Search

Abstract The production of low-mass dielectrons is one of the most promising tools for the investigation of chiral symmetry restoration and thermal radiation from the QGP created in heavy-ion collisions. To single out the signal characteristics of the QGP, it is crucial to understand the primordial e+e− pair production in vacuum, i.e. in inelastic proton-proton (pp) collisions. Low-mass dielectrons have been measured with ALICE at the LHC in pp collisions at s = 7 and 13 TeV , and in Pb–Pb collisions at s NN = 2.76 TeV. An overview of the results on dielectron production is presented, together with their implications for the direct-photon and heavy-quark production. [ABSTRACT FROM AUTHOR]

Abstract Microscopic transport approaches are the tool to describe the non-equilibrium evolution in low energy collisions as well as in the late dilute stages of high-energy collisions. Here, a newly developed hadronic transport approach, SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) is introduced. The overall bulk dynamics in low energy heavy ion collisions is shown including the excitation function of elliptic flow employing several equations of state. The implications of this new approach for dilepton production are discussed and preliminary results for afterburner calculations at the highest RHIC energy are presented and compared to previous UrQMD results. A detailed understanding of a hadron gas with vacuum properties is required to establish the baseline for the exploration of the transition to the quark-gluon plasma in heavy ion collisions at high net baryon densities. [ABSTRACT FROM AUTHOR]

Abstract We investigate the production of direct photons in heavy ion collisions within the modified bottom–up thermalization scenario, which we show to be related to the thermalizing Glasma framework. The dynamics of the parton system up to thermalization/equilibration can be described by two momentum scales, by means of which we express the photon invariant yield excluding the prompt pQCD contribution. We derive an analytic formula, which provides an estimate of photon production from thermalizing matter for a wide range of collision systems and energies. We compare the yield with that measured in the PHENIX Au + Au run04 and the combined run07 + run10 data sets at s N N = 200 GeV. We also make theory-data comparisons for Pb + Pb at 2760 GeV and d + Au at 200 GeV collision energies. Finally, we make predictions for the direct photon invariant yield from collisions of U + U , Cu + Au , He 3 + Au at 200 GeV and Pb + Pb at 5020 GeV. [ABSTRACT FROM AUTHOR]

Photons and dileptons are emitted throughout the evolution of the deconfined nuclear medium produced in heavy ion collisions. As such they can provide valuable information about the different phases of the medium, and complement hadronic measurements and other observables. In this work, recent developments related to electromagnetic emissions at early times, in the crossover region, and at late times are reviewed. The space-time description of the nuclear medium from a hydrodynamic model of heavy ion collisions is used to provide context and guide the discussion. [ABSTRACT FROM AUTHOR]

Dilepton and direct photon measurements in heavy ion collisions provide access to the early stages of the collision. Dilepton continuum measurements at SPS and RHIC show an excess consistent with broadening of the rho meson spectral function as a result of hadronic interactions. Direct photon measurements at RHIC and the LHC show an excess of low p T photons over the expected yield. This excess has a large azimuthal anisotropy with respect to the collision plane. Understanding the direct photon excess and its anisotropy has become an outstanding question in the field of heavy ion physics. This proceeding consists of a summary of new dilepton and direct photon experimental results shown at the 2017 International Conference on Ultrarelativistic Nucleus-Nucleus Collisions and presents them in the context of our current understanding of these electromagentic probes. [ABSTRACT FROM AUTHOR]

Starting from IP-Glasma initial conditions, we investigate the effects of bulk pressure on low mass dilepton production at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) energies. Though thermal dilepton v 2 is affected by the presence of both bulk and shear viscosity, whether or not these effects can be measured depends on the dilepton “cocktail” contribution to the the low mass dilepton v 2 . Combining the thermal and “cocktail” dileptons, the effects of bulk viscosity on total dilepton v 2 is investigated. [ABSTRACT FROM AUTHOR]

Direct photon production in heavy-ion collisions provides a valuable set of observables to study the hot QCD medium. The direct photons are produced at different stages of the collision and escape the medium unaffected. In heavy-ion collisions, the direct photon yield at high transverse momentum ( p T > 5 GeV / c ) is dominated by prompt photons produced in hard scattering of incoming partons and provides information on nuclear parton distribution functions and on the initial parton dynamics. The low momentum component ( p T ≲ 5 GeV / c ) of the direct photon production is dominated by thermal radiation by the hot and dense matter created, carrying information on its space-time evolution, collective flow and temperature. We present recent ALICE results on direct photon production Pb-Pb collisions at 2.76 TeV and on direct photon production in pp at 7 TeV using isolation techniques. The results are compared to theoretical predictions and previous measurements. [ABSTRACT FROM AUTHOR]

Thermal and prompt photon production in heavy ion collisions is evaluated and compared with measurements from both RHIC and the LHC. An event-by-event hydrodynamical model of heavy ion collisions that includes shear and bulk viscosities is used, along with up-to-date photon emission rates. Larger tension with measurements is observed at RHIC than at the LHC. The center-of-mass energy and centrality dependence of thermal and prompt photons is investigated. [ABSTRACT FROM AUTHOR]

We review the current state of research on electromagnetic probes in the context of heavy-ion collisions. The focus is on thermal photons and dileptons which provide unique insights into the properties of the created hot and dense matter. This review is intended to provide an introductory overview of the topic as well as a discussion of recent theoretical and experimental results. In particular, we discuss the role of vector-meson spectral functions in the calculation of photon and dilepton rates and present recent results obtained from different frameworks. Furthermore, we will highlight the special role of photons and dileptons to provide information on observables such as the temperature, the lifetime, the polarization and the electrical conductivity of the produced medium as well as their use to learn about chiral symmetry restoration and phase transitions. [ABSTRACT FROM AUTHOR]

We consider the possibility that quark and gluon distributions in the medium created in high energy heavy ion collisions may be modified by a power law tail at energies much higher than the temperature. We parametrize such a tail by Tsallis distributions with an exponent motivated by phenomenology. These distributions are characterized by an effective temperature scale that we assume to evolve in time like the temperature for thermal distributions. We find that including such a tail increases the rates for photon production and significantly delays the emission times for photons of a fixed energy. We argue that these effects should modify photon yields and flow patterns in a way that will help the agreement of theoretical calculations with data from LHC and RHIC experiments. [ABSTRACT FROM AUTHOR]

In this article we discuss the properties of electromagnetic fields in heavy-ion collisions and consequences for observables. We address quantitatively the issue of the magnetic field lifetime in a collision including the electric and chiral magnetic conductivities. We show that for reasonable parameters, the magnetic field created by spectators in a collision is not modified by the presence of matter. [ABSTRACT FROM AUTHOR]

Abstract: Several sources of direct photons are known to contribute to the total photon yield in high energy nuclear collisions. All of these photons carry characteristic and important information on the initial nuclei or the hot and dense fireball created in the collision. We investigate the possibility to separate photons from back-scattering of high momentum quarks off quark gluon plasma from other sources. Their unique kinematics can be utilized through high energy jet triggers on the away-side. We discuss the basic idea and estimate the feasibility of such a measurement at RHIC and LHC. [Copyright &y& Elsevier]

Abstract: In this work we discuss the emission of low mass dilepton radiation from a hydrodynamic evolution model of Au–Au collisions and make comparisons with recent PHENIX measurements. The dilepton emission rates from the hadronic phase are treated at finite temperature and Baryon density and are completely constrained by broken chiral symmetry in a density expansion. The rates are expressed in terms of vacuum correlators which are measured in annihilation, τ decays and photo-reactions on nucleons and nuclei. We consider two possibilities for the hadronic phase: A chemical equilibrated an off equilibrium hadronic gas. We find that while chemical off-equilibrium helps explain part of the low mass () enhancement seen in the data there is still a large discrepancy. [Copyright &y& Elsevier]

Abstract: A radiative deuteron–proton capture experiment was carried out at KVI using polarized-deuteron beams at incident energies of 55, 66.5, and 90 MeV/nucleon. Vector and tensor-analyzing powers were obtained for a large angular range. The results are interpreted with the help of Faddeev calculations, which are based on modern two- and three-nucleon potentials. Our data are described well by the calculations, and disagree significantly with the observed tensor anomaly at RCNP. [Copyright &y& Elsevier]

A new set of electromagnetic probes has been designed for the RFX modified toroidal assembly. Typology and layout of the probes have been revised based on the analysis carried out in previous experiments, and taking into account the foreseen scenarios of operation. A large number of probes are required to correctly identify the complex spatial structure of the plasma column. An accurate calibration and a careful probe mounting is required. Probes will be installed inside the stabilising shell, which will be placed very close to the vessel. Very little space is available. A two axes pick-up probe has been developed: probe materials and construction techniques have been optimised in order to minimise coupling between the two coils and the dependence of the measurement on the temperature. Extensive tests have been carried out on probe prototypes to characterise their electrical and thermal behaviour. In the paper, the design of the new set of probes is presented, and the results of the tests performed on prototypes are reported. [Copyright &y& Elsevier]

Dielectrons are an excellent probe for the QCD matter created in created in ultra-relativistic heavy-ion collisions, since they are emitted during the whole evolution of the collision and do not interact strongly with the medium. To isolate the QGP signals, measurement of the dielectron production in vacuum and its modifications due to the presence of cold nuclear matter is necessary. We present and discuss results from a low magnetic field detector setup in proton-proton collisions at s = 13 TeV , as well as the measurement of dielectron production in pp, p–Pb, and Pb–Pb collisions at s NN = 5 TeV. [ABSTRACT FROM AUTHOR]

Electromagnetic and electroweak probes are the most versatile probes in the study of heavy-ion collisions. Produced at every stage in the evolution of QCD matter, its messengers are practically inert to the strongly interacting medium they travel through. In this contribution, I will discuss a selection of new results from experiments at the LHC, RHIC, and SIS facilities, spanning almost four orders of magnitude in beam energy. I will conclude with a brief overview of the experimental landscape in the near future. [ABSTRACT FROM AUTHOR]

Cross sections for photon production in hadronic scattering processes have been calculated according to an effective chiral field theory. For π + ρ → π + γ and π + π → ρ + γ processes, these cross sections have been implemented into a novel hadronic transport approach (SMASH), which is suitable for collisions at low and intermediate energies. The implementation is verified by systematically comparing the thermal photon rate to theoretical expectations. The photon rates we obtain are compared to previous works, where scattering processes mediated by ω mesons are found to contribute significantly to the total photon production. Finally, the impact of considering the finite width of the ρ meson is investigated, and a significant enhancement of photon production in the low-energy region is observed. This work is the first step towards a consistent treatment of photon emission in hybrid hydrodynamics+transport approaches. The quantification of the importance of the hadronic stage for the resolution of the direct photon flow puzzle is a next step and can be applied to identify equilibrium and non-equilibrium effects in the hadronic afterburner. [ABSTRACT FROM AUTHOR]

An overview of the latest theoretical developments and results on electromagnetic and weak probes in relativistic heavy-ion collisions is presented. The possibilities to use electromagnetic probes, i.e., photons and dileptons, as a spectrometer, thermometer, chronometer, polarimeter, barometer, and multimeter of the collision system, as well as on their application to explore the QCD phase diagram and to locate phase transitions such as the critical endpoint, are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Due to their penetrating nature, dileptons are a valuable probe for the properties of the hot and/or dense medium created in relativistic heavy-ion collisions. Dilepton invariant-mass spectra provide direct access to the properties of the electromagnetic current-correlation function in strongly interacting matter. In this paper an overview is given of our current theoretical understanding of the dilepton phenomenology in comparison to recent data in heavy-ion collisions at the CERN SPS. [Copyright &y& Elsevier]