Highlights from the STAR Experimental Program

1
Some

• Highlights from the STAR Experimental Program
• at RHIC
• Jim Thomas
• Lawrence Berkeley National Laboratory
• for the STAR Collaboration
• ICHEP Amsterdam
• July 27th, 2002

2
The RHIC Accelerator Facility

• RHIC
• Two independent accelerator rings
• 3.83 km in circumference
• Accelerates everything, from p to Au
• Ös L
• p-p 500 1032
• Au-Au 200 1026
• GeV cm-2 s-1
• Polarized protons
• STAR is the Hadronic Signals experiment
• At its heart is a large
  Time Projection Chamber

h
3
The STAR Detector at RHIC
STAR uses the worlds largest Time Projection
Chamber
4
Au on Au Event at CM Energy 130 GeVA
Data Taken June 25, 2000
5
Au on Au Event at CM Energy 130 GeVA
Pictures from Level 3 online display. Data
Taken June 25, 2000
6
Spectra Measured .vs. Centrality (impact
parameter)
Peripheral Collision
(near) Central Collision
central collision ? high multiplicity in CTB
low multiplicity in Zcal
7
Whats New? Identified Particle Spectra at 200 GeV
K-
p
p, p-, K, K- spectra versus centrality ( 130
GeV/N data in nucl-ex/0206008 )
8
Anti-Proton Spectra at 200 130 GeV / N
Au Au ? ?p X
?p
p and?p spectra versus centrality 130 GeV data
in PRL 87 (2002)
9
Anti-Particle to Particle Ratios
Excellent agreement between experiments at y 0,
Ös 130

• STAR results on the?p/p ratio
• ?p/p 0.11 0.01 _at_ 20 GeV
• ?p/p 0.71 0.05 _at_ 130 GeV
• Previously reported as 0.60 0.06
• ?p/p 0.80 0.05 _at_ 200 GeV

10
Anti-Baryon/Baryon Ratios versus ?sNN

• In the early universe
• ?p / p ratio 0.999999
• At RHIC, pair-production increases with ?s
• Mid-rapidity region is not yet baryon-free!
• Pair production is larger than baryon transport
• 80 of protons from pair production
• 20 from initial baryon number transported over 5
  units of rapidity

? pp ?p/p ISR
In HI collisions at RHIC, more baryons are pair
produced than are brought in by the initial state
11
Particle Ratios at RHIC
?p/p 0.71 0.02(stat) 0.05 (sys) 0.60
0.04(stat) 0.06 (sys) 0.64 0.01(stat)
0.07 (sys) 0.64 0.04(stat) 0.06
(sys) ??/? 0.73 0.03(stat) X/X-
0.83 0.03(stat.)0.05(sys.) K-/ ?- 0.15
0.01 (stat) 0.02 (sys) K/ ? 0.16
0.01 (stat) 0.02 (sys) ??/?? 1.00
0.01(stat) 0.02 (sys) 0.95
0.03(stat) 0.05 (sys)

• ?/h- 0.021 0.001 (stat) 0.005 (sys)
• ?/h- 0.060 0.001 (stat) 0.006 (sys)
• ?? / h- 0.043 0.001 (stat) 0.004(sys)
• K0s / h- 0.124 0.001 (stat)
• (?KK ) / 2 h- 0.032 0.003(stat.) 0.008
  (sys.)
• 2 ?/(?KK ) 0.64 0.06 (stat) 0.16
  (sys)

K-/K 0.89 0.008(stat) 0.05 (sys)
0.91 0.07(stat) 0.06 (sys) 0.89
0.07(stat) 0.05 (sys) K/K- 1.08
0.03(stat) 0.22(sys) min. bias K-/ ?-
0.15 0.01 (stat) 0.02 (sys) K/ ? 0.16
0.01 (stat) 0.02 (sys) ?K/K 0.92
0.14(stat.)
Good agreement between the 4 experiments STAR,
PHOBOS, PHENIX, BRAHMS
12
Chemical Freeze-out from a thermal model
( P. Braun-Munzinger et al hep-ph/105229)

• Assume
• Thermally and chemically equilibrated fireball
  at hadro-chemical freeze-out
• Law of mass action is applicable
• Recipe
• Grand canonical ensemble to describe partition
  function ?
• density of particles of species ?i
• Fixed by constraints Volume V, strangeness
  chemical potential ?S, and isospin

input measured particle ratios output
temperature T and baryo-chemical potential ?B
13
Putting STAR on the Phase Diagram

• Final-state analysis suggests RHIC reaches the
  phase boundary
• Hadron resonance ideal gas (M. Kaneta and N. Xu,
  nucl-ex/0104021 QM02)
• TCH 175 10 MeV
• ?B 40 10 MeV
• ltEgt/N 1 GeV
• (J. Cleymans and K. Redlich, Phys.Rev.C, 60,
  054908, 1999 )

We know where we are on the phase diagram but
now we want to know what other features are on
the diagram
14
The Phase Diagram for Nuclear Matter
K. Rajagopol

• The goal is to explore nuclear matter under
  extreme conditions T gt mpc2 and r gt
  10 r0

15
Chemical and Kinetic Freeze-out

• Chemical freeze-out (first)
• End of inelastic interactions
• Number of each particle species is frozen
• Useful data
• Particle ratios

• Kinetic freeze-out (later)
• End of elastic interactions
• Particle momenta are frozen
• Useful data
• Transverse momentum distributions
• and Effective temperatures

16
Transverse Flow
AuAu at 200 GeV
? -
STAR Preliminary
K -
?p
17
Kinetic Freezeout from Transverse Flow
STAR Preliminary
ltßrgt (RHIC) 0.55 0.1 c TKFO (RHIC) 100
10 MeV
Thermal freeze-out determinations are done with
the blast-wave model to find ltpTgt
Explosive Transverse Expansion at RHIC ? High
Pressure
18
Anisotropic (Elliptic) Transverse Flow

• The overlap region in peripheral collisions is
  not symmetric in coordinate space
• Almond shaped overlap region
• Easier for particles to emerge in the
• direction of x-z plane
• Larger area shines to the side
• Spatial anisotropy ? Momentum anisotropy
• Interactions among constituents generates
• a pressure gradient which transforms the initial
  spatial anisotropy into the observed momentum
  anisotropy
• Perform a Fourier decomposition of the momentum
  space particle distributions in the x-y plane
• v2 is the 2nd harmonic Fourier coefficient of the
  distribution of particles with respect to the
  reaction plane

19
v2 vs. Centrality

• v2 is large
• 6 in peripheral collisions
• Smaller for central collisions
• Hydro calculations are in reasonable agreement
  with the data
• In contrast to lower collision energies where
  hydro over-predicts anisotropic flow
• Anisotropic flow is developed by
  rescattering
• Data suggests early time history
• Quenched at later times

Anisotropic transverse flow is large at RHIC
20
v2 vs. pT and Particle Mass

• The mass dependence is reproduced by hydrodynamic
  models
• Hydro assumes local thermal equilibrium
• At early times
• Followed by hydrodynamic expansion

D. Teaney et al., QM2001 Proc.P. Huovinen et
al., nucl-th/0104020
Hydro does a surprisingly good job!
21
v2 for p, K, K0, ?p and L
Preliminary
Preliminary
Preliminary
22
v2 for High pt Particles
v2 is large but at pt gt 2 GeV/c the data starts
to deviate from hydrodynamics
23
Centrality Dependence of v2(pT)
130 GeV
peripheral
central
200 GeV (preliminary)

• v2 is saturated at high pT and it does not come
  back down as rapidly as expected
• What does v2 do at very high pT ?

24
v2 up to 12 GeV/c
v2 remains saturated
25
Hard Probes in Heavy-Ion Collisions

• New opportunity using Heavy Ions at RHIC ? Hard
  Parton Scattering
• ?sNN 200 GeV at RHIC
• 17 GeV at CERN SPS
• Jets and mini-jets
• 30-50 of particle production
• High pt leading particles
• Azimuthal correlations
• Extend into perturbative regime
• Calculations reliable (?)
• Scattered partons propagate through matter
• radiate energy (dE/dx x) in colored medium
• Interaction of parton with partonic matter
• Suppression of high pt particles jet quenching
• Suppression of angular correlations

26
Scaling pp to AA including the Cronin Effect

• At SPS energies
• High pt spectra evolves systematically from pp
  ? pA ? AA
• Hard scattering processes scale with the number
  of binary collisions
• Soft scattering processes scale with the number
  of participants
• The ratio exhibits Cronin effect behavior at
  the SPS
• No need to invoke QCD energy loss

27
Inclusive pT Distribution of Hadrons at 200 GeV

• Scale Au-Au data by the number of binary
  collisions
• Compare to UA1?pp reference data measured at 200
  GeV

28
Comparison of AuAu / pp at 130 GeV
29
RAA Comparison to pT 6 GeV/c
130 GeV nucl-ex/0206011
Preliminary ?sNN 200 GeV
Similar Suppression at high pT in 130 and 200 GeV
data
30
Flow vs. Inclusive Hadron Spectra
Different views of same physics?
Evidence for hadron suppression at high pT
Partonic interaction with matter? dE/dx?
31
Jet Physics it is easier to find one in ee-
Jet event in ee- collision
STAR AuAu collision
32
Identifying jets on a statistical basis in Au-Au

• You can see the jets in p-p data at RHIC

• Identify jets on a statistical basis in Au-Au
• Given a trigger particle with pT gt pT (trigger),
  associate particles with pT gt pT (associated)

STAR Preliminary AuAu _at_ 200 GeV/c 0-5 most
central 4 lt pT(trig) lt 6 GeV/c 2 lt pT(assoc.) lt
pT(trig)
33
Peripheral AuAu data vs. ppflow

• Ansatz
• A high pT triggered
• AuAu event is a superposition of a high pT
  triggered
• pp event plus anisotropic transverse flow
• v2 from reaction plane analysis
• A is fit in non-jet region (0.75lt??lt2.24)

34
Central AuAu data vs. ppflow
35
Jets at RHIC

• The backward going jet is missing in central
  Au-Au collisions when compared to p-p data
  flow
• Other features of the data
• High pT charged hadrons dominated by jet
  fragments
• Relative charge
• Azimuthal correlation width
• Evolution of jet cone azimuthal correlation
  strength with centrality
• Other explanations for the disappearance of
  back-to-back correlations in central Au-Au?
• Investigate nuclear kT effects
• Experiment pAu or dAu
• Theory Add realistic nuclear kT to
  the models

Surface emission?
Suppression of back-to-back correlations in
central AuAu collisions
36
Conclusions About Nuclear Matter at RHIC

• Its hot
• Chemical freeze out at 175 MeV
• Thermal freeze out at 100 MeV
• The universal freeze out temperatures are
  surprisingly flat as a function of ?s
• Its fast
• Transverse expansion with an average velocity of
  0.55 c
• Large amounts of anisotropic flow (v2) suggest
  hydrodynamic expansion and high pressure at
  early times in the collision history
• Its opaque
• Saturation of v2 at high pT
• Suppression of high pT particle yields relative
  to p-p
• Suppression of the away side jet
• And its nearly in thermal equilibrium
• Excellent fits to particle ratio data with
  equilibrium thermal models
• Excellent fits to flow data with hydrodynamic
  models that assume equilibrated systems

37

• Encore Slides

38
STAR Institutions

• U.S. Labs
• Argonne, Brookhaven, and Lawrence Berkeley
  National Labs
• U.S. Universities
• UC Berkeley, UC Davis, UCLA, Carnegie
  Mellon, Creighton, Indiana, Kent
  State, Michigan State, CCNY, Ohio State,
  Penn State, Purdue,
  Rice, UT Austin, Texas AM,
  Washington, Wayne State, Yale
• Brazil
• Universidade de Sao Paolo
• China
• IPP - Wuhan, IMP - Lanzhou USTC, SINR,
  Tsinghua University, IHEP - Beijing

England University of Birmingham France
IReS - Strasbourg SUBATECH -
Nantes Germany Max Planck Institute - Munich
University of Frankfurt India Institute of
Physics - Bhubaneswar IIT - Mumbai, VECC -
Calcutta Jammu University, Panjab
University University of Rajasthan Poland
Warsaw University of Technology Russia
MEPHI - Moscow, IHEP - Protvino LPP LHE
JINR - Dubna

39
The STAR Collaboration
40
Two-particle azimuthal correlations

• Identify jets on a statistical basis
• Given a trigger particle with pT gt pT (trigger),
  
• associate particles with pT gt pT (associated)
• Efficiency for finding trigger particle cancels
• C2 is probability to find another particle at
  (??,??)
• pT (associated) gt 2 GeV/c
• pT (trigger) 4-6 GeV/c, 3-4 GeV/c, 6-8 GeV/c
• ? lt 0.7 ? ?? lt 1.4
• STAR analysis for 200 GeV data
• pp Minbias 10 M events
• AuAu Minbias 1.7 M events
• AuAu central 1.5 M events

41
Relative Charge Dependence
Strong dynamical charge correlations in jet
fragmentation ? Compare and ? ? charged
azimuthal correlations to ? azimuthal
correlations
AuAu
pp
0lt??lt1.4
STAR Preliminary _at_ 200 GeV/c 0-10 most central
AuAu pp minimum bias 4 lt pT(trig) lt 6 GeV/c 2 lt
pT(assoc.) lt pT(trig)
Same particle production mechanism for pT gt 4
GeV/c in pp and central AuAu
42
Comparing AuAu and pp

• Ansatz high pT triggered AuAu event is a
  superposition of high pT triggered pp event plus
  anisotropic transverse flow
• v2 from reaction plane analysis
• A fit in non-jet region (0.75lt??lt2.24)
• Quantify deviations for jet cone region ( ?? lt
  0.75 ) and back-to-back region ( 2.24 lt ?? lt
  3.14 )

43
Ratio vs. participants
44
Central/Peripheral Normalized by ?Nbin?
suppression
suppression
45
Mass Dependence of Slopes
Mass ( GeV/c2 )
Mass ( GeV/c2 )
N. Xu, Nucl. Phys. A 610, 175c (1996)
46
STAR from the Inside - Out
4 meters
47
?0 pt Spectra for ?sNN 130 GeV
Even in a high multiplicity event, rare processes
can be found
48
Prelude to a Typical Data Sample

• AuAu at 200 GeV
• 3M min-bias events
• 9 centrality bins
• y lt 1.5
• AuAu at 130 GeV 100K min-bias events 8
  centrality bins y lt 1.5
• pp at 200 GeV 12 M min-bias events y lt
  1.5

60
50
40
30
20
10
5
Yield
Number of Charged Particles
Centrality measures impact parameter
49
Particle ID in STAR
50
Time Evolution of Anisotropic Flow
Zhang, Gyulassy, Ko, Phys. Lett. B455 (1999) 45
Mainly sensitive to the early stages of the
expansion
51
Extracting ?0 ? ? ? Yields
One photon rotated by ? in ?, 2nd order polynomial
Two photon invariant mass spectrum, Gaussian
Nbg (2nd poly)
52
The Blast Wave model vs Rout/Rside

• Pion
• Kaon
• Proton

53
Particle ratios thermal approach

• Statistical Thermal Model
• F. Becattini
• P. Braun-Munzinger, J. Stachel, D. Magestro
• Assume
• Thermally and chemically
• equilibrated fireball at hadro-
• chemical freeze-out
• Law of mass action is applicable
• Recipe
• Grand canonical ensemble to
• describe partition function ? density
• of particles of species ?i
• Fixed by constraints Volume V, ,
• strangeness chemical potential ?S,
• and isospin
• input measured particle ratios
• output temperature T and baryo-
• chemical potential ?B

54
Comparison with data
M. Kaneta, N. Xu
P. Braun-Munzinger et al. hep-ph/105229
Central
Chemical freeze-out parameters Tch 1794 MeV,
ms -0.82.0 MeV mB 514 MeV, gs 0.99
0.03
Why do these simple pictures work so well ?
55
Hadron Resonance Ideal Gas Fit to the Data
56
The STAR Detector

• STAR is a Time Projection Chamber with good
  vertex tracking, surrounded by Calorimeters
• The worlds biggest TPC
• The Hadronic Signals experiment
• Outstanding hadronic signals coverage with some
  leptonic coverage
• Complementarity with the rest of the RHIC program
• STAR is designed to handle a HUGE Multiplicity of
  tracks
• 1000s charged particles into -1 lt ??lt 1
• Demonstrated capability to handle high density
  tracks with dE/dx info
• Built upon experience with the MPS, EOS NA49
  experiments
• Large angular coverage
• TPC Excellent coverage -1.5 lt ??lt 1.5
  extended by FTPC 2.5 lt ??lt 4
• SVT SDD Four layer coverage -1 lt ??lt 1
• EMCal Barrel coverage -1 lt ??lt 1 extended by
  one endcap 1 lt ??lt 2
• Event by Event electronic readout with a
  sophisticated trigger
• Event by event physics analysis
• A new degree of freedom!

57
The STAR Physics program

• Exploring the properties of quarks and gluons are
  the motivation for the STAR physics program
• It breaks down into four fundamental categories
• Dense Matter
• Thermodynamic effects due to extra degrees of
  freedom
• Screening and deconfinement J/? melting
• Chiral Symmetry
• Changing quark masses ???gt K K- mass,
  width, BR
• Isospin fluctuations
• Hard QCD processes
• Jets mini-jets, jet quenching, dE/dx in the
  plasma
• Direct photons ( Jets) gt parton distribution
  functions
• Spin
• Spin structure function of the proton and neutron

58
Centrality Selection at RHIC common to all Exp
59
Nuclear Physics

• Low energy nuclear physics is dominated by
  potential interactions and mutual excitations

The UrQMD Collaboration

• Ultra-relativistic energies probe the fundamental
  interactions of the constituent particles

60
Feynmans Wisdom

• Feynman invented partons to explain features of
  high energy reactions that werent explained by
  Gell-Manns quarks.
• The bare electron and photon are the partons of
  QED
• We now know that quarks and gluons are the
  partons of QCD
• The physics of very high energy collisions ( e.g.
  100 Bev ) is determined by the transverse
  momentum, Q, and the longitudinal momentum
  fraction x Pz / Pbeam
• Lorentz contraction forces the problem to
  factorize
• Cross sections obey scaling laws
• Exclusive vs inclusive reactions
• (Feynman, Third Int. Conf. On High Energy
  Collisions, SUNYSB, 1969).

Feynman founded a school of thought based on
studying reactions as a function of xf
61
Rapidity vs xf

• xf pz / pmax
• A natural variable to describe physics at forward
  scattering angles
• Rapidity is different. It is a measure of
  velocity but it stretches the region around v c
  to avoid the relativistic scrunch
• Rapidity is relativistically invariant and
  cross-sections are invariant

Rapidity is the natural kinematic variable for HI
collisions ( y is approximately the lab angle
where y 0 at 90 degrees )
62
Bjorkens Wisdom

• Bjorken emphasized the central rapidity region
• Highly relativistic nucleus-nucleus collisions
  The central rapidity region, J.D. Bjorken, Phys.
  Rev. D27, 140 (1983).
• He went exploring and assumed
• Approximate 1- dimensional hydrodynamic
  expansion
• Invariance in y along central rapidity plateau
  (i.e., flat rapidity distribution)
• Boost-invariance of distributions,
  or
• and Factorization

Bjorken founded a school of thought based on
studying the central region as a function of
rapidity