Bulletin of the American Physical Society
2021 Fall Meeting of the APS Division of Nuclear Physics
Volume 66, Number 8
Monday–Thursday, October 11–14, 2021; Virtual; Eastern Daylight Time
Session FH: Mini-Symposium: Hadron Production and Interactions in Jets and Media III |
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Chair: Ivan Vitev, LANL Room: Whittier |
Tuesday, October 12, 2021 2:00PM - 2:12PM |
FH.00001: Correlations between semi-inclusive jet production and event activity in $\sqrt{s_{NN}}$ = 200 GeV p+Au collisions at STAR David Stewart Jets are formed early in high energy collisions and are causally uncorrelated with soft particle production (event activity, EA) sufficiently separated in phase space. This makes jets ideal probes for quark-gluon plasma (QGP) formation in A+A collisions: EA and jets simultaneously measure centrality and medium conditions, with jet suppression indicating QGP formation. In the ongoing effort to understand QGP-like flow signals in small system ($p$/$d$/He+A) collisions, corresponding small system jet measurements provide critical inputs. In this talk we present correlations between semi-inclusive R~=~0.4 jets and EA from $p$+Au collisions at STAR, with EA azimuthally transverse to the jets at mid-rapidity and/or at Au-going high rapidity. These are the first semi-inclusive small system jet spectrum measurements at RHIC energies and show a clear jet spectra dependence on EA. Suggestively, this dependence is at comparable $x_p$ at which jet nuclear modification factor ($R_{p \mathrm{Pb}}$) at LHC is measured to be EA dependent. We conclude that the observed modification to jet yields most likely results from phase space constraints of the initial collision system and/or early time dynamics, and discuss how to address that bias in this semi-inclusive measurement. |
Tuesday, October 12, 2021 2:12PM - 2:24PM |
FH.00002: Progress in Precison Nuclear PDFs: Fredrick Olness Proper treatment of nuclear effects are essential to extract precison |
Tuesday, October 12, 2021 2:24PM - 2:36PM |
FH.00003: Angular distances between different jet-axis definitions Reynier Cruz Torres The development of state-of-the-art particle detectors, as well as the progress made in precision perturbative Quantum Chromo-Dynamics (pQCD) calculations have made the jet-substructure field grow significantly in the past few years. Jets are rich in substructure, and many different observables are needed to fully characterize their formation and evolution from the partonic to the hadronic state that we observe in particle detectors. In this talk we present the experimental study of the angular distance between different definitions of the jet axis in proton-proton collisions at $\sqrt{s}=5.02$ TeV in ALICE. These observables are infrared and collinear safe, which makes them calculable in pQCD. The measurement is also sensitive to non-perturbative physics such as hadronization and underlying-event effects and can be used as a baseline to compare to heavy-ion collisions, where a deconfined state of matter is predicted by lattice QCD calculations. Differences between the standard and soft-drop axes probes the effect that grooming has on the jet direction. Similarly, differences between the standard and winner-takes-all axes can describe how precisely the leading hadron in a jet can be used as a proxy for the direction of the jet itself. These new experimental results are compared to predictions from different event generators. |
Tuesday, October 12, 2021 2:36PM - 2:48PM |
FH.00004: Using Event-Shape Engineering to Constrain Jet Energy Loss Mechanisms in Pb-Pb Collisions at √sNN = 5.02 TeV in ALICE Caitlin Beattie It has long been understood that jets, the hadronic showers generated by hard-scattered partons, lose energy to the Quark-Gluon Plasma (QGP). The relative contributions of the energy-loss mechanisms, however, remain to be quantified. One approach to constraining these values is to investigate the link between hard and soft observables. By applying the technique of Event-Shape Engineering (ESE), we can assess how the soft behaviour of the medium is correlated with the level of jet suppression. Notably, this technique selects ensembles of events within a centrality class, allowing direct access to observables that depend on the shape of the QGP droplet while minimizing effects from a varying temperature. In this talk, I will present the status of the charged-jet RAA analysis for central and semi-central collisions using Pb-Pb data at 5.02 TeV. I will then discuss plans for the application of event-shape selection to this sample of jets. |
Tuesday, October 12, 2021 2:48PM - 3:00PM |
FH.00005: Modelling the Impact of Path-Length and Energy-Loss Fluctuations on Jet-Like Correlations Michael H Oliver The question of how QCD jets interact with a QGP medium as a function of path-length is a significant ongoing research effort in the relativistic heavy ion community. However, it has been demonstrated that jet-by-jet fluctuations in the jet-medium interactions may dominate measurements intended to study the path-length dependence, such as dijet asymmetry. In this study, we use the JEWEL model of jet-medium interaction to study how this applies to two-particle correlations as a function of the angle with the reaction plane. In particular, we study dihadron correlations in dijet events and direct photon-hadron correlations in photon-jet events. In the latter case, the contribution from jet-medium interaction fluctuations to observables is suppressed, as there is only one jet and the direct photon leaves the medium unaffected. We demonstrate that such direct photons can provide an optimal window into path-length dependence. By studying the yield and widths of the awayside and nearside peaks, we make predictions for such measurements in heavy ion collisions at RHIC and LHC energies. |
Tuesday, October 12, 2021 3:00PM - 3:12PM |
FH.00006: Applying hard-soft factorized parton energy loss model beyond weak coupling Tianyu Dai, J-F Paquet, Derek Teaney, Steffen A Bass The quark-gluon plasma produced in heavy ion collisions is understood to be strongly coupled, so that even the energy loss of a very energetic parton can be affected by non-perturbative effects. Generally, soft interactions with small momentum transfer are expected to suffer from much larger non-perturbative effects than hard interactions. Because of the large number of soft interactions, they can be described as a diffusion process with a few transport coefficients; these transport coefficients can capture any non-perturbative effects, agnostic to the strongly- or weakly-coupled nature of the underlying deconfined plasma. In this work, we implement a systematic factorization of the hard and soft interactions which combines a stochastic description of soft interactions and rate-based modelling of hard scatterings. We introduce a scale to estimate the regime of validity of the stochastic description, allowing for a better understanding of the model's applicability at small and large coupling. We compare this model to measurements using realistic hydrodynamic simulation of the strongly-coupled plasma. |
Tuesday, October 12, 2021 3:12PM - 3:24PM |
FH.00007: PHENIX Results on Jet Modification Using π0-Hadron Correlations in 200 GeV Au+Au Collisions Anthony M Hodges In the initial stages of heavy ion collisions, the constituent partons of the colliding nuclei can undergo collisions with large momentum transfer. These hard-scattered partons then traverse the hot dense medium known as the Quark Gluon Plasma (QGP) and lose energy, leading to the modification of the jets they produce relative to jets in baseline p+p collisions. Measurement of jet modification can, therefore, allow us to quantify the properties of the QGP. |
Tuesday, October 12, 2021 3:24PM - 3:36PM |
FH.00008: Study of systematic uncertainties in jet shape measurements in $\sqrt{s_{NN}} = 200$ $\mathrm{GeV}$ Au+Au collisions at STAR Tanmay Pani Jets are highly collimated sprays of particles emitted by energetic partons from hard scatterings. In relativistic heavy-ion collisions, these hard-scattered partons undergo various interactions with the surrounding Quark-Gluon Plasma (QGP) medium, resulting in the emitted jet being modified of its energy and substructure relative to a baseline reference in pp collisions. Jet shape observables study the radial distributions of energy and particles in a jet, and aid us in understanding how jets are modified in the presence of the QGP medium. Recently, preliminary uncorrected measurements of these observables have been reported by the STAR experiment at RHIC energies. To study potential impact of the medium on these observables, further corrections due to detector effects need to be established. We embed PYTHIA 8 pp events into minimum-bias Au+Au events at $\sqrt{s_{NN}} = 200$ $\mathrm{GeV}$, and pass them through a GEANT simulation of the STAR detector to estimate correction factors and associated systematic uncertainities for the jet shape measurements. This talk provides a status update of the jet shape measurements in $\sqrt{s_{NN}} = 200$ $\mathrm{GeV}$ Au+Au collisions by the STAR experiment. |
Tuesday, October 12, 2021 3:36PM - 3:48PM |
FH.00009: Event-shapes and the presence of jets in e+e- and pp collisions Mike Sas
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Tuesday, October 12, 2021 3:48PM - 4:00PM |
FH.00010: Exploring the challenges of using machine learning for ALICE jet measurements Hannah J Bossi Ongoing challenges for the use of machine learning (ML) for jet physics include the interpretability and the quantification of uncertainties/bias in such measurements. One use case of ML that illustrates its potential is the jet measurements in the ALICE detector discussed in this talk. In this case, ML is used in order to correct the measured jets for the large fluctuating background in heavy-ion collisions. The ML estimator utilizes properties of jets from simulation in training, including the constituents of the jet. This analysis does not utilize deep learning methods but instead utilizes a shallow neural network for increased interpretability. The bias of interest in this approach is introduced by including constituent information in training. This introduces a dependence on the fragmentation pattern in simulation, which has been shown to differ from the fragmentation in Pb--Pb data. In an effort to understand and quantify this bias, three modifications of the fragmentation utilized in training were studied, illustrating the degree to which the ML estimator is robust to the biases explored. Such results demonstrate ways in which ML approaches in jet physics can overcome the contemporary challenges of the field in order to make useful, interpretable measurements. |
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