Jet-like Correlations Research Articles

Addressing the physics of the ridge by 2- and 3-particle correlations at STAR

We present new results on 2-particle azimuthal (Δ ϕ) correlation relative to event plane and 3-particle pseudorapidity (Δ η) correlation at mid-rapidity in Au+Au collisions at s N N = 200 GeV , measured by the STAR experiment. While jet-like correlation is symmetric, ridge is found to be asymmetric when trigger particle azimuth is between in- and out-of-plane. The charge ordering properties between associated and trigger particles are exploited to separate jet-like and ridge contributions in 3-particle Δ η-Δ η correlations. We found that like-sign triplets are dominated by ridge. The separated ridge, while narrow in Δ ϕ, is extremely broad in Δ η. The ridge particles are not only uncorrelated to the trigger particle in Δ η, but also uncorrelated between themselves.

Modification of jet-like correlations in Pb-Au at the SPS

A study of high- p t two-particle azimuthal correlations in Pb-Au at S NN = 17 , 2 GeV by the CERES experiment at the CERN-SPS, is presented. A broad away-side correlation with significant dip at Δ ϕ ≈ π is observed. The shape and magnitude of the correlation is similar to measurements at RHIC. In comparison to PYTHIA calculations, we observe a significant excess of soft particles at the away-side. A study of charge correlations between trigger and associated particles disfavors vacuum fragmentation of the away-side jet and suggests significant energy transfer of the hard-scattered parton to the medium.

Modification of jet-like correlations in Pb–Au collisions at [formula omitted

Results of a two-particle correlation analysis of high-pt charged particles in Pb–Au collisions at 158A GeV/c are presented. The data have been recorded by the CERES experiment at the CERN-SPS. The correlations are studied as function of transverse momentum, particle charge and collision centrality. We observe a jet-like structure in the vicinity of a high-pt trigger particle and a broad back-to-back distribution. The yields of associated particles per trigger show a strong dependence on the trigger/associate charge combination. A comparison to PYTHIA confirms the jet-like pattern at the near-side but suggests a strong modification at the away-side, implying significant energy transfer of the hard-scattered parton to the medium.

Cumulant versus jet-like three-particle correlations

Two-particle jet-like azimuthal correlations have revealed intriguing modifications to the away-side of high ${p}_{\ensuremath{\perp}}$ trigger particles in relativistic heavy-ion collisions. Three-particle jet-like azimuthal correlation and three-particle azimuthal cumulant have been analyzed in experiments in attempt to distinguish conical emission of jet-correlated particles from other physics mechanisms. We investigate the difference between three-particle jet-like correlation and three-particle cumulant in azimuth. We show, under the circumstance where the away-side two-particle correlation is relatively flat in azimuth and similar in magnitude to the azimuthal average of the two-particle correlation signal, that the three-particle cumulant cannot distinguish conical emission from other physics mechanisms. The three-particle jet-like correlation, on the other hand, retains its discrimination power.

Azimuthal anisotropy and formation of an extreme state of strongly interacting matter at the relativistic heavy-ion collider (RHIC)

Experimental results obtained by studying the azimuthal anisotropy of final states in nucleus-nucleus interactions at the energies of the relativistic heavy-ion collider (RHIC) are systematized. The medium is found to exhibit a pronounced collective behavior, which is likely to be formed at an early, parton, stage of the spacetime evolution of product hot and dense matter. Experimental data on the azimuthal anisotropy indicate that strongly interacting matter produced in the final state under extreme conditions behaves as a nearly ideal liquid rather than an ideal gas of quarks and gluons. The experimentally observed suppression of high-transverse-momentum jets and substantial modification of jetlike azimuthal correlations in heavy-ion collisions suggest that the energy loss of partons propagating in high-temperature matter featuring a high density of color charges is extremely large. The dependence of the amount of hardjet suppression in nucleus-nucleus collisions on the orientation of a jet with respect to the reaction plane was first discovered experimentally at RHIC. A strong suppression of the production of high-transverse-momentum particles and jets at RHIC is a unique phenomenon, which was discovered experimentally at lower energies.

Conical emission in heavy-ion collisions

A broadened or double humped away-side structure was observed in 2-particle azimuthal jet-like correlations at RHIC and SPS. This modification can be explained by conical emission, from either Mach-cone shock waves or Čerenkov gluon radiation, and by other physics mechanisms, such as large angle gluon radiation, jets deflected by radial flow and path-length-dependent energy loss. Three-particle jet-like correlations are studied for their power to distinguish conical emission from other mechanisms. This paper discusses Mach-cone shock waves, Čerenkov gluon radiation and the experimental evidence for conical emission from RHIC and SPS.

System size dependence of di-hadron correlations and fragmentation functions in Cu+Cu and Au+Au collisions at GeV

We present a study of di-hadron jet-like correlations in Cu+Cu and Au+Au collisions in an attempt to investigate the path length dependence of energy loss. The near- and away-side jet yields as a function of number of participants are investigated. The results are found to be equal within errors for the two systems at similar Npart. The di-hadron fragmentation functions are found to be unmodified on the near side, whereas on the away side they are suppressed but have the same shape in the heavy ion collisions as in d+Au collisions.

Analysis method for jet-like three-particle azimuthal correlations

Jet-like three-particle azimuthal correlations can discriminate various physical scenarios that have been proposed to explain the observed strong modification to two-particle azimuthal correlations. The three-particle correlation analysis is notoriously difficult in heavy-ion collisions due to the large combinatorial backgrounds. We describe the general idea behind the jet-like three-particle azimuthal correlation analysis, with emphasis on the subtraction of the combinatorial backgrounds. We discuss in detail various sources of systematic uncertainties in such an analysis.

System size dependence of strange particle correlations in Cu + Cu and Au + Au collisions at 200 GeV in STAR at RHIC

Two-particle azimuthal correlations with strange trigger particles (K0S, Λ, Ξ) are presented in Cu + Cu and Au + Au collisions at 200 GeV in STAR at RHIC. The near-side associated yield is investigated as a function of centrality, pT, and strangeness content of the trigger to investigate possible flavour or meson/baryon differences. The system size dependence of both the jet-like correlations and the long-range pseudorapidity correlations is studied to give insight into the origin of these two components.

Baryon to meson ratios on the near and away side of jets and their centrality dependence at STAR

We measure relative abundances of K0S, Λ and in near-side and away-side cones correlated with triggered high-pT particles in 197Au + 197Au collisions at = 200 GeV. The centrality dependence of identified particles in the triggered particle cones is also presented. Particle yields and ratios are extracted on the near side and away side of the trigger particle. The associate-particle ratios are studied as a function of the angle relative to the trigger particle azimuth Δϕ. Such studies should help elucidate the origin of the modifications in the jet-like correlations observed in Au+Au collisions relative to p+p collisions. And these studies will also help us in understanding the variation of local parton densities at the away side. We discuss how these measurements might be related to several scenarios for interactions of fast partons with the medium created in Au+Au collision.

Jet-like correlations between forward- and mid- rapidity in p+p, d+Au and Au+Au collisions from STAR at

In these proceedings we present STAR measurements of two-particle azimuthal correlations between trigger particles at mid-rapidity (|η| < 1) and associated particles at forward-rapidities (2.7 <|η|< 3.9) in p+p, d+Au and Au+Au collisions at . Two-particle azimuthal correlations between a mid-rapidity trigger particle and forward-rapidity-associated particles preferably probe large-x quarks scattered off small-x gluons in RHIC collisions. Comparison of the separate d- and Au-side measurements in d+Au collisions may potentially probe gluon saturation and the presence of colour glass condensate. In Au+Au collisions quark energy loss can be probed at large rapidities, which may be different from gluon energy loss measured at mid-rapidity.

In-medium properties of jets

Modifications of jet-like azimuthal correlations have revealed novel properties of the medium created in relativistic heavy-ion collisions. Experimental results on jet-like 2- and 3-particle correlations, specifically ‘punch-through’ at high transverse momentum, broadening at low and modest transverse momentum, and particularly the possible experimental evidence for conical flow, are reviewed. Future prospects of jet-like correlations and their physics potential are discussed.

Jet-Like Correlations between Forward and Mid-Rapidity

Mid-rapidity azimuthal correlations probe di-jets originating mainly from gluon-gluon hard-scattering. Measurements of such correlations have revealed significant (gluon-)jet modification in central Au+Au collisions. Azimuthal correlations at forward rapidity with a mid-rapidity high- p T hadron, on the other hand, are sensitive primarily to quark-gluon hard-scattering and may probe quark-jet modification in nuclear medium. They may be also sensitive to the Color Glass Condensate by separating d-side and Au-side in d+Au collisions. We present the first results on correlations of charged hadrons at forward rapidity in the STAR FTPCs ( 2.7 < | η | < 3.9 , p T < 2 GeV / c ) with high- p T charged hadrons at mid-rapidity from the main TPC ( | η | < 1 , p T > 3 GeV / c ) . Preliminary results from pp, d+Au, and Au+Au collisions at s NN = 200 GeV are compared. Implications of the results are discussed.

Multiplicity distributions inside parton cascades developing in a medium

The explanation of the suppression of high-pT hadron yields at RHIC in terms of jet-quenching implies that the multiplicity distributions of particles inside a jet and jet-like particle correlations differ strongly in nucleus–nucleus collisions at RHIC or at the LHC from those observed at e+e- or hadron colliders. We present a framework for describing the medium-induced modification, which has a direct interpretation in terms of a probabilistic medium-modified parton cascade, and which treats leading and subleading partons on an equal footing. We show that our approach can account for the strong suppression of single inclusive hadron spectra measured in Au–Au collisions at RHIC, and that this implies a characteristic distortion of the single inclusive distribution of soft partons inside the jet. We determine, as a function of the jet energy, to what extent the soft fragments within a jet can be measured above some momentum cut.

Leading-Particle Suppression and Surface Emission in Nucleus–Nucleus Collisions

After a short summary of the predictions of the Parton Quenching Model (PQM) for the nuclear modification factor and its centrality dependence in Au−Au collisions at RHIC, we concentrate on back-to-back jet-like correlations at high transverse momentum. We illustrate how this probe is biased by the surface effect.

Multiplicity distributions for jet parton showers in a medium

The “jet-quenching” interpretation of suppressed high- p T hadron production at RHIC implies that jet multiplicity distributions and jet-like particle correlations in heavy-ion collisions at RHIC and LHC differ strongly from those seen at e + e − or p p ( p p ¯ ) colliders. Here, we present an approach for describing the changes induced by the medium, which implements jet quenching as a probabilistic medium-modified parton shower, treating leading and subleading parton splittings on an equal footing. We show that the strong suppression of single inclusive hadron spectra measured in Au–Au collisions at RHIC implies a characteristic distortion of the single inclusive distribution of soft partons inside the jet. We determine, as a function of jet energy, to what extent the soft jet fragments can be measured above some momentum cut.

Dissipation and Fragmentation of Low- Q 2 Scattered Partons in Au–Au Collisions at RHIC

Two-particle correlations and event-wise fluctuations in transverse momentum p_t are reported for Au-Au collisions at sqrt{s_{NN}} = 62 and 200 GeV on pseudorapidity (eta) and azimuth (phi). Distributions of all pairs of particles (no leading trigger particle) reveal jet-like correlations, or peaks at pair-wise opening angles of order 1 radian or less. The width of this same-side correlation peak increases dramatically on pseudorapidity and decreases on azimuth for increasing collision centrality. Evolution of the same-side peak with centrality suggests dissipation of low-Q^2 partons via strong coupling to an expanding bulk medium. p_t correlations, which provide access to temperature and/or velocity distributions in the colliding system, are also presented.

STAR Highlights

We present selected results from the STAR collaboration at RHIC. Recent results on jet-like correlations and nuclear modification factors of identified hadrons at intermediate and transverse momentum are discussed. The expansion of the created system is characterized by the measurements of two particle interferometry and the transverse elliptic flow of multi-strange baryons Ξ and Ω.

Semihard scattering unraveled from collective flow at the SPS

A study of elliptic flow and two-particle azimuthal correlations of charged particles (\( 0.5 \) 1.2 GeV/c exceed the v2 values by about 60\(\%\) in semicentral collisions. This non-flow component is attributed to near-side and away-side jetlike correlations. While the near-side peak remains constant with centrality 0.23\(\pm\)0.03 rad, as expected for fragmentation, the away-side peak experiences broadening and disappears in central collisions.

Studying intermediate pT hadron production with fluctuations

Mechanisms for particle production at intermediate pT in nuclear collisions at RHIC are discussed, emphasizing the differences in associated jet-like correlations between colour-neutral and coloured production. An alternative production mechanism involving both recombination and fragmentation is suggested, which might simultaneously lead to an enhancement of baryons and to jet-like correlations. To gain more insight into the relative importance of different mechanisms a study of constrained distributions of associated multiplicity is proposed. In a simple model it is shown that these multiplicity distributions may change significantly, if the nature of the production mechanism fluctuates from event to event.