Chun Shen

Production of photons in relativistic heavy-ion collisions

In this work it is shown that the use of a hydrodynamical model of heavy-ion collisions which incorporates recent developments, together with updated photon emission rates, greatly improves agreement with both ALICE and PHENIX measurements of direct photons, supporting the idea that thermal photons are the dominant source of direct photon momentum anisotropy. The event-by-event hydrodynamical model uses the impact parameter dependent Glasma model (IP-Glasma) initial states and includes, for the first time, both shear and bulk viscosities, along with second-order couplings between the two viscosities. The effect of both shear and bulk viscosities on the photon rates is studied, and those transport coefficients are shown to have measurable consequences on the photon momentum anisotropy.

A hybrid approach to relativistic heavy-ion collisions at the RHIC BES energies

Using a hybrid (viscous hydrodynamics + hadronic cascade) framework, we model the bulk dynamical evolution of relativistic heavy-ion collisions at Relativistic Heavy Ion Collider (RHIC) Beam Energy Scan (BES) collision energies, including the effects from non-zero net baryon current and its dissipative diffusion. This framework is in full (3+1)D, which allows us to study the non-trivial longitudinal structure and dynamics of the collision systems, for example baryon stopping and transport, as well as longitudinal fluctuations. For the first time, the quantitative effect of net-baryon diffusion on hadronic observables is addressed. Finally, we propose a dynamical initialization scheme to study the importance of the pre-equilibrium stage at the RHIC BES energies.

Multistage Monte-Carlo simulation of jet modification in a static medium

The modification of hard jets in an extended static medium held at a fixed temperature is studied using three different Monte-Carlo event generators (LBT, MATTER, MARTINI). Each event generator contains a different set of assumptions regarding the energy and virtuality of the partons within a jet versus the energy scale of the medium, and hence, applies to a different epoch in the space-time history of the jet evolution. For the first time, modeling is developed where a jet may sequentially transition from one generator to the next, on a parton-by-parton level, providing a detailed simulation of the space-time evolution of medium modified jets over a much broader dynamic range than has been attempted previously in a single calculation. Comparisons are carried out for different observables sensitive to jet quenching, including the parton fragmentation function and the azimuthal distribution of jet energy around the jet axis. The effect of varying the boundary between different generators is studied and a theoretically motivated criterion for the location of this boundary is proposed. The importance of such an approach with coupled generators to the modeling of jet quenching is discussed.

Direct-Photon Spectra and Anisotropic Flow in Heavy Ion Collisions from Holography

The thermal-photon emission from strongly coupled gauge theories at finite temperature is calculated by using holographic models for QCD in the Veneziano limit (V-QCD). These emission rates are then embedded in hydrodynamic simulations combined with prompt photons from hard scattering and the thermal photons from hadron gas to analyze the spectra and anisotropic flow of direct photons at RHIC and LHC. The results from different sources responsible for the thermal photons in the quark gluon plasma (QGP) including the weakly coupled QGP (wQGP) from perturbative calculations, strongly coupled

Solving the qˆ puzzle with x and scale dependence

mathcal{N}

Proton structure fluctuations: constraints from HERA and applications to p + A collisions

=4 super Yang-Mills (SYM) plasma (as a benchmark for reference), and Gubsers phenomenological model mimicking the strongly coupled QGP (sQGP) are then compared. It is found that the direct-photon spectra are enhanced in the strongly coupled scenario compared with the ones in the wQGP, especially at intermediate and high momenta, which improve the agreements with data. Moreover, by using IP-glassma initial states, both the elliptic flow and triangular flow of direct photons are amplified at high momenta (

Bulk viscous effects on flow and dilepton radiation in a hybrid approach

p_T

A Detailed Study and Synthesis of Flow Observables in the IP-Glasma+MUSIC+UrQMD Framework

>2.5 GeV) for V-QCD, while they are suppressed at low momenta compared to wQGP. The distinct results in holography stem from the blue-shift of emission rates in strong coupling. In addition, the spectra and flow in small collision systems were evaluated for future comparisons. It is found that thermal photons from the deconfined phase are substantial to reconcile the spectra and flow at high momenta.

Physics perspectives of heavy-ion collisions at very high energy

Abstract We present here a possible resolution of the jet quenching parameter q ˆ puzzle by exploring the momentum fraction x and scale Q2 dependence of q ˆ , in addition to temperature dependence. We explore the momentum broadening of the jet due to Glauber gluon exchange with the quark-gluon plasma (QGP). This has led us to define the momentum fraction x for the partons in the QGP. We also show here, for the first time, possible forms of the parton distribution function (PDF) of the QGP. The QGP-PDF as input in q ˆ has been used to fit the PHENIX and CMS data simultaneously. We observe that the scale evolution of the QGP-PDF and the energy of the jet are the missing ingredients responsible for the enhancement of q ˆ at the same temperature in RHIC collisions compared to LHC collisions.

Electromagnetic Radiation from QCD Matter: Theory Overview: The XXVth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions

Abstract We constrain proton structure fluctuations at small- x by comparing with the HERA diffractive vector meson production data, and find that large geometric fluctuations in the proton wave function are needed. Hydrodynamical simulations of proton-nucleus collisions including the fluctuating proton structure are found to be compatible with the momentum anisotropies measured at the LHC.