1. Directed Flow Measurements at STAR
Aihong Tang for the Collaboration
Aihong Tang Moriond QCD March 2011

2. A few useful concepts in heavy ion collisionsx y z
Reaction plane : Defined by the beam
and the line connecting two colliding nuclei
Peripheral collisions : collisions in which
two nuclei barely graze each other.
Central collisions : head-on collisions.
Aihong Tang Moriond QCD March 2011

3. Definitions: directed flow (v1), elliptic flow (v2)X Z
XZ – the reaction plane
Picture: © UrQMD
rapidity
<px> or v1=<px/pt> x y z py px
initial spatial anisotropy
anisotropy in momentum space
directed
elliptic
isotropic
It is a convention to
define the sign of v1
of spectators at forward
rapidity as positive.
Aihong Tang Moriond QCD March 2011

4. Anti-flow / 3rd flow component
8 A GeV
flow
antiflow
Brachmann, Soff, Dumitru, Stocker, Maruhn, Greiner Bravina, Rischke , PRC 61 (2000)
L.P. Csernai, D. Roehrich PLB 458, 454 (1999) M.Bleicher and H.Stocker, PLB 526,309(2002)
Anti-flow/3rd flow component : Flat v1 at midrapidity due to 1st order phase transition
Caution : Seeing anti-flow does not necessarily mean that there is a QGP EoS. (refer to UrQMD). In following slides, anti-flow only means zero or negative slope at midrapidity, due to the fast expansion of a tilted source.
Aihong Tang Moriond QCD March 2011

5. Aihong Tang Moriond QCD March 2011
v1 from baryon stopping
RQMD v2.4
AuAu 200 GeV
R.Snellings, H.Sorge, S.Voloshin, F.Wang, N. Xu, PRL (84) 2803(2000)
Baryon stopping + positive space-momentum correlation  v1 wiggle.
No QGP necessary
Aihong Tang Moriond QCD March 2011

6. v1 and early thermalization
Piotr Bozek and Iwona Wyskiel-Piekarska, PRC 83, (2011)
Sensitive to the tilt, the initial time (not shown) and the relaxation time.
Aihong Tang Moriond QCD March 2011

7. Aihong Tang Moriond QCD March 2011
STAR Experiment
EMC Barrel
MRPC ToF Barrel
EMC End Cap
FMS
BBC
FPD
TPC
ZDC &
ZDCSMD
ZDC &
ZDCSMD
FTPC
PMD
Aihong Tang Moriond QCD March 2011

8. v1 at RHIC, viewed with η/ybeam
Preliminary
Clear patterns for spectators and “transported + produced” particles.
Aihong Tang Moriond QCD March 2011

9. v1 at RHIC, viewed in η-ybeam
Preliminary
Phobos, PRL (2006)
Similar if viewed in the projectile frame – Limiting Fragmentation Hypothesis (It remains as a hypothesis, unfortunately !)
Aihong Tang Moriond QCD March 2011

10. v1 at RHIC, system size independence
Piotr Bozek and Iwona Wyskiel-Piekarska, PRC 81, (2010)
STAR, PRL (2008)
AMPT failed to describe data. Hydro+”anti-flow” works (but failed in describing PID flow – see later).
Aihong Tang Moriond QCD March 2011

11. Aihong Tang Moriond QCD March 2011
PID v1 at RHIC
Preliminary
Anti-proton slope has the same sign of pions – consistent with anti-flow
Kaon suffers less shadowing effect due to smaller k/p cross section, yet we found negative v1 slope for Kshort – consistent with anti-flow
Aihong Tang Moriond QCD March 2011

12. Aihong Tang Moriond QCD March 2011
PID v1 at RHIC
AuAu 200 GeV
Preliminary
Preliminary
Difference seen between v1 of protons and anti-protons in 5-30% central collisions.
No difference is seen in “Hydro + anti-flow” model (not shown).
Aihong Tang Moriond QCD March 2011

13. Centrality dependence of v1 slope
AuAu 200 GeV
Preliminary
Negative v1 slope for protons is observed in 30-80% centralities.
Large difference seen between v1 of protons and anti-protons in 5-30% centralities.
Anti-flow alone cannot explain the data.
Aihong Tang Moriond QCD March 2011

14. Proton v1 Excitation Function
Preliminary
5-10% /-0.1(stat)+/-0.1(sys) fm
10-20% 5.9 +/-0.2(stat)+/-0.2(sys) fm
20-30% 7.6 +/- 0.2(stat)+/-0.2(sys) fm
Rapidity window used in STAR : [-0.6,0.6]
Proton v1 in mid-central collisions at RHIC stays small
Aihong Tang Moriond QCD March 2011

15. Aihong Tang Moriond QCD March 2011
Summary
Directed flow probes the early dynamics.
Directed flow has been measured by STAR over a wide η/ybeam range at
different energies. The directed flow is found to be independent of
system size, and its value at forward region is confirmed to scale with
η-ybeam for a wide energy range.
Anti-flow alone cannot explain the difference in v1 between protons and
anti-protons.
So far no models can explain all features simultaneously.
Aihong Tang Moriond QCD March 2011

16. Aihong Tang Moriond QCD March 2011
BackUp
Aihong Tang Moriond QCD March 2011

17. Aihong Tang Moriond QCD March 2011
v1 at low energies
6 A GeV Au+Au
e895, PRL (2000)
E895, PRL (2001)
Aihong Tang Moriond QCD March 2011

18. Aihong Tang Moriond QCD March 2011
v1 at low energies
peripheral
mid-central
central
NA49, NPA 715, 583 (2003)
Aihong Tang Moriond QCD March 2011

19. Data set for PID v1 analysis
Run7 , 62 M events.
Preliminary
Preliminary
Tracks used for TPC :
No. of fit hits ∈[15, 50]
Global DCA∈[0.0, 1.0]
No. of fit hits/No.of possible hits
∈[0.52, 1.05]
0.1 < pT < 12.0 GeV/c | η | < 1.
PID achieved by TPC dE/dx
Pt cut for protons/anti-protons [0.4,1 GeV/c]
Aihong Tang Moriond QCD March 2011

20. Aihong Tang Moriond QCD March 2011
Pion v1 at RHIC
Au+Au 200GeV 10%-70%
RQMD: Phys. Rev. Lett. 84 (2000) 2803;
UrQMD: Phys. Lett. B 526 (2002) 309;
AMPT: Phys. Rev. C 81, (2010);
QGSM+ Partonic recombination:
Phys. Rev . C 76 (2007) ;
hydro + tilted source: Piotr Bo˙zek and Iwona Wyskiel, arXiv: [nucl-th] (2010).
Preliminary
Hydro+tilted source describes the data the best
Aihong Tang Moriond QCD March 2011

21. v1 and 1st order phase transition
Central
Peripheral
dv1/dy at y=0
Aihong Tang Moriond QCD March 2011