# Presentation: 073760

## Anisotropic flow: Achievements, Difficulties, Expectations 

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*              **Sergei A. Voloshin**** ***
*    **Wayne State University, Detroit, Michigan*

- Anisotropic flow: Achievements, Difficulties, Expectations

- Three major RHIC discoveries:
- 1. _Large elliptic flow_
- 2. Jet quenching
- 3. _Constituent quark scaling_
- Note the importance of item #3 observed in anisotropic flow “sector”
- (the observation in spectra would not constitute that strongly
- “partonic flow” - deconfinement !).
- At present, we are in the second phase of RHIC – quantitative
- understanding of sQGP, and I concentrate mostly
- on “what is happening now”.

- _Outline_
- 1. v2/eps:
-     - Advances in hydro calculations
-     - ...including viscous effects.
-     - Understanding initial conditions
-     - and fluctuations in those
- 2. Non-flow and flow fluctuations.
-     Flow fluctuations in the Gaussian model     and beyond
- 3. Anisotropic initial conditions; v2, v1,     and more.
- 4. Future: RHIC low energy scan, LHC
- 5. Conclusions

## Elliptic flow and sQGP

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- - How well we measure v2?
- (Non-flow, Flow fluctuations,
- participant plane vs reaction plane, etc.

- - Did we use “right” initial conditions in hydro calculations?   (do we compare apples to apples   when comparing data to the “hydro limits”?)
- - What is the role of viscosity?
- - Have we missed important effects in our    description/calculations?

- There are many components in this plot.
- We are now questioning each and any of them.

- Is there a saturation?

## How (in)complete is the thermalization?

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- note that there is a difference of a factor of “2” in the definitions of S

- Even central Au+Au collisions are about 30-50% away from
- ideal hydro limit!
- Note: assumed  constant speed of sound - no phase transitions, change in initial conditions with energy, 2d, boost invariance, etc..

- 0.46, 4.3
- 0.25, 5.7

## Ideal Hydro  Viscous hydro

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- Ideal Hydro  Viscous hydro

- Ideal hydro, if tuned to spectra, over predicts elliptic flow!  Including viscosity might improve agreement with data.

## viscosity II

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## Initial eccentricity

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- Gluon saturation picture (CGC) leads to a noticeably larger ( ~ 50%)initial eccentricity, and consequently larger elliptic flow.

## Larger eccentricity – larger elliptic flow

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- “late viscosity” was simulated by hydro+_cascade_ MC.

- The details depend in particular on transversecoordinate dependence of the saturation scale,
- if entropy or energy density is used as a weight,...
- Lappi, Venugopalan **Phys.Rev.C74:054905,2006**

## Flow and Non-flow

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- Non-flow = (a few particle) correlations
-         not related to the orientation of
-         the reaction plane: e.g.
-         - inter and intra jet correlations
-         - resonance decays, etc.

- Non-flow

- There are different methods to “fight” non-flow,including correlations with different charge combinations, rapidity difference, fitting 1d and 2d(φ) correlation functions, with “known” shape of non-flow correlations. Among those the “cleanest” is many particle correlations, which, unfortunately, have its own drawback.

## v2{2}, v2{4}, non-flow, and flow fluctuations

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- v2{2}, v2{4}, non-flow, and _flow fluctuations_

- Several reasons for *v* to fluctuate in a centrality bin:
- 1. “trivial” variation in impact parameter in a centrality bin
- (easy to take out)
- 2. Real flow fluctuations (due to fluctuations in the initial conditions or in the dynamics of the system evolution)   Different event classes, e.g  Correlation of jet
- quenching  with the strength of radial (and elliptic) flow.
- Mrowczynski-Shuryak, Sorensen QM08

- Non-flow

- Flow fluctuations

- Might have non-trivial dependence on σv

- “History” of flow fluctuations:
- STAR first papers (in particular, PRC2002)
- QM01 _Kodama_-Aguiar-Hama-Osada talk
- Snellings-Miller  ‘04
- Manly (PHOBOS) QM05
- Bhalerao-Ollitrault ‘06
- Grassi et al  QM06
- S.V.-Poskanzer-Tang-Wang ‘07
- PHOBOS-Heinz Collaboration ‘07

## Gaussian model of eccentricity fluctuations

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- Note the “evolution” of notion of anisotropic flow.
- Fluctuations in the initial geometry lead to deviation
- of the symmetry plane in a given event from the “true” reaction plane.

- Model assumes Gaussian form for the distributions
- in εx and εy, (which is a very good approximationof MC Glauber calculations).

- There is no possibility to separate fluctuations and non-effects  (which might be traced to the fact that
- a Gaussian distribution has all higher than rank 2  cumulants equal to zero)

- See poster of A. Tang at this conference

## Higher cumulant results, data and UrQMD calculations

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- v2{2}=v2{4}=v2{6}=v2{LYZ}=v2{ZDC-SMD)  and it looks like
- we now know why.

## Eccentricity fluctuations

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- Note the universal value of the ratio
- in the limit of very central collisions

- Key word

## Modification of the initial conditions

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- Role of viscosity? Can we measure it?

- Initial flow field is non-isotropic. How important is this effect?
- Revival of interest o the picture of rotating system (Dremin, Kharzeev, ...)

- Contribution to the in-planeexpansion due to initial
- (longitudinal) velocity gradient

## Hydro: “antiflow”, “third flow component”

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- Brachmann, Soff, Dumitru, Stocker, Maruhn, Greiner
- Bravina, Rischke ,  PRC 61 (2000) 024909

## Directed flow

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- G. Wang (STAR), QM2006

- Not explained by any model (even close)!
- Error-bars at the level of  < 10-3 !

- Measurements done
- with STAR ZDC-SMD (“spectator neutrons”
- and 3 –particle correlations (mixed harmonics).

## Parity violation: the effect and the observable

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*a* > 0   preferential emission along the angular momentum, *a**+**= **–** **a**–** **,*
- The sign of *Q* **can vary** event to event one has to measure correlations, <*a**a**a**b*>,
- P-even quantity (! ) - possibility of contribution from effects not related to P-violation

- Evremov, Kharzeev, PLB366:311(1996)
- Kharzeev, Pisarsky, Tytgat, PRL81:512(1998)
- Kharzeev, Pisarsky, PRD61:111901(2000)
- Voloshin, PRC62:044901(2000)
- Kharzeev, Krasnitz, Venugopalan, PLB545:298(2002)
- Finch, Chikanian, Longacre, Sandweiss, Thomas, PRC65:014908(2002)
- _Kharzeev,  Phys.Lett. B633 260 (2006) [hep-ph/0406125]_
- _Voloshin,    PRC70 057901 (2004) [hep-ph/0406311]._
- Selyuzhenkov (STAR) Poster at QM05
- Kharzeev, Zhitnitsky, Nucl.Phys, A797:67(2007)
- Kharzeev, McLerran, Warringa, ArXiv:0711.0950

## Probe for parity violations

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- Voloshin (STAR), poster # 205, QM2008

- The largest “background” from
- “flowing” clusters( e.g. decay of flowing resonances)
- Note, no contribution from

## Nearest future: RHIC 

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- Nearest future: RHIC

- _Low energy scan_: Search for the phase transition1. Collapse of directed flow
- 2. Non-monotonic behavior of v2/eps, and/or   scaling violation (may also  reflect change in the initial    conditions (e.g. participant Glauber  CGC)

- _U=U Collisions_

- Do not forget that with
- upgrades we will have
- 1. Higher pt reach
- 2. Better PID, and more

## LHC

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- “Naive” extrapolations lead to elliptic flow values at LHC
- up to 50% larger than at RHIC.

## LHC II

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- Hirano, et al., QM2006

- - Predictions are made.
- - Collaborations ready
- Looking forward for the first collisions

## Not discussed

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- - hadronic vs partonic flow (phi flow)
- - “other scalings”
- - v2 at high pt
- - v4/v2^2- latest results
-   on const. quark scaling.
- - photon flow

- PRL 99, 052301 (2007)

## Conclusions

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- 1. Significant progress in understanding of
-    - the role of viscosity
-    - initial conditions
-    - flow fluctuations
- 2. New ideas for the anisotropic flow are being developed, such as
-    - role of non-zero vorticity (system rotation)
-    - flow fluctuations due to dynamical fluctuations (instabilities, “jet” events”, etc...)
- 4. Directed flow still remains under explored.
- 3. We need 3d viscous hydro!
- 5. Azimuthal correlations analyses of non-central collisions established to be one of the most    informative direction in HIC studies (note also Global polarization, parity violation studies, etc.)
-  Bright and exciting future at RHIC and LHC!

## Backup slides

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## v2/   scaling

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*v*2/**   scaling

- Hydro curves are obtained from  calculationsKolb, Sollfrank, Heinz, PRC 62 (2000) 054909,
- made at b=7 fm and rescaled by ‘optical’ eccentricity value. The centrality dependence is not fullyreflected by these curves, as it is more ‘flat’ at eachgiven collision energy (very roughly indicatedby strait lines)

## v2{FTPC}(pt) , 20-60% centrality

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- v2{FTPC}(pt) , 20-60% centrality

- There still indications of non-flow contribution in ***v******2******{FTPC},*** especially at high transverse momenta. The green points show the results “AA-pp” [G.Wang (STAR), QM2005]

**AuAu 200 GeV**

**CuCu 200 GeV**

- Non-flow contribution in ***v******2******{2},*** relatively small
- in AuAu @200 GeV, increases with pt

## CuCu 200 GeV, v2{FTPC}(pt)

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- CuCu 200 GeV, v2{FTPC}(pt)

- Indicates strong non-flow contribution in ***v******2******{2}*** (measured within Main TPC).
- Non-flow might be still present at high pt in ***v******2******{FTPC}***

## Effect of fluctuations in eccentricity

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- If compare to the “original”  *v*2 */***  plot (page 2) note the difference in eccentricity
***optical, old* (using parameterization made for SPS energies) and ***part*    (***optical, old* ~ (1.1– 1.2) * ***part* )  ! )

- Systematic error on dn/dy ~10%, similarly on* v**2* and *S.*
- Hydro results (Kolb, Sollfrank, Heinz, PRC 62 (2000) 054909 )are rescaled with ***optical*

- Scaling with ε should be used ifv2’s in the Main and ForwardTPCs are not correlated.

##  v2{ZDC-SMD} and eccentricity scaling

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-  *v*2{ZDC-SMD} and eccentricity scaling

- Note that under assumption that the* directed*flow of spectator neutrons is not correlatedto the  *elliptic *shape of the system at midrapidty,
*v**2*{ZDC-SMD} should follow* ***std .
- (This assumption requires further study with MC Glauber model).