# Presentation: 223365

## Hadronic Signals of Deconfinement at RHIC

- Riken-BNL “Discovery” Workshop
- May 14-15, 2004

- Berndt Müller Duke University

- Hadrons encode essential properties of the partonic phase of dense, hot matter created in RHI reactions.

## Special thanks to...

- M. Asakawa
- S.A. Bass
- R.J. Fries
- C. Nonaka
- T. Renk
- PRL 90, 202303
- PRC 68, 044902
- PLB 583, 73
- PRC 69, 031902
- nucl-th/0404015

- DUKE PHYSICS

## What would it really take...?

- Question (from a friend):
- 	*What would it take to convince you that the quark-gluon plasma has been discovered at RHIC?*

- Answer – If we could show that:
*Hadrons are emitted in universal equilibrium abundances;*
*Hadrons are produced by recombination of quarks;*
*Hadrons show clear evidence of collective flow *(v0* and *v2);
*Flow pattern is not universal for hadrons, but universal for the constituent quarks.*

## Chemical equilibrium – not new, but...

*T*ch does not grow from SPS to RHIC !

## Equilibrium fits work...

- 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
- 	(STAR Preliminary)

## Strangeness in Au+Au at RHIC

- The strangeness “enhancement” is less than at SPS energy, as expected from chemical equilibrium paradigm!

## Conclusions (1)

- Clear evidence for a *universal* hadronization temperature *T*ch ≡ *T*c is seen in RHIC data;
- Already visible at SPS, but only RHIC data make the evidence compelling;
- Strangeness equilibration is critical discriminator between phase space dominance (*pp*, *e*+*e*-) and equilibration (*AA*).

## Issues to be clarified...

- Are hadron abundances described by *universal* parameters [*T*c* = T*c* = T*c()] or by *A*- and *E*CM-dependent parameters describing chemical freezeout conditions?
- If the hadron abundances are determined by equilibrium at *T*c, then only the *deviations* from equilibrium should be non-universal. Do the data confirm this?
- Can these deviations from equilibrium be *quantitatively* described by microscopic models of final-state hadronic rescattering (e.g. by UrQMD)?

## Hadronization mechanisms

- Recombination was predicted in the 1980’s – but a surprise after all

## Recombination “wins” ...

- ... always for a thermal source

- Fragmentation still wins for a power law tail

## Recombination of thermal quarks

- Relativistic formulation using hadron light-cone frame:

- For a thermal distribution,
- hadron wavefunctions can be integrated out, eliminating the model dependence of predictions.

## Recombination vs. Fragmentation

## Recombination & statistical model

- In stat. model, hadron distributions at freeze-out are given by:

- For pt , hadron ratios are identical to those in recombination!
- 	(only determined by hadron degeneracy factors & chem. pot.)
-  recombination provides a microscopic basis for the apparent chemical    	equilibration among hadrons at (moderately) large pt

**But: **The elliptic flow velocity is approximately additive in valence quark number, showing partonic, rather than hadronic origin of the elliptic flow.

## Recombination vs. Fragmentation

## Suppression: Baryons vs. mesons

## Conclusions (2)

- Strong evidence for dominance of hadronization by quark recombination from a thermal, deconfined phase comes from:
- Large baryon/meson ratios at moderately large *p*T;
- Compatibility of measured abundances with statistical model predictions;
- Collective flow (v0) still visible at large *p*T.
-  -meson is an excellent test case (if not from *KK*).

## Parton Number Scaling of Elliptic Flow

- In the recombination regime, meson and baryon v2 can be obtained from the parton v2 :

- Neglecting quadratic and cubic terms, a simple scaling law holds:

## Hadron v2 reflects quark flow !

## Conclusions (3)

- Recombination model works nicely for v2:
- v2(*p*T) curves for different hadrons collapse to *universal* curve for constituent quarks;
- Saturation value of v2 for large *p*T is *universal* for quarks and agrees with expectations from anisotropic energy loss;
- Vector mesons (, *K**) permit test for influence of mass versus constituent number (but note the *effects of hadronic rescattering on resonances*!)

## Issues to be clarified...

**T. Ochiai, Prog. Theor. Phys. 75 (1986) 1184**

- Baryon enhancement by two-quark scattering and di-quark formation is possible, **but** does **not** naturally imply a statistical ratio of hadron yields (baryons vs. mesons).
- Recombination from a **thermal **quark ensemble does!

- The “pQCD approach” to parton recombination

## Hadron-hadron correlations

- The fly in the ointment... ?

- “Jet”-like correlations persist in the recombination regime, for baryons and mesons!

## How serious is this?

- _Not_ a “theorist’s excuse”:
- Existing recombination models are based on the assumption of a one-body quark density. Two-hadron correlations are determined by quark correlations, which are not included (except by Hwa et al.).
- Two- and multi-quark correlations are a natural result of jet quenching by energy loss of fast partons.
- Incorporation of quark correlations is straightforward, but introduces many new parameters: *C*(*p*1, *p*2).

## Parton correlations ...

**Parton correlations trivially translate into hadron correlations.**
**Soft-hard recombination naturally gives such correlations.**
**Parton correlations even in the thermal regime?**

**...****provide a simple way out?**

## Correlations - formalism 

- Model studies are in progress; please stay tuned...

## Conclusions – at last!

- How the Q(G)P (“a deconfined phase of QCD matter”) was discovered at RHIC:
*Hadrons are emitted in ****universal**** equilibrium abundances;*
*Most hadrons are produced by ****recombination of quarks****;*
*Hadrons show evidence of ****collective flow**** *(v0* and *v2);
*Flow pattern *(v2)* is not universal for hadrons, but ****universal for the constituent quarks****.*
- THE END