thermal plasma characterizing its dissociation and recombination rates. This gluon distribution can be written in terms of a correlation function of two chromoelectric fields connected by an adjoint Wilson line. We formulate and calculate this object in <math><mi>N</mi><mo>=</mo><mn>4</mn></math> supersymmetric Yang-Mills theory at strong coupling using the AdS/CFT correspondence, allowing for a nonzero center-of-mass velocity <math><mi>v</mi></math> of the heavy quark pair relative to the medium. The effect of a moving medium on the dynamics of the heavy quark pair is described by the simple substitution <math><mi>T</mi><mo>→</mo><msqrt><mi>γ</mi></msqrt><mi>T</mi></math> in agreement with previous calculations of other observables at strong coupling, where <math><mi>T</mi></math> is the temperature of the plasma in its rest frame, and <math><mi>γ</mi><mo>=</mo><mo>(</mo><mn>1</mn><mo>−</mo><msup><mi>v</mi><mn>2</mn></msup><msup><mo>)</mo><mrow><mo>−</mo><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msup></math> is the Lorentz boost factor. Such a velocity dependence can be important when the quarkonium momentum is larger than its mass. Contrary to general expectations for open quantum systems weakly coupled with large thermal environments, the contributions to the transition rates that are usually thought of as the leading ones in Markovian descriptions vanish in this strongly coupled plasma. This calls for new theoretical developments to assess the effects of strongly coupled non-Abelian plasmas on in-medium quarkonium dynamics. Finally, we compare our results with those from weakly coupled QCD, and find that the QCD result moves toward the <math><mi>N</mi><mo>=</mo><mn>4</mn></math> strongly coupled result as the coupling constant is increased within the regime of applicability of perturbation theory. This behavior makes it even more pressing to develop a non-Markovian description of quarkonium in-medium dynamics.

Bruno Scheihing-Hitschfeld

Xiaojun Yao

Generalized gluon distribution for quarkonium dynamics in strongly coupled <math><mrow><mi>N</mi><mo>=</mo><mn>4</mn></mrow></math> Yang-Mills theory

Ultracentral heavy ion collisions due to their exceptionally large multiplicity probe an interesting regime of quark-gluon plasma where the size is (mostly) fixed and fluctuations in the initial condition dominate. Spurred by the recent measurement of the CMS collaboration we investigate the driving factors of the increase of transverse momentum, including a complete analysis of the influence of the QCD equation of state. Particularly interesting is the influence of the centrality selection as well as the initial energy deposition.

Ultracentral heavy ion collisions, transverse momentum and the equation of state

Previous studies have shown that a gauge-invariant correlation function of two chromoelectric fields connected by a straight timelike adjoint Wilson line encodes crucial information about quark-gluon plasma (QGP) that determines the dynamics of small-sized quarkonium in the medium. Motivated by the successes of holographic calculations to describe strongly coupled QGP, we calculate the analog gauge-invariant correlation function in strongly coupled   <math> <mi>N</mi> </math>  $$ \mathcal{N} $$  = 4 supersymmetric Yang-Mills theory at finite temperature by using the AdS/CFT correspondence. Our results indicate that the transition processes between bound and unbound quarkonium states are suppressed in strongly coupled plasmas, and moreover, the leading contributions to these transition processes vanish in both the quantum Brownian motion and quantum optical limits of open quantum system approaches to quarkonia.

Chromoelectric field correlator for quarkonium transport in the strongly coupled   <math> <mi>N</mi> </math>  $$ \mathcal{N} $$  = 4 Yang-Mills plasma from AdS/CFT

As particles traverse the quark-gluon plasma (QGP) formed during a heavy ion collision they undergo energy loss depending on the distance traveled. We study several temperature- and velocity-weighted path length distributions of non-interacting particles as they traverse the plasma using the Trajectum heavy ion code, including those of back-to-back path lengths. We use event-shape engineering (ESE) in combination with in-plane versus out-of-plane selection to accurately control these path lengths. Lastly, we show how soft observables depend on the different ESE classes.

Caitlin Beattie

Hard probe path lengths and event-shape engineering of the quark-gluon plasma

Ab initio methods using weakly interacting nucleons give a good description of condensed nuclear matter up to densities comparable to the nuclear saturation density. At higher densities strong interactions between overlapping nucleons become important; we propose that the interactions will continuously switch over to follow a holographic model in this region. In order to implement this, we construct hybrid equations of state (EOSs) where various models are used for low-density nuclear matter, and the holographic V-QCD model is used for nonperturbative high-density nuclear matter as well as for quark matter. We carefully examine all existing constraints from astrophysics of compact stars and discuss their implications for the hybrid EOSs. Thanks to the stiffness of the V-QCD EOS for nuclear matter, we obtain a large family of viable hybrid EOSs passing the constraints. We find that quark matter cores in neutron stars are unstable due to the strongly first-order deconfinement transition and predict bounds on the tidal deformability as well as on the radius of neutron stars. By relying on universal relations, we also constrain characteristic peak frequencies of gravitational waves produced in neutron star mergers.

Finland

Niko Jokela

Israel

South Korea

Matti Järvinen

Jere Remes

Unified weak and strong coupling framework for nuclear matter and neutron stars

We investigate the combined effects of anisotropy and a magnetic field in strongly interacting gauge theories by the gauge/gravity correspondence. Our main motivation is the quark-gluon plasma produced in off-central heavy-ion collisions which exhibits large anisotropy in pressure gradients as well as large external magnetic fields. We explore two different configurations, with the anisotropy either parallel or perpendicular to the magnetic field, focusing on the competition and interplay between the two. A detailed study of the RG flow in the ground state reveals a rich structure where depending on which of the two, anisotropy or magnetic field, is stronger, intermediate geometries with approximate AdS4 × ℝ and AdS3 × ℝ2 factors arise. This competition is also manifest in the phase structure at finite temperature, specifically in the dependence of the chiral transition temperature on anisotropy and magnetic field, from which we infer the presence of inverse magnetic and anisotropic catalyses of the chiral condensate. Finally, we consider other salient observables in the theory, including the quark-antiquark potential, shear viscosity, entanglement entropy and the butterfly velocity. We demonstrate that they serve as good probes of the theory, in particular, distinguishing between the effects of the magnetic field and anisotropy in the ground and plasma states. We also find that the butterfly velocity, which codifies how fast information propagates in the plasma, exhibits a rich structure as a function of temperature, anisotropy and magnetic field, exceeding the conformal value in certain regimes.

Umut Gürsoy

United Kingdom

Juan Pedraza

On the interplay between magnetic field and anisotropy in holographic QCD

We point out minor errors appearing in the published version of our paper [1].

Erratum to: Inverse anisotropic catalysis in holographic QCD

Topological insulators in odd dimensions are characterized by topological numbers. We prove the well-known relation between the topological number given by the Chern character of the Berry curvature and the Chern-Simons level of the low energy effective action for a general class of Hamiltonians bilinear in the fermion with general U(1) gauge interactions including nonminimal couplings by an explicit calculation. A series of Ward-Takahashi identities are crucial to relate the Chern-Simons level to a winding number, which could then be directly reduced to Chern character of Berry curvature by carrying out the integral over the temporal momenta.