11th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions

Hadronic collisions produce prompt photons that are characterized by a large transverse momentum and absence of event activity in their vicinity. Photons are a robust probe of cold nuclear matter effects in small and large collision systems because they do not interact strongly and are thus insensitive to medium-induced final-state effects. Prompt photon production is dominated by the Compton...

Relativistic heavy-ion beams at the LHC are accompanied by a large flux of equivalent photons, leading to multiple photon-induced processes. This talk presents a series of measurements of dilepton production from photon fusion performed by the ATLAS Collaboration, which provide strong constraints on the nuclear photon flux, its dependence on the impact parameter and photon energy, and can also...

An important sign of the creation of the Quark-Gluon Plasma in heavy-ion collisions is the observation of jet energy-loss. Energetic, high transverse momentum ($p_{T}$) partons produced at the moment of initial hard scattering are influenced by the evolution history of the medium and lose energy via interactions. In this work we compare two models of low virtuality radiative energy loss:...

Electromagnetic probes such as photons and dielectrons are a unique tool to study the space-time evolution of the hot and dense matter created in ultrarelativistic heavy-ion collisions. At low dielectron invariant mass ($m_{\rm ee}$), thermal radiation from the hot hadronic phase contributes to the dielectron spectrum via decays of $\rho$ mesons, whose spectral function is sensitive to...

We present a consistent photon production calculation from hadronic cross sections, including bremsstrahlung and 2-to-2 reactions, matching the usually employed thermal rates [1,2]. Using the hadronic transport approach SMASH as the afterburner for the hadronic stage at RHIC and LHC energies, we find a significant increase in the calculated momentum anisotropies of these photons due to...

Photon-induced reactions in ultra-peripheral collisions (UPCs) of heavy nuclei at the LHC have been studied using the ALICE detector for several years. The ALICE detector can measure the photoproduction cross section for vector mesons at various rapidities, centre-of-mass energies and collision systems. Beyond the recent ALICE studies of the rapidity and momentum transfer dependence of...

PHENIX presents the simultaneous measurement of high pt (8-18 GeV/c) direct γ and π0 production in d+Au collisions at 200 GeV. The analysis is performed for different events samples selected by event activity. The direct γ-to-π0 ratio is independent of event activity, except for events with the highest activity where the ratio is slightly enhanced. Final state effects are expected to be small...

Relativistic heavy-ion beams at the LHC are accompanied by a large flux of equivalent photons, leading to multiple photon-induced processes. This talk presents searches for physics beyond the standard model enabled by photon-photon processes in both di-tau and di-photon final states. The tau-pair production measurements can constrain the tau lepton’s anomalous magnetic dipole moment (g-2), and...

Photons are emitted at all stages of relativistic heavy-ion collisions and do not interact with the medium strongly. With access to the versatility of RHIC, measurements of low momentum direct photons are made possible across different system size and beam energies. An excess of direct photons, above prompt photon production from hard scattering processes, is observed for a system size...

Electroweak bosons produced in hard-scattering processes at the early stage of the collision, are efficient probes of the initial state of the collision. While the W measurements in pp collisions are a stringent test of perturbative QCD-based calculations and production mechanisms, they can constrain the nuclear parton distribution functions in p-Pb and Pb-Pb collisions.

Electroweak bosons...

Thermal photons are vital tool to study Quark-Gluon Plasma The photon production rate from the plasma at some temperature T is proportional to the transverse spectral function $\rho_T(\omega=|\vec k|,\vec k)$. One can calculate the photon production rate also from the difference between $\rho_T(\omega,\vec k)$ (transverse) and $\rho_L(\omega,\vec k)$ (longitudinal) correlator as $\rho_L$...

Measurements of direct photons can provide valuable information on the properties and dynamics of the quark-gluon plasma (QGP) by comparing them to model calculations that describe the whole evolution of the system created in heavy-ion collisions, from the initial conditions to the pre-equilibrium, QGP, and hadronic phases. In the ALICE experiment, photons can be reconstructed via conversion...

We present recent results on the in-medium spectral function of the rho(770) vector meson and the a1(1260) axial-vector meson in nuclear matter, as well as on the resulting thermal dilepton rate. As an effective description of the thermodynamics and the phase structure of nuclear matter we use a chiral baryon-meson model, taking into account the effects of fluctuations from scalar mesons,...

Photons are radiated throughout heavy-ion collisions, including from the hot and dense quark-gluon plasma (QGP). In this talk, we consider the polarization of QGP photons. The polarization gives detailed information about how the pressure anisotropy of the QGP medium evolves and thus how the medium isotropizes during the initial stages of collisions. We calculate for the first time the...

We use QCD kinetic theory to compute photon production in the chemically equilibrating out-of-equilibrium Quark-Gluon Plasma created in the early stages of high-energy heavy-ion collisions. We compare the non-equilibrium rates to the production in a thermal QGP and extract the dependence of pre-equilibrium photon production on the kinetic and chemical equilibration time. This allows us to...

Electromagnetic probes ($\gamma, \gamma^{\ast}$) offer a unique opportunity to study the conditions during heavy-ion collisions. They are produced throughout the whole evolution of the colliding system and can penetrate the strongly interacting medium to bring direct information from their origins to the detector. In this manner, it is possible to not only probe freeze-out, but also earlier...