Hard-scattered Partons Research Articles

Transverse momentum spectra and suppression of charged hadrons in deformed Xe–Xe collisions at $$\sqrt{s_{NN}}$$ = 5.44 TeV using HYDJET++ model

In this study, we systematically investigate deformed Xe-Xe collisions at 5.44 TeV center of mass energy. We exploit the Monte Carlo HYDJET++ model to compute transverse momentum ($p_{T}$) distribution, nuclear modification factor $R_{AA}$ and relative suppression in terms of $R_{CP}$ as a function of transverse momentum and centrality of collision of charged hadrons in body-body and tip-tip geometrical configurations, respectively. We have compared HYDJET++ model results to those from ALICE experimental data and AMPT model (String-Melting version) results. Minimum bias Xe-Xe collisions show a suitable match with ALICE experimental data at midrapidity. Average transverse momentum ($\langle p_{T} \rangle$) show strong centrality dependence. The contribution of hard parton scatterings to the total charged hadron yield is discussed. Both nuclear modification factor $R_{AA}$ and relative suppression $R_{CP}$ show a clear dependence on centrality as well as transverse momentum. The charged hadron observables show a clear dependence on the geometrical configuration of the collisions. HYDJET++ model justifies experimental data more closely than the AMPT model.

Medium-Induced Modification of Z-Tagged Charged Particle Yields in Pb+Pb Collisions at 5.02TeV with the ATLAS Detector.

The yield of charged particles opposite to a Z boson with large transverse momentum (p_{T}) is measured in 260 pb^{-1} of pp and 1.7 nb^{-1} of Pb+Pb collision data at 5.02TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The Z boson tag is used to select hard-scattered partons with specific kinematics, and to observe how their showers are modified as they propagate through the quark-gluon plasma created in Pb+Pb collisions. Compared with pp collisions, charged-particle yields in Pb+Pb collisions show significant modifications as a function of charged-particle p_{T} in a way that depends on event centrality and Z boson p_{T}. The data are compared with a variety of theoretical calculations and provide new information about the medium-induced energy loss of partons in a p_{T} regime difficult to measure through other channels.

Reconstruction of neutral-triggered charged recoil jets in √s = 200 GeV p+p collisions at the STAR experiment

Jets — collimated sprays of hadrons — are produced by hard-scattered partons during the early stages of heavy ion collisions. Hence, they provide a valuable probe of the complex multi-particle dynamics within the hot, dense medium produced in such collisions. In particular, the study of jets recoiling from direct photons (γdir+jet) and those recoiling from energetic π0 (π0+jet) may shed light on the path-length and initial flavor (quark vs. gluon) dependence of the energy-loss experienced by a parton as it traverses the medium. We present here measurements of the yields of charged recoil jets tagged by γdir and π0 in p+p collisions at √S = 200 GeV. These measurements will serve as a vacuum fragmentation reference for an upcoming measurement in Au + Au collisions.

Jet measurements in heavy ion physics

A hot, dense medium called a Quark Gluon Plasma (QGP) is created in ultrarelativistic heavy ion collisions. Hard parton scatterings generate high momentum partons that traverse the medium, which then fragment into sprays of particle called jets. Experimental measurements from high momentum hadrons, two particle correlations, and full jet reconstruction at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) continue to improve our understanding of energy loss in the QGP. Run 2 at the LHC recently began and there is a jet detector at RHIC under development. Now is the perfect time to reflect on what the experimental measurements have taught us so far, the limitations of the techniques used for studying jets, how the techniques can be improved, and how to move forward with the wealth of experimental data such that a complete description of energy loss in the QGP can be achieved. Measurements of jets to date clearly indicate that hard partons lose energy. Detailed comparisons of the nuclear modification factor between data and model calculations led to quantitative constraints on the opacity of the medium to hard probes. While there is substantial evidence for softening and broadening jets through medium interactions, the difficulties comparing measurements to theoretical calculations limit further quantitative constraints on energy loss mechanisms. We call for an agreement between theorists and experimentalists on the appropriate treatment of the background, Monte Carlo generators that enable experimental algorithms to be applied to theoretical calculations, and a clear understanding of which observables are most sensitive to the properties of the medium, even in the presence of background. This will enable us to determine the best strategy for the field to improve quantitative constraints on properties of the medium in the face of these challenges.

Measurements of charm hadron production and anisotropic flow in Au+Au collisions at 200 GeV with the STAR experiment at RHIC

Heavy flavor quarks, owing to their large masses, are predominantly produced through initial hard parton scatterings in heavy-ion collisions, and thus are excellent probes to study properties of the strongly coupled Quark Gluon Plasma (sQGP) medium produced in these collisions. Measurements of anisotropic flow harmonics of heavy flavor hadrons can provide information on the properties of the medium, including the heavy flavor transport coefficient. Charm quark hadronization mechanism in the sQGP medium can be studied through measurements of yields of different charm hadrons. In these proceedings we report on the measurements of elliptic and triangular flow harmonics of D0 mesons as well as the yield ratios of D±s/D0 and Λ±c/D0 in Au+Au collisions at [see formula in PDF] = 200 GeV at RHIC with the STAR detector. These measurements use the STAR Heavy Flavor Tracker (HFT) to reconstruct charm hadrons via their hadronic decay channels. Results are compared to model calculations and the implications on the understanding of charm quark dynamics in the medium are discussed.

Investigation of the properties of jets from p-Pb and Pb-Pb collisions with ALICE

Jets from hard scattering processes allow to study the properties of strongly interacting matter produced in ultra-relativistic heavy-ion collisions. The hot and dense medium created in such collisions is expected to cause energy loss of hard-scattered partons via elastic scattering and gluon radiation. Eventually, these processes modify the parton fragmentation. We report measurements of charged jets from lead-lead (Pb-Pb) and proton-lead (p-Pb) collisions at J= 2.76 TeV and 5.02 TeV. To estimate cold nuclear matter effects, the jet production in p-Pb collisions is studied for different centrality classes and is compared to that in proton-proton (pp) collisions via the nuclear modification factor. In addition, we discuss the measurement of (charged) jets recoiling from a high-pT trigger hadron, which allows to remove the contribution of combinatorial jets without introducing a bias on the jet population. Furthermore, we report about the measurement of strange hadrons (A, Kg) in association with charged jets from Pb-Pb and p-Pb collisions. The results are expected to clarify the role of the fragmentation process in the anomalous baryon-to-meson ratio observed at intermediate pT in A-A collisions. In particular, the measurement allows disentangling the contributions from jet fragmentation and other hadronisation processes.

Measurement of the dependence of transverse energy production at large pseudorapidity on the hard scattering kinematics of proton-proton collisions at [formula omitted] with the ATLAS detector

The relationship between jet production in the central region and the underlying event activity in a pseudorapidity-separated region is studied in 4.0 pb−1 of s=2.76 TeVpp events recorded with the ATLAS detector at the LHC. The hard scattering is characterised by the average transverse momentum and pseudorapidity of the two highest transverse momentum jets in the event. Results are also presented as a function of the scaled longitudinal momenta of the hard-scattered partons in the target and projectile beam-protons. Transverse energy production at large pseudorapidity is observed to vary strongly with the longitudinal momentum fraction in the target proton and only weakly with that in the projectile proton.

Measurement of the dependence of transverse energy production at large pseudorapidity on the hard-scattering kinematics of proton–proton collisions at s=2.76 TeV with ATLAS

The relationship between jet production in the central region and the underlying-event activity in a pseudorapidity-separated region is studied in 4.0 pb$^{-1}$ of $\sqrt{s} = 2.76$ TeV $pp$ collision data recorded with the ATLAS detector at the LHC. The underlying event is characterised through measurements of the average value of the sum of the transverse energy at large pseudorapidity downstream of one of the protons, which are reported here as a function of hard-scattering kinematic variables. The hard scattering is characterised by the average transverse momentum and pseudorapidity of the two highest transverse momentum jets in the event. The dijet kinematics are used to estimate, on an event-by-event basis, the scaled longitudinal momenta of the hard-scattered partons in the target and projectile beam-protons moving toward and away from the region measuring transverse energy, respectively. Transverse energy production at large pseudorapidity is observed to decrease with a linear dependence on the longitudinal momentum fraction in the target proton and to depend only weakly on that in the projectile proton. The results are compared to the predictions of various Monte Carlo event generators, which qualitatively reproduce the trends observed in data but generally underpredict the overall level of transverse energy at forward pseudorapidity.

Jet production in pp, p–Pb and Pb–Pb collisions measured by ALICE

Particle jets, formed when a hard scattered parton fragments into a jet of hadrons, are an ideal probe of the medium formed in heavy-ion collisions. The hard-scattered partons that produce them come from early in the collision, prior to the medium formation. These partons lose energy as they traverse the medium, and eventually fragment into jets of hadrons, which exhibit a modification when compared to jets produced in pp and p-Pb collisions. At LHC energies, the parton production cross-section is much larger than at RHIC, allowing jets to be reconstructed over a much wider kinematic range. Jet reconstruction allows for a more differential investigation of the parton energy loss than single hadrons, which have been used as jet proxies in the past, as the jets collect a larger percentage of the final state energy, which means their kinematics are more closely correlated to the kinematics of the initial parton.Jets are reconstructed in ALICE either using information from the tracking systems, or by combining this with the ALICE electromagnetic calorimeter (EMCal). In these proceedings, jet spectra from 2.76 TeV Pb-Pb and pp collisions will be presented. In particular, the centrality and event-plane dependence of the measured spectra and the background will be discussed. Jets from different centrality classes and event-plane orientations provide additional information necessary for understanding the path-length and temperature dependence of energy loss mechanisms. The reconstruction and correction procedures for jets will be shown. Results from Pb-Pb events will be compared to the baseline pp and p-Pb results, which allows the initial state and cold nuclear matter effects to be disentangled from hot medium effects. The jet nuclear modification, which quantifies the suppression, will be compared to energy-loss models.

Measurement of jet pT spectra and RAA in pp and Pb–Pb collisions at sNN=2.76 TeV with the ALICE detector

Hard-scattered partons provide an ideal probe for the study of the Quark–Gluon Plasma because they are produced prior to the formation of the QCD medium in heavy-ion collisions. Jet production is therefore susceptible to modifications induced by the presence of the medium (“jet quenching”). Both RHIC and LHC experiments have provided compelling evidence of jet quenching. Jets are reconstructed in ALICE utilizing the central tracking system for the charged constituents and the Electromagnetic Calorimeter for the neutral constituents. Jet spectra are reported for central (0–10%) and semi-central (10–30%) Pb–Pb events at s NN = 2.76 TeV . The nuclear modification factor, determined using a pp baseline measured at the same collisional energy, shows a strong suppression of jet production in central Pb–Pb collisions with the expected centrality ordering. Observations are in qualitative agreement with medium-induced energy loss models. Furthermore, indication of a path-length dependence of jet suppression is inferred from measurements of the yields relative to the orientation of the event plane.

Jet Quenching at RHIC and the LHC

Relativistic heavy ion collisions allow one to study Quantum Chromodynamics (QCD) under extreme temperature and density conditions. A new form of matter formed at energy densities above ~ 1 GeV/fm3, Quark-Gluon Plasma (QGP), is predicted in Lattice QCD calculations. One of the most interesting experimental signatures of QGP formation is jet-quenching due to in-medium energy loss when hard-scattered partons pass through the QGP. Direct jet reconstruction in heavy ion collisions has opened a new era of precision studies of jet-quenching. In this summary talk, recent jet measurements in heavy ion collisions and their implications are reviewed and discussed.

Two particle correlation measurements at PHENIX

Measurements of two particle azimuthal correlations in relativistic heavy ion collisions are useful tools to dissect the interplay between hard-scattered partons and hot dense medium. Correlations with trigger particle selection relative to second order event plane are sensitive to the path-length dependence of parton energy loss and to the influence of the medium on jet for high and intermediate transverse momenta pairs, respectively. To study the parton-medium coupling, it is also crucial to obtain correlations with rejection of contributions from higher harmonic flow. We present current results of second order event plane dependent correlations as well as correlations in which contributions from higher harmonic flow have been excluded in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV measured by PHENIX.

Suppression of high pT hadron spectra in p+A collisions

Multiple hard and semi-hard parton scatterings in high-energy $p+A$ collisions involve multi-parton correlation in both momentum and flavor inside the projectile proton which will lead to modification of the final hadron spectra relative to that in $p+p$ collisions. Such modification of final hadron transverse momentum spectra in $p+A$ collisions is studied within HIJING 2.1 Monte Carlo model which includes nuclear shadowing of the initial parton distributions and transverse momentum broadening. Multi-parton flavor and momentum correlation inside the projectile are incorporated through flavor and momentum conservation which are shown to modify the flavor content and momentum spectra of final partons and most importantly lead to suppression of large $p_{T}$ hadron spectra in $p+A$ collisions at both RHIC and LHC energies.

Reconstructed Jet Results in p + p, d + Au and Cu + Cu collisions at 200 GeV from PHENIX

Abstract Jet reconstruction in heavy ion collisions at RHIC and the LHC is becoming a popular tool to explore medium effects including the energy loss and modified fragmentation of hard-scattered partons. In p + A and d + A collisions, reconstructed jets are important for evaluating cold nuclear matter effects such as the impact parameter dependence of nuclear parton distribution functions and initial state energy loss. We present current PHENIX results from p + p, d + Au, and Cu + Cu collisions at 200 GeV using the Gaussian filter and anti- k T algorithms. The systematic study of direct jet reconstruction across a variety of collisions systems at PHENIX will help to tell a coherent story of jet physics at RHIC.

Jets and Jet-like correlations in STAR

The propagation and modification of hard-scattered partons in the QGP can be studied using various types of jet and jet-like correlation measurements. The STAR detector with its full azimuthal and large pseudorapidity acceptance, as well as its wide transverse momentum (pT) coverage, is well-suited for these measurements. At mid-rapidity, azimuthal correlations of charged hadrons with the axis of a reconstructed trigger jet are used to study the modification of jet shapes and associated hadron yields from p+p to Au+Au. Dihadron correlations with back-to-back high-pT hadron pairs are used to investigate dijets and fragmentation biases. STAR's increased particle identification capabilities due to the Time-Of-Flight detector are utilized to investigate the differences between jet-related and bulk-related particle production. Dihadron correlations with identified trigger particles provide experimental tests of simple recombination theories. The comprehensive set of STAR jet-quenching measurements can be used to further constrain theories of parton energy loss at RHIC.

Charged particle production in Pb–Pb collisions at the LHC with the ALICE detector

The ALICE collaboration measured charged particle production in $\sqrt{s_{NN}}=2.76$ TeV Pb--Pb collisions at the LHC. We report on results on charged particle multiplicity and transverse momentum spectra. All the results are presented as a function of the centrality of the collision, estimated with a Glauber Monte Carlo fit to multiplicity distributions reconstructed in various detectors. The applicability of the Glauber model at LHC energies, the precision of the centrality determination and the related systematic uncertainties are discussed in detail. Particles are tracked in the pseudorapidity window $|\eta| \lesssim 0.9$\ with the silicon Inner Tracking System (ITS) and the Time Projection Chamber (TPC), over the range $0.15 < \pt \lesssim 50$ GeV/$c$. The low-$p_t$ cut-off is further reduced in the multiplicity measurement using "tracklets", reconstructed in the 2 innermost layers of the ITS. The charged particle multiplicity is measured in $|\eta| < 0.5$ to be $\mathrm{d}N_{ch}/\mathrm{d}\eta = 1601 \pm 60$ in 5% most central Pb--Pb collisions, indicating an energy density a factor $\sim 3$ higher than at RHIC. Its evolution with centrality shows a pattern strikingly similar to the one measured at RHIC. Intermediate ($5 \lesssim \pt \lesssim 15$ GeV/$c$) transverse momentum particles are found to be most strongly suppressed with respect to pp collisions, consistent with a large energy loss of hard-scattered partons in the hot and dense medium. The results are presented in terms of the nuclear modification factor $R_{\mathrm{AA}}$ and compared to theoretical expectations.

Exploring cold nuclear matter effects in d + Au with high-pT reconstructed jets at PHENIX

Proton–nucleus (p + A) collisions can be used to investigate cold nuclear matter effects on hard-scattered partons and serve as an important baseline for heavy–ion collisions. In particular, p + A collisions at different centrality selections can probe the impact parameter dependence of nuclear parton distribution functions, initial state energy loss and final state parton interactions in the cold nucleus. Jet reconstruction can better determine the initial parton kinematics and recent improvements in analysis techniques allow the exploration of these effects over a wide p T range. We present the latest jet reconstruction measurements performed with the PHENIX detector at RHIC in deuteron–gold ( d + Au) collisions at 200 GeV using the Gaussian filter algorithm and discuss the possible implications on descriptions of cold nuclear matter.

Momentum dependences of charmonium properties from lattice QCD

Charmonia produced in initial hard parton scatterings during heavy ion collisions move with respect to the medium rather than flow with the medium. Lattice studies suggest that charmonium bound states at the rest are dissociated at $T\gtrsim 1.5 T_c$. We present results on momentum dependences of charmonium properties in a hot medium from lattice QCD Monte Carlo simulations. The dispersion relation of the screening mass and the change of correlation and spectral functions at various temperatures and momenta are discussed.

Reconstructed Jet Results in p+p, d+Au and Cu+Cu collisions at 200 GeV from PHENIX

Jet reconstruction in heavy ion collisions at RHIC and the LHC is becoming a popular tool for exploring medium effects including the energy loss and modified fragmentation of hard-scattered partons. In p+A and d+A collisions, reconstructed jets are important for evaluating cold nuclear matter effects such as the impact parameter dependence of nuclear parton distribution functions and initial state energy loss. We present current PHENIX results from p+p, d+Au, and Cu+Cu collisions at 200 GeV using the Gaussian filter and anti-kT algorithms. The systematic study of direct jet reconstruction across a variety of collisions systems at PHENIX will help to tell a coherent story of jet physics at RHIC.

Evolution ofπ0Suppression inAu+AuCollisions fromsNN=39to 200 GeV

Neutral-pion π(0) spectra were measured at midrapidity (|y|<0.35) in Au+Au collisions at √(s(NN))=39 and 62.4 GeV and compared with earlier measurements at 200 GeV in a transverse-momentum range of 1<p(T)<10 GeV/c. The high-p(T) tail is well described by a power law in all cases, and the powers decrease significantly with decreasing center-of-mass energy. The change of powers is very similar to that observed in the corresponding spectra for p+p collisions. The nuclear modification factors (R(AA)) show significant suppression, with a distinct energy, centrality, and p(T) dependence. Above p(T)=7 GeV/c, R(AA) is similar for √(s(NN))=62.4 and 200 GeV at all centralities. Perturbative-quantum-chromodynamics calculations that describe R(AA) well at 200 GeV fail to describe the 39 GeV data, raising the possibility that, for the same p(T) region, the relative importance of initial-state effects and soft processes increases at lower energies. The p(T) range where π(0) spectra in central Au+Au collisions have the same power as in p+p collisions is ≈5 and 7 GeV/c for √(s(NN))=200 and 62.4 GeV, respectively. For the √(s(NN))=39 GeV data, it is not clear whether such a region is reached, and the x(T) dependence of the x(T)-scaling power-law exponent is very different from that observed in the √(s(NN))=62 and 200 GeV data, providing further evidence that initial-state effects and soft processes mask the in-medium suppression of hard-scattered partons to higher p(T) as the collision energy decreases.