Δη Correlations Research Articles

Measurement of elliptic and higher order flow from ATLAS experiment at the LHC

We present a differential measurement of the azimuthal anisotropy of charged hadron production in Pb+Pb collisions at = 2.76 TeV. This azimuthal anisotropy is expanded into a Fourier series in azimuthal angle, where the coefficient for each term, vn, characterizes the magnitude of the anisotropy at a particular angular scale. We extract v2–v6 via a discrete Fourier analysis of the two-particle Δϕ–Δη correlation with a large Δη gap (|Δη| > 2), and via an event-plane method based on the forward calorimeter. Significant v2–v6 values are observed over a broad range in , η and centrality, and they are found to be consistent between the two methods in the transverse momentum region GeV. This suggests that the measured v2–v6 obtained from two-particle correlations at low with a large Δη gap are consistent with the collective response of the system to the initial-state geometry fluctuations, and is not the result of jet fragmentation or resonance decay.

Leading hadron PID effects in di-hadron angular correlations from STAR

We present preliminary results of a di-hadron correlation analysis with identified triggers, measured as a function of relative azimuth (Δϕ) and pseudorapidity (Δη). The relativistic rise in the momentum dependence of the ionization energy loss of particles measured in the STAR TPC is used to statistically separate charged pions from kaons and protons. On the near side, the long range Δη correlation known as the ridge is revealed to have a strong trigger-type dependence, providing new constraints for its theoretical description. The jet-like cone structure above the ridge, while also trigger-dependent, shows little modification compared to d+Au measurements, challenging expectations from recombination models.

Untriggered di-hadron correlations in Pb–Pb collisions at TeV from ALICE

We present measurements of untriggered di-hadron correlations as a function of centrality in Pb–Pb TeV collisions, for charged hadrons with pT > 0.15 GeV/c. These measurements provide a map of the bulk correlation structures in heavy-ion collisions. Contributions to these structures may come from jets, initial density fluctuations, elliptic flow, resonances and/or momentum conservation. We decompose the measured correlation functions via a multi-parameter fit in order to extract the nearside Gaussian, the longer range Δη correlation often referred to as the soft ridge. The effect of including higher harmonics (v3 and v4) in this procedure will be discussed. We investigate how the nearside Gaussian scales with the number of binary collisions. Finally, we show the charge dependence of the nearside Gaussian.

Three-Particle Coincidence of the Long Range Pseudorapidity Correlation in High Energy Nucleus-Nucleus Collisions

We report the first three-particle coincidence measurement in pseudorapidity (Δη) between a high transverse momentum (p⊥) trigger particle and two lower p⊥ associated particles within azimuth |Δϕ|<0.7 in square root of s(NN)=200 GeV d+Au and Au+Au collisions. Charge ordering properties are exploited to separate the jetlike component and the ridge (long range Δη correlation). The results indicate that the correlation of ridge particles are uniform not only with respect to the trigger particle but also between themselves event by event in our measured Δη. In addition, the production of the ridge appears to be uncorrelated to the presence of the narrow jetlike component.

Identifying the underlying physics of the ridge via 3-particle Δη–Δη correlations

We present the first results on 3-particle Δη–Δη correlations in minimum bias d + Au, peripheral and central Au+Au collisions at = 200 GeV measured by the STAR experiment. The analysis technique is described in detail. The ridge particles, observed at large Δη in dihadron correlations in central Au+Au collisions, appear to be uniformly distributed over the measured Δη–Δη region in 3-particle correlation. The results, together with theoretical models, should help further our understanding of the underlying physics of the ridge.

High pT correlations with strange particles in STAR

We present the highlights of the current identified strange particles Δϕ and Δη correlations analyses, including system-size and trigger-p⊥ of the jet and ridge, jet, ridge and away-side meson/baryon ratios, and the current state of the multi-strange baryon analysis. We see clear azimuthal peaks of comparable strength for all strange baryons and K0S mesons. We see no observable species dependence on the same-side jet or ridge yields as a function of p⊥. However, while the away side and the ridge have the Λ0 to K0S ratio similar to that of the bulk, the jet-only ratio is similar to that in p+p. The implications of these findings on current in-medium jet theoretical explanations are discussed.

Near-side Δη correlations of high-pt hadrons from STAR

Systematic measurements of pseudorapidity (Δη) and azimuthal (Δϕ) correlations between high-pt charged hadrons and associated particles from the high statistics 200 GeV Au+Au and Cu+Cu datasets will be presented. In previous measurements differences in the near-side Δη correlation between central Au+Au and the lighter systems, d+Au and p+p, were observed, including a long-range near-side correlation in Au+Au collisions. Studies to characterize the properties of the long-range correlation will be presented.

Recent high pT measurements in STAR

After five years of data taking, the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory provides precise measurements of particle production at high transverse momentum in p-p, d-Au, and Au-Au collisions at √SNN = 200 GeV. We review recent results on the flavor dependence of high pT particle suppression and hadron particle spectra at √SNN = 62.4 GeV. New results on two-particle angular correlations for identified trigger particles and for low momentum associated charged hadrons in p-p and Au-Au as well as near-side .Δη correlations will be presented and discussed.