Bulletin of the American Physical Society
2020 Fall Meeting of the APS Division of Nuclear Physics
Volume 65, Number 12
Thursday–Sunday, October 29–November 1 2020; Time Zone: Central Time, USA
Session EB: Heavy Ions and Jets II |
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Chair: Laura Havener, Yale |
Friday, October 30, 2020 10:30AM - 10:42AM |
EB.00001: Inclusive Jet Measurements in Pb-Pb Collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV with ALICE using Machine Learning Techniques Hannah Bossi Jets in relativistic heavy-ion collisions interact with the Quark-Gluon Plasma, leading to effects such as a suppression of jet yields and modification of internal jet structure that are used to measure the properties of the QGP. Measurements of the jet spectra and jet yield suppression will be presented for inclusive charged-particle jets and inclusive full jets (containing both charged and neutral particles) in Pb--Pb and pp collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV recorded with the ALICE detector. These measurements use a machine learning based background correction that has been shown to reduce residual fluctuations. Reduced fluctuations may allow for measurements at lower transverse momenta and larger jet radii ($R$) than before with ALICE. In this method, machine learning techniques are used to correct the jet transverse momentum using jet parameters, such as information about the constituents of the jet. Studies that investigate and estimate the fragmentation bias of this machine learning approach will also be presented. The $R$-dependence of the jet yield suppression will be shown and compared to models. These data could help disentangle competing mechanisms of jet energy loss and recovery of that energy due to the response of the medium. [Preview Abstract] |
Friday, October 30, 2020 10:42AM - 10:54AM |
EB.00002: The use of Machine Learning techniques for the identification of pi0s in the MPC-EX Analysis of d+Au collisions at sqrt(s_NN) = 200 GeV Liankun Zou The Muon Piston Calorimeter Extension (MPC-EX), is a combination of high resolution charged particle tracker and electromagnetic pre-shower detector, which sits in front of a PbW crystal calorimeter located at forward rapidities ($3.1 < |\eta|< 3.8$) in the PHENIX detector. It is uniquely positioned in d+Au collisions to measure phenomena related to low-x partons in the target nucleus and high-x partons in the projectile hadron [1]. The eight Si-W layers have a spatial resolution of 1.8 mm and work to capture the structure of shower profile. In recent years, machine learning techniques have achieved great success in many fields, particularly in image recognition. The shower structures taken by the MPC-EX can be viewed as images. In this talk, we apply these techniques to identify neutral $\pi^{0}$s in the MPC-EX at high energies where the showers overlap, and distinguish them from the pileup of electromagnetic showers from other sources. [Preview Abstract] |
Friday, October 30, 2020 10:54AM - 11:06AM |
EB.00003: Fully Reconstructed Jets in p$+$p and p$+$Au Collisions at PHENIX Jonathan Runchey Fully Reconstructed Jets in p$+$p and p$+$Au Collisions at PHENIX Jets are an excellent observable for the study of quantum chromodynamics in high energy collisions as they are the products of the initial hard, partonic scattering. Comparisons of fully reconstructed jet~measurements made in p$+$p and p$+$Au collisions are powerful tools that can be used to probe cold nuclear matter effects. These measurements will also provide information about the initial state in A$+$A collisions and therefore, help disentangle which effects on jet production are due to the presence of the quark gluon plasma. Using the PHENIX Central Arm detectors, jet production is measured at mid-rapidity using the anti-k\textunderscore T algorithm with R$=$0.3 in both p$+$Au and p$+$p collisions at sqrt(s)$=$200 GeV from datasets recorded in 2015 to determine the jet nuclear modification factor, R\textunderscore pAu. The latest results from this analysis will be presented. [Preview Abstract] |
Friday, October 30, 2020 11:06AM - 11:18AM |
EB.00004: Inclusive jet measurements in small system collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV in STAR Tong Liu With the observation of flow-like correlations in small system collisions (p+Pb, p+Au, and d+Au) at the LHC and RHIC, the existence of the quark-gluon plasma (QGP) in small systems, initially assumed to be absent, became an open question and has been actively investigated over recent years. High momentum partons produced at early stages of heavy-ion collisions generate collimated sprays of hadrons called jets. These partons lose energy when passing through the QGP medium, experiencing a phenomenon usually known as jet quenching. As a successful probe of the QGP in heavy-ion collisions, jet quenching has been widely studied in small systems across experiments. While most results in minimum-bias events are consistent with expectations from the absence of the QGP, indications of suppression/enhancement are observed in different event activity (EA) bins. In this talk, we present investigations of p/d+Au collisions at $\sqrt{s_\mathrm{NN}}$=200 GeV at STAR for possible evidence of jet quenching by studying the inclusive jet yield. Progress towards the resultant nuclear modification factor $R_{\mathrm{p/d+Au}}$ will be presented. Efforts towards a definition of EA suitable for the system, as well as the comparison between jet yields in high and low EA bins, will also be discussed. [Preview Abstract] |
Friday, October 30, 2020 11:18AM - 11:30AM |
EB.00005: Modification of semi-inclusive jet spectra in high event activity $p$+Au collisions at $\sqrt{s_\mathrm{NN}}=\SI{200}{GeV}$ $p$+Au David Stewart Observations of event activity (EA) dependent modification of jet spectra per binary collision in $p/d$+A collisions at the LHC and RHIC have raised questions about the nature of hard scatterings in ``small'' (non-A+A) system collisions. All jet measurements using inclusive event samples in small system collisions are consistent with $pp$ results; the spectra modification is observed only when binned by EA and for high Bjorken-$x$ jets. Semi-inclusice jet spectra at RHIC energies also show clear spectra dependence on EA at backward-rapidity. This has been shown to not result from diject kinematics. It also does not appear to result from jet quenching, particularly when taken in conjunction with measurements in these collisions which show no EA-dependence of jet substructure. In this talk we present results by which we argue that the EA dependence of the semi-inclusive jet spectra is a consequence of phase space constraints, not present in A+A collisions, which correlate the scale of the hard scattering with EA at both high rapidity and transverse in azimuth to the hard scattering. [Preview Abstract] |
Friday, October 30, 2020 11:30AM - 11:42AM |
EB.00006: Implementation of heavy ion analyses in Rivet Christine Nattrass, Antonio Da Silva Rivet (Robust Independent Validation of Experiment and Theory) is a valuable framework for the comparison of data and simulations. Since features required for heavy ion analyses were only recently available, there is a backlog of analyses which need to be implemented. We discuss implementation of heavy ion analyses in Rivet by undergraduates in a Course-Based Undergraduate Research Experience (CURE) in order to address this backlog. This both provides a valuable educational experience for undergraduates while also assisting collaborations and the field with data preservation and comparisons to models. We show results from the implementation of heavy ion analyses in Rivet and discuss the next steps for wide-spread community adoption of Rivet. [Preview Abstract] |
Friday, October 30, 2020 11:42AM - 11:54AM |
EB.00007: Implementing Jet Analyses in Heavy-Ion Collisions in Rivet William Witt, Christine Nattrass, Antonio Silva Rivet (Robust Independent Validation of Experiment and Theory) is an analysis framework for validating Monte Carlo event generators and comparing simulated results to experimental data. New features like background subtraction and event plane determination allow for the analysis of jets in heavy ion collisions in Rivet. This contribution will present an overview on writing and contributing new Rivet heavy-ion analyses, and show results from jet $R_{AA}$ and $v_{2}$ Rivet analyses on Pb--Pb collisions simulated in Jewel, Angatyr, and Jetscape. [Preview Abstract] |
Friday, October 30, 2020 11:54AM - 12:06PM |
EB.00008: Novel Methods for Measuring the Fragmentation Function of Jets in Heavy Ion Collisions Using Jet-Hadron Correlations Charles Hughes, Alex Aukerman, Thomas Krobatsch, Adam Matyja, Christine Nattrass, James Neuhaus, William Witt Detailed investigation of low momentum (10-60 GeV) jet fragmentation functions may complement previous studies by providing more information on partonic energy loss. The main difficulty in studying low momentum jets in heavy ion collisions is the significant uncorrelated background of low momentum hadrons from soft processes. One way to deal with this background is to use jet-hadron azimuthal correlations to fit and subtract the soft, flow correlated background information from the jet (on the average). This technique allows one to measure the near side yield in the correlation function after background subtraction for a large number of jets binned in jet transverse momentum (pT) and hadron transverse momentum. From these yields, one can construct an uncorrected fragmentation function. We discuss the specifics of this proposed method of measuring the fragmentation function including corrections for detector effects. We present the results of a Monte Carlo study using Pythia and a custom made Heavy Ion Background Generator (with mocked up detector effects) that demonstrate the feasibility of this method. [Preview Abstract] |
Friday, October 30, 2020 12:06PM - 12:18PM |
EB.00009: High pT gamma/pi-zero ratios in Au+Au collissions at sqrt(sNN) = 200 GeV Ananya Paul Photons, being color neutral can traverse unscathed through any colored matter hence serving as an excellent penetrating probe. The sources of real photons in heavy ion collisions are mainly direct photons and decay photons coming from electromagnetic decays of \piz , $\eta$ . High transverse momentum direct photons (\pt$>$4-5\,\gevc) come almost exclusively from initial hard parton scattering. Extracting the direct photon yield $\gamma_{dir}(\pt)$ from the inclusive $\gamma_{inc}(\pt)$ yield can be done using different techniques, all quite involved. Our aim is to characterize high \pt direct photon production and its dependence on collision centrality in a simpler way, using the inclusive $\gamma_{inc}(\pt)/\piz(\pt)$ ratio. It is well known, that at moderate and high \pt the ratio of \piz decay $\gamma_{dec}$ to \piz is constant $\star$. \footnote{R.M. Sternheimer, Phys. Rev. 99, 1, 1955} Since $\gamma_{inc}=\gamma_{dir}+\gamma_{dec}$, and most $\gamma_{dec}$ comes from \piz, deviations of the ratio from a constant at higher \pt will be a measure of direct photon production. In this talk we will present the status of the analysis of centrality dependent $\gamma_{inc}(\pt)/\piz(\pt)$ ratios in $\sqrt{s_{NN}}$ = 200 GeV \auau collisions based on PHENIX data taken in 2014. [Preview Abstract] |
Friday, October 30, 2020 12:18PM - 12:30PM |
EB.00010: Neutral Pion-Hadron Correlations in Pb-Pb Collisions at $\sqrt{s_{NN}}=5.02$ TeV at the LHC Measured with ALICE Michael Oliver One of many ways to measure the modification of jets in heavy ion collisions is through two-particle correlations of hadrons, especially when the trigger is a high $p_{\rm T}$ hadron. Then, one can study how the distributions of particles in the near-side and recoiling away-side jets are modified by the nuclear medium found in such collisions. This analysis pursues this goal by measuring $\pi^0$ -hadron correlations with high $p_{\rm T}$ $\pi^0$s using the ALICE electromagnetic calorimeter and charged particles measured with the ALICE central barrel trackers. These correlations are measured with varying trigger momenta and event centralities. Additionally, the correlations are measured in separate bins of trigger angle with respect to the 2nd order event plane, both in order to apply the reaction plane fit method for background subtraction and in order to measure path-length dependent modification of jets. [Preview Abstract] |
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