1. How To See the Quark-Gluon Plasma
And Now For Something Completely Different…
Matthew Wysocki
University of Colorado at Boulder
July 22
Hot Quarks 2004

2. The Quark-Gluon Plasma
The quark-gluon plasma is a hot, dense state of matter in which partons have become deconfined (within the region of the plasma) from the normal hadronic state.
Characterized by high energy density (e > few GeV/fm^3) and high temperature (T > MeV) or baryon density (n > 0.72/fm^3) relative to normal nuclear matter.

3. Signature Required
Q: How can we tell the difference between states of matter?
A: We need a signature to check!
Proposed signatures include:
Dilepton production
Photon production
Hanbury-Brown-Twiss Effect
Strange matter content
J/Y suppression

4. Debye Screening These quarks effectively cannot “see” each other!
The charge of one particle is cancelled (screened) by the surrounding charges.
Debye Screening Radius (lD): The distance at which the effective charge is reduced by 1/e.
Originally defined for electromagnetic plasma, later extended to plasma of color charges.
These quarks effectively
cannot “see” each other!

5. Charmonia states as given in the Particle Data Book
Charmonium and the J/Y
T=Tc
Charmonia ( ) is represented fairly well (at T≈0) by the potential:
T=0
State
Mass (GeV)
Charmonia states as given in the Particle Data Book
h c
2.98
J/Y
3.1
c c Y’
But at T=Tc:

6. J/Y in the QGP ∞ Matsui & Satz (Phys. Lett. B178 (1986) 416):
In the QGP the screening radius could become smaller than the J/Y radius, effectively screening the quarks from each other!
The charm and anti-charm become unbound, and may combine with light quarks to emerge as “open charm” mesons.
T=0
T=200
aeff
0.52
0.20
0.41 fm
1.07 fm ∞
0.59 fm
From Introduction to High-Energy Heavy-Ion Collisions, C.Y. Wong 1994

7. Bam! J/Y Suppression!
In a QGP at T > Tc, we should see less J/Y’s than we would expect without the plasma (J/Y suppression).

8. The villain of our story: Dilepton Backgrounds
Drell-Yan
Dominant background in J/Y mass region and higher
Thermal dileptons
Dominated by Drell-Yan in heavy ion collisions such as those at RHIC.
M emitted at Tc= 200 MeV
Q emitted at To = 300 MeV
Hadronic dileptons
Provides background in low mass region (r,w,f)
RHIC
energies
*Dileptons from open charm decay were not
examined by Matsui & Satz

9. The hero fights back: Background Subtraction
Dimuon invariant mass plot from PHENIX Run4
Signal after background subtraction.
*Slide courtesy of
David Silvermyr

10. Whoa there, cowboy… A lot has happened since Matsui & Satz:
J/Y suppression from other sources:
Nuclear absorption
Shadowing
J/Y enhancement!
Recombination
More recent lattice calculations show J/Ys remaining bound as high as 1.5Tc (Petreczky et al, hep-lat/ )

11. NA50: Keeping it real

12. Cliffs Notes
In the quark-gluon plasma, the J/Y bound state will be suppressed.
How hot? How much?
High-energy heavy ion collisions (such as those at RHIC and soon the LHC) offer the best chance of observing this effect, and hence evidence of the QGP.

13. Fin Many thanks to the Hot Quarks Organizing
Committee for putting this workshop together!