# Presentation: 690997

## Are We There Yet?

***Are We There Yet?***

***The STAR Collaboration’s Critical Evaluation of the Evidence Regarding Formation of a Quark Gluon Plasma in RHIC Collisions***

***Steve Vigdor for the White Paper Drafting Team***

## Drafting Committee

**Jamie Dunlop (BNL)**
**Huan Huang (UCLA)**
**Peter Jacobs (LBNL)**
**Mike Lisa (Ohio State U.)**
**Raimond Snellings (NIKHEF)**
**Steve Vigdor (Indiana U. -- Chair)**
**Nu Xu (LBNL)**
**Zhangbu Xu (BNL)**

***As per our charge, we have produced “a draft White Paper to serve as the starting point for a focused discussion by the STAR Collaboration...”, not yet a publication-quality paper!***

## Choices, Choices ...

*** ******Two audiences:  RHIC community (************ concise summary and bottom line – at end); larger physics community (************ ~ jargon-free background info + framing of question – most of the text!)***
*** ******Include data from all 4 RHIC experiments, but STAR-centric***
*** ******Data from first 3 RHIC runs only, published at least in conference proceedings***
*** ******Tone:  aim for skeptical, but inspirational ***
*** ******Overall goal:  honest scientific (~ apolitical) discussion, guided by consideration of convincing physicists with high standards outside of our community***

**Question for Collaboration(s): What do we want to do with these documents?  This decision determines real audience.**

## Table of Contents

***Table of Contents***

## Start by Defining QGP :

**M. Gyulassy**

**M. Gyulassy & L. McLerran**

***Approximately thermalized matter at energy densities so large that the simple degrees of freedom are quarks and gluons.  This energy density is that predicted by LGT for the existence of a QGP, ************ 2 GeV/fm******3******.***

## Lattice QCD Still Predicts a RAPID Transition!

**in entropy density, hence pressure**

**in heavy-quark screening mass**

**in chiral condensate**

***The most realistic calcs. ************ no discontinuities in thermodynamic proper-ties @ RHIC conditions (i.e., no 1******st******- or 2******nd******-order phase transition), but still crossover transition with rapid evolution vs. temperature near T******c************ 160 – 170 MeV.***

## Community was Led to Expect an Interesting Energy-Dependence:

**Early mixed-phase hydro evolution ...**

****** ****Soft EOS dip in v****2****(********s****NN****)?**

**Energy density ******

- Harris & Müller AnnRevNuclPartSci ‘96

***Community was Led to Expect an Interesting Energy-Dependence:***

## But What We Observe (at least in the soft sector) Appears Smooth :

***But What We Observe (at least in the soft sector) Appears Smooth :***

**No exp’tal smoking gun! ******** Rely on theory-exp’t comparison ******** Need critical evaluation of both! Theory must eventually explain the smooth energy- and centrality-dependences.**

**Charged particle pseudo-rapidity density**

**HBT parameters**

**p****T****-integrated elliptic flow**

**p****T****-integrated elliptic flow, scaled by initial spatial eccentricity**

## Soft Sector:  Evidence for Thermalization and EOS with Soft Point?

*** ******Systematic m-dependence of v******2******(p******T******) suggests common transverse vel. field***
*** ******m******T****** spectra and v******2****** systematics for mid-central collisions at low p******T****** are well (~20-30% level) described by hydro expansion of ideal relativistic fluid***
*** ******Hydro success suggests early thermalization, very short mean free path***
*** ******Best agreement with v******2****** and spectra for ************therm****** < 1 fm/c and soft (mixed-phase-dominated) EOS ~ consistent with LQCD expectations for QGP ************ hadron***

- Hydro calculations: Kolb, Heinz and Huovinen

## How Unique & Robust is Hydro Account in Detail?

**Sharp freezeout ******** dip**

**Hydro+RQMD ******** no dip?**

- Teaney, Lauret & Shuryak

**Hydro vs. STAR HBT *****R******out******/R******side***

*** ******Are we sure that observed v******2****** doesn’t result alternatively from harder EOS (no transition) and late thermalization?***
*** ******How does sensitivity to EOS in hydro calcs. compare quantitatively to sensitiv-ity to other unknown features: e.g., freezeout treatment ******(compare figures at right)******, thermaliz’n time, longitudinal boost non-invariance?***
*** ******What has to be changed to understand HBT (below), and what effect will that change have on soft EOS conclusion?***

## Soft Sector:  Hadron Yield Ratios

**STAR**
**O PHENIX**

*** ******p******T******-integrated yield ratios in central Au+Au collisions consistent with Grand Canonical stat. distribution @ T******ch****** = (160 ******± 10) MeV,   ************B****** ************ 25 MeV, ******across u, d and s sectors******.***
*** ******Inferred T******ch****** consistent with T******crit****** (LQCD) ************ T******0****** ************ T******crit ******.***
*** ******Does result point to thermodynamic and chemical equilibration, and not just phase-space dominance?***

## Intermediate pT: Hints of Relevant Degrees of Freedom

*** ******For 1.5 ************ p******T ************ 6 GeV/c, see clear meson vs. baryon (rather than mass-dependent) differences in central-to-mid-central yields and v******2******.***
*** ******v******2******/n******q****** vs. p******T ******/n******q****** suggestive of constituent-quark scaling.  If better established exp’tally, would give direct evidence of degrees of freedom relevant at hadronization, and suggest collective flow @ constituent quark level.***
*** ******N.B.  Constituent quarks ************ partons!  Constituent quark flow does not prove QGP: what is ************therm****** vs. ************constituent q****** ?***

## Questions for Coalescence Models

***Questions for Coalescence Models***

*** ******Can one account simultaneously for spectra, v******2****** and di-hadron ************ correlations at intermediate p******T****** with mixture of quark recombination and fragmentation contributions?  Do observed jet-like near-side correlations arise from small vacuum fragmentation component, or from “fast-slow” recombination?  ***
*** ******Are thermal recomb., “fast-slow” recomb. and vacuum fragment-ation treatments compatible?  Double-counting, mixing d.o.f., etc.?***
*** ******Do coalescence models have predictive power?  E.g., can they predict centrality-dependences? ***

**Duke-model recomb. calcs.**

**Duke-model recomb. calcs.**

## Hard Sector:  Quantitative Indication of Early Gluon Density

*** ******Inclusive hadron and away-side cor-relation suppression in central Au+Au, but ******not****** in d+Au, clearly establish ******jet quenching****** as final-state phenomenon, indicating very strong interactions of hard-scattered partons or their fragments with ******dense, dissipative medium****** produced in central Au+Au.***

**PHENIX**

## Questions for Parton Energy Loss Models

***Questions for Parton Energy Loss Models***

***Can pQCD models account for orientation- dependence of dihadron correlation? Should be sensitive to both path length and matter expansion rate variation with (************R******).***
*** ******~p******T******-independence of measured R******CP****** ************ unlikely that hadron absorption dominates jet quenching.***

*** ******pQCD parton energy loss fits to observed central suppression ************ dN******gluon******/dy ~ 1000 at start of rapid expansion, i.e., ~30-50 times cold nuclear matter gluon density.  ******How sensitive is this quantitative con-clusion to: assumptions of fact-orization in-medium and vacuum fragmentation following degrada-tion; treatments of expansion and initial-state cold energy loss preceding hard collision?***

## Gluon Saturation: a QCD Scale for Initial Gluon Density + Early Thermaliz’n Mechanism?

*******s******NN****** = 130 GeV Au+Au***

**Saturation model curves use optical         Glauber**

*** ******Does the high initial gluon density inferred from parton E loss fits ******demand****** a deconfined initial state? Can QCD illuminate the initial conditions?***
*** ******Assuming initial state dominated by g+g below the saturation scale (con-strained by HERA e-p), Color Glass Condensate approaches ~account for RHIC bulk rapidity densities ************ dN******g******/dy ~ consistent with parton E loss.***
*** ******How robust is agreement, given optical vs. MC Glauber ambiguity in calcu-lating N******part****** , and assumption of ~one charged hadron per gluon?***
*** ******CGC applies @ SPS too?  If not, why is measured dN******ch******/d************(************s******NN******) so smooth?***

## Soft-Hard Correlations:  Partial Approach Toward Thermalization?

***Away side not  jet-like!  ******In central Au+Au, the balancing hadrons are greater in number, softer in p******T******, and distributed ~statistically [~ cos(************)] in angle, relative to pp or peripheral Au+Au.***
********* ******away-side products seem to approach equilibration with bulk medium traversed, making thermalization of the bulk itself quite plausible.***

## The Five Pillars of RHIC Wisdom

**pQCD parton E loss**

***The Five Pillars of RHIC Wisdom***

**Ideal hydro**

**Quark recombination ******** ****constituent q d.o.f.**

**CGC**

**Statistical model**

**Early thermalization + soft EOS**

**Very high inferred initial gluon density**

**Very high anticipated initial gluon density**

**u, d, s equil-ibration near T****crit**

## The State of RHIC Theory

**Emerging description of beautiful evolution from one new state of matter to another!**

**AND/ OR**

**A theoretical patchwork, with parameters adjusted independ-ently at each successive stage!**

***In order to rely on theory for compelling QGP discovery claim, we need: ******greater coherence; fewer adjusted parameters; quantitative estimates of theoretical uncertainties; quantitative PREdictions whose subsequent comparison to experiment is “make-or-break”.***

## Critical Future Exp’t Needs: Short-Term (some data already in the bag from run 4)

**Establish v****2**** scaling more definitively: *****better statistics, more particles (incl. ************, ************, resonances), include ************ correlations in recomb.-model fits.***
**Establish that jet quenching is an indicator of parton, not hadron, E loss: *****higher p******T******; better statistics dihadron correlations vs. reaction plane; away-side punchthrough? charmed meson suppression?***
**Extend RHIC Au+Au meas’ments down toward SPS energy,  search for possible indicators of a rapid transition in measured properties: *****determine turn-on of jet suppression vs. ************s; pp reference data crucial.***

**Measure charmonium yields + open charm yields and flow, to search for signatures of color screening and partonic collectivity: *****charmed hadrons in chem. equil.? Coalescence vs. frag-mentation? D-meson flow; J/************ sup-pression? (eventually ************ , other “onia”)***
**Measure hadron correlations with far forward high-energy hadrons in d+Au: *****search for monojet signature of interaction with classical gluon field.***

## Some Critical Future Exp’t Needs: Longer-Term

**Develop thermometers for the early stage of the collision, when thermal equilibrium is first established: *****direct photons (************ HBT for low E), thermal dileptons.***
**Quantify parton E loss by measurement of mid-rapidity jet fragments tagged by hard direct photon, a heavy-quark hadron, or a far forward energetic hadron: *****constrain E loss of light quarks vs. heavy quarks vs. gluons in bulk matter.***
**Test quantitative predictions for elliptic flow in U+U collisions:**

***Considerable extrapolation away from Au+Au ************ significant test for hydro predictive power @ RHIC.***
**Measure hadron multiplicities, yields, correlations and flow at LHC & GSI, and compare to quantitative predictions based on models adjusted to work at RHIC: *****test viability and falsifiability of QGP-based theoretical framework.***
**Devise tests for the fate of fundamental QCD symmetries in RHIC collision matter: *****chiral & U******A******(1) restoration? CP violation? Look especially at the strongly affected particles opposite a high-p******T****** hadron tag.***

## Summary

**RHIC’s major advances from runs 1-3:**
***Extended reach in energy density appears to reach simplifying conditions in central collisions -- *****~ideal fluid expansion; approx. local thermal equilibrium.**
***Extended reach in p******T****** gives probes for behavior inaccessible at lower energies – *****jet quenching; ~constituent quark scaling.**
**Bottom Line:    *****In the absence of a direct “smoking gun” signal of deconfinement revealed by experiment alone, a QGP discovery claim must rest on the comparison with a promising, but still not yet mature, theoretical framework.  In this circumstance, clear predictive power with quantitative assessments of theoretical uncertainties are necessary for the present appealing picture to survive as a lasting one.***
**RHIC has produced striking and important results, with many more to come! But a QGP discovery claim would be premature.**

## Keep the Faith!  A personal note:

**Consider the analogy with neutrino oscillations:**
** ****~midway through 3 decades of solar ******** research, most  exp’talists and theorists working in the field thought the underlying explanation for the observed deficit was probably ******** mass and oscillations.**
** ****The probable explanation could not support a discovery claim, in part because the theory was a not yet completely constrained m****élange of thermodynamics, nuclear reaction theory and novel effects of a “dense” medium on neutrino interactions.  The exp’t’l observations were strongly suggestive,  but still fairly narrow in scope.**
** ****The case got more interesting, but not yet definitive, by coupling it to a somewhat weaker case for another interesting anomaly (atmospheric ********e****/************) that could also be explained by ******** oscillations.**
** ****A new generation of definitive experiments (SuperK, SNO) to test clear, make-or-break theoretical predictions then led to indisputable discovery claims, 10-15 years after most “suspected” the answer.**
** ****The delay in a discovery claim was science at work: not politics, not Hamlet-like indecision, not “too much democracy” in collaborations.  It has not lessened, but rather deepened, the impact of the eventual discovery.  It has not dissipated the recognition of Nobel physics.**

## End of Presentation

## Implications for STAR Near-Term Upgrades and Run Scenario Plan

** ****MUST find a way to get to charm sector physics with reasonable efficiency on reasonable time scale *****– e.g., for 2008 next long full-energy Au+Au run?  Funding initiation for pixel detector + upgraded inner tracker before FY07 is getting unlikely.  May need a clever interim solution!  If so, physics justification for subsequent full microvertex + inner tracker stage needs to be carefully constructed.***
** ****DAQ/FEE upgrade should be in place for next long Au+Au to maximize physics impact of that run.**
** ****Develop ideas for QCD symmetry tests in greater depth, and see if can’t get good start during next long Au+Au.**
*** *****MUST be ready for another long d+Au run sometime during 2005-7 period *****– forward physics upgrades needed for crucial CGC tests must be integrated into overall STAR plan and fast-tracked.***** **

## Identified particle spectra

- Mass dependence of particle spectra described reasonably well by ideal hydrodynamics

- Hydro (P. Kolb & U. Heinz)
- With initial flow kick

- Central AuAu
- √s = 200 GeV

## Angular correlations of hard and soft hadrons in STAR explore  transverse momentum balance opposite a high-pT particle, in the light of jet quenching (F. Wang):

***Angular correlations of hard and soft hadrons in STAR explore  transverse momentum balance opposite a high-p******T****** particle, in the light of jet quenching (F. Wang):***

***Away side not  jet-like! ***
***In central Au+Au, the balancing hadrons are greater in number, softer in p******T******, and distributed ~statistic-ally [~ cos(************)] in angle, relative to pp or peripheral Au+Au.***
********* ******away-side products approach equilibration with bulk medium traversed.***

**STAR PRELIMINARY**

**STAR PRELIMINARY**