# Presentation: 562825

## “Soft physics in STAR”

- Lee Barnby for the STAR CollaborationJuly 21st 2005.  HEP2005, Lisboa, Portugal

## Overview

- Bulk particle production
- Spectra, chemistry - thermal fits, dynamics - blast-wave
- Elliptic flow
- Probing the medium
- Fluctuations
*Auto*correlations
- Final state
- HBT
- Resonances

- L.Barnby@bham.ac.uk - HEP2005

## STAR Experiment

- L.Barnby@bham.ac.uk - HEP2005

## Particle spectra measurements

- Most comprehensive identified particle species dataset.
- Integrate (extrapolated) spectra to get yields.
- Look at shape of transverse momentum spectrum to learn about dynamics.

- Nucl. Phys. A 757 (2005) 102. Fig. 13 & refs. therein

- L.Barnby@bham.ac.uk - HEP2005

## Yields: thermal model

- Ratios of many particle yields successfully reproduced
- Is there true thermalization or is this statistical filling of phase space?

- Nucl. Phys. A 757 (2005) 102. Fig. 12 & refs. therein

- L.Barnby@bham.ac.uk - HEP2005

## Thermal Model Chemical fits

- 4 parameters Tch, b, s & s
- Tch ~ 160 GeV (not shown)
- constant with centrality
-  same for 200 & 62 GeV
- B changes with energy due to stopping
-  shows dependence on Nch at small values of Nch
- s factor shows some centrality dependence
- fully saturated in most central collisions

- π,K,p,ΛΞ** **

- π,K,p (200 GeV)** **

-  62 GeV

*STAR preliminary*

- L.Barnby@bham.ac.uk - HEP2005

## Spectra Shapes:Radial Flow

- Blast wave fit to , K, p,  (and anti-particle) spectra.
- Able to model changing shape with mass

- 

- K (dE/dx)

- p

- 

- K (kink)

- Tth=107±8 [MeV]
- <t>=0.55±0.08 [c]
- n=0.65±0.09
- 2/dof=106/90

- solid lines: fit range

-  - 
-  - K0S
-  - K±
-  - p
-  - 

- L.Barnby@bham.ac.uk - HEP2005

## Kinetic Freezeout: Blast-wave

- Bulk particles show lower Tfo with more central collisions.
- , ,  with (assumed) lower hadronic cross-sections have higher Tfo *but* still show strong flow.

- L.Barnby@bham.ac.uk - HEP2005

## Elliptic Flow: low pT

- Mass ordering described by hydro model
- Indicates collective motion
- What are relevant degrees of freedom
- Hadrons or quarks and gluons
- Detailed agreement depends on equation of state

- L.Barnby@bham.ac.uk - HEP2005

## Elliptic Flow: moderate pT

- v2 re-plotted per constituent quark i.e.
- Divide v2 by n
- Divide pT by n
- n=2 for mesons, n=3 for baryons
- This quark number scaling *may* indicate that constituent quarks are the relevant degrees of freedom

- L.Barnby@bham.ac.uk - HEP2005

## Spectra summary

- Thermal models used to fit yields at chemical freezeout.
- Blast-wave used to fit spectra and extract thermal freezeout parameters.
- Azimuthal anisotropies following hydrodynamic behaviour.

- L.Barnby@bham.ac.uk - HEP2005

## , autocorrelations

- Correlate all particle pairs
- No (or all) triggers
- Construct , 
- Subtract cos() and cos(2) terms.
- Away side excess disappears even in peripheral bin
- Peaks become elongated in  and narrower in  with increasing centrality.

- p+p 200 GeV

- Au+Au 130 GeV

- Central

- Peripheral

- nucl-ex/0411003

- L.Barnby@bham.ac.uk - HEP2005

## Centrality dependence of correlation structure

- Correlation amplitude and
- volume per final state particle.

- Linear amp. increase

- amplitude & volume

- widths

- Factor 2.3 width increase

**amplitude**

- Alternative centrality measure:

- L.Barnby@bham.ac.uk - HEP2005

## Minijets: velocity/temperature correlation structures on ()
Strong elongation on  and new negative same-side structure

- Strong elongation on  and new *negative* same-side structure

- 70-80%

- 20-30%

- 0-5%

- subtract multipoles

*STAR preliminary*

- centrality

- partons and velocity correlations

-  *p**t*  Fluctuations and* p**t* Correlations – 200 GeV Au-Au

*STAR preliminary*

- L.Barnby@bham.ac.uk - HEP2005

## data  fit peak

**20-30%**

- data  fit peak

- recoil?

- fit residuals

- fit

- data

- fit peak

*STAR preliminary*

- collision centrality 

- HIJING Au-Au 200 GeV
- 0-10% centrality

- Quench off & on

- widths

- L.Barnby@bham.ac.uk - HEP2005

## Event-by-event fluctuations

- Looking for “excess” fluctuations
- No large fluctuation: chemical & kinetic equilibrium in every event?
- √s dependence looks smooth
- Net Charge reflects hadronic d.o.f, no Koch *et al*. QGP.
- Cross-over? Are we way above the transition?

- 

- L.Barnby@bham.ac.uk - HEP2005

## Correlations and Fluctuations Summary

- No unusual energy dependence in integral fluctuation measurements
- Correlations in -show interesting structures which change with centrality indicating interaction with medium.

- L.Barnby@bham.ac.uk - HEP2005

## Femtoscopy (HBT)

- q decomposed:
- qlong - beam direction
- qout - pair kT direction
- qside - perp. to above
- Radii relations to source:
- Ro - emission time
- Rs - transverse extent
- Rl - longitudinal extent
- Radii as a fn. of energy shown
- Surprising lack of s dependence given sensitivity to EoS & soft point

- L.Barnby@bham.ac.uk - HEP2005

## Radii and models

- Hydro models are not successful at reproducing radii as a function of kT
- Especially Rout/Rside ratio
- Although they are good for pT and v2.

- Experimental data agree!
- Model comparisons shown

- L.Barnby@bham.ac.uk - HEP2005

## HBT w.r.t reaction plane

- Change of source shape - due to expansion
- Still out of plane extended though - another indication of short time scale?

- L.Barnby@bham.ac.uk - HEP2005

## System size & Expansion

- Smooth growth of radii
- Factor 2 expansion for central Au+Au

- L.Barnby@bham.ac.uk - HEP2005

## Resonance studies

- Resonance decay products re-scattered
- prevents reconstruction
- Sensitive to lifetime of system
- However additional effects of regeneration to consider

*STAR preliminary*

- L.Barnby@bham.ac.uk - HEP2005

## Conclusions

- Bulk phenomena successfully described by collective behaviour
- Particle yields fit well by thermal model
- Changing shape of transverse spectra modelled by blast-wave
- Elliptic flow follows hydro-like behaviour
- Number of constituent quarks relevant d.o.f?
- Medium can be probed via correlation structures
- Reveals mini-jet like effects, coupled to bulk and possible evidence of recoil.
- Femtoscopic investigations are currently throwing up some interesting puzzles.

- L.Barnby@bham.ac.uk - HEP2005

## Acknowledgements

**Funding Agencies**
- We thank the RHIC Operations Group and RCF at BNL, and the NERSC Center at LBNL for their support. This work was supported in part by the HENP Divisions of the Office of Science of the U.S.DOE; the U.S. NSF; the BMBF of Germany; IN2P3, RA, RPL, and EMN of France; EPSRC of the United Kingdom; FAPESP of Brazil;the Russian Ministry of Science and Technology; the Ministry of Education and the NNSFC of China; IRP and GA of the Czech Republic,FOM of the Netherlands, DAE, DST, and CSIR of the Government of India; Swiss NSF; the Polish State Committee for Scientific Research; STAA of Slovakia, and the Korea Sci. & Eng. Foundation

**STAR Collaboration Institutions**
- Argonne National Laboratory, Institute of High Energy Physics - Bejing, University of Bern, University of Birmingham, Brookhaven National Laboratory, California Institute of Technology, University of California - Berkeley, University of California - Davis, University of California - Los Angeles, Carnegie Mellon University, Creighton University, Nuclear Physics Inst., Academy of Sciences - Prague, Laboratory of High Energy Physics - Dubna, University of Frankfurt, Institute of Physics - Bhubaneswar, Indian Institute of Technology - Mumbai, Indiana University Cyclotron Facility, Institut de Recherches Subatomiques de Strasbourg, University of Jammu, Kent State University, Institute of Modern Physics. Lanzhou, Lawrence Berkeley National Laboratory, Massachusetts Institute of Technology, Max-Planck-Institut fuer Physics, Michigan State University, Moscow Engineering Physics Institute, City College of New York, NIKHEF and Utrecht University, Ohio State University, Panjab University, Pennsylvania State University, Institute of High Energy Physics - Protvino, Purdue University, Pusan National University, University of Rajasthan, Rice University, Instituto de Fisica da Universidade de Sao Paulo, University of Science and Technology of China - USTC, Shanghai Institue of Applied Physics, SUBATECH, Texas A&M University, University of Texas - Austin, Tsinghua University, Valparaiso University, Variable Energy Cyclotron Centre - Kolkata, Warsaw University of Technology, University of Washington, Wayne State University, Institute of Particle Physics - Wuhan, Yale University, University of Zagreb

- L.Barnby@bham.ac.uk - HEP2005