Very soon the LHC will provide beams for heavy ion collisions at 5.52 TeV/nucleon. This center-of-mass energy results in a large cross-section for producing high-$E_T$ ($>$ 50 GeV) jets that are distinct from the soft, underlying event. This brings with it the possibility of performing full jet reconstruction to directly study jet energy loss in the medium produced in heavy ion collisions. In this note, we present the current state of jet reconstruction performance studies in heavy ion events using the ATLAS detector. We also discuss the possibilities of energy loss measurements available with full jet reconstruction: single jet $R_{AA}$ and di-jet and $γ$-jet correlations.
Jets at the LHC are expected to provide the testing ground for studying QCD energy loss. In this contribution, we briefly outline the strategy that will be used to measure jets in ATLAS and how we will go about studying energy loss. We describe the utility of measuring the jet $R_{AA}$, the fragmentation function, and heavy flavor jets. Utilizing the collision energy provided by the LHC and the nearly hermetic and highly segmented calorimeter, ATLAS is expected to make important contributions to the understanding of parton energy loss using fully reconstructed jets.
Jet quenching measurements using leading particles and their correlations suffer from known biases, which can be removed via direct reconstruction of jets in central heavy ion collisions. In this talk, we discuss several modern jet reconstruction algorithms and background subtraction techniques that are appropriate to heavy ion collisions.
The main goals of relativistic heavy-ion experiments is to study the properties of QCD matter under extreme temperatures and densities. The focus of this talk is the studies that are underway at the Relativistic Heavy Ion Collider (RHIC), located at the Brookhaven National Laboratory (BNL) on Long Island, New York, U.S.A. I discuss selected highlights from the past couple of years that are key to elucidating the characteristics of the new state of matter created in these heavy-ion collisions, called a Quark Gluon Plasma.
At the Relativistic Heavy Ion Collider, jets have been a useful tool to probe the properties of the hot, dense matter created. At the Large Hadron Collider, collisions of Pb+Pb at $\sqrt{s_{NN}}$ = 5.5 TeV will provide a large cross section of jets at high $E_T$ above the minimum bias heavy ion background. Simulations of the Compact Muon Solenoid (CMS) experiment's capability to measure jets in heavy ion collisions are presented. In particular, $γ$-jet measurements can estimate the amount of energy lost by a jet interacting strongly with the medium, since the tagged photon passes through unaff…
Full jet reconstruction in heavy-ion collisions enables a complete study of the modification of jet structure due to energy loss in hot and dense QCD matter, but is challenging due to the high multiplicity environment. The STAR and PHENIX collaborations at RHIC have recently presented measurements of fully reconstructed jets from p+p, Cu+Cu and Au+Au collisions at $\sqrt{s_\mathrm{NN}} = 200 \mathrm{GeV}$. We review the first results on inclusive jet spectra, di-jets and fragmentation functions and discuss their implications on understanding of jet quenching.
Reconstructed jets in heavy ion collisions are a crucial tool for understanding the quark-gluon plasma. The separation of jets from the underlying event is necessary particularly in central heavy ion reactions in order to quantify medium modifications of the parton shower and the response of the surrounding medium itself. There have been many methods proposed and implemented for studying the underlying event substructure in proton-proton and heavy ion collisions. In this paper, we detail a method for understanding underlying event contributions in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV …
Full jet reconstruction has traditionally been thought to be difficult in heavy ion events due to large multiplicity backgrounds. The search for new physics in high luminosity p+p collisions at the LHC similarly requires the precise measurement of jets over large backgrounds caused by pile up; this has motivated the development of a new generation of jet reconstruction algorithms which are also applicable in the heavy ion environment. We review the latest results on jet-medium interactions as seen in A+A collisions at RHIC, focusing on the new techniques for full jet reconstruction.
Mach-cone shock waves were proposed to explain the broad and perhaps double-peaked away-side 2-particle jet-correlations at RHIC; however, other mechanisms cannot be ruled out. Three-particle jet-correlation is needed in order to distinguish various physics mechanisms. In this talk the 3-particle jet-correlation measurements are presented and their implications are discussed.
Methodology is presented for analysis of three-particle correlation functions obtained in heavy ion collisions at ultra-relativistic energies. We show that harmonic correlations can be removed and jet driven correlations reliably extracted. Results from detailed Monte Carlo simulations are used to demonstrate the efficacy of this technique for the study of modifications to away-side jet topologies. Such modifications are an essential probe of the properties of the quark gluon plasma produced in heavy ion collisions.
In the particle-flow approach information from all available sub-detector systems is combined to reconstruct all stable particles. The global event reconstruction has been shown to improve, in particular, the resolution of jet energy and missing transverse energy in pp collisions compared to purely calorimetric measurements. This improvement is achieved primarily by combining the precise momentum determination of charged hadrons in the silicon tracker with the associated energy depositions in the calorimeters. By resolving individual particles inside jets, particle flow reduces the sensitivity…
Measurements of inclusive hadron suppression and di-hadron azimuthal correlations have provided important insights into jet quenching in hot QCD matter. However, they do not provide access to the energy of the hard scattering and are limited in their sensitivity since they can be affected by biases toward hard fragmentation and small energy loss. Full jet reconstruction in heavy-ion collisions enables a complete study of the modification of jet structure due to energy loss, but is challenging due to the high-multiplicity environment. We present measurements of fully reconstructed di-jets at mi…
A modification of the internal structure of jets is expected due to the production of a dense QCD medium, the Quark Gluon Plasma, in heavy-ion collisions. We discuss some aspects of jet reconstruction in p+p and A+A collisions and emphasize the dramatically increased contribution of the underlying event in nucleus-nucleus collisions as compared with the vacuum case. We conclude with its consequences on the full jet spectrum and fragmentation function extraction at LHC.
We present the first measurement of reconstructed jets in ultra-relativistic heavy ion collisions. Utilizing the large coverage of the STAR Time Projection Chamber and Electromagnetic Calorimeter, we apply several modern jet reconstruction algorithms and background subtraction techniques and explore their systematic uncertainties in heavy ion events. The differential spectrum for inclusive jet production in central Au+Au collisions at $\sqrt {s_{NN}}= 200$ GeV is presented. In order to assess the jet reconstruction biases, this spectrum is compared with the jet cross section measured in $\sqrt…
These proceedings present a brief overview of the main results on jet-modifications in heavy ion collisions at RHIC. In heavy ion collisions, jets are studied using single hadron spectra and di-hadron correlations with a high-\pt{} trigger hadrons. At high pt, a suppression of the yields due to parton energy loss is observed. A quantitative confrontation of the data with various theoretical approaches to energy loss in a dense QCD medium is being pursued. First results from $γ$-jet events, where the photon balances the initial jet energy, are also presented and compared to expectations from mo…
The presence of large event-by-event flow fluctuations in heavy ion collisions at RHIC and the LHC provides an opportunity to study a broad class of flow observables. This paper explores the correlations among harmonic flow coefficients $v_n$ and their phases $Φ_n$, and the rapidity fluctuation of $v_n$. The study is carried out usin Pb+Pb events generated by the AMPT model with fixed impact parameter. The overall ellipticity/triangularity of events is varied by selecting on the eccentricities $ε_n$ or the magnitudes of the flow vector $q_n$ for n=2 and 3, respectively. The responses of the $v…
Measurements of strong suppression of inclusive hadron distributions and di-hadron correlations at high $p_{T}$, while providing evidence for partonic energy loss, also suffer from geometric biases due to the competition of energy loss and fragmentation. The measurements of fully reconstructed jets is expected to lack these biases as the energy flow is measured independently of the fragmentation details. In this article, we review the recent results from the heavy ion collisions collected by the STAR experiment at RHIC on direct jet reconstruction utilizing the modern sequential recombination …
Full jet reconstruction in relativistic heavy ion collisions provides new and unique insights to the physics of parton energy loss. Because of the large underlying event multiplicity in $A+A$ collisions, random and correlated fluctuations in the background can result in the reconstruction of fake jets. These fake jets must be identified and rejected to obtain the purest jet sample possible. A large but reducible fake rate of jets reconstructed using an iterative cone algorithm on HIJING events is observed. The absolute rate of fake jets exceeds the binary-scaled p+p jet rate below 50 GeV and i…
Multi-particle correlation techniques are frequently used to study jet shapes and yields in hadronic and nuclear collisions. To date, a standard assumption applied in such analyses is that the observed correlations arise from either jets and associated hard scattering phenomena, or from a background component due to combinatorial pairs connected only through whole even correlations. Within this assumption of two essentially independent sources, a fundamental problem centers around determining the relative contributions of each component. We discuss the methods commonly used to establish the ba…
In ultra-relativistic heavy-ion collisions at $\sqrt{s_{NN}}$ = 5.5 TeV at the ALICE experiment at the LHC, interactions between the high-$p_{T}$ partons and the hot, dense medium produced in the collisions, are expected to lead to jet energy loss (jet-quenching) resulting in changes in the jet fragmentation functions as compared to the unquenched case. In order to reconstruct jet fragmentation functions, accurate information on the jet energy, direction and momentum distribution of the jet particles is needed. This thesis presents first results on jet reconstruction in simulated Pb+Pb collisi…
Jets in relativistic heavy-ion collisions (HICs) interact with the quark-gluon plasma (QGP), leading to effects such as a suppression of jet yields and a modification of internal jet structure that are used to measure the properties of the QGP. These proceedings show 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 momenta down to 40 GeV/$c$. The analysis utilizes machine learning techniques to correct the large background in HICs, ex…
Measurements of inclusive hadron suppression and di-hadron azimuthal correlations in ultra-relativistic nuclear collisions at RHIC have provided important insights into jet quenching in hot QCD matter, but are limited in their sensitivity due to well-known biases. Full jet reconstruction in heavy-ion collisions would conceptually provide a direct measurement of the energy of the scattered parton before energy loss, alleviating such biases and allowing a measurement of the energy loss probability distribution in a model-independent way from hard probes. In these proceedings we utilize recent pr…
These proceedings review recent results from two and three particle correlations in heavy ion collisions. In particular we discuss the modified structure of the away side jet correlations. Under the assumption that the away side can be decomposed into a punch through component at $Δϕ=π$ and a shoulder component with a peak displaced from $π$ many similar properties are observed between the ridge and shoulder. The particle ratios, yields and $p_T$ spectra are in near agreement. We also highlight important future measurements, including investigating if the decomposition of the away side jet cor…
Jets are produced from hard scatterings in the early stages of heavy-ion collisions. It is expected that these high-$p_T$ partons travel through the hot and dense medium before fragmenting. Therefore they are expected to suffer energy loss in the QGP via gluon radiation and elastic collisions along their path. Measurements of fully reconstructed jets help understand the effect of the energy loss on the jet structure and energy profile. A data-driven characterization of the background in Au+Au is needed in order to compare the results to p+p. The full azimuthal coverage of STAR Time Projection …
The Quark-Gluon Plasma (QGP) is created in high energy heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). This medium is transparent to electromagnetic probes but nearly opaque to colored probes. Hard partons produced early in the collision fragment and hadronize into a collimated spray of particles called a jet. The partons lose energy as they traverse the medium, a process called jet quenching. Most of the lost energy is still correlated with the parent parton, contributing to particle production at larger angles and lower momenta relative…
A new technique of elucidating $β$-decay schemes of isotopes with large density of states at low excitation energies has been developed, in which a Broad Energy Germanium (BEGe) detector is used in conjunction with coaxial hyper-pure germanium detectors. The power of this technique has been demonstrated on the example of 183Hg decay. Mass-separated samples of 183Hg were produced by a deposition of the low-energy radioactive-ion beam delivered by the ISOLDE facility at CERN. The excellent energy resolution of the BEGe detector allowed $γ$ rays energies to be determined with a precision of a few…
We present a measurement of dijet asymmetry and dijet azimuthal correlations in Pb+Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV using the ATLAS detector. This measurement provides the first evidence of a strong jet quenching in relativistic heavy ion collisions at TeV energies. The jet reconstruction procedure is discussed as well as studies which have been performed to check that the observed asymmetry is not produced by detector effects and underlying event backgrounds.
Measurements of jet substructure in heavy-ion collisions may provide key insight to the nature of jet quenching in the quark-gluon plasma. Jet grooming techniques from high-energy physics have been applied to heavy-ion collisions in order to isolate theoretically controlled jet observables and explore possible modification to the hard substructure of jets. However, the grooming algorithms used have not been tailored to the unique considerations of heavy-ion collisions, in particular to the experimental challenge of reconstructing jets in the presence of a large underlying event. We report a se…
We present data-driven methods for the full reconstruction of jets in heavy ion collisions, for inclusive and co-incidence jet measurements at both RHIC and LHC. The complex structure of heavy ion events generates a large background of combinatorial jets, and smears the measured energy of the true hard jet signal. Techniques to correct for these background effects can induce biases in the reported jet distributions, which must be well controlled for accurate measurement of jet quenching. Using model studies, we evaluate the proposed methods for measuring jet distributions accurately while mini…
Full jet reconstruction in heavy ion events has been thought to be difficult due to large multiplicity backgrounds. A new generation of jet reconstruction algorithms to search for new physics in high luminosity p+p collisions at the LHC is developed to precisely measure jets over large backgrounds caused by pile up. From simulations it turns out, this new generation of reconstruction algorithms are also applicable in the heavy ion environment. We review the latest results on jet-medium interactions as seen in A+A collisions at RHIC, focusing on the new techniques for full jet reconstruction.
Jet quenching has been one of the most important indicators that ultra-relativistic heavy-ion collisions produce a deconfined state of quarks and gluons, known as the Quark-Gluon Plasma. While the quenching of jets traditionally refers to the energy loss of high-momentum partons, the study of jet quenching has grown into a multi-pronged field where the measurement of jets and their modification in heavy-ion collisions is used as an important tool to study many aspects of QCD deconfinement. This contribution reviews the current experimental status of jets at the LHC and RHIC, and reports recent…
The energy loss of high-p_T partons provides insight into the transport properties of the medium created in relativistic heavy ion collisions. Evidence for this energy loss was first experimentally established through observation of high-p_T hadron suppression at RHIC. More recently, measurements of fully reconstructed jets have been performed at the LHC. In this summary the latest experimental results from the ATLAS collaboration on jet suppression are presented. In particular the jet suppression in inclusive jet yields, path length dependence of the jet suppression, photon-jet and Z^0-jet co…
We present an update to the analysis of fully reconstructed jets in heavy ion collisions by the STAR Collaboration at RHIC. We analyse the response of the anti-$k_{T}$ algorithm in the presence of background, and present a new observable for the measurement of inclusive jet production that is expected to be more robust against background model assumptions than previous jet analyses at RHIC and LHC.
The precise reconstruction of jet transverse momenta in heavy-ion collisions is a challenging task. A major obstacle is the large number of (mainly) low-$p_{\rm T}$ particles overlaying the jets. Strong region-to-region fluctuations of this background complicate the jet measurement and lead to significant uncertainties. In this paper, a novel approach to correct jet momenta (or energies) for the underlying background in heavy-ion collisions is introduced. The proposed method makes use of common Machine Learning techniques to estimate the jet transverse momentum based on several parameters, inc…
Jet reconstruction in heavy-ion collisions is strongly affected by soft background from the underlying event. For an appropriate interpretation of the jet observables it is essential to understand the influence of the background and its fluctuations on the reconstructed jets. With this purpose we study random cones and the response of a known probe embedded in a heavy-ion events. The embedded probe can be a single high-p_T track or a jet from a simulated or real pp event. This allows a detailed study of background fluctuations and verification of the performance of background subtraction metho…
Jet substructure, defined by observables constructed from the distribution of constituents within a jet, provides the versatility to tailor observables to specific regions of QCD radiation phase space. This flexibility provides exciting new opportunities to study jet quenching in heavy-ion collisions and to ultimately help reveal the nature of the quark-gluon plasma. The ALICE detector is particularly well-suited to jet substructure measurements in heavy-ion collisions due to its high-precision tracking system. In these proceedings, we report several new jet substructure measurements in Pb-Pb …
Jet studies provide an experimental method to explore the features of energy loss in the strongly interacting quark-gluon plasma. Recent jet results from 2.76 TeV PbPb and pp collisions measured with the CMS detector are presented. Jets in the most head-on (central) PbPb collisions are quenched in comparison to pp, and the jets fragment in different ways in the two systems. Measurements from pPb collisions at 5.02 TeV show that the jet and charged particle suppression seen in central PbPb measurements are not due to initial state nuclear effects.
These proceedings report recent studies performed by the ALICE, ATLAS and CMS experiments on jet substructure in pp and Pb+Pb collisions at 5.02 TeV. The dependence of the modification of the jet shape on the dijet transverse moment um asymmetry is presented. Measurements of particles recoiling against the Z boson without being constrained to jet cone delimitations are shown. Fully-corrected measurements of the N-subjettiness within anti-kt R = 0.4 charged-particle jets are also described.
Over the past 30 years, jet observables have proven to provide important information about the quark-gluon plasma created in heavy-ion collisions. I review the challenges, results, and open problems of jet physics in heavy-ion collisions, discussing the main ideas as well as some most recent results focussing on two major jet observables, the nuclear modification factor and the high-pT elliptic flow.
Jet quenching in high-energy heavy-ion collisions can be used to probe properties of hot and dense quark-gluon plasma. We provide a brief introduction to the concept and framework for the study of jet quenching. Different approaches and implementation of multiple scattering and parton energy loss are discussed. Recent progresses in the theoretical and phenomenological studies of jet quenching in heavy-ion collisions at RHIC and LHC are reviewed.
The production of a strongly interacting medium in heavy-ion collisions is identified through suppression of high transverse momentum jets leading to an effect known as jet quenching. Detailed measurements of nuclear modification factors and energy flow of the aforementioned quenched jets are performed with the CMS detector. These new results extend previous measurements to large angles with respect to the quenched jets and increase reach in the transverse momentum range. High statistics pp, pPb and PbPb data taken in 2011-13 are used in the following analysis. A new data-driven method to esti…
Modifications of jets in the existence of a hot and dense QCD medium have recently attracted a lot of attentions. In this talk, we demonstrate how jet-medium interactions change the behavior of jets by offering examples of inclusive jet and dijet productions at $ {\cal O}(α_s^3)$ in heavy ion collisions including initial-state cold nuclear effects and especially the final-state parton energy loss effect. The suppression of inclusive jet spectrum varying with jet radii and a flatter dijet momentum imbalance as compared to those in hadron-hadron collisions are observed in high-energy nuclear col…
In the first talk I discuss the usefulness of jet grooming for testing jet quenching mechanisms, and I present a calculation of soft-drop jet mass distribution in proton-proton and heavy ion collisions. In the second talk I discuss the comprehensive studies of quark and gluon jet substructure as a way to search for qualitatively new jet modification signatures. I emphasize the importance of precise understanding of jet substructure as well as powerful analysis tools in future heavy ion jet physics studies, with direct applications to hard probes using heavy flavors and hadronically-decaying el…
We investigate the effect of PT-broadening on jet substructure observables in heavy-ion collisions at the LHC. As an example, we focus on the opening angle of the two branches that satisfy the soft drop grooming condition in a highly energetic jet. The medium modification of the angular distribution can provide important information on the jet transport properties of hot QCD matter. In addition, we take into account a change of the overall fraction of quark and gluon jets in heavy-ion collisions. We comment on the comparison to a recent measurement from the ALICE Collaboration.
We review a recently proposed phenomenological framework to establish the notions of QCD factorization and universality of jet cross sections in the heavy-ion environment. First results of a global analysis of the nuclear modification factor of inclusive jets are presented where we extract medium modified jet functions using a Monte Carlo sampling approach. We observe that gluon jets are significantly more suppressed than quark jets. In addition, we study the jet radius dependence of the inclusive jet cross section in heavy-ion collisions and comment on a recent measurement from CMS. By consid…
The di-jet asymmetry --- the measure of the momentum imbalance in a di-jet system --- is a key jet quenching observable. Using the event generator \jewel we show that the di-jet asymmetry is dominated by fluctuations both in proton-proton and in heavy ion collisions. We discuss how in proton-proton collisions the asymmetry is generated through recoil and out-of-cone radiation. In heavy ion collisions two additional sources contribute to the asymmetry, namely energy loss fluctuations and differences in path length. The latter is shown to be a sub-leading effect. We discuss the implications of o…
Based on a multi-phase transport model, the measured jet fragmentation function ratio of Pb+Pb collisions to p+p collisions in CERN Large Hadron Collider experiments is decomposed into two parts, corresponding to the two contributions of jet hadronization from fragmentation and coalescence. The results suggest an existence of distinct competitions between two jet hadronization mechanisms for different $ξ$=ln(1/$z$) ranges in different centrality bins. The jet fragmentation functions for different types of hadrons (mesons and baryons) are proposed as a good probe to study the competition betwee…
The nature of a jet's fragmentation in heavy-ion collisions has the potential to cast light on the mechanism of jet quenching. However the presence of the huge underlying event complicates the reconstruction of the jet fragmentation function as a function of the momentum fraction z of hadrons in the jet. Here we propose the use of moments of the fragmentation function. These quantities appear to be as sensitive to quenching modifications as the fragmentation function directly in z. We show that they are amenable to background subtraction using the same jet-area based techniques proposed in the…
Understanding the inner working of the quark-gluon plasma requires complete and precise jet substructure studies in heavy ion collisions. In this proceeding we discuss the use of quark and gluon jets as independent probes, and how their classification allows us to uncover regions of QCD phase space sensitive to medium dynamics. We introduce the telescoping deconstruction (TD) framework to capture complete jet information and show that TD observables reveal fundamental properties of quark and gluon jets and their modifications in the medium. We draw connections to soft-drop subjet distributions…
We propose the lepton-jet correlation in deep inelastic scattering as a unique tool for nucleon/nucleus tomography at the electron-ion collider. The azimuthal angular correlation between the final state lepton and jet depends on the transverse momentum dependent quark distributions. We take the example of single transverse spin asymmetries to show the sensitivity to the quark Sivers function. When the correlation is studied in lepton-nucleus collisions, transverse momentum broadening effects can be used to explore cold nuclear matter effects. These features make lepton-jet correlations an impo…
We present predictions and projections for hadron-in-jet measurements and electron-jet azimuthal correlations at the future Electron-Ion Collider (EIC). These observables directly probe the three-dimensional (3D) structure of hadrons, in particular, the quark transversity and Sivers parton distributions and the Collins fragmentation functions. We explore the feasibility of these experimental measurements by detector simulations and discuss detector requirements. We conclude that jet observables have the potential to enhance the 3D imaging EIC program.
We propose a new phenomenological approach to establish QCD factorization of jet cross sections in the heavy-ion environment. Starting from a factorization formalism in proton-proton collisions, we introduce medium modified jet functions to capture the leading interaction of jets with the hot and dense QCD medium. A global analysis using a Monte Carlo sampling approach is performed in order to reliably determine the new jet functions from the nuclear modification factor of inclusive jets at the LHC. We find that gluon jets are significantly more suppressed due to the presence of the medium tha…
