# Presentation: 621184

## A  NEARLY  PERFECT  INK:The quest for the quark-gluon plasma at the Relativistic Heavy Ion Collider 

*Berndt Mueller (Duke University)*
- LANL Theory Colloquium
- 2 June 2005

## The Road to the Quark-Gluon Plasma...

- ...Is Hexagonal and 2.4 Miles Long

- Insights and Scientific Challenges       from the RHIC Experiments

## The quest for simplicity

- Before the 1975, matter at high energy density was considered a _*real mess*_!
- QCD predicts that hot matter becomes *simple – *the* QGP* (not necessarily weakly interacting!).
- Characteristic features: deconfinement and chiral symmetry restoration.

- The equation of state of strongly interacting matter according to lattice QCD

- Quark-gluon plasma

- Tc ≈ 160 MeV

## The QCD phase diagram

- Tc

## Space-time picture of a r.h.i.c.

- Equilibration

- Hadronization

- Thermal freeze-out

## RHIC data gathering

- RHIC has had runs with:
- Au+Au at 200, 130, 62 GeV
- Cu+Cu at 200, 62 GeV
- d+Au at 200 GeV
- p+p at 200, 130 GeV

- Charged particle tracks from a central Au+Au collision

## Frequently Asked Questions

- How do we know that we have produced *equilibrated* *matter*, not just a huge bunch of particles ?
- What makes this matter *special* ?
- How do we measure its *properties* ?
- Which evidence do we have that quarks are *deconfined* for a brief moment (about 10-23 s) ?
- Which evidence do we have for temporary *chiral symmetry restoration* ?
- What do we still need to learn ?
- Translation: When can RHIC be closed down ?

## FAQ #1

- How do we know that we produced *equilibrated* *matter*,
- not just a bunch of particles ?
- Answer:
- Particles are thermally distributed and flow collectively !

## Chemical equilibrium

- Chemical equilibrium fits work, *except* where they should not (resonances with large rescattering).
-  RHIC Au+Au @ 200 GeV
- Tch = 160  10 MeV
- μB  =  24  5 MeV

## Elliptic flow

- Two-particle correlations
- dN/d(1- 2) 1 + 2v22cos(2[1- 2])

## FAQ #2

- What makes this matter *special* ?
- Answer:
- It flows astonishingly smoothly !
- “The least viscous non-superfluid ever seen”

## v2 requires low viscosity

- Relativistic viscous fluid dynamics:

- Shear viscosity *** *tends to smear out the effects of sharp gradients

## Viscosity must be ultra-low

- D. Teaney

- Quantum lower bound on /s :
- /s = 1/4   (Kovtun, Son, Starinets)
- Realized in strongly coupled (g1)                *N *= 4 SUSY YM theory, also in QCD ?

- /s = 1/4 implies f ≈ (5 T)-1 ≈ 0.3 d

- QGP(T≈Tc) = sQGP

- v2 data comparison with (2D) relativistic hydrodynamics results suggests /s  0.1

- Recent excitement:

## FAQ #3

- How do we measure its *properties* ?
- Answer:
- With hard QCD probes,
- such as jets, photons, or heavy quarks

## “Jet quenching” = Parton energy loss

- Radiative energy loss:

- “Jet quenching” = Parton energy loss

- q

- q

- g

- Scattering centers = color charges

***L***

- Scattering power of the QCD medium:

- Density of scattering centers

- Range of color force

## Suppression of fast pions (0)

## Energy loss at RHIC

- Data are described by a very large loss parameter for central collisions:

- (Dainese, Loizides, Paic, hep-ph/0406201)

## Does QCD perturbation theory work?

- →  What is the appropriate value of QCD coupling s ?

- 0

## FAQ #4

- Which evidence do we have that quarks are *deconfined*
- for a brief moment (about 10-23 s) ?
- Answer:
- Baryons and mesons are formed
- from independently flowing quarks

## Suppression Patterns: Baryons vs. Mesons

- What makes baryons different from mesons ?

## Hadronization Mechanisms

**This is not coalescence from a dilute medium !**

## Recombination “wins” ...

- ... always for a thermal source

- Fragmentation still wins for a power law tail

## Recombination vs. Fragmentation

- Baryon enhancement

## Hadron v2 reflects quark flow !

## FAQ #5

- Which evidence do we have for temporary
*chiral symmetry restoration* ?

## Strangeness in Au+Au at RHIC

- Flavor

- Mass (MeV)

## FAQ #6: What do we still need to (or want to) learn ?

- Number of degrees of freedom:
- via energy density – entropy relation.
- Color screening:
- via dissolution of heavy quark bound states (J/).
- Chiral symmetry restoration:
- modification of hadron masses via e+e- spectroscopy.
- Quantitative determination of transport properties:
- viscosity, stopping power, sound velocity, etc.
- What exactly is the “s”QGP ?

## QCD equation of state

**Challenge: Devise method for determining ********from data**

## Alternative method

- Eliminate *T*  from ** and *s* :

- Lower limit on  requires lower limit on *s* and upper limit on .

- BM & K. Rajagopal, hep-ph/0502174
- Eur. J. Phys. C (in print)

## Measuring  and s

- Entropy is related to produced particle number and is conserved in the expansion of the (nearly) ideal fluid:             dN/dy →  S*  *→  s = S/V.
- Energy density is more difficult to determine:
- Energy contained in transverse degrees of freedom is _not_* *conserved during hydrodynamical expansion.
- Focus in the past has been on obtaining a _lower_ limit on ; here we need an _upper_ limit.
- New aspect at RHIC: parton energy loss. dE/dx is telling us something important – but what exactly?

## Entropy

- Two approaches:
- Use inferred particle numbers at chemical freeze-out from statistical model fits of hadron yields;
- Use measured hadron yields and HBT system size parameters as kinetic freeze-out (Pratt & Pal).
- Method 2 is closer to data, but requires more assumptions (HBT radii = geometric radii, isentropic expansion of _hadronic_ gas).
- Good news: results agree within errors:
- dS/dy = 5100 ± 400  for Au+Au (6% central, 200 GeV/NN)

## State of the art

**6% central s****NN****1/2**** = 200 GeV Au+Au collisions (********0**** = 1 fm/c):**
- d*S*/dy = 5100 ± 400     → *s* = (d*S*/dy)/(R20) = 33 ± 3 fm-3
- d*E*T/dy = 650 GeV/fm3 → ***Bj* = (d*E*T/dy)/(R20) = 4.2 GeV/fm3
** > **Bj due to longitudinal hydrodynamic work done.                             PHOBOS estimate is ** > 5 GeV/fm3 at 0 = 1 fm/c.                                     Some examples:
** = 5 GeV/fm3 →  = 71 ± 22
** = 7 GeV/fm3 →  = 26 ± 8
** = 9 GeV/fm3 →  = 12 ± 4
- Improved determination of ** must be an immediate goal.

## Heavy quarks

- Heavy quarks* *(*c*, *b*) provide a hard scale via their mass. Three ways to make use of this:
-  Color screening of (Q-Qbar) bound states;
-  Energy loss of “slow” heavy quarks;
-  D-, B-mesons as probes of collective flow.
- RHIC program: *c*-quarks and J/;
- LHC-HI program: *b*-quarks and .
-  RHIC data for J/ are forthcoming (Runs 4 & 5).

## The Baier plot - again

- Plotted against ,    is the same for a  gas and for a perturbative QGP.
- Suggests that    is really a measure of the energy density.
- Data suggest that    may be larger than compatible with Baier plot.
- Better calculations are needed.
- One approach (Turbide et al.) based on complete LO HTL transport theory, gets RAA right (hep-ph/0502248)

## J/ suppression ?

*V*qq is screened at scale (*gT*)-1  heavy quark bound states dissolve above *some* *T*d.

*Color singlet free energy*

**Challenge: Compute J/******** spectral function in unquenched QCD**

## Di-hadron correlations

- Correlations depend on selected momentum windows

## “Waking” the sQGP

- v=0.55c

## Outlook

- The “discovery phase” of RHIC is just hitting its full stride.
- Several important observables still waiting to be explored.
- Run-4 and -5 data are eagerly anticipated.
- More than 109 events will provide many answers and help us to refine the questions.
- Many well defined theoretical challenges.