# Presentation: 693402

## New Experimental Results from the Relativistic Heavy Ion Collider -The Perfect Fluid

- Gary D. Westfall
*Michigan State University*

*Fermilab Colloquium*
*July 19, 2006*

## RHIC Collisions

- RHIC is the Relativistic Heavy Ion Collider located at Brookhaven National Laboratory on Long Island, New York
- Collisions of heavy nuclei at very high energies offer the possibility of transforming the protons and neutrons in the nuclei into a fluid consisting of quarks and gluons
- We will call this state of matter thequark-gluon plasma (QGP)
- It is thought that the universe existed as a QGP a few s after the Big Bang

## Protons, Neutrons, and Quarks

## Relation to the Big Bang

- Benchmarks
- Energy density
- 1 GeV/fm3 
- 1.81015 g/cm3
- Temperature
- 170 MeV 
- 2.01012 K

## Strategy

- Look for a novel state of matter created in central Au+Au collisions at top RHIC energies
- Compare to situations in which the novel state of matter should not be created
- Lower incident energies
- Peripheral Au+Au collisions
- p+p collisions
- d+Au collisions

## Central Au+Au Collision at RHIC

- Study state of matter created early in central Au+Au collisions
- Equilibrium and bulk properties
- Elliptic flow and hydrodynamics
- Jet suppression

- 14 fm = 1410-15 m

-  = 100

- t = 10 fm/c = 3.310-23 s

## Lattice QCD

- O. Kaczmarel *et al*., Phys. Rev. **D 62**, 034021 (2000)

**T****c****=170 MeV**
**e****C****=0.5 GeV/fm****3**
**F. Karsch, Nucl. Phys. A698, 199c (2002).**

## The Relativistic Heavy Ion Collider

## The White Papers

- All four groups concurrently published "white paper" summaries of their work in the journal Nuclear Physics A
- These papers describe the RHIC experimental groups’ perspectives on discoveries at RHIC based on the first three years of RHIC running
- Quark-gluon plasma and color glass condensate at RHIC? The perspective from the BRAHMS experiment
- Nucl. Phys. **A757**, 1 (2005).
- The PHOBOS perspective on discoveries at RHIC
- Nucl. Phys. **A757**, 28 (2005).
- Experimental and theoretical challenges in the search for the quark-gluon plasma: The STAR Collaboration’s critical assessment of the evidence from RHIC Collisions
- Nucl. Phys. **A757**, 102 (2005).
- Formation of dense partonic matter in relativistic nucleus-nucleus collisions at RHIC: Experimental evaluation by the PHENIX Collaboration
- Nucl. Phys. **A757**, 184 (2005).

## Equilibrium in RHIC Collisions

- All of our ideas about the QGP rest on the idea that the system is in equilibrium
- Chemical equilibrium
- Represented by the relative yield of particles
- Temperature, chemical potential, partition function
- Kinetic equilibrium
- Represented by pt spectra
- Temperature, expansion velocity
- “Blast Wave”
- Colliding nuclei together at RHIC energies produces very hot, dense, and expanding matter

## Chemical Equilibrium at RHIC

- Assume thermally (constant T) and chemically (constant density ni) equilibrated system at chemical freeze-out
- Assume that the system is composed of non-interacting hadrons and resonances
- The assumption of constant ni leads to chemical potentials 
- Given T and ’s and the system size, ni’s can be calculated as a grand canonical ensemble

## Chemical Equilibrium

- 200 GeV p+p

- 200 GeV Au+Au

-  In p+p, particle ratios are well described
-  In Au+Au, only stable particle ratios are well described

*Resonance suppression*

## Chemical Freeze-out @ 200 GeV

*200 GeV Au+Au*

- ,K,p** **

- ,K,p,** **

- Close to chemical equilibrium !

- Close to net-baryon free

## Blast Wave

- E.Schnedermann et al,  PRC48 (1993) 2462.

- Blast-wave model:

******, K, p  ********  T= 90MeV, ******** **** **
** ******** ****T=160MeV, ******

## Kinetic Freeze-out @ 200 GeV

**Radial flow velocity**

**Kinetic  FO  temperature**

- ,K,p: Tkin decreases with centrality
-  Tkin = const
- ,  and flow

## Temperature and Energy Density

- Very high temperatures are created in RHIC collisions
- Very high energy densities are created in RHIC collisions

## The QGP Shines

- The QGP can be thought of as a blackbody radiating photons
- However, there is a huge background of photons from the decay of particles like the 0
- With some work, PHENIX has extracted a spectrum of direct photons from central Au+Au collisions at 200 GeV
- To get a feeling for what we are seeing, let’s go to the Wien Displacement Law

## Equilibrium Conclusions

- We reach temperatures and densities consistent with QGP formation
- We see chemical equilibration with some exceptions
- Multi-strange baryons seem to freeze-out a a different time
- We see strong, bulk flow characterized by a kinetic temperature and flow velocity
- Even multi-strange baryons flow
- Suggestive of hydrodynamic-like behavior

## Hydrodynamic Flow

- Now we will try to quantify the hydrodynamic behavior suggested by the observed blast wave
- We will study hydrodynamic behavior using elliptic flow, also called the azimuthal anisotropy
- We start by defining a reaction plane and an impact parameter

## Reaction Plane and Impact Parameter

- Reaction Plane

- Impact Parameter

## Elliptic Flow

- Look at non-central collisions
- Overlap region is not symmetric in coordinate space
- Almond shaped overlap region
- Larger pressure gradient in *x*-*z* plane than in *y* direction
- Spatial anisotropy -> momentum anistropy
- Process quenches itself -> sensitive to early time in the evolution of the system
- Sensitive to the equation of state
- Perform a Fourier decomposition of the momentum space particle distributions in the *x*-*y* plane
*v**n* is the *n**th* harmonic Fourier coefficient of the distribution of particles with respect to the reaction plane
*v*1: directed flow
- v2: elliptic flow

## Hydrodynamic Expansion

- In this hydrodynamic calculation, the initial coordinate space is transformed into momentum space anisotropy (arrows)
- Because the hydro calculations have no viscosity, the resulting azimuthal anisotopy is as large as it can be

- x (fm)

- y (fm)

## Elliptic Flow

- Note agreement of hydro model     absolute value     scaling with mass of particle

## The Perfect Liquid

- The hydro model reproduces v2 for the bulk of the produced particles
- Hydro overpredicts v2 at lower incident energies
- The hydro model incorporates zero viscosity
- Zero mean free path
- Evidence for the production of a perfect liquid in RHIC collisions

## How Perfect?

- Calculations of viscosity of hadron gas and quark gluon plasma compared with water

- Hadron
- Gas

- QGP

- Csernai, Kapusta, McLerran, nucl-th/0604032

## Hydro Over-Predicts v2 at Lower Energies

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

-  is the eccentricity of the	overlap region
- S is the area of the overlap region

- Colorpercolationpoint

- (based on mean-free path arguments)

- Pb+Pb 17 GeV

## Quark Coalescence in Flow

- Evidence that v2 arises at the quark/gluon level

## Elliptic Flow

- From M. Gyulassy

**Notes:**
"It's a far stretch over the horizon of theoretical imagination that links RHIC data currently to superstring theory, but in our "Elegant Universe" anything is still possible."

## Conclusions from Elliptic Flow

- We see hydrodynamic flow for low *p**t* particles
- Agreement with hydrodynamics with no viscosity implies we see a perfect fluid at RHIC
- At RHIC energies, hydrodynamics predicts the magnitude of elliptic flow
- At intermediate *p**t*, we seem to see quark coalescence in elliptic flow
- Evidence for partonic origin of flow
- Hydrodynamic behavior not observed at high *p**t* and away from mid-rapidity
- New calculations are coming out using color glass condensate to predict initial conditions for elliptic flow
- Initial eccentricity is higher
- May need some viscosity to reproduce experimental results
- Stay tuned....

## Jets in p+p Collisions

- In p+p collisions, hard quark/gluon scattering can produce back-to-back jets

## Jets in Au+Au Collisions

- In Au+Au collisions, hard scattering can also produce jets
- These jets become an internal probe for the newly created state of matter
- The produced particles must now traverse the matter produced in the collision to be observed
- In case the presence of the gold nucleus causes strong effects, we will compare with d+Au

## Jet Suppression - Nuclear Modification Factor

- We can study jet suppression using leading hadrons
- We define a nuclear modification factor, *R**AA*, in terms of the ratio of the pt spectra in nucleus-nucleus collisions divided by the pt spectra in p+p collisions
- We also define a nuclear modification factor, *R**CP*, in terms of the ratio of the pt spectra in central nucleus-nucleus collisions divided by the pt spectra in peripheral nucleus-nucleus collisions
- If naïve binary scaling applies, we should get 1 for these factors as a function of *p**t*

## Suppression of Leading Hadrons

- The combined data from Runs 1-3 at RHIC on p+p, Au+Au and d+Au collisions establish that ***a new effect*** (a new state of matter?) is produced in central Au-Au collisions

- Au + Au Experiment

- d + Au Control Experiment

- Preliminary Data

- Final Data

## Detecting charm/beauty via semileptonic D/B decays

-  Hadronic decay channels: D0KD*D0D+/-K
-  *Non-photonic* electrons:
-  Semileptonic channels:
- c  e+ + anything 	(B.R.: 9.6%)
- D0  e+ + anything	(B.R.: 6.87%)
- D  e + anything	(B.R.: 17.2%)
- b  e+ + anything	(B.R.: 10.9%)
- B  e + anything	(B.R.: 10.2%)
- Drell-Yan (small contribution for pT < 10 GeV/c)
-  *Photonic* electron background:
-  conversions (0   ee)
- 0, ’  Dalitz decays
- ,  ... decays (small)
- Ke3 decays (small)

## Charm: Electron suppression

- Suppression is approximately the same as for hadrons!
- Indicates necessity of collisional energy loss mechanism?

**Charged Hadrons**

**Electrons**

**STAR, H. Zhang @ SQM06**

**STAR, Phys Rev Lett 91 (2003) 072304**

## Jet Suppression - Azimuthal Correlations

- We can study jet suppression using azimuthal correlations
- We start with a high pt trigger particle
- We look at the azimuthal correlations of particles in coincidence with the trigger particles
- Jets will show up as correlated particles at an azimuthal angle of 180 from the trigger particle
- “back-to-back”
- The jets particles on the away-side must travel through the produced matter
- Can provide information about the produced matter

## Suppression of Away-Side Jets

## Azimuthal Dependence of Jet Suppression

- out-of-plane

- in-plane

##  Emergence of Dijets with Increasing pT(assoc)

-  correlations (not background subtracted)

- Narrow peak emerges cleanly above vanishing background

**p****T****(assoc) > 2 GeV/c**

**p****T****(assoc) > 3 GeV/c**

**p****T****(assoc) > 4 GeV/c**

**p****T****(assoc) > 5 GeV/c**

**p****T****(assoc) > 6 GeV/c**

**p****T****(assoc) > 7 GeV/c**

**p****T****(assoc) > 8 GeV/c**

## Hadron-triggered fragmentation functions

- Away-side D(zT) suppressed, but shape unchanged

- ~0.54

- ~0.25

**Scaling**
_**factors**_

- 8 < pT(trig) < 15 GeV/c

## Conclusions

- The four RHIC experiments have produced strong, consistent results, when combined with theoretical understanding, provide overwhelming evidence new state of matter based on
- Equilibrium
- Hydrodynamic behavior
- Jet suppression
- This new state of matter is clearly not what we expected when we started our journey, a weakly interacting QGP
- This new state of matter seems to be a strongly interacting, nearly-perfect fluid, a strongly interacting quark gluon plasma, something qualitatively new, totally unexpected
- The study of this new state may lead us in new and unexpected directions such string theory applications to RHIC collisions

## Links for Further Information

- Today’s Colloquium
- _[http://www.nscl.msu.edu/~westfall/Westfall_FNAL.pdf](http://www.nscl.msu.edu/~westfall/Westfall_FNAL.pdf)_
- RHIC
- http://www.bnl.gov/rhic
- STAR
- http://www.star.bnl.gov
- PHENIX
- http://www.phenix.bnl.gov
- PHOBOS
- http://www.phobos.bnl.gov
- BRAHMS
- http://www4.rcf.bnl.gov/brahms/WWW/
- Quark Matter 2005
- http://qm2005.kfki.hu/
- My home page
- http://www.nscl.msu.edu/~westfall

## Extra Slides

## RCP at Lower Energies

- Suppression much less at lower energies

## RAA Direct Photons

## Elliptic Flow vs Pseudorapidity and Energy

***Au+Au 0-40% central***

****

**v****2**

**PHOBOS nucl-ex/0406021(PRL in press)**

## Miklos Gyulassy

- AdS/CFT=Maldecena conjectured duality between
- weak gravity in 5D anti-de Sitter space and Infinite coupled conformal
- N=4 Supersymmetry field theory in 4D

- NSAC Presentation
- 4/5/05

**Notes:**
"It's a far stretch over the horizon of theoretical imagination that links RHIC data currently to superstring theory, but in our "Elegant Universe" anything is still possible."

## J/ Suppression

- Compare SPSand RHIC results
- Very similar
- No model canreproduce bothenergies

## Elliptic Flow of Heavy Quarks

- v2 for non-photonic electrons

## RdAu at Forward Rapidities

- PRL 93 242303 (2004)

- Cronin-like enhancement at  = 0
- Clear suppression as  changes up to 3.2
- Consistent with gluon saturation models

## Heavy Quark Suppression

- High-pT electron suppression is comparable to inclusive charged hadron suppression

- Gluons and light quarks

- Heavy quarks