Two-particle Azimuthal Correlations Research Articles

Azimuthal Angle Correlations for Rapidity Separated Hadron Pairs ind+AuCollisions atsNN=200 GeV

Deuteron-gold (d+Au) collisions at the Relativistic Heavy Ion Collider provide ideal platforms for testing QCD theories in dense nuclear matter at high energy. In particular, models suggesting strong saturation effects for partons carrying small nucleon momentum fraction (x) predict modifications to jet production at forward rapidity (deuteron-going direction) in d+Au collisions. We report on two-particle azimuthal angle correlations between charged hadrons at forward/backward (deuteron/gold going direction) rapidity and charged hadrons at midrapidity in d+Au and p+p collisions at square root of sNN=200 GeV. Jet structures observed in the correlations are quantified in terms of the conditional yield and angular width of away-side partners. The kinematic region studied here samples partons in the gold nucleus with x~0.1 to ~0.01. Within this range, we find no x dependence of the jet structure in d+Au collisions.

Decomposition of harmonic and jet contributions to particle-pair correlations at ultrarelativistic energies

Methodology is presented for analysis of two-particle azimuthal angle correlation functions obtained in collisions at ultra-relativistic energies. We show that harmonic and di-jet contributions to these correlation functions can be reliably decomposed by two techniques to give an accurate measurement of the jet-pair distribution. Results from detailed Monte Carlo simulations are used to demonstrate the efficacy of these techniques in the study of possible modifications to jet topologies in heavy ion reactions.

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.

Erratum: Inclusive two-gluon and valence-quark–gluon production in DIS andpAcollisions [Phys. Rev. D70, 114017 (2004)

We calculate production cross sections of a forward quark-gluon pair and of two gluons at mid-rapidity in Deep Inelastic Scattering and in high energy proton-nucleus collisions. The calculation is performed in the framework of the Color Glass Condensate formalism. We first calculate the cross sections in the quasi-classical approximation, which includes multiple rescatterings in the target. We then proceed to include the effects of non-linear small-x evolution in the production cross sections. It is interesting to note that our result for the two-gluon production cross section appears to be in direct violation of AGK cutting rules, which is the first example of such violation in QCD. The calculated quark-gluon and gluon-gluon production cross sections can be used to construct theoretical predictions for two-particle azimuthal correlations at RHIC and LHC (I^{p(d)A}) as well as for Deep Inelastic Scattering experiments at HERA and eRHIC.

Semihard scattering unraveled from collective dynamics at

We present a study of elliptic flow (v2) and two-particle azimuthal correlations of charged hadrons and high transverse momentum (pT) pions measured by the CERES experiment in Pb+Au collisions at . Azimuthal anisotropy v2 increases linearly with pT to a value of 10% at pT = 1.5 GeV/c. Beyond this pT the slope decreases indicating a possible saturation at high pT. Two-pion azimuthal anisotropies (pT > 1.2 GeV/c) exceed v2 by about 60% in semicentral collisions and reveal non-harmonic contributions at close (Δϕ ≈ 0) and back-to-back (Δϕ ≈ π) angles that can be attributed to semihard processes. While the close-angle peak remains unchanged, the back-to-back peak is broadened and disappears in central collisions.

Modification of jet properties at the Relativistic Heavy Ion Collider

The two-particle azimuthal correlation function in p+p and Au+Au at is studied. It is shown that the away-side correlation width is a measure of the product of parton mean transverse momentum ⟨kT⟩ and the average momentum fraction of the trigger particle ⟨ztrig⟩. The phenomenological parametrization of the parton final-state spectrum is used for the estimation of the slope of the fragmentation function. Using the knowledge of the fragmentation function the ⟨kT⟩ values in p+p collisions are deduced. The broadening of the away-side peak in Au+Au collisions is discussed.

Inclusive two-gluon and valence-quark-gluon production in DIS andpAcollisions

We calculate production cross sections of a forward quark-gluon pair and of two gluons at midrapidity in deep inelastic scattering and in high energy proton-nucleus collisions. The calculation is performed in the framework of the color glass condensate formalism. We first calculate the cross sections in the quasiclassical approximation, which includes multiple rescatterings in the target. We then proceed to include the effects of nonlinear small-$x$ evolution in the production cross sections. It is interesting to note that our result for the two-gluon production cross section appears to be in direct violation of Abramovsky-Gribov-Kanchelli cutting rules, which is the first example of such violation in QCD. The calculated quark-gluon and gluon-gluon production cross sections can be used to construct theoretical predictions for two-particle azimuthal correlations at the Relativistic Heavy Ion Collider and LHC (${I}^{p(d)A}$) as well as for deep inelastic scattering experiments at the Hadron Electron Ring Accelerator and the Electron-Relativistic Heavy Ion Collider.

Elliptic flow contribution to two-particle correlations at different orientations to the reaction plane

Collective anisotropic particle flow, a general phenomenon present in relativistic heavy-ion collisions, can be separated from direct particle-particle correlations of different physics origin by virtue of its specific azimuthal pattern. We provide expressions for flow-induced two-particle azimuthal correlations, if one of the particles is detected under fixed directions with respect to the reaction plane. We consider an ideal case when the reaction plane angle is exactly known, as well as present the general expressions in case of finite event-plane resolution. We foresee applications for the study of generic two-particle correlations at large transverse momentum originating from jet fragmentation.

Semihard Scattering Unraveled from Collective Dynamics by Two-Pion Azimuthal Correlations in158A GeV/cPb+AuCollisions

Elliptic flow and two-particle azimuthal correlations of charged hadrons and high-p(T) pions (p(T)>1 GeV/c) have been measured close to midrapidity in 158A GeV/c Pb+Au collisions by the CERES experiment. Elliptic flow (v(2)) rises linearly with p(T) to a value of about 10% at 2 GeV/c. Beyond p(T) approximately 1.5 GeV/c, the slope decreases considerably, possibly indicating a saturation of v(2) at high p(T). Two-pion azimuthal anisotropies for p(T)>1.2 GeV/c exceed the elliptic flow values by about 60% in midcentral collisions. These nonflow contributions are attributed to nearside and back-to-back jetlike correlations, the latter exhibiting centrality dependent broadening.

Flow and non-flow event anisotropies at the SPS

A study of differential elliptic event anisotropies ( ν 2) of charge particles and high- p T pions in 158 AGeV/c Pb+Au collisions is presented. Results from correlations with respect to the event plane and from two-particle azimuthal correlations are compared. The latter give systematically higher ν 2 values at p T >1.2 GeV/c providing possibly an evidence of a non-flow semihard component.

Flow measurements via two-particle azimuthal correlations in Au + Au collisions at sqrt [s(NN)]=130 GeV.

Two-particle azimuthal correlation functions are presented for charged hadrons produced in Au+Au collisions at the Relativistic Heavy Ion Collider (sqrt [s(NN)]=130 GeV). The measurements permit determination of elliptic flow without event-by-event estimation of the reaction plane. The extracted elliptic flow values (v2) show significant sensitivity to both the collision centrality and the transverse momenta of emitted hadrons, suggesting rapid thermalization and relatively strong velocity fields. When scaled by the eccentricity of the collision zone epsilon, the scaled elliptic flow shows little or no dependence on centrality for charged hadrons with relatively low p(T). A breakdown of this epsilon scaling is observed for charged hadrons with pT >1.0 GeV/c.

Is the analysis of flow at the CERN Super Proton Synchrotron reliable?

Several heavy ion experiments at SPS have measured azimuthal distributions of particles with respect to the reaction plane. These distributions are deduced from two-particle azimuthal correlations under the assumption that they result solely from correlations with the reaction plane. In this paper, we investigate other sources of azimuthal correlations: transverse momentum conservation, which produces back-to-back correlations, resonance decays, HBT correlations and final state interactions. These correlations increase with impact parameter: most of them vary with the multiplicity N like 1/N. When they are taken into account, the experimental results of the NA49 collaboration at SPS are significantly modified. These correlations might also explain an important fraction of the pion directed flow observed by WA98. Data should be reanalyzed taking into account carefully these non--flow correlations.

Disappearance of rotational flow and reaction plane dispersions in Kr+Au collisions

Two-particle azimuthal correlations have been used to extract reaction plane dispersion free triple-differential cross sections for $d$, $t$, and $\ensuremath{\alpha}$ particles for the midcentral collisions of ${}^{84}\mathrm{Kr}{+}^{197}\mathrm{Au}$ at $E/A=35$, 55, and 70 MeV. Both experimental measurements and extrapolations from lower incident energies suggest that rotational flow disappears at $E/A\ensuremath{\approx}100$ MeV for light charged particles and that reaction plane dispersions introduce large uncertainties in extracting the disappearance of rotational flow.

Azimuthal distribution, azimuthal correlation, and reaction plane dispersion in the reaction 10.6 MeV/nucleon84Kr on27Al

The azimuthal distribution and azimuthal correlation of the emitted light charge particles in coincidence with the forward-going heavy fragments in the reaction of 10.6 MeV/nucleon ${}^{84}\mathrm{Kr}$ on ${}^{27}\mathrm{Al}$ have been studied in detail. A novel way to extract the reaction plane dispersion by combining the azimuthal distribution constructed using the experimental measured reaction plane which contains a reaction plane dispersion and a two-particle azimuthal correlation which is free of reaction plane dispersion was described. This method was applied to the low-energy fission reaction ${}^{84}\mathrm{Kr}$+${}^{27}\mathrm{Al}$ at 10.6 MeV/nucleon for light particles, $p$ and $\ensuremath{\alpha}$, with success.

Two-particle azimuthal and rapidity correlations in intervals of transverse momentum in π+ p-interactions at 250 GeV/c

A multi-dimensional analysis of two-particle correlations in π+p-interactions at 250 GeV/c shows interesting structure. Particularly strong positive shortrange rapidity and azimuthal correlations are observed for low-pT like sign pairs. This observation is not reproduced by models used for comparison (FRITIOF, DPM, quark gluon (multi)string model). A possible explanation is Bose-Einstein interference not included in these models.

Two-particle azimuthal correlations in light nuclei collisions at 4.2 GeV/c per nucleon.

Two-particle azimuthal correlations are studied in 4.2 GeV/c per nucleon dC, \ensuremath{\alpha}C, and CC collisions with a propane bubble chamber at JINR Dubna Synchrophasotron. It is found that the azimuthal correlations are different for different pairs of secondary particles, and that they also depend on the mass of the projectiles and the collision centrality. The majority of the observed characteristics can be accounted for by the kinematic correlations calculated from the model of independent nucleon-nucleon collisions. The only exception is the effect of close pairing of like particles considered to be due to the identical particle effect for pion pairs and to short-range strong interactions for proton pairs.

ON THE PROJECTILE MASS DEPENDENCE OF AZIMUTHAL CORRELATIONS IN INTERACTIONS WITH EMULSION TARGET NUCLEI

We have observed strong two-particle azimuthal correlations in different groups of charged particles in Kr-Em interactions at 1.4 A GeV . We also found a systematic growth of correlation with the increase of projectile mass and also with respect to participating target mass. The parameters studied are the asymmetry coefficients (A and <α>) and intergroup correlation parameter <Φij>. The colinearity coefficients (B and <β>) were also studied but no statistically significant signal, except for Z = 2 (He) fragments from the projectile was found.

Effect of Bose-Einstein statistics on two-particle azimuthal and rapidity correlations

The short-range effect in azimuthal and rapidity correlations can be explained within the independent cluster model taking into account the cluster transverse momentum and the Bose-effect (identical particle effect) in the single as well as the two-cluster term. Good agreement with data from Dubna, NAL and ISR is obtained. From the data we determine parameters related to the longitudinal and transverse dimensions of the clusters.

Two-Particle Azimuthal Correlations in High-Energy Collisions

The angular correlations between the transverse momenta of particles produced in high-energy collisions are studied. An explicit expression is derived for the so-called minimal azimuthal correlations, in terms of the total cross section and the single-particle distribution function. A simple model is also considered for illustration. Experimental distributions for several ${\ensuremath{\pi}}^{+}p$ reactions at 8.5 GeV/c and at 20 GeV/c are presented and compared with the minimal correlations.