Session EF: Jets in Heavy-ion Collisions I

Measurements of inclusive jet suppression in Pb--Pb collisions with ALICE

Jets in relativistic heavy-ion collisions interact with the quark-gluon plasma (QGP), leading to effects such as a suppression of inclusive jet yields and modification of internal jet structure that can be used to constrain properties of the QGP. The dependence of inclusive jet suppression on the resolution parameter ($R$) and jet $p_{\rm T}$ is a powerful observable to disentangle competing energy loss mechanisms with a high discriminating power when compared to models. Due to the presence of the large underlying event in heavy-ion collisions, measurements at large $R$ and low $p_{\rm T}$ are limited by the $p_{\rm T}$ resolution achieved using traditional techniques. A new method using machine learning techniques is used to correct the large background in heavy-ion collisions and extend the measurement of inclusive jet yields to lower $p_{\rm T}$ than previously achieved in heavy-ion collisions at the LHC. This talk will present the inclusive jet nuclear modification factors in Pb--Pb collisions in various centrality classes at $\sqrt{s_{\rm NN}} = 5.02$ TeV recorded with the ALICE detector for resolution parameters up to $R$ = 0.6 for jet transverse momenta down to 40 GeV/$c$. These results will also be compared to jet quenching models and other experimental measurements of inclusive jet suppression.   

Measurement of jet-axis differences in pp and PbPb collisions with ALICE

Jets are a powerful tool to study quantum chromodynamics (QCD) and can be used to study the physical properties of quark-gluon plasma (QGP) created in heavy-ion collisions. In this talk, we review recent jet-substructure measurements carried out in ALICE - a systematic study of the opening angle between different jet axes denoted $\Delta R_{\rm axis}$. Studies of $\Delta R_{\rm axis}$ are an attractive experimental strategy since these observables are infrared and collinear safe, sensitive to transverse momentum dependent (TMD) physics and analytically calculable in perturbative QCD (pQCD). In consequence, these measurements in pp collisions can be used to test the universality of the TMD evolution kernel constrained from processes such as semi-inclusive deep-inelastic scattering and Drell-Yan. Moreover, comparisons between distributions obtained in PbPb and pp collisions provide a novel perspective on the relationship between competing effects induced by the QGP such as parton energy loss and intra-jet transverse momentum broadening. The new experimental results are compared to predictions from a selection of event generators and pQCD calculations.   

Current and future measurements of the Lund jet plane density in pp and Pb-Pb collisions with ALICE

Jets are excellent probes of the quark-gluon plasma (QGP) produced in heavy-ion collisions because the partons inside jets interact with the medium, leading to jet energy loss and substructure modification - a phenomenon called jet quenching. In particular, observables that focus on the hard parton splittings inside the jet can be used to isolate different jet-medium interactions and answer questions about the microscopic structure of the QGP. The primary Lund jet plane density is a two-dimensional visual representation of the parton splittings within the jet that can be used to isolate different regions of the QCD phase space. In heavy-ion collisions it provides a map of the space-time structure of the QGP. This talk will highlight recent measurements from ALICE that probe specific regions of the Lund plane. Some regions remain inaccessible due to current experimental constraints such as the heavy-ion background and precision. This talk will discuss how the ALICE Run 3 upgrades and the future ALICE 3 detector design will allow us to overcome some of these challenges and provide access to some of the previously inaccessible regions of the Lund plane.   

Isolated photon-jet correlations in Pb+Pb collisions at the LHC

Jets recoiling from isolated photons are a promising channel to study jet quenching in heavy-ion collisions, as photons do not interact strongly and therefore constrain the $Q^2$ of the initial hard scattering. We present isolated photon-jet correlations measured in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV by the ALICE collaboration. We study correlations of isolated photons above 20 GeV/$c$ with charged-particle jets above 10 GeV/$c$, reconstructed with the anti-kT algorithm with R=0.2. The correlations probe the lowest $p_\mathrm{T}$ range for photon-jet correlations measured so far at LHC energies. As the energy loss is expected to weakly depend on the parton energy, larger relative modifications due to the QGP are expected in the lower $p_\mathrm{T}$ regime.   

Measurements of jet shape observables in √sNN = 200 GeV Au+Au collisions at STAR

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 jets having their energy and substructure modified relative to those in $pp$ collisions. Jet shape observables probe the energy and multiplicity distributions within a jet and are sensitive to the dynamics of the parton shower. Recently, raw distributions of some of these observables have been reported by the STAR experiment at RHIC for Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV. To study the hot medium effects on these observables, corrections due to detector effects need to be established. Monte Carlo simulations of $pp$ events from PYTHIA 8, after passing through a GEANT simulation of the STAR detector, are embedded into minimum-bias real Au+Au events at $\sqrt{s_{NN}} = 200$ GeV. This talk presents a status update of quantifying detector effects in measurements of jet shape observables by comparing particle- and detector-level jets from the aforementioned embedding sample.   

An Investigation of Flavor Dependence of Jet Shape Modifications in Au+Au Collisions at √sNN = 200 GeV

Partons, i.e, quarks and gluons, in heavy-ion collisions interact strongly with the Quark-Gluon Plasma (QGP), and hence have their energy and shower structure modified compared to those in vacuum, e.g., those produced in proton-proton collisions. Theoretical calculations predict that the radiative energy loss, which is the dominant mode of energy loss for gluons and light quarks in the QGP, is suppressed for heavy quarks, such as charm and bottom, at low transverse momenta (p_T). The measurement of the D⁰ meson radial profile in jets from the CMS experiment at the LHC hints at its modification at low D⁰ p_T in heavy-ion collisions, which is qualitatively different from that of the lighter hadrons. The excellent secondary vertex resolution provided by the Heavy Flavor Tracker in the STAR experiment at RHIC enables reconstruction of D⁰ mesons at low p_T with high signal significance over the background.

We report the first measurements of the D⁰ meson tagged jet p_T spectra and the D⁰ meson radial profile in jets reconstructed from Au+Au collisions at √s_NN = 200 GeV, collected by the STAR experiment in 2014. Such measurements can shed light on parton flavor and mass dependencies of jet energy loss, and therefore constrain theoretical models.   

A Systematic Study of In-Medium Hadronization of Jet Showers with JETSCAPE and Hybrid Hadronization

“Hybrid Hadronization” is a Monte Carlo package that hadronizes systems of partons. It smoothly combines quark recombination, applicable when distances between partons in phase space are small, and string fragmentation appropriate for dilute parton systems, following the picture outlined by Han et al. [PRC 93, 045207 (2016)]. Hybrid Hadronization can be applied to a variety of systems from e⁺+e^- to A+A collisions. It takes systems of partons and their color flow information, for example from a Monte Carlo parton shower generator, as input. In addition, if for A+A collisions a thermal background medium is provided, the package allows to sample thermal partons that contribute to hadronization. Hybrid Hadronization is available for use as a standalone code and is part of JETSCAPE since the 2.0 release.

In this presentation we review the physics concepts underlying Hybrid Hadronization. We then quantify the effects that an ambient medium has on the hadronization process of jet showers. We focus in particular on the dependence of medium effects on the flow and size of the medium. Jet showers are simulated in JETSCAPE either in vacuum or in an ambient medium. We systematically vary parameters like jet energy, jet flavor, medium size, medium flow velocity, and medium flow direction. Among other observables we compute hadron fragmentation functions, ratios of identified hadrons, properties of clustered jets, and jet shapes. We see clear signatures of the medium interacting with jets during hadronization. Our results clarify the effect we expect from in-medium hadronization of jets on observables like fragmentation functions, hadron chemistry, jet shape, or acoplanarity. They could be useful to further constrain our understanding of the interaction of jet showers in the parton phase extracted from experimental data.