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Register a new userIsolation of Flow and Nonflow Correlations by Two- and Four-Particle Cumulant Measurements of Azimuthal Harmonics in $sqrt{s_{_{rm NN}}} =$ 200 GeV Au+Au Collisions
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A data-driven method was applied to measurements of Au+Au collisions at $sqrt{s_{_{rm NN}}} =$ 200 GeV made with the STAR detector at RHIC to isolate pseudorapidity distance $Deltaeta$-dependent and $Deltaeta$-independent correlations by using two- and four-particle azimuthal cumulant measurements. We identified a component of the correlation that is $Deltaeta$-independent, which is likely dominated by anisotropic flow and flow fluctuations. It was also found to be independent of $eta$ within the measured range of pseudorapidity $|eta|<1$. The relative flow fluctuation was found to be $34% pm 2% (stat.) pm 3% (sys.)$ for particles of transverse momentum $p_{T}$ less than $2$ GeV/$c$. The $Deltaeta$-dependent part may be attributed to nonflow correlations, and is found to be $5% pm 2% (sys.)$ relative to the flow of the measured second harmonic cumulant at $|Deltaeta| > 0.7$.
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We present measurements of elliptic and triangular azimuthal anisotropy of charged particles detected at forward rapidity $1<|eta|<3$ in Au$+$Au collisions at $sqrt{s_{_{NN}}}=200$ GeV, as a function of centrality. The multiparticle cumulant technique is used to obtain the elliptic flow coefficients $v_2{2}$, $v_2{4}$, $v_2{6}$, and $v_2{8}$, and triangular flow coefficients $v_3{2}$ and $v_3{4}$. Using the small-variance limit, we estimate the mean and variance of the event-by-event $v_2$ distribution from $v_2{2}$ and $v_2{4}$. In a complementary analysis, we also use a folding procedure to study the distributions of $v_2$ and $v_3$ directly, extracting both the mean and variance. Implications for initial geometrical fluctuations and their translation into the final state momentum distributions are discussed.
We present two-dimensional (2D) two-particle angular correlations on relative pseudorapidity $eta$ and azimuth $phi$ for charged particles from Au-Au collisions at $sqrt{s_{rm NN}} = 62$ and 200 GeV with transverse momentum $p_t geq 0.15$ GeV/$c$, $|eta| leq 1$ and $2pi$ azimuth. Observed correlations include a {same-side} (relative azimuth $< pi/2$) 2D peak, a closely-related away-side azimuth dipole, and an azimuth quadrupole conventionally associated with elliptic flow. The same-side 2D peak and away-side dipole are explained by semihard parton scattering and fragmentation (minijets) in proton-proton and peripheral nucleus-nucleus collisions. Those structures follow N-N binary-collision scaling in Au-Au collisions until mid-centrality where a transition to a qualitatively different centrality trend occurs within a small centrality interval. Above the transition point the number of same-side and away-side correlated pairs increases rapidly {relative to} binary-collision scaling, the $eta$ width of the same-side 2D peak also increases rapidly ($eta$ elongation) and the $phi$ width actually decreases significantly. Those centrality trends are more remarkable when contrasted with expectations of jet quenching in a dense medium. Observed centrality trends are compared to {sc hijing} predictions and to the expected trends for semihard parton scattering and fragmentation in a thermalized opaque medium. We are unable to reconcile a semihard parton scattering and fragmentation origin for the observed correlation structure and centrality trends with heavy ion collision scenarios which invoke rapid parton thermalization. On the other hand, if the collision system is effectively opaque to few-GeV partons the observations reported here would be inconsistent with a minijet picture.
Yields, correlation shapes, and mean transverse momenta pt{} of charged particles associated with intermediate to high-pt{} trigger particles ($2.5 < pt < 10$ GeVc) in d+Au and Au+Au collisions at $snn=200$ GeV are presented. For associated particles at higher $pt gtrsim 2.5$ GeVc, narrow correlation peaks are seen in d+Au and Au+Au, indicating that the main production mechanism is jet fragmentation. At lower associated particle $pt < 2$ GeVc, a large enhancement of the near- ($dphi sim 0$) and away-side ($dphi sim pi$) associated yields is found, together with a strong broadening of the away-side azimuthal distributions in Au+Au collisions compared to d+Au measurements, suggesting that other particle production mechanisms play a role. This is further supported by the observed significant softening of the away-side associated particle yield distribution at $dphi sim pi$ in central Au+Au collisions.
Dihadron azimuthal correlations containing a high transverse momentum ($p_T$) trigger particle are sensitive to the properties of the nuclear medium created at RHIC through the strong interactions occurring between the traversing parton and the medium, i.e. jet-quenching. Previous measurements revealed a strong modification to dihadron azimuthal correlations in Au+Au collisions with respect to p+p and d+Au collisions. The modification increases with the collision centrality, suggesting a path-length or energy density dependence to the jet-quenching effect. This paper reports STAR measurements of dihadron azimuthal correlations in mid-central (20-60%) Au+Au collisions at $sqrt{s_{_{rm NN}}}=200$ GeV as a function of the trigger particles azimuthal angle relative to the event plane, $phi_s=|phi_t-psi_{rm EP}|$. The azimuthal correlation is studied as a function of both the trigger and associated particle $p_T$. The subtractions of the combinatorial background and anisotropic flow, assuming Zero Yield At Minimum (ZYAM), are described. The correlation results are first discussed with subtraction of the even harmonic (elliptic and quadrangular) flow backgrounds. The away-side correlation is strongly modified, and the modification varies with $phi_s$, with a double-peak structure for out-of-plane trigger particles. The near-side ridge (long range pseudo-rapidity $Deltaeta$ correlation) appears to drop with increasing $phi_s$ while the jet-like component remains approximately constant. The correlation functions are further studied with subtraction of odd harmonic triangular flow background arising from fluctuations. It is found that the triangular flow, while responsible for the majority of the amplitudes, is not sufficient to explain the $phi_s$-dependence of the ridge or the away-side double-peak structure. ...
The azimuthal anisotropic flow of identified and unidentified charged particles has been systematically studied in Cu+Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV for harmonics $n=$ 1-4 in the pseudorapidity range $|eta|<1$. The directed flow in Cu+Au collisions is compared with the rapidity-odd and, for the first time, the rapidity-even components of charged particle directed flow in Au+Au collisions at $sqrt{s_{_{NN}}}$ = 200~GeV. The slope of the directed flow pseudorapidity dependence in Cu+Au collisions is found to be similar to that in Au+Au collisions, with the intercept shifted toward positive $eta$ values, i.e., the Cu-going direction. The mean transverse momentum projected onto the spectator plane, $langle p_xrangle$, in Cu+Au collision also exhibits approximately linear dependence on $eta$ with the intercept at about $etaapprox-0.4$, closer to the rapidity of the Cu+Au system center-of-mass. The observed dependencies find natural explanation in a picture of the directed flow originating partly due the tilted source and partly due to the rapidity dependent asymmetry in the initial density distribution. Charge-dependence of the $langle p_xrangle$ was also observed in Cu+Au collisions, indicating an effect of the initial electric field created by charge difference of the spectator protons in two colliding nuclei. The rapidity-even component of directed flow in Au+Au collisions is close to that in Pb+Pb collisions at $sqrt{s_{_{NN}}}$ = 2.76 TeV, indicating a similar magnitude of dipole-like fluctuations in the initial-state density distribution. Higher harmonic flow in Cu+Au collisions exhibits similar trends to those observed in Au+Au and Pb+Pb collisions and is qualitatively reproduced by a viscous hydrodynamic model and a multi-phase transport model. For all harmonics with $nge2$ we observe an approximate scaling of $v_n$ with the number of constituent quarks.
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