The Letter “S”

1. The Letter “S” sQGP Brought to you by the letter “S” Jamie Nagle, University of Colorado

2. Paradigm Shift? I would like to develop a common and consistent understanding of our terms and physics conclusions. A part of the discussion relates to what the letter "s" actually stands for (and of course what it means). sQGP I started this quest by asking people what it meant !

3. Miklos Gyulassy: “I still have the original small piece of paper that TD Lee slipped under my door last year with his hand written suggestion ‘strongly interacting Quark Gluon Plasma’ (which I dubbed sQGP).” sQGP “The name sQGP helps to distinguish that matter from ordinary hadronic resonance matter (as described e.g. by RQMD) and also from the original 1975 asymptotically free QGP (which I dubbed wQGP) that is now theoretically defined in terms of resummed thermal pQCD.” Public list discussion: http://lists.bnl.gov/pipermail/rhicii-new-l/

4. “Our criteria for the discovery of QGP are: • Matter at energy densities so large that the simple degrees of freedom are quarks and gluons. This energy density is that predicted by lattice gauge theory for the existence of a QGP in thermal systems, and is about 2 GeV/fm3. • The matter must be to a good approximation thermalized. • The properties of the matter associated with the matter while it is hot and dense must follow QCD computations based on hydrodynamics, lattice gauge theory results, and perturbative QCD for hard processes such as jets.” Gyulassy and McLerran [nucl-th/0405013 v2]

5. First time hydrodynamics without any viscosity describes heavy ion reactions. v2 pT (GeV) Thermalization time t=0.6 fm/c and e=20 GeV/fm3 * We have heard many talks on the caveats on this conclusion, and particularly the current lack of constraint on the lattice EOS.

6. “All of the above are satisfied from the published data at RHIC. … This leads us to conclude that the matter produced at RHIC is a strongly coupled QGP (sQGP) contrary to original expectations that were based on weakly coupled plasma estimates.” [Gyulassy, McLerran] Strongly interacting or strongly coupled or is there no difference? If coupling, which coupling?

7. Plasma Coupling G Used as a measure of the interaction strength in electromagnetic plasmas. G << 1 Weakly coupled. Behave as gases. G >> 1 Strongly coupled. Behave as low viscosity liquids, and at large enough G as solids. S. Ichimaru, Univ. of Tokyo

8. Insights from ElectroMagnetic Plasmas? • Be careful of analogies… • QCD always has screening of long range color magnetic fields. • However, on short distances, color E and B fields can be of equal order. • May have low viscosity like a liquid, but cannot define, for example, compressibility since P and V are independent for mB = 0. • QGP is not a liquid or plasma, but may share some properties. Analogy is only good if it yields insights!

9. Strongly Interacting Does Not Imply Plasma In the case of strongly coupled or strongly interacting, we also use these terms to describe the cold atom results. Strongly interacting (yes), but not the strong interaction. Also, not a plasma in the sense that the atoms are still together.

10. QGP G After fixing 4p units error, and factor of 2 for color magnetic interaction. “Considerations such as these have led some to denote QGP in this regime as ‘sQGP’ for strongly interacting QGP.” [PHENIX Whitepaper]

11. QCD Potential “Coulomb” “Confining” V(r) r

12. QGP = Polymers, Crystals ? This was what Shuryak was talking about at Quark Matter 2005.

13. cQGP? • Calculation seems to show QGP with liquid behavior at large G and even solid at larger G. • What does the “c” stand for? • “c” = Cookie • “c” = Classical How do we know if this non-relativistic, non-quantum case gives us any insight? Gelman, Shuryak, Zahed, nucl-th/0601029

14. Cannot Use G ? Unfortunately, this criterion is not applicable to matter composed of (nearly) massless, relativistic particles, such as QGP. Potential energy is not a well defined concept in a relativistic Quantum Field Theory (QFT), QED or QCD. However, there is a well defined measure of the interaction strength, the ratio between its shear viscosity (a measure of the mean free path of particles) and its entropy density (measure of the inter particle distance).

15. PHENIX preliminary Low h/s Fluid? Hydrodynamics does not need to be a “perfect” fit (i.e. EOS) to still be a “near-perfect” fluid. Need viscous 3-d hydro. Au+Au Moore & Teaney PRC71, 064904, ‘05

16. Utility of String Theory? “A test of this prediction comes from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, which has been colliding gold nuclei at very high energies. A preliminary analysis of these experiments indicates the collisions are creating a fluid with very low viscosity. Even though Son and his co-workers studied a simplified version of chromodynamics, they seem to have come up with a property that is shared by the real world. Does this mean that RHIC is creating small five-dimensional black holes? It is really too early to tell, both experimentally and theoretically.” [Maldacena]

17. Near-Perfect Fluid Motivated by AdS/CFT calculated lower viscosity bound • Interesting Side Note (learned from Zajc). • Superfluidity is at ~ 2 Kelvin • Gas-Liquid Phase transition ~ 4 Kelvin • Thus, the minimum is associated not with the minimum viscosity, but the sudden change in entropy! ? Critical goal to put the QGP data point on this plot

18. Strongly Coupled = as ? Does “s” mean "strongly coupled“ just implying that as is large? Does weak coupling (wQGP) simply mean that a perturbative calculation describes the data? And thus in contrast strong coupling means pQCD calculations do not?

19. Running of the Coupling aS(Q) 0.2 fm 0.02 fm 0.002 fm

20. BSQGP? “The interaction between quasiparticles is strong enough to bind them.” [Shuryak] However, lattice calculation of Baryon number – Electric charge correlation shows no such quasiparticles with these quantum numbers. [Karsch] Conclusion: (qq) and (q-qbar) ruled out. Can this method be extended to (qg) & (gg)?

21. Jet Quenching and Strong Coupling? Radiative energy loss calculations are done perturbatively! In fact, they are effectively leading order (so I am told). GLV assumes correct pQCD interaction strength, and determines the color charge density. One then gets dN/dy(gluons) ~ 1000 or dN/dy(quarks,gluons) ~ 2000. The final entropy density dS/dy ~ 5000, and thus since the entropy cannot be larger at earlier times  dN(qg)/dy < 1300. Perhaps more than radiative is going on? Or?

22. RHIC data sQGP QGP Pion gas Cold nuclear matter Interpretations This may suggest that the parton-parton cross sections in medium must be significantly larger than predicted by pQCD. Density of scatterings Range of color force Baier’s plot

23. Strongly Interacting? I am not sure what "strongly interacting" might mean since it is the strong interaction and strong is also a relative term. For cold atoms case (Li6) they are referred to as "strongly interacting" which means very large scattering cross section. One could use this terminology in the sense that parton scattering calculations by Molnar et al. imply much higher cross sections than the perturbatively calculated ones. This would be a definition related to hydrodynamics and very small mean free paths. Is it always really one of the above "strongly coupled" terms?

24. Perturbative calculations of gluon scattering lead to long equilibration times (> 2.6 fm/c) and small v2. R. Baier, A.H. Mueller, D. Schiff, D. Son, Phys. Lett. B539, 46 (2002). MPC 1.6.0, D. Molnar, M. Gyulassy, Nucl. Phys. A 697 (2002). v2 2-2 processes with pQCD s = 3 mb pT (GeV/c) Perturbative Transport Calculation This is not a perturbative QGP? (no wQGP) This underscores why ggggg contribution is critical to resolve!

25. e/T4 hadrons quark/gluon T/Tc Why did “we” ever think it would be a wQGP or PP (Perturbative Plasma)?

26. Berndt Mueller, Proc. of NATO Advanced Study Institute (1992) • “For plasma conditions realistically obtainable in nuclear collisions (T ~250 MeV, g = (4pas) = 2) the effective gluon mass mg* ~ 300 MeV.  • We must conclude, therefore, that the notion of almost free gluons (and quarks) in the high temperature phase of QCD is quite far from the truth. Certainly one has mg* << T when g<<1,but this condition is never really satisfied in QCD, because • g ~ 1/2 even at the Planck scale (1019 GeV).”

27. “I do not really care what the ''s'' in sQGP means. However, I am worried and partly also disappointed about the way this new name is used. The disappointment, of course, arises from the fact that suddenly a new name seems to be necessary to describe the properties of QCD in a temperature regime which LGT calculations since a long time have identified as ''not being an ideal gas'' and ''impossible to be described by perturbation theory''. F. Karsch Public list discussion: http://lists.bnl.gov/pipermail/rhicii-new-l/

28. Now for Something Completely Different! sQGP And in good fun…

29. Who Invented the letter “S”? “I still have the original small piece of paper that TD Lee slipped under my door last year with his hand written suggestion ‘strongly interacting Quark Gluon Plasma’ (which I dubbed sQGP).” sQGP “From archive it is clear that the abbreviationappeared in a number of well known talks by Miklos in 2004. I want to point out that we have not get that piece of paper from T.D. and yet used the ‘strongly coupled QGP’ consistently in several published papers of 2003.” [Shuryak] Public list discussion: http://lists.bnl.gov/pipermail/rhicii-new-l/

30. “Dear Edward and others: I quote from my plenary talk at PANIC'02 ‘Perturbative mechanisms seem unable to explain the phenomenological required very short thermalization time scale, pointing to strong non-perturbative dynamics in the QGP even at or above 2*T_c.’ T.D.Lee and Miklos put a catchy label on this (‘sQGP’), but that was long after it had become clear that the QGP behaved as a fluid and therefore needed to be a strongly coupled plasma. I think realizing the basic physics is at least as important as putting a catchy name on it, and should deserve proper recognition.” [Uli Heinz] Public list discussion: http://lists.bnl.gov/pipermail/rhicii-new-l/

31. “Dear Uli and Edward, This "priority discussion" is ridiculous. If you want to read really ‘old quark matter hydro’, go to my papers from 1980 etc, concerned with quark-hydro at the AGS... ;-) please come back down to earth, yours old friend horst” Public list discussion: http://lists.bnl.gov/pipermail/rhicii-new-l/

32. Positive Statement: Many exciting discoveries and more to come from RHIC and LHC. Young people thinking in new directions is the key.

33. sdQGP sd = scientifically defined sd = scientifically discovered sd = super duper !