Jet-hadron Correlations Research Articles

Medium Response and Jet–Hadron Correlations in Relativistic Heavy-Ion Collisions

The study of high-energy heavy-ion collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider has evolved from a qualitative understanding to the precise extraction of the properties of the quantum chromodynamics medium at extremely high temperatures. Jet quenching has offered unique insights into the transport properties of the quark–gluon plasma (QGP) created in these energetic collisions. Apart from medium modification of jets, jet-induced medium excitation constitutes another crucial aspect of jet–QGP interaction and is indispensable in understanding the soft components of jets. We review recent theoretical and phenomenological developments regarding medium response to jet energy loss, including an overview of both weakly and strongly coupled approaches for describing the thermalization and propagation of energy deposition from jets, effects of medium response on jet observables, and exploration of its unique signatures in jet–hadron correlations. Jet-induced medium excitation is shown to be an essential component in probing the in-medium dynamics of jets and a critical step toward precise extraction of the QGP properties.

Measurement of jet radial profiles in Pb–Pb collisions at [formula omitted] TeV

The jet radial structure and particle transverse momentum (pT) composition within jets are presented in centrality-selected Pb–Pb collisions at sNN=2.76 TeV. Track-based jets, which are also called charged jets, were reconstructed with a resolution parameter of R=0.3 at midrapidity |ηch jet|<0.6 for transverse momenta pT,ch jet=30–120 GeV/c. Jet–hadron correlations in relative azimuth and pseudorapidity space (Δφ,Δη) are measured to study the distribution of the associated particles around the jet axis for different pT,assoc-ranges between 1 and 20 GeV/c. The data in Pb–Pb collisions are compared to reference distributions for pp collisions, obtained using embedded PYTHIA simulations. The number of high-pT associate particles (4<pT,assoc<20 GeV/c) in Pb–Pb collisions is found to be suppressed compared to the reference by 30 to 10%, depending on centrality. The radial particle distribution relative to the jet axis shows a moderate modification in Pb–Pb collisions with respect to PYTHIA. High-pT associate particles are slightly more collimated in Pb–Pb collisions compared to the reference, while low-pT associate particles tend to be broadened. The results, which are presented for the first time down to pT,ch jet=30 GeV/c in Pb–Pb collisions, are compatible with both previous jet–hadron-related measurements from the CMS Collaboration and jet shape measurements from the ALICE Collaboration at higher pT, and add further support for the established picture of in-medium parton energy loss.

Event plane dependence of the flow modulated background in dihadron and jet-hadron correlations in heavy ion collisions

Di-hadron and jet-hadron correlations are commonly used in relativistic heavy ion collisions to study the soft component of jets in a quark gluon plasma. There is a large correlated background which is described by the Fourier decomposition of the azimuthal anisotropy where $v_n$ is the $n$th order coefficient. The path length dependence of partonic energy loss can be studied by varying the angle of the high momentum trigger particle or jet relative to a reconstructed event plane. This modifies the shape of the background correlated with that event plane. The original derivation of the shape of this background only considered correlations relative to the second order event plane, which is correlated to the initial participant plane. We derive the shape of this background for an event plane at an arbitrary order. There is a phase shift in the case of jets restricted to asymmetric regions relative to the event plane. For realistic correlations between event planes, the correlation between the second and fourth order event planes leads to a much smaller effect than the finite event plane resolution at each order. Finally, we assess the status of the rapidity even $v_1$ term due to flow, which has been measured to be comparable to $v_2$ and $v_3$ terms.

Jet-hadron correlations relative to the event plane at the LHC with ALICE

In ultra relativistic heavy-ion collisions at the Large Hadron Collider (LHC), conditions are met to produce a hot, dense and strongly interacting medium known as the Quark Gluon Plasma (QGP). Quarks and gluons from incoming nuclei collide to produce partons at high momenta early in the collisions. By fragmenting into collimated sprays of hadrons, these partons form 'jets'. The outgoing partons scatter and interact with the medium, leading to a manifestation of medium modifications of jets in the final state, known as jet quenching. Within the framework of perturbative QCD, jet production is well understood in pp collisions. We use jets measured in pp interactions as a baseline reference for comparing to heavy-ion collision systems to detect and study jet quenching. The jet quenching mechanism can be studied through the angular correlations of jets with charged hadrons and is examined in transverse momentum (pT) bins of the jets, pT bins of the associated hadrons, and as a function of collision centrality. A robust and precise background subtraction method is used in this analysis to remove the complex, flow dominated, heavy-ion background. The analysis of angular correlations for different orientations of the jet relative to the event plane allows for the study of the path-length dependence of medium modifications to jets. The event plane dependence of azimuthal angular correlations of charged hadrons with respect to the axis of an R=0.2 reconstructed full (charged + neutral) jet in Pb–Pb collisions at sNN=2.76 TeV in ALICE is presented. Results are compared for three angular bins of the jet relative to the event plane in mid-peripheral events. The yields relative to the event plane are presented and then quantified through yield ratio calculations. The results show no significant path-length dependence on the medium modifications.

Jet-hadron correlations relative to the event plane at the LHC with ALICE

Jet quenching is observed at both RHIC and LHC energies. This suggests that partons lose energy as they traverse the medium. When the trigger jet is studied relative to the event plane, the path length dependence of medium modifications can be studied. We present measurements of the angular correlations relative to the event plane between reconstructed jets and charged hadrons in Pb–Pb collisions at = 2.76 TeV in ALICE. A newly implemented, robust background subtraction method to remove the complex, flow dominated, combinatorial background is used in this analysis.

Background subtraction methods for precision measurements of di-hadron and jet-hadron correlations in heavy ion collisions

Di-hadron correlations and jet-hadron correlations are frequently used to study interactions of the Quark Gluon Plasma with the hard partons that form jets. The existing background subtraction methods for these studies depend on several assumptions and independent measurements of the Fourier coefficients of the combinatorial background. In this paper, we present two background subtraction methods which use the observation that the near-side correlation signal is negligible for large separations between the associated particle and the trigger. The combinatorial background is fit in this background-dominated region. Including the reaction plane dependence of the raw correlation improves these results. To test the accuracy of these methods, a simple model is used to simulate di-hadron and jet-hadron correlations with a combinatorial background similar to that observed in the data. The true signal is compared to the extracted signal. The results are compared to results from two variants of the Zero-Yield-At-Minimum (ZYAM) method. We test these methods for mid-peripheral and central collisions and for di-hadron and jet-hadron correlations. These methods are more precise than the ZYAM method with fewer assumptions about the shape and level of the combinatorial background, even in central collisions where the experimental resolution on the measurement of the reaction plane dependence is poor. These methods will allow more accurate studies of modifications of the away-side jet and will be particularly useful for studies of jet-hadron correlations, where the combinatorial background is poorly constrained from previous studies.

Overview of STAR results on correlations, jets and heavy-flavor production

Measurements of jets and heavy-flavor production play an important role in understanding properties of hot and dense nuclear matter created in high energy heavy-ion collisions. As direct measurements of jets are difficult due to large underlying background, jet properties can be also studied via di-hadron and multihadron correlations. In this contribution to these proceedings, an overview of recent STAR results on correlations, jet-hadron correlations, and heavy-flavor production in Au+Au collisions at top RHIC energy ( = 200 GeV) is given. In order to draw quantitative conclusions on properties of the hot and dense nuclear matter created in Au+Au collisions, reference measurements in elementary proton-proton collisions and d+Au collisions are essential to estimate contributions due to cold nuclear matter effects and are discussed as well.

Jet-hadron correlations in √[s(NN)]=200 GeV p+p and central Au+Au collisions.

Azimuthal angular correlations of charged hadrons with respect to the axis of a reconstructed (trigger) jet in Au+Au and p+p collisions at √[s(NN)]=200 GeV in STAR are presented. The trigger jet population in Au+Au collisions is biased toward jets that have not interacted with the medium, allowing easier matching of jet energies between Au+Au and p+p collisions while enhancing medium effects on the recoil jet. The associated hadron yield of the recoil jet is significantly suppressed at high transverse momentum (pTassoc) and enhanced at low pTassoc in 0%-20% central Au+Au collisions compared to p+p collisions, which is indicative of medium-induced parton energy loss in ultrarelativistic heavy-ion collisions.

Jet-Hadron Correlations in pp and Pb-Pb Collisions with ALICE

Jet quenching has been observed at both RHIC and LHC energies, indicating that partons lose energy as they traverse the medium. To probe the effects of this partonic energy loss, measurements of the angular correlations between fully reconstructed jets and charged tracks in Pb-Pb collisions are studied. Fully reconstructing a jet provides access to the kinematics of the initial hard scattering while allowing us to study the distribution of hadrons on the away side from the modified recoil jet. Here we present first measurements of jet-hadron correlations in pp collisions at =2.76 TeV and an outlook for Pb-Pb collisions. The jets in this analysis are reconstructed from the 2011 data set using both charged tracks and neutral energy measured in the ALICE tracking system and the electromagnetic calorimeter respectively.

Prompt Photon Production and Photon–Jet–Hadron Correlations in PHENIX

Abstract A variety of heavy-ion data from RHIC and recently also from the LHC on hard direct photon production testifies that this non-interacting probe of the densely colored QGP continues to lend new insights to understanding jet suppression and energy loss. In this talk, we report on new results of high p T single direct photon production in both p + p and Heavy Ion systems. Additionally, new PHENIX results on fragmentation functions of the direct photon–jet photon–hadron correlations from the 2010 dataset, when combined with the previous 2007 dataset result, now show an enhancement that is statistically significant in regions of low z . It is found that this enhancement appears to occur over relatively wide angles from back-to-back with respect to the direct photon trigger.

Theoretical assessment of jet-hadron correlations

With high-statistics data coming from both RHIC and LHC, the experimentally available selection of hard tomographic probes for the medium created in ultrarelativistic heavy-ion (A-A) collisions is rapidly expanding. Jet-hadron (jet-h) correlation measurements as introduced by the STAR collaboration are a promising avenue to study the structure of highly modified jets in a differential way since the away side correlation can be measured down to very low transverse momenta (P_T) in an essentially hydrodynamical regime. At the same time the geometry bias introduced by the trigger condition ensures that the away side shower has propagated a long distance in the medium. The aim of this paper is to provide a theoretical overview of the observable, discuss similarities and differences to other correlation observables such as hadron-hadron (h-h) and gamma-hadron (gamma-h) correlations and to provide recommendations for future measurements.

Jet–hadron correlations in STAR

Advancements in full jet reconstruction have made it possible to use jets as triggers in azimuthal angular correlations to study the modification of hard-scattered partons in the medium created in ultrarelativistic heavy-ion collisions. This increases the range of parton energies accessible in these analyses and improves the signal-to-background ratio compared to dihadron correlations. Results of a systematic study of jet–hadron correlations in central Au–Au collisions at GeV are indicative of a broadening and softening of jets which interact with the medium. Furthermore, jet–hadron correlations suggest that the suppression of the associated hadron yield at high-pT is balanced in large part by low-pT enhancement.

Jet-hadron correlations in STAR

In recent years, the study of dihadron correlations has been one of the primary methods used to investigate the propagation and modification of hard-scattered partons through the QGP. Due to recent advances in jet-finding algorithms, it is now possible to use reconstructed jets in these correlation studies, extending the kinematic reach compared to dihadron analyses. The results of the jet-hadron correlation analysis indicate a broadening and softening of jets that interact with the medium. Jet-hadron correlations can also be used to assess the systematics of other jet-like correlation analyses, such as 2+1 correlations. It is shown that the jets selected in 2+1 correlations are relatively unmodified. Future work will include an analysis of jet-hadron correlations with respect to the event plane to measure the pathlength dependence of parton energy loss. The first steps in this analysis indicate that complications arise when calculating the event plane in the presence of a jet as well as in calculating jet v2. The data analyzed were collected by the STAR detector in = 200 GeV Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC).

Measurements of di-jets in p+p and Au+Au in the STAR experiment

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 or elastic collisions along their path. Measurements from full jet reconstruction help in the understanding of energy loss and its effect 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 large coverage of the STAR detector along with an online trigger also allows for reconstructing di-jets. Suitable selection of trigger jets allows for studying a subset of jets on the recoil side which are biased towards higher energy loss because of a larger, on average, in-medium path length traversed. Trigger jets are also used to study jet-hadron correlations as an independent measurement to assess the effect of energy loss on the recoil side. We present measurements of di-jets and jet-hadron correlations in the presence of reconstructed di-jets in Au+Au and p+p at 200 GeV in the STAR experiment.