Bulletin of the American Physical Society
2019 Fall Meeting of the APS Division of Nuclear Physics
Volume 64, Number 12
Monday–Thursday, October 14–17, 2019; Crystal City, Virginia
Session FJ: Mini-Symposium: Quantitative Understanding of QGP Properties II |
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Chair: Rosi Reed, Lehigh University Room: Salon C |
Tuesday, October 15, 2019 10:30AM - 10:42AM |
FJ.00001: Photons associated with jets in p-p and A-A collisions Chathuranga Sirimanna, Abhijit Majumder Jet modification is now understood to be a multistage effect: a parton produced in a high virtuality initial state, radiates a multitude of partons, giving way to a variety of lower virtuality stages. Hadrons produced in the fragmentation from these partons are clustered within jets. Modeling of these multistage effects involves several parameters. We consider a set of these parameters which have been tuned to successfully describe a variety of jet-based data. Quarks inside jets can radiate photons along with gluons. Photons are also produced in the hard scattering, via the quark-gluon Compton scattering process. In this work, we study the correlation of photons with jets in p-p and A-A collisions. Photon radiation from the hard scattering, along with radiation from a PYTHIA based hadronization model are included in this analysis. We focus on the photon jet transverse momentum and angular balance. The calculations of photon production from each stage are calculated in close analogy to gluon radiation, with the exact same approximations, i.e., no new parameters are introduced or tuned either in the p-p or A-A collisions. The level of agreement with experimental data provides independent verification of the multi-stage theory of jet modification. [Preview Abstract] |
Tuesday, October 15, 2019 10:42AM - 10:54AM |
FJ.00002: Measurement of $\pi^0$-Hadron Correlations in Pb-Pb Collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with ALICE Michael Oliver In heavy ion collisions at sufficiently high energies, it has been observed that jets (collimated sprays of mostly hadronic particles originating from a quark or gluon) appear to interact with a quark-gluon plasma, losing energy and transferring momentum to the medium. Such jets are typically created back-to-back, an arrangement called a dijet, but are sometimes created opposite a high energy photon, which can pass through a quark-gluon plasma unhindered. Measuring correlations between high momentum neutral pions and associated hadrons allows measurements of the dijet arrangement with high statistical precision and also can be used to calculate a background for measurements of the photon-jet arrangement. In ALICE, we can measure both high energy photons and high momentum $\pi^0$'s with the Electromagnetic Calorimeter and correlate them with charged hadrons measured with ALICE's Inner Tracking System and Time Projection Chamber. The environment of the heavy ion collision produces unique challenges to such an analysis, particularly for identifying the $\pi^0$'s and disentangling the component of jet-like correlations from the effects of collective flow. We present progress towards and results from a measurement of $\pi^0$-hadron correlations in this collision system. [Preview Abstract] |
Tuesday, October 15, 2019 10:54AM - 11:06AM |
FJ.00003: Measurement of Jets Recoiling from Direct-photon and $\pi^0$ Triggers in Au+Au Collisions at $\sqrt{s_{NN}}$ = 200 GeV in the STAR Experiment Justin Ewigleben Jets recoiling from a direct photon have long been seen as a golden probe of the quark gluon Plasma created in relativistic heavy ion collisions, due to the ability to tightly constrain the initial hard scattering kinematics and the partonic flavor bias. Until recently, the ability to measure this channel and the ensuing observables at RHIC were largely statistics-limited, owing to the small cross-section of direct photon production compared to for example the most abundant di-jet cross-section. In this talk, we will present measurements of semi-inclusive recoil jets for both direct-photon and $\pi^0$ triggers, using the 13 nb$^{-1}$ of data recorded in 2014 by the STAR experiment. An outlook towards future direct-photon measurements from STAR, including the transverse momentum imbalance ($x_{j\gamma} = p_{T,Jet}/p_{T,\gamma}$) as previously measured by the ATLAS and CMS experiments will also be discussed. [Preview Abstract] |
Tuesday, October 15, 2019 11:06AM - 11:18AM |
FJ.00004: Neutral Pion v$_2$ in Central d+Au Collisions Measured with PHENIX Carlos Perez Lara The observation of multiparticle correlations in Heavy Ion collisions have long been related to collective behavior in the formed medium. Recent results at RHIC provide strong arguments for QGP formation in smaller systems. In this talk, I present the status of the Neutral Pion second harmonic coefficient v$_2$ as a function of transverse momentum for very central d+Au collisions at 200 GeV. The data was recorded during the 2016 operational period in PHENIX. The analysis makes use of the central rapidity electromagnetic calorimeter. The results provides strong insight into the dynamics of the produced hadrons at such scales. [Preview Abstract] |
Tuesday, October 15, 2019 11:18AM - 11:30AM |
FJ.00005: Recent results on correlations and fluctuations in pp, p+Pb, and Pb+Pb collisions from the ATLAS Experiment at the LHC Soumya Mohapatra The azimuthal anisotropies of particle yields observed in relativistic heavy-ion collisions are traditionally considered as a strong evidence of the formation of a deconfined quark-gluon plasma. However multiple recent measurements in pp and p+Pb systems show similar features as those observed in heavy-ion collisions, indicating the possibility of the production of such a deconfined medium in smaller systems. This talk presents a summary of recent ATLAS measurements in pp and p+Pb collisions. It includes measurements of two-particle hadron-hadron and muon-hadron correlations in pp and p+Pb collisions, with a template fitting procedure used to subtract the dijet contributions. Measurements of multi-particle cumulants $c_n${2-8} are also presented. The standard cumulant measurements confirm presence of collective phenomena in p+Pb collisions, but are biased by non-flow correlations and are not able to provide evidence for collectivity in pp collisions. To address this, measurements from a new sub-event cumulant method that suppresses the contribution of non-flow effects are presented. Recent studies of longitudinal flow decorerelations, and higher-order cumulants in Pb+Pb collisions are also presented, and provide insight into the details of the geometry of the initial state. [Preview Abstract] |
Tuesday, October 15, 2019 11:30AM - 11:42AM |
FJ.00006: Transport coefficients in the linear sigma model with massive particles Matthew Heffernan, Sangyong Jeon, Charles Gale The remarkable achievements of the RHIC and LHC programs have raised the hope that transport coefficients of QCD can be extracted from heavy-ion data. Now that simulations have shown that the value of shear and bulk viscosity influence measured spectra, we must study the transport coefficients quantitatively and restrict the parameter space. We present an extended framework for calculating transport coefficients in the linear sigma model, which incorporates vacuum masses and thermal masses arising from mean field effects and showcases many features of strongly interacting systems. We calculate the electrical conductivity and the shear and bulk viscosity of strongly interacting matter in the relaxation time approximation, and the shear viscosity and electrical conductivity using a variational expansion. Our calculations with vacuum sigma masses consistent with the that of f0(500) correspond well to the results and behavior of previous pion gas calculations obtained via completely different techniques, and with expectations from pQCD. We discuss subtleties arising in exact calculations of the bulk viscosity within our framework. Finally, we present a review of existing results and point out avenues of future research. [Preview Abstract] |
Tuesday, October 15, 2019 11:42AM - 11:54AM |
FJ.00007: Measurement of the Azimuthal Anisotropy of Charged Particle Production in Xe+Xe Collisions at 5.44 TeV with the ATLAS Detector Pengqi Yin ATLAS measurements of flow harmonics ($v_{n}$) and their fluctuations in Pb+Pb and Xe+Xe collisions covering a wide range of transverse momenta, pseudorapidity and collision centrality are presented. The $v_{n}$ are measured up to $n=7$ using the two-particle correlations, multi-particle cumulants and scalar product methods. The measurements are also performed using non-flow subtraction techniques -- recently developed for measurements in proton-nucleus and proton-proton collisions -- to improve the understanding of flow in peripheral Pb+Pb and Xe+Xe collisions. The effects of geometric fluctuations and of viscous effects, both of which are stronger in the smaller Xe+Xe system are demonstrated. A universal scaling in the transverse-momentum dependence of the $v_{n}$ is observed for both systems. [Preview Abstract] |
Tuesday, October 15, 2019 11:54AM - 12:06PM |
FJ.00008: Shear Viscosity over Entropy Ratio of Hot Hadronic Matter Rainer Fries, Zhidong Yang The ratio of shear viscosity to entropy density $\eta/s$ typically exhibits a minimum at phase transitions, which is prominent in the case of true phase transitions and is smooth if the transition is a crossover [1]. For quark gluon plasma both lattice QCD and extractions from data hint at a very small value of $\eta/s$, close to the conjectured lower bound $1/4\pi$, as the temperature approches $T_c$ from above. On the other hand, hadronic transport is predicting $\eta/s$ to be as large as $10/4\pi$ when the temperature approaches $T_c$ from below [2]. This discrepancy is uncomfortably large. In this talk we review the current results for $\eta/s$ from the literature and discuss the importance of a reliable estimate of $\eta/s$ for hot hadronic matter. We introduce the idea of extracting $\eta/s$ from data through an analysis of the freeze-out process using a Navier-Stokes approximation [3]. We quantify uncertainties from this extraction and correct for known biases. This method suggests a smooth minimum of $\eta/s$ at $T_c$ and a gradual rise below $T_c$. We discuss possible consequences of this scenario. [1] L.P. Csernai, J.I. Kapusta, L.D. McLerran, PRL 97, 152303 (2006) [2] J.-B. Rose et al., PRC97, 055204 (2018) [3] Z. Yang, R.J. Fries, arXiv:1807.03410 [Preview Abstract] |
Tuesday, October 15, 2019 12:06PM - 12:18PM |
FJ.00009: PHENIX measurement of system size dependence of low pT photon production Roli Esha Relativistic heavy ion collisions lead to the formation of a deconfined system of quarks and gluons, called the Quark Gluon Plasma. This thermalized medium emits photons as all the other stages of the collisions do. Photons, as opposed to partons, are color blind and do not interact strongly with the medium, thereby, carrying out the information about the partonic and hadronic phases including the dynamic evolution of the QGP. PHENIX has shown a large excess of direct photons at low transverse momentum compared to the $N_{coll}$-scaled pQCD expectations which is interpreted as thermal radiation of the system. The measurements of the integrated yield of low momentum direct photons through different collision species (Au+Au and Cu+Cu) have shown a universal scaling as a function of charged particle multiplicity. Data from small systems (p/d/$^3$He+Au) suggests a rapid turn on of the scaling behavior. This may imply that the bulk of the thermal photons are produced near the transition from QGP to hadron gas combined with an onset of QGP formation at low $dN_{ch}/d\eta$. [Preview Abstract] |
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