Anisotropic flow in AuAu and CuCuat 62 GeV and 200 GeV

Introduction Flow Fourier coeffs (Oldenburg talk + others) v1(y) structure ZDC upgrade Flow Study with ZDC-SMD Physics results v1 in 62 GeV AuAu, 10% - 70% centrality v1 in 62 GeV AuAu, centrality dependence v1 in 200 GeV AuAu, centrality dependence v2 in 200 GeV AuAu, 20% - 70% centrality v2 in 200 GeV CuCu, 20% - 60% centrality v2 in 200 GeV CuCu, centrality dependence

Summary

Gang Wang (Kent State, Ohio) for the STAR Collaboration

23/4/19 Gang Wang 2

v1(y) structure

Models predict that anti-flow/3rd flow component, with QGP v1(y) flat or a wiggle structure.

L.P. Csernai, D. RohrichPLB 458, 454 (1999)

J. Brachmann et al.PRC 61, 024909 (2000)

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v1(y) structure

Models without QGP also predict a wiggle structure in peripheral or mid-peripheral events.

M. Bleicher and H. Stöcker, PLB 526, 309 (2002)

RQMD

uRQMD

R. Snellings, H. Sorge, S. Voloshin, F. Wang, N. Xu, PRL 84, 2803 (2000); also H. Liu et al., PRC 59, 348 (1999).

Baryon stopping and positive space-momentum correlation

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v1 - experimental complications

v1{3} -- the three-particle cumulant method;

v1{EP1, EP2} -- the event plane method with mixed harmonics;

v1{ZDC-SMD} -- the standard method with the 1st-order event plane determined from the sideward deflection (“bounce-off”) of spectator neutrons in the Shower Max detector (SMD) of the Zero Degree Calorimeters (ZDCs).

Directed flow is small at RHIC, especially in the central rapidity region, and analyses are easily confused by non-flow effects (azimuthal correlations not related to the reaction plane orientation).

To reduce sensitivity to non-flow [recent review in PRC 72, 014904 (2005) ]:

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STAR ZDC-SMD

SMD is 8 horizontal slats & 7 vertical slats located at 1/3 of the depth of the ZDC

• New knowledge of the direction of the impact ~parameter vector• Minimal, if any, non-flow effects• Minimal, if any, effects from flow fluctuations• Worse resolution than from TPC, but that ~disadvantage is minor

ZDC side view

Scintillator slats of Shower Max Detector

Transverse plane of

ZDC

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Flow study with ZDC-SMD: 4 terms

In analysis:

For example:

To systematically study the method with ZDC-SMD, we can use the sub event plane from only east or west ZDC-SMD, instead of the full event plane, and even break down the correlation into X and Y direction.

Definitions of 4 terms:

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Charged hadrons: v1 in 62 GeV Au +Au

STAR preliminaryAu +Au 62 GeV

Models do well at larger || but their v1() is too flat near =0

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Charged hadrons : v1 in 62 GeV Au +Au

STAR preliminary

directed flow of charged particles is opposite to spectator bounce-off at all centralities, and three methods agree.

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Au +Au 200 GeV

Charged hadrons: v1 in 200 GeV Au +Au

STAR

preliminar

y

NA49, Phys. Rev. C 68 (2003) 034903

Similar trends in AuAu v1() vs. centrality from SPS thru top RHIC energy; 200 GeV central collisions allow v1 sign-change to be probed

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Charged hadrons: v2 in 200 GeV Au +Au

Au +Au 200 GeV

STAR preliminary

Systematic check on v2 measurements using ZDC-SMD 1st harmonic event plane shows substantial non-flow effects in peripheral collisions.

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v2 – minimizing non-flow for CuCu analysis

In this talk, we use this subset of elliptic flow analysis methods to reduce sensitivity to non-flow for CuCu [recent review in PRC 72, 014904 (2005)]:

v2{4} -- four-particle cumulant; v2{AA-pp} -- the scalar product method;

v2{qdist} -- involves fit to distribution of q = flow vector Q /√M ; the fit contains a term to account for non-flow.

Sum over multiplicity, M

particle azim from given pt bin

assume can be obtained from pp

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Charged hadrons: v2 in 200 GeV CuCu

v2 for CuCu is smaller than for AuAu but large relative size of CuCu non-flow & divergence among methods at higher pt requires further investigation

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Charged hadrons: v2 in 200 GeV CuCu

Integrated v2(0.15 < pt < 2 GeV/c) again shows lower flow for CuCu & agreement on non-flow correction

23/4/19 Gang Wang 14

Summary

STAR ZDC-SMD allows the reaction plane to be determined with useful resolution from the “bounce-off” of spectator neutrons. This has been used to study charged particle v1 and v2 for 62 and 200 GeV AuAu & 200 GeV CuCu.

Charged particles in the -region covered by the STAR TPC and FTPCs (up to || = 4.0) predominantly have opposite directed flow from the spectator nucleons. For 62 GeV AuAu, several models agree well in FTPC region, but their v1() is too flat at central pseudorapidities.

62 & 200 GeV AuAu show a complex structure in v1 vs. vs. centrality, and present a major challenge to models. 200 GeV central collisions allow v1 sign change near = 3.5 to 4 to be probed. For 200 GeV AuAu, v2{ZDC-SMD} allows nonflow-corrected elliptic flow to be extended to more peripheral centralities.

The relative importance of non-flow is greater for CuCu analysis than for AuAu – initial v2 estimates for 200 GeV CuCu based on v2{4}, v2{CuCu-pp} and v2{qdist} show reasonable consistency across broad regions of centrality and pT .

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• Backup Slides

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Limiting Fragmentation in Au +Au

The hypothesis holds here.

It has been observed that particle emission(both spectra and flow) as a function of rapidity in the vicinity of beam rapidity appears unchanged over a wide range of beam energies, a pattern known as limiting fragmentation.

STAR preliminary

ycm = 0