Jet-like Correlations Research Articles

Jet-like correlations with respect to textrm{K}^{0}_mathrm{{S}} and Lambda (overline{Lambda }) in pp and central Pb–Pb collisions at mathbf {sqrt{s_textrm{NN}}} = 5.02 TeV

Two-particle correlations with textrm{K}^{0}_mathrm{{S}}, Lambda /overline{Lambda }, and charged hadrons as trigger particles in the transverse momentum range 8{<}p_{{textrm{T}},{textrm{trig}}}{<}16 GeV/c, and associated charged particles within 1{<}p_{{textrm{T}},{textrm{assoc}}}{<}8 GeV/c, are studied at midrapidity in pp and central Pb–Pb collisions at a centre-of-mass energy per nucleon–nucleon collision sqrt{s_{textrm{NN}}}~=~5.02 TeV with the ALICE detector at the LHC. After subtracting the contributions of the flow background, the per-trigger yields are extracted on both the near and away sides, and the ratio in Pb–Pb collisions with respect to pp collisions (I_{textrm{AA}}) is computed. The per-trigger yield in Pb–Pb collisions on the away side is strongly suppressed to the level of I_{textrm{AA}}approx 0.6 for p_{{textrm{T}},{textrm{assoc}}}>3 GeV/c as expected from strong in-medium energy loss, while an enhancement develops at low p_{{textrm{T}},{textrm{assoc}}} on both the near and away sides, reaching I_{textrm{AA}}approx 1.8 and 2.7 respectively. These findings are in good agreement with previous ALICE measurements from two-particle correlations triggered by neutral pions (pi ^{0}–h) and charged hadrons (h–h) in Pb–Pb collisions at sqrt{s_{textrm{NN}}}~=~2.76 TeV. Moreover, the correlations with textrm{K}^{0}_mathrm{{S}} mesons and Lambda /overline{Lambda } baryons as trigger particles are compared to those of inclusive charged hadrons. The results are compared with the predictions of Monte Carlo models.

Recent Highlights from the PHENIX Collaboration

These proceedings present a set of PHENIX highlight results using recent high-luminosity data sets for p+p, p (d, 3He)+A, and A+A(B) collisions at RHIC top energies with a focus on: (1) light-flavor particle productions; (2) hard-probes such as jet-like correlations, direct photon, and heavy-quark productions; and (3) azimuthal anisotropy in small collision systems.

Measurement of away-side broadening with self-subtraction of flow in Au+Au collisions at GeV * * This work was supported in part by the Office of Nuclear Physics within the U.S. DOE Office of Science, the U.S. National Science Foundation, the Ministry of Education and Science of the Russian Federation, National Natural Science Foundation of China, Chinese Academy of Science, the

High transverse momentum ( ) particle production is suppressed owing to the parton (jet) energy loss in the hot dense medium created in relativistic heavy-ion collisions. Redistribution of energy at low-to-modest has been difficult to measure, owing to large anisotropic backgrounds. We report a data-driven method for background evaluation and subtraction, exploiting the away-side pseudorapidity gaps, to measure the jetlike correlation shape in Au+Au collisions at GeV in the STAR experiment. The correlation shapes, for trigger particles and various associated particle ranges within , are consistent with Gaussians, and their widths increase with centrality. The results indicate jet broadening in the medium created in central heavy-ion collisions.

Data-driven subtraction of anisotropic flows in jet-like correlation studies in heavy-ion collisions

Measurements of two-particle azimuthal angle correlations are a useful tool to study the distribution of jet energy loss, however, they are complicated because of the significant anisotropic flow background. We devise a data-driven method for subtracting anisotropic flow background in jet-like correlation analysis. We first require a large recoil momentum ($P_x$) within a given pseudo-rapidity ($\eta$) range from a high-transverse momentum particle to enhance in-acceptance population of away-side jet-like correlations. Then we take the difference of two-particle correlations in the close-region and far-region with respect to the $\eta$ region of $P_x$ to subtract the anisotropic flow background. We use a toy model which contains only anisotropic flow and PYTHIA8 which have jets to demonstrate the validity of our data-driven method. The results indicate that the data-driven method can subtract anisotropic flow effectively.

Methods for separation of deuterons produced in the medium and in jets in high-energy collisions

Coalescence has long been used to describe the production of light (anti-)nuclei in heavy ion collisions. The same underlying mechanism may also exist in jets when a proton and a neutron are close enough in phase space to form a deuteron. We model deuteron production in jets by applying an afterburner to protons and neutrons produced in PYTHIA for $p$+$p$ collisions at a center of mass energy $\sqrt{s} =$ 7 TeV. PYTHIA provides a reasonable description of the proton spectra and the shape of the deuteron spectrum predicted by the afterburner is in agreement with the data. We show that the rise in the coalescence parameter $B_2$ with momentum observed in data is consistent with coalescence in jets. We show that di-hadron correlations can be used to separate the contributions from the jet and the underlying event. This model predicts that the conditional coalescence parameter in the jet-like correlation should be independent of the trigger momentum.

Multiplicity dependence of jet-like two-particle correlations in pp collisions at [formula omitted] and 13 TeV with ALICE

Two-particle correlations in relative azimuthal angle (Δϕ) and pseudorapidity (Δη) have been used to study heavy-ion collision dynamics, including medium-induced jet modification. Further investigations also showed the importance of Multiple Parton Interactions (MPI) in high-multiplicity pp collisions, which are often described by pQCD-inspired models. In these proceedings the latest ALICE measurements of two-particle correlations as a function of multiplicity in pp collisions are presented using the data from Run I and Run II at the LHC.

Collisional broadening of angular correlations in a multiphase transport model

Systematic comparisons of jetlike correlation data to radiative and collisional energy loss model calculations are essential to extract transport properties of the quark-gluon medium created in relativistic heavy ion collisions. This paper presents a transport study of collisional broadening of jetlike correlations, by following parton–parton collision history in a multiphase transport (AMPT) model. The correlation shape is studied as functions of the number of parton–parton collisions suffered by a high transverse momentum probe parton (Ncoll) and the azimuth of the probe relative to the reaction plane (ϕfin.probe). Correlation is found to broaden with increasing Ncoll and ϕfin.probe from in- to out-of-plane direction. This study provides a transport model reference for future jet-medium interaction studies.

Jet-like correlations with neutral pion triggers in pp and central Pb–Pb collisions at 2.76 TeV

We present measurements of two-particle correlations with neutral pion trigger particles of transverse momenta $8 < p_{\mathrm{T}}^{\rm trig} < 16~\mathrm{GeV}/c$ and associated charged particles of $0.5 < p_{\mathrm{T}}^{\rm assoc} < 10~\mathrm{GeV}/c$ versus the azimuthal angle difference $\Delta\varphi$ at midrapidity in pp and central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV with ALICE. The new measurements exploit associated charged hadrons down to $0.5~\mathrm{GeV}/c$, which significantly extends our previous measurement that only used charged hadrons above $3~\mathrm{GeV}/c$. After subtracting the contributions of the flow background, $v_2$ to $v_5$, the per-trigger yields are extracted for $|\Delta\varphi|<0.7$ on the near and for $|\Delta\varphi-\pi| < 1.1$ on the away side. The ratio of per-trigger yields in Pb--Pb to those in pp collisions, $I_{\mathrm{AA}}$, is measured on the near and away side for the $0$--$10$\% most central Pb--Pb collisions. On the away side, the per-trigger yields in Pb--Pb are strongly suppressed to the level of $I_{\mathrm{AA}} \approx 0.6$ for $p_{\mathrm{T}}^{\rm assoc} > 3~\mathrm{GeV}/c$, while with decreasing momenta an enhancement develops reaching about $5$ at low $p_{\mathrm{T}}^{\rm assoc}$. On the near side, an enhancement of $I_{\mathrm{AA}}$ between $1.2$ at the highest to $1.8$ at the lowest $p_{\mathrm{T}}^{\rm assoc}$ is observed. The data are compared to parton-energy-loss predictions of the JEWEL and AMPT event generators, as well as to a perturbative QCD calculation with medium-modified fragmentation functions. All calculations qualitatively describe the away-side suppression at high $p_{\mathrm{T}}^{\rm assoc}$. Only AMPT captures the enhancement at low $p_{\mathrm{T}}^{\rm assoc}$, both on the near and away side. However, it also underpredicts $I_{\mathrm{AA}}$ above $5$ GeV/$c$, in particular on the near-side.

Near-side azimuthal and pseudorapidity correlations using neutral strange baryons and mesons ind+Au, Cu + Cu, and Au + Au collisions atsNN=200GeV

We present measurements of the near-side of triggered di-hadron correlations using neutral strange baryons ($\Lambda$, $\bar{\Lambda}$) and mesons ($K^0_S$) at intermediate transverse momentum (3 $<$ $p_T$ $<$ 6 GeV/$c$) to look for possible flavor and baryon/meson dependence. This study is performed in $d$+Au, Cu+Cu and Au+Au collisions at $\sqrt{s_{{NN}}}$ = 200 GeV measured by the STAR experiment at RHIC. The near-side di-hadron correlation contains two structures, a peak which is narrow in azimuth and pseudorapidity consistent with correlations due to jet fragmentation, and a correlation in azimuth which is broad in pseudorapidity. The particle composition of the jet-like correlation is determined using identified associated particles. The dependence of the conditional yield of the jet-like correlation on the trigger particle momentum, associated particle momentum, and centrality for correlations with unidentified trigger particles are presented. The neutral strange particle composition in jet-like correlations with unidentified charged particle triggers is not well described by PYTHIA. However, the yield of unidentified particles in jet-like correlations with neutral strange particle triggers is described reasonably well by the same model.

Effect of radial flow on two particle correlations with identified triggers at intermediate p T in p–Pb collisions atsNN=5.02 TeV

Results from the two-particle correlation between identified triggers (pions ($\pi^{\pm}$) , protons (p/$\bar{p}$))) and un-identified charged particles at intermediate transverse momentum ($p_{T}$) in p-Pb collisions at $\sqrt{s_{NN}} = $ 5.02 TeV have been presented. The events generated from a hybrid Monte-Carlo event generator, EPOS 3.107 that implements a flux-tube initial conditions followed by event by event 3+1D viscous hydrodynamical evolution, have been analysed to calculate two-dimensional correlation functions in $\Delta\eta$-$\Delta\phi$. The strength of angular correlations at small relative angles (jet-like correlations), quantified in terms of near-side jet-like per-trigger yield has been calculated as a function of the event multiplicity. The yield associated with pion triggers exhibit negligible multiplicity dependence, while the proton-triggered yield shows a gradual suppression from low to high multiplicity events. In small collision systems like p-Pb where jet modification is expected to be less dominant, the observed suppression may be associated with the hydrodynamical evolution of the bulk system that generates an outward radial flow. Analogous measurements in Au-Au collisions at RHIC energy have shown a hint of dilution in proton-triggered correlation at its highest multiplicity suggesting that the soft physics processes in p-Pb and heavy ion collisions may have qualitative similarity.

Forward-central two-particle correlations in p–Pb collisions at [formula omitted

Two-particle angular correlations between trigger particles in the forward pseudorapidity range (2.5<|η|<4.0) and associated particles in the central range (|η|<1.0) are measured with the ALICE detector in p–Pb collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV. The trigger particles are reconstructed using the muon spectrometer, and the associated particles by the central barrel tracking detectors. In high-multiplicity events, the double-ridge structure, previously discovered in two-particle angular correlations at midrapidity, is found to persist to the pseudorapidity ranges studied in this Letter. The second-order Fourier coefficients for muons in high-multiplicity events are extracted after jet-like correlations from low-multiplicity events have been subtracted. The coefficients are found to have a similar transverse momentum (pT) dependence in p-going (p–Pb) and Pb-going (Pb–p) configurations, with the Pb-going coefficients larger by about 16±6%, rather independent of pT within the uncertainties of the measurement. The data are compared with calculations using the AMPT model, which predicts a different pT and η dependence than observed in the data. The results are sensitive to the parent particle v2 and composition of reconstructed muon tracks, where the contribution from heavy flavour decays is expected to dominate at pT>2 GeV/c.

Jet-quenching and correlations

This article reviews recent advances in our understanding of the experimental aspects of jet-quenching and correlations in relativistic heavy-ion collisions at RHIC and LHC. Emphasis is put on correlation measurements, namely jet-like correlations with anisotropic flow subtraction in heavy-ion collisions and long-range pseudorapidity correlations in small systems. Future path on correlation studies is envisioned which may elucidate jet–medium interactions and the properties of the hot dense medium in QCD.

Effect of event selection on jetlike correlation measurement in d + Au collisions at [formula omitted

Dihadron correlations are analyzed in sNN=200 GeVd+Au collisions classified by forward charged particle multiplicity and zero-degree neutral energy in the Au-beam direction. It is found that the jetlike correlated yield increases with the event multiplicity. After taking into account this dependence, the non-jet contribution on the away side is minimal, leaving little room for a back-to-back ridge in these collisions.

Multiplicity dependence of jet-like two-particle correlation structures in p–Pb collisions at [formula omitted

Two-particle angular correlations between unidentified charged trigger and associated particles are measured by the ALICE detector in p–Pb collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV. The transverse-momentum range 0.7<pT,assoc<pT,trig<5.0 GeV/c is examined, to include correlations induced by jets originating from low momentum-transfer scatterings (minijets). The correlations expressed as associated yield per trigger particle are obtained in the pseudorapidity range |η|<0.9. The near-side long-range pseudorapidity correlations observed in high-multiplicity p–Pb collisions are subtracted from both near-side short-range and away-side correlations in order to remove the non-jet-like components. The yields in the jet-like peaks are found to be invariant with event multiplicity with the exception of events with low multiplicity. This invariance is consistent with the particles being produced via the incoherent fragmentation of multiple parton–parton scatterings, while the yield related to the previously observed ridge structures is not jet-related. The number of uncorrelated sources of particle production is found to increase linearly with multiplicity, suggesting no saturation of the number of multi-parton interactions even in the highest multiplicity p–Pb collisions. Further, the number scales only in the intermediate multiplicity region with the number of binary nucleon–nucleon collisions estimated with a Glauber Monte-Carlo simulation.

Pseudorapidity dependence of long-range two-particle correlations in pPb collisions at 5.02 TeV with CMS

CMS measurements of two-particle correlations of charged particles have previously been published for proton–proton (pp), proton–lead (pPb), and lead–lead (PbPb) systems. This analysis studies the pseudorapidity dependency of the two-particle correlations by restricting one of the particles in a narrow η range. The near-side (Δϕ≈0) correlation in high-multiplicity collisions is decomposed into a short-range jet-like component and a long-range ridge component in pseudorapidity. The jet-like correlations are subtracted, and the remaining ridge correlations are studied versus Δη. Furthermore, the Fourier coefficients of the two-particle correlations are extracted for high-multiplicity collisions, with and without subtraction of jet contributions approximated by the scaled low-multiplicity collision data. The η dependence of the azimuthal anisotropy parameters is reported.

Search for “ridge” in d+Au collisions at RHIC by STAR

Dihadron correlations are measured in d+Au collisions at 200 GeV by the STAR detector. The correlated yields with uniform background subtraction are studied in high- and low-multiplicity collisions. The effects of multiplicity selection bias on jet-like correlations are discussed. Finite correlated yields are observed on the near-side at large pseudo-rapidity separation in high-multiplicity collisions.

Dihadron correlations in d+Au collisions from STAR

Dihadron correlations are reported for peripheral and central d+Au collisions at 200 GeV from STAR. The ZYAM background subtracted correlation yields are larger in central than peripheral collisions. The difference is mainly caused by centrality biases to jet-like correlations. Fourier coefficients of the raw dihadron correlations are also reported. It is found that the first harmonic coefficient is approximately inversely proportional to event multiplicity, whereas the second harmonic coefficient is approximately independent of event multiplicity.

Study of h-h and V0-h angular correlations in Pb-Pb collisions at = 2.76 TeV

Two-particle angular correlations provide a powerful tool to study jets and their modification in ultra-relativistic heavy-ion collisions. The study of the particle species dependence of correlation structures as a function of transverse momentum provides additional information on particle production mechanisms. In this contribution we present studies of a near-side jet-like correlation peak in h-h correlations in Pb-Pb collisions at = 2.76 TeV. We observe significant broadening of the jet-like peak that increases with decreasing ptrigT and passocT and with increasing centrality and additional broadening of the jet-like peak in Δη with respect to broadening in Δϕ in most central collisions.The prospects of expanding these studies to include strange particles (Λ, and K0S) is explored.

Very High-pT Triggered Dihadron Correlations in PbPb Collisions at 2.76 TeV with CMS

Measurements of dihadron correlations triggered by very high-pT particles in 2.76 TeV PbPb collisions are shown. The data set, which corresponds to an integrated luminosity of 150 μb−1, collected by the CMS detector is utilized. Long-range correlations are measured up to pT ~ 50 GeV/c. These correlations are driven by single-particle azimuthal anisotropies and characterized by the Fourier harmonics, vn. Once the V2 − v4 harmonic components are subtracted the associated particle yields on the near and away side of the jet-like correlations are studied. These measurements are done over a wide kinematic range in the associated and trigger particle pT and as a function of centrality. The data are compared to pp collisions at the same energy. A suppression of about 50% from the pp reference in the away-side associated yield is observed for passocT > 3 GeV/c. For lower momentums, passocT ~ 0.5 GeV/c, the yield is found to be enhanced significantly by up to a factor of 3–4 on the away-side. On the near side, there is some evidence of a moderate enhancement.

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.