Photons from thermalizing matter in heavy ion collisions

Direct real photons are arguably the most versatile tools to study relativistic heavy ion collisions. They are produced, by various mechanisms, during the entire space-time history of the strongly interacting system. Also, being colorless, most the time they escape without further interaction, i.e. they are penetrating probes. This makes them rich in information, but hard to decypher and interpret. This review presents the experimental and theoretical developments related to direct real photons since the 1970s, with a special emphasis on the recently emerged ‘direct photon puzzle’, the simultaneous presence of large yields and strong azimuthal asymmetries of photons in heavy ion collisions, an observation that so far eluded full and coherent explanation.

Direct photons are a powerful tool for elucidating the properties of the hot QCD matter in heavy-ion collisions. They are conventionally estimated by taking into account prompt photon contributions in proton-proton collisions and thermal and prompt photon contributions in heavy-ion collisions. However, there could also be other sources of photons such as pre-equilibrium photons. I investigate prompt, pre-equilibrium and thermal photons and their effects on the direct photon $p_T$ spectra at the CERN Large Hadron Collider energies.

Recent measurements of the azimuthal anisotropy of direct photons in heavy-ion collisions at the energies of RHIC showed that it is of the same order as the hadronic one. This finding appears to contradict the expected dominance of photon production from a quark-gluon plasma at an early stage of a heavy-ion collision. A possible explanation of the strong azimuthal anisotropy of the photons, given recently, is based on the presence of a large magnetic field in the early phase of a collision. In this talk, we consider a novel photon production mechanism stemming from the conformal anomaly of QCDxQED and the existence of strong (electro)magnetic fields in heavy ion collisions. Using the hydrodynamical description of the bulk modes of QCD plasma, we show that this mechanism leads to the photon production yield that is comparable to the yield from conventional sources. The comparison of the results to the data from the PHENIX collaboration, show that this mechanism might be the most responsible for the observed azimuthal anisotropy of photons.

The NICA (Nuclotron-based Ion Collider fAcility) project is under realization at the Joint Institute for Nuclear Research (JINR, Dubna). The main goal of the project is an experimental study of hot and dense strongly interacting matter in heavy ion (up to Au) collisions at center-of-mass energies up to 11 GeV per nucleon. The physics program will be performed at two experiments, BM@N (Baryonic Matter at Nuclotron) at beams extracted from the Nuclotron, and MPD (Multi-Purpose Detector) at the NICA collider. The aim of the BM@N experiment is to study interactions of relativistic heavy ion beams with fixed targets. The scientific program comprises studies of nuclear matter in the intermediate energy range between experiments at the SIS and NICA/FAIR facilities. The BM@N experiment has recorded first experimental data in the carbon, argon and krypton beams of kinetic energy per nucleon ranging from 2.3 to 4.5 GeV per nucleon. The first measurement of short range correlations of nucleons in carbon nucleus was performed in inverse kinematics with the carbon beam and liquid hydrogen target. The MPD detector is under construction to study hot and baryon rich QCD matter in heavy ion collisions at the NICA collider in the energy range $\sqrt{s_{NN}} = 4 \div 11$ GeV. Physics program includes the study of collective phenomena, $\Lambda$ polarization, dilepton, hyperon and hypernuclei production under extreme conditions of highest baryonic density.

Toward more efficient schemes for achieving deeply degenerate molecular Fermi gases, we study anisotropic thermalization in dilute gases of microwave shielded polar molecular fermions. For collision energies above the threshold regime, we find that thermalization is suppressed due to a strong preference for forward scattering and a reduction in total cross section with energy, significantly reducing the efficiency of evaporative cooling. We perform close-coupling calculations on the effective potential energy surface derived by Deng [] to obtain accurate two-body elastic differential cross sections across a range of collision energies. We use Gaussian process regression to obtain a global representation of the differential cross section over a wide range of collision angles and energies. The route to equilibrium is then analyzed with cross-dimensional rethermalization experiments, quantified by a measure of collisional efficiency toward achieving thermalization. Published by the American Physical Society 2024

The PHENIX experiment at the relativistic heavy ion collider (RHIC) finished data taking in 2016. However, large datasets collected in different collision systems (p+p, p+A and A+A) at different energies (√sNN = 19-500 GeV) during the last years of the detector operation are actively analysed by the collaboration and bring a wealth of new experimental results. This paper reviews the most recent PHENIX results on the light flavour hadron production, yields and angular correlations of the direct photons in heavy-ion collisions as well as on the search for the onset of collectivity in high multiplicity p+p and p+A collisions.

We introduce a novel photon production mechanism stemming from the conformal anomaly of QCD×QED and the existence of strong (electro)magnetic fields in heavy ion collisions. Using the hydrodynamical description of the bulk modes of QCD plasma, we show that this mechanism leads to the photon production yield that is comparable to the yield from conventional sources. This mechanism also provides a significant positive contribution to the azimuthal anisotropy of photons, v(2), as well as to the radial "flow." We compare our results to the data from the PHENIX Collaboration.

In this work, we reconstruct the $\gamma$-photon energy spectrum which is in good agreement with the experimental data of $^{86}$Kr + $^{12}$C at $E/A$ = 44 MeV in the framework of our modified EQMD model. The directed flow and elliptic flow of free protons and direct photons have been investigated by taking $\alpha$-clustering structure of $^{12}$C into account. Comparing with the free proton, the direct photon flows give a clearer information about early stage of nuclear reaction. Difference of collective flows between different configurations of $^{12}$C is observed in this work. This indicates that collective flows of direct photons are sensitive to the initial configuration, therefore the $\gamma$ bremsstrahlung process might be taken as an alternative probe to investigate $\alpha$-clustering structure in light nucleus from heavy ion collisions at Fermi-energy region.

Transition to hot quark matter in relativistic heavy-ion collision

Results obtained from the search for the critical point of strongly interacting matter in relativistic heavy-ion collisions at the CERN-SPS (experiments NA49 and NA61/SHINE) and the RHIC beam energy scan program (experiments STAR and PHENIX) are discussed. Although some intriguing signals were found, establishing firm evidence for the critical point remains a challenging enterprise.

Direct photon emission in heavy-ion collisions is calculated within a relativistic micro+macro hybrid model and compared to the microscopic transport model UrQMD. In the hybrid approach, the high-density part of the collision is calculated by an ideal 3+1-dimensional hydrodynamic calculation, while the early (pre-equilibrium-) and late (rescattering-) phase are calculated with the transport model. Different scenarios of the transition from the macroscopic description to the transport model description and their effects are studied. The calculations are compared to measurements by the WA98-collaboration and predictions for the future CBM-experiment are made.

The lund monte carlo for e +e − jet physics

Heavy ion event generator HYDJET++ (HYDrodynamics plus JETs)

Recent measurements of the azimuthal anisotropy of direct photons in heavy-ion collisions at the energies of Relativistic Heavy Ion Collider show that it is of the same order as the hadronic one. This finding appears to contradict the expected dominance of photon production from a quark-gluon plasma at an early stage of a heavy-ion collision. A possible explanation of the strong azimuthal anisotropy of the photons, given recently, is based on the presence of a large magnetic field in the early phase of a collision. In this Letter, we propose a method to experimentally measure the degree to which a magnetic field in heavy-ion collisions is responsible for the observed anisotropy of photon production. The experimental test proposed in this Letter may potentially change our understanding of the nonequilibrium stage and possible thermalization in heavy-ion collisions.

Results from the PHENIX experiment at RHIC on direct photon production in p+p, d+Au, and Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV are presented. In p+p collisions, direct photon production at high pT behaves as expected from perturbative QCD calculations. The p+p measurement serves as a baseline for direct photon production in Au+Au collisions. In d+Au collisions, no effects of cold nuclear matter are found within the large uncertainty of the measurement. In Au+Au collisions, the production of high pT direct photons scales as expected for particle production in hard scatterings. This supports jet quenching models, which attribute the suppression of high pT hadrons to the energy loss of fast partons in the medium produced in the collision. Low pT direct photons, measured via e+e- pairs with small invariant mass, are possibly related to the production of thermal direct photons.