# Presentation: 684470

## Recent progress at RHIC(assessment of results from the first three years of RHIC running)

- Raimond Snellings
- For the STAR Collaboration

## Introduction

- We are in the process of assessing what the overall quality of evidence is for the formation of a QGP, based on the data obtained in three successful years of RHIC running
- Time scale: collaboration discussion June 12th , available to the public mid-June
- The outline of this talk (given):
- What we have learned so far in the heavy-ion program at RHIC (from STAR’s perspective)
- The enormous success of the first RHIC years makes it impossible to summarize all the beautiful results from STAR and what we learned from them in just 15 min
- What are unanswered questions/issues which could be addressed in the future RHIC program over the next 5-10 years or at the LHC (see Tim Hallman’s talk).

- NSAC Subcommittee on Heavy-Ion Physics

## The STAR Collaboration: 51 Institutions, ~ 500 People

- 	University of Birmingham
**France:****     **
- 	Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes
**Germany:****  **
- 	Max Planck Institute – Munich University of Frankfurt
**India:**
**	**Bhubaneswar, Jammu, IIT-Mumbai, Panjab, Rajasthan, VECC
**Netherlands:**
- 	NIKHEF
**Poland:**
- 	Warsaw University of Technology
**Russia:  **
- 	MEPHI – Moscow,  LPP/LHE JINR – Dubna,  IHEP – Protvino
**Switzerland:**
- 	University of Bern
**	**

**U.S. Labs:**
- 	Argonne,   Lawrence Berkeley,  and Brookhaven National Labs
**U.S. Universities:**
- 	UC Berkeley, UC Davis,  UCLA,  Caltech**,** Carnegie Mellon, Creighton,  Indiana, Kent State, MIT, MSU,  CCNY, Ohio State, Penn State,  Purdue,  Rice, Texas A&M, UT Austin, Washington, Wayne State, Valparaiso, Yale
**Brazil:****	**
- 	Universidade de Sao Paolo
**China:****	****    **
- 	IHEP - Beijing, IPP - Wuhan, USTC,
- 	Tsinghua, SINR, IMP Lanzhou
**Croatia:****	****    **
- 	 Zagreb University
**Czech Republic:****	****    **
- 	 Nuclear Physics Institute

**The STAR Collaboration: 51 Institutions, ~ 500 People**

- NSAC Subcommittee on Heavy-Ion Physics

## The STAR Detector

**Magnet**
**Coils**
**Central **
**Trigger**
**Barrel **
**(CTB)**
**ZCal**
**Time **
**Projection **
**Chamber**
**(TPC)**
**Year 2000**

**Barrel EM Cal (BEMC)**
**Silicon Vertex Tracker (SVT)****Silicon Strip Detector (SSD)**
**FTPC****Endcap EM Cal****FPD**
**TOFp, TOFr****Year 2001+**

***DAQ:  currently running at ~100x design rate for TPC***

- NSAC Subcommittee on Heavy-Ion Physics

## Au+X Datasets

-  This year: extremely successful run, all BUR goals substantially met
-  Greater than order of magnitude increase in dataset

- NSAC Subcommittee on Heavy-Ion Physics

## Leading particle spectra

**Au+Au**

**d+Au**

- Charged hadron pT distributions measured up to 12 GeV in Au+Au, d+Au and p+p reference

- NSAC Subcommittee on Heavy-Ion Physics

## Suppression of the particle yield at high transverse momenta

- Suppression of particle yield is a final state effect!
- Consistent with expectations from parton energy loss in a dense medium!

- NSAC Subcommittee on Heavy-Ion Physics

## Di-hadron correlations

- Adams *et al*., Phys. Rev. Let. 91 (2003)

- Suppression of associated particles on away side

- No ‘jet’-suppression

- p+p, peripheral Au+Au: 2 jets

- Central Au+Au: no away-side jet

- Adler *et al*., PRL90:082302 (2003)

- d+Au control: 2 jets

- Correlation of
- trigger particles 4<pT<6.5 GeV withassociated particles 2<pT<pT,trig

- Associated particles

- Back to back suppression is a final state effect, consistent with expectations of parton energy loss in a dense medium

- NSAC Subcommittee on Heavy-Ion Physics

## Back to back correlations versus the reaction plane

- The back to back correlation depends on the average distance traveled through the medium!

**STAR Preliminary**

- Au+Au centrality: 20-60%

- NSAC Subcommittee on Heavy-Ion Physics

## Discovery of the suppression phenomena at RHIC

- The observed high-pt phenomena can be described efficiently by parton energy loss in matter starting with large energy and gluon densities
- Large observed suppression of the particle yield
- The suppression of the back to back correlation (broadening at low-pt)
- The large elliptic flow
- The path length dependence of the back to back azimuthal correlation

- NSAC Subcommittee on Heavy-Ion Physics

## Baryon Enhancement

- p+pbar/h enhancement in Au + Au; Not fully explained by Cronin effect
-  Strong baryon/meson modification in Au + Au also in /K0s ratio***  ***
-  Simple fragmentation picture fails for pT less than ~6 GeV/c

- J. Adams et al., nucl-ex/0309012

- PHENIX

- STAR preliminary

- NSAC Subcommittee on Heavy-Ion Physics

## Identified particles at intermediate to high-pt

- Two groups, baryons and mesons
- At ~5 GeV/c baryons and mesons seem to approach each other
- Suggesting relevance of constituent quarks for hadron production
- Coalescence/recombination provides a description between ~1.5-5 GeV/c

- NSAC Subcommittee on Heavy-Ion Physics

## Elliptic flow at intermediate to high-pt

- Elliptic flow at intermediate to high-pt

- Two groups, baryons and mesons
- Suggesting relevance of constituent quarks for hadron production
- Coalescence/recombination provides a description above 1.5-5 GeV/c
- suggestive of collective flow at the constituent quark level

- NSAC Subcommittee on Heavy-Ion Physics

## Identified particle spectra

- STAR and PHENIX data

- PHENIX Phys. Rev. C 69, 034909 (2004)

- NSAC Subcommittee on Heavy-Ion Physics

## Particle yields/thermal model

- Chemical freeze-out 160 +/- 10 MeV, close to expected critical temperature, particle ratios similar in pp for most abundant species
- Deviations of the resonance yields compared to thermal model predictions indicative of hadronic phase after chemical freeze-out

**STAR**
**O**** PHENIX**

- NSAC Subcommittee on Heavy-Ion Physics

## 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

- NSAC Subcommittee on Heavy-Ion Physics

## Charged hadron elliptic flow

- First time in Heavy-Ion Collisions a system created which at low pt is in quantitative agreement with ideal hydrodynamic model predictions for v2 up to mid-central collisions

- PHOBOS: Phys. Rev. Lett.** 89**, 222301 (2002)** **

- STAR: Phys. Rev. Lett. **86**, 402 (2001)

- PHENIX: Phys. Rev. Lett. 89, 212301 (2002)

- RQMD

- NSAC Subcommittee on Heavy-Ion Physics

## Energy dependence of v2

- See: Peter F. Kolb and Ulrich Heinz, review for 'Quark Gluon Plasma 3', nucl-th/0305084. Pasi Huovinen, review for 'Quark Gluon Plasma 3', nucl-th/0305064. D. Teany, J. Lauret and E. V. Shuryak, nucl-th/0110037; Phys. Rev. Lett  86, 4783 (2001)
- Rather smooth dependence versus beam energy (important to realize; pion v2 for AGS and SPS, charged hadrons at RHIC)

- NA49

- Phys.Rev. C68 (2003) 034903

- NSAC Subcommittee on Heavy-Ion Physics

## v2(m,pt)

- v2(m,pt)

- Clear mass dependence; signature of collective flow (not only in hydro)
- Hydrodynamics gives reasonable description of various mass particles at low transverse momenta
- Hydro calculation constrained by particle spectra

- Hydro calculations: Kolb, Heinz and Huovinen

- 130 GeV

- Hydro; Huovinen, private communication

- NSAC Subcommittee on Heavy-Ion Physics

## v2(m,pt)

- PHENIX

- v2(m,pt)

- Consistent measurements from PHENIX and STAR
- Blast-wave fit as well as full hydro calculation gives reasonable description of mass dependence from pions to cascade

- NSAC Subcommittee on Heavy-Ion Physics

## Space-time information

- Dynamical models which describe the relatively well the spectra and elliptic flow  give a rather poor description of the HBT “radii”

- D. Zschiesche, S. Schramm,
- H. Stoecker, and W. Greiner,  Phys. Rev. C65,064902(02)

- NSAC Subcommittee on Heavy-Ion Physics

## The first three years of RHIC

- A lot of progress made in the first three years of RHIC running
- Large volumes of high quality heavy-ion and reference data
- Consistent results from all RHIC experiments
- Significant advances in theoretical treatment – many of the striking results have been described and sometimes even predicted to a reasonable quantitative level
- At higher pt: strong suppression phenomena
- Confirmation of theory predictions and evidence of high densities
- How are the uncertainties, of the density for example, amplified by the expansion of the system? Does this observation demand and explanation in terms of traversal through deconfined matter?
- At intermediate pt: baryon meson scaling in v2 and Rcp
- Described by coalescence/recombination model – constituent quark collectivity
- What about predictive power? Centrality dependence of scaling, correlations and dynamical treatment?
- At low-pt: strong collective behavior, manifest in all observables
- Collisions at RHIC energies appear to have attained conditions which simplify the theoretical treatment at lower pt – ideal fluid expansion and approximate local thermalization
- Unique QGP EOS? How does the HBT “puzzle” fit in?

- NSAC Subcommittee on Heavy-Ion Physics

## Questions from RBRC

- NSAC Subcommittee on Heavy-Ion Physics

## Discovery of the suppression phenomena at RHIC

- The observed strong suppression can be described efficiently by parton energy loss in matter starting with large energy and gluon densities
- Does the magnitude of parton energy loss inferred from these observations ***demand*** an explanation in terms of traversal through deconfined matter?
- What are the uncertainties due to factorization in-medium, in-medium fragmentation versus vacuum fragmentation?
- How are possible uncertainties amplified due to the longitudinal (and also transverse) expansion of the system?
- Can we ***prove****** *** from the inferred densities that deconfined matter has been created?

- NSAC Subcommittee on Heavy-Ion Physics

## Identified particles at intermediate to high-pt

- Baryons and mesons scaling is suggestive of importance of constituent quark degree of freedom in hadronization and suggestive of collective flow at the constituent quark level
- This scaling is compactly described in a coalescence/recombination model
- Aside from providing an organizing principle, what predictive power do these models have? Can they predict the correct centrality-dependence of these ratios, or meson vs. baryon correlations (angular or otherwise) at moderate pt? Does it also still work when applied on models with a more complete space-time evolution?

- NSAC Subcommittee on Heavy-Ion Physics

## Is the system in approximate local thermal equilibrium?

- The unprecedented success of hydrodynamics calculations assuming ideal relativistic fluid behavior in accounting for RHIC elliptic flow results has been interpreted as evidence for both early attainment of local thermal equilibrium and softening of the equation of state, characteristic of the predicted phase transition.
- How do we know that the observed elliptic flow can not result alternatively from a harder EOS coupled with incomplete thermalization? (D. Teaney, J. Lauret, E.V. Shuryak; Phys. Rev. Lett  86, 4783 (2001))

- NSAC Subcommittee on Heavy-Ion Physics

## HBT, spectra and v2; the soft sector

- The indirect evidence for a phase transition in the elliptic flow results comes primarily from the sensitivity in hydrodynamic calculations of the magnitude and hadron mass-dependence of v2 to the EOS
- How does the level of this EOS sensitivity compare quantitatively to that of uncertainties in the calculations, based the range of adjustable parameters and the failure to describe the spectra, elliptic flow and HBT at the same time?

- NSAC Subcommittee on Heavy-Ion Physics

## Initial conditions (CGC)

- If there is a truly universal gluon density saturation scale, determined already from HERA *e*-p deep inelastic scattering measurements, why has it been necessary to refit parameters of the saturation scale to RHIC A+A particle multiplicities? Is not the A-dependence of the gluon densities at the relevant Bjorken x-ranges predicted in gluon saturation treatments

- NSAC Subcommittee on Heavy-Ion Physics

## Critical behavior?

- Is it naïve to expect non-monotonic behavior?
- Can we make a convincing QGP discovery claim without a rapid change in an observable characteristic of a phase transition?
- Where is the smoking gun?
- Can we predict, based on what we now know from SPS and RHIC, at what energies or under what conditions we produce matter below the critical temperature and which observables from those collisions will show a non-monotonic behavior?

- NSAC Subcommittee on Heavy-Ion Physics