Session Q18: Heavy Flavor and the Initial State in Heavy Ion Collisions

On Demand

Many prior studies of in-medium quarkonium suppression have implicitly made use of an adiabatic approximation in which it was assumed that the heavy quark potential is a slowly varying function of time. In the adiabatic limit, one can separately determine the in-medium breakup rate and the medium time evolution, folding these together only at the end of the calculation. In this paper, we relax this assumption by solving the 3d Schrodinger equation in real-time in order to compute quarkonium suppression dynamically. We compare results obtained using the adiabatic approximation with real-time calculations for both harmonic oscillator and realistic complex heavy quark potentials. Using the latter, we find that, for the Y(1s), the difference between the adiabatic approximation and full real-time evolution is at the few percent level, however, for the Y(2s), we find that the correction can be as large as 18{\%} in low temperature regions. For the J/Psi, we find a larger difference between the dynamical evolution and the adiabatic approximation, with the error reaching approximately 36{\%}.   

On Demand

Muon pairs produced via two-photon scattering processes in hadronic Pb+Pb collisions provide a potentially sensitive electromagnetic probe of the quark gluon plasma. First measurements by ATLAS and STAR of dileptons produced via two-photon scattering in non-ultra-peripheral (non-UPC) nucleus-nucleus collisions showed an unexpected centrality-dependent broadening of the angular correlation betweenthe two leptons and/or of the two-lepton $p_T$ distribution. ATLAS has recently measured dimuons produced via two-photon scattering in non-UPC Pb+Pb collisions at 5.02 TeV using data collected during the 2018 Pb+Pb run at the LHC corresponding to am integrated luminosity of $1.73~{\mathrm{nb}^{-1}$. This data set represents a factor of four increase in statistics over the 2015 data set used for the first ATLAS measurement. The increased statistics allow new features to be observed in the data,as well as differential studies of the dependence of the pair-distribution on the transverse-momentum and pseudorapidity of the two muons. The results of the new measurement and the possible physics implications will be discussed.   

On Demand

ATLAS measurements are presented on the production of muons from heavy-flavor decays in pp and Pb+Pb collisions at 2.76 TeV and in p+Pb collisions at 5.02 TeV. The measurements are performed over the transverse momentum range 4$ [Preview Abstract]

On Demand

Measurements of long-range, collective azimuthal correlations involving heavy-flavor quarks provide a powerful tool for unraveling the origin of the collectivity observed in small collision systems. In particular, these measurements are sensitive to the early stages of the collisions. As compared to light quarks, heavy flavor (charm and bottom) quarks can only be produced at initial stages via hard process, and evolve with the whole system, thus they are rare probes to explore the initial conditions of the collisions. Based on long-range, two-particle correlations, recent measurements by CMS showed large azimuthal anisotropy signals for charm hadrons, including prompt $D^{0}$ and J/$\psi$ particles, in high multiplicity pPb collisions, while for nonprompt $D^{0}$ mesons from b hadron decay, a flavor hierarchy is indicated between $p_{T}$ $2-5$ GeV. To further explore the system size dependence of collectivity for charm hadrons in small systems, elliptic azimuthal anisotropy ($v_2$) of prompt $D^{0}$ meson in 8.16 TeV pPb and 13 TeV pp collisions are presented as a function of multiplicity over wide rapidity ($|y|<1$) ranges. The results provide key insights to the heavy quark collectivity developed in pp and pPb systems.   

On Demand

In Pb+Pb collisions at the Large Hadron Collider, fast-moving charged ions generate intense electromagnetic fields. At high center of mass energies, these fields are effectively modelled as a flux of quasi-real photons. These intense fluxes allow for photon-nucleus and photon-photon interactions to occur at significant rates, even without significant contributions from nucleus-nucleus interactions. This talk will present recent measurements performed by the ATLAS collaboration which use these "ultra-peripheral" collisions as a new experimental tool. It will focus on studies of dilepton and diphoton photoproduction as well as jet production via photon-nucleus scattering.   

Not Participating

Nearly 20 years after its discovery, the structure of the exotic hadron X(3872) is still not clear. ~Models have considered compact tetraquarks, hadronic molecules, and mixtures of exotic and conventional hadrons with varying levels of agreement with experimental data. ~Interactions of X(3872) hadrons with the dense QCD environment created in high-multiplicity pp collisions and heavy ion collisions offer a new method of probing the structure experimentally. ~Here we will present LHCb results on the relative production of X(3872) and psi(2S) hadrons as a function of charged particle multiplicity in pp collisions at 8 TeV, and discuss how the observed modifications can discriminate between models of exotic hadron structure.