# Presentation: 123919

## Open Questions in the “Perfect Fluid”

**Open Questions in the “Perfect Fluid”**

**Aihong Tang**

- Aihong Tang                                           The Berkeley School 05

## Outline

*v*2/ scaling
-  “Elliptic flow” in pp and dAu
*v*1 wiggle
- Puzzle at intermediate pt
- Photon flow

- Aihong Tang                                           The Berkeley School 05

## v2/ scaling Universal curve?

***v*****2****/******** scaling**** ****Universal curve?**

- Aihong Tang                                           The Berkeley School 05

## v2/eccentricity : universal curve? 

- STAR, Phys. Rev. C(66), 034904 (2002)
- NA49,Phys. Rev. C(68), 034903 (2003)

- Aihong Tang                                           The Berkeley School 05

## “v2”in pp and dAu Don’t ask again, we have done it. Case closed.

**“*****v*****2****”in pp and dAu**** ****Don’t ask again, we have done it. Case closed.**

- Aihong Tang                                           The Berkeley School 05

## How does elliptic flow evolve from pp to dAu, and AuAu collisions ? 

**How does elliptic flow evolve from pp to dAu, and AuAu collisions ? **

- STAR, nucl-ex/0409033 PRC accepted.

- Aihong Tang                                           The Berkeley School 05

## How does elliptic flow evolve from pp to dAu, and AuAu collisions ? 

- STAR, nucl-ex/0409033 PRC accepted.

- Correlation in dAu increases as a function of multiplicity, which is a pattern that is opposite to that in AuAu collisions.

- In high multiplicity events, the Cronin effect is expected to produce more collective motion among soft particles in order to generate a particle with higher pt .

- Aihong Tang                                           The Berkeley School 05

## Directed Flow What wiggle #$!?

**Directed Flow**** ****What wiggle #$!?**

- Aihong Tang                                           The Berkeley School 05

## Wiggle : RQMD 

- R.Snellings, H.Sorge, S.Voloshin, F.Wang, N. Xu, PRL (84) 2803(2000)

- Positive space-momentum correlation, no QGP necessary  *v**1* wiggle.

- Aihong Tang                                           The Berkeley School 05

## Wiggle : uRQMD 

- Marcus Bleicher, Horst Stocker PLB 526, (2002) 309-314

- The non-interacting strings employed in the UrQMD approach result in a smaller space-momentum correlations and thus in a smaller “anti-flow” around midrapidity.

- Aihong Tang                                           The Berkeley School 05

## Wiggle : phase transition 

**Wiggle : phase transition **

- Brachmann, Soff, Dumitru, Stocker, Maruhn, Greiner Bravina, Rischke ,  PRC 61 (2000) 024909. L.P. Csernai, D. Roehrich PLB 458, 454 (1999)   M.Bleicher and H.Stocker, PLB 526,309(2002)

- Anti-flow/3rd flow component, with QGP  *v**1* flat at middle rapidity.

- Aihong Tang                                           The Berkeley School 05

## Wiggle : to summarize 

- _Nucleon wiggle,_
- Both RQMD and UrQMD give NO wiggle for central collisions (b~3fm), and both give a wiggle at peripheral collisions (b~8-11fm).
- Only the one-fluid hydro model gives a wiggle for central collisions but only IF there is a phase transition of first order.
- _Pion wiggle,_
- Different model give different results.

- Aihong Tang                                           The Berkeley School 05

## Chasing the wiggle : where we are? 

- Aihong Tang                                           The Berkeley School 05

## Chasing the wiggle : where we are? 

**Charged particles (mostly pions) !**

- STAR Preliminary

- Aihong Tang                                           The Berkeley School 05

## Puzzles at intermediate Pt Recombination and/or Energy loss, is that all?

**Puzzles at intermediate P****t**** ****Recombination and/or Energy loss, is that all?**

- Aihong Tang                                           The Berkeley School 05

## Puzzles at Itermediate pt

- Althogh R+F reproduces the centrality dependence, still it misses the magnitude. What is the uncertainty in models?

- A.Tang (for the STAR Collaboration) J. Phys. G : Nucl. Part. Phys. 31 s35-39 (2005)
- C. Nonaka et al., J. Phys. G : Nucl. Part. Phys. 31 s429-435 (2005)

- Aihong Tang                                           The Berkeley School 05

## Puzzles at Itermediate pt

- Can we at least get the sign of v4 right?

- STAR, nucl-ex/0409033 PRC accepted.

- Aihong Tang                                           The Berkeley School 05

## Does direct photon flow? Show me the light !

**Does direct photon flow?**** ****Show me the light !**

- Aihong Tang                                           The Berkeley School 05

## Does direct photon flow?

- Argument A: phtons are produced isotropically in the momentum space, they do free stream after being produced : v2 = 0 (widerly accepted, but never confirmed !)

- Argument B: Strong space-momentum correlation cause photons to be emitted out-of-plane : v2 < 0

- Aihong Tang                                           The Berkeley School 05

## Does direct photon flow?

- Phenix, nucl-ex/0503003

- Aihong Tang                                           The Berkeley School 05

## Summary

**Summary**

***v*****2****/******** scaling needs to be tested with different system sizes and collision energies.**
** ****There are some collectivities in dAu collisions, especially at Au side, however the “flow” is much weaker (a few hundred times less) than that is in AuAu.**
***v*****1**** wiggle - let’s see how the identified particle analysis says.**
***v*****2**** at intermediate pt is still too large to be fit in models. The magnitude of *****v*****4**** and its sign needs special attention.**
**Photon flow - Be more open mind. **

- Aihong Tang                                           The Berkeley School 05