Results from STAR

1
Results from STAR
MPI Colloquium October 22, 2002

• Markus D. Oldenburg

Munich, Germany
For the STAR Collaboration
2
Outline

• Physics introduction
• STAR _at_ RHIC
• Analyses Results
• Particle spectra
• Anisotropic Flow
• Jets at RHIC
• Ultra-Peripheral Collisions
• Summary
• Outlook

3
Phase Diagram for Nuclear Matter

• Study of nuclear matter under extreme conditions
  (high temperature and/or high pressure)

K. Rajagopol
4
The Relativistic Heavy Ion Collider

• Two independent accelerator rings
• 3.83 km in circumference
• Accelerates everything from p to Au
• Running conditions Au-Au 2001
• 55-56 bunches per ring (tested up
  to 110)
• 7.5?108 Au/bunch _at_ storage energy
• Storage energy 100 GeV/A
• Peak luminosity 5?1026 cm-2 s-1
• Running conditions p?p? 2001
• 55 bunches per ring
• 0.8?1011 p?/bunch
• Energy/beam 100 GeV
• Peak luminosity 1.5?1030 cm-2 s-1
• Beam polarization 25 (? AGS)

Long Island
5
The STAR experiment at RHIC
STAR uses the worlds largest Time Projection
Chamber
6
One of the first Au on Au Events at CM Energy of
200 GeV?A
7
Spectra Measured vs. Centrality (impact parameter)
Peripheral Collision
(near) Central Collision

• central collision ?
• high multiplicity in CTB
• low multiplicity in ZDcal

8
Identified Particle Spectra at 200 GeV
K-
p
p, p-, K, K- spectra versus centrality (130
GeV/N data in nucl-ex/0206008)
9
Anti-Proton Spectra at 200/130 GeV/N
Au Au ? ?p X
?p

• p and p-bar spectra versus centrality
• (130 GeV data in PRL 87 (2002))

10
Anti-Particle to Particle Ratios
K/K- ratios
?
p-bar/p ratios
Excellent agreement between experiments at y 0,
Ös 130

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

11
Anti-Baryon/Baryon Ratios vs. ?sNN

• In the early universe
• p-bar/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
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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
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Chemical Freeze-out (thermal model)
(P. Braun-Munzinger et al. hep-ph/105229)

• Assume - Thermally and chemically equilibrated
  fireball at hadro-chemical freeze-out
• 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
14
Putting STAR on the Phase Diagram

• Final-state analysis suggests RHIC reaches the
  phase boundary
• Hadron yields cannot probe higher temperatures
• ltEgt/N 1 GeV
• (J. Cleymans and K. Redlich, Phys. Rev. Lett.,
  81, 5284, 1998)

Lattice results
Neutron STAR
We know where we are on the phase diagram but now
we want to know what other features are on the
diagram
15
Chemical and Kinetic Freeze-out

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

• 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
The transverse radial expansion of the source
(flow) adds kinetic energy to the particle
distribution. So the classical expression for
ETot suggests a linear relationship
K -
?p

• Slopes decrease with mass. ltpTgt and the
  effective temperature increase with mass.

17
Kinetic Freezeout from Transv. Flow
ltßrgt (RHIC) 0.55 0.1 c TKFO (RHIC) 100
10 MeV
Explosive Transverse Expansion at RHIC ?
High Pressure
18
Anisotropic Flow

• Look at peripheral collisions
• Overlap region 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 ? Momemtum anisotropy
• Interactions among constituents generate a
  pressure 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
• vn is the n-th harmonic Fourier coefficient of
  the distribution of particles with repsect to the
  reaction plane
• v1 directed flow
• v2 elliptic flow

19
v2 vs. Centrality (130 GeV)

• 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 (130 GeV)

• 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 very good job
21
v2 for p, K, K0, ?p and L
Preliminary
Preliminary
22
v2 for High pt Particles (130 GeV)

• pQCD inelastic energy loss parameterized hydro
  component (M. Gyulassy, I. Vitev and
  X.N. Wang, PRL 86 (2001) 2537)
• value of v2 at high pt sensitive to the initial
  gluon density
• saturation and decrease of v2 as a function of pt
  at higher pt
• data starts to deviate from hydrodynamics at pt gt
  2 GeV/c

Adler et al., nucl-ex/0206006
Data is in qualitative agreement with
jet-quenching scenario
23
Centrality dependence of v2(pt)
130 GeV

• 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 ?

200 GeV (preliminary)
24
v2 up to 12 GeV/c
v2 seems to remain 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
Inclusive pt Distribution of Hadrons

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

27
Comparison AuAu / pp at 130 GeV
28
Flow vs. Inclusive Hadron Spectra
Different views of same physics?
Evidence for hadron suppression at high pt
Partonic interaction with matter? dE/dx?
29
Jets in Hadronic Collisions
AuAu ???? (STAR_at_RHIC)
pp ?jetjet (STAR_at_RHIC)
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Identifying jets on a statistical basis in Au-Au
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)

• Given a trigger particle with pt gt pt (trigger),
  associate particles with pt gt pt (associated)
• AuAu
• flow
• pp and AuAu collisions
• dijets
• momentum conservation
• jets
• resonances

All ??
Small ??
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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 (0.75 lt ?? lt
  2.24)

32
Central AuAu data vs. ppflow
33
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

Surface emission?

• 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

?
Suppression of back-to-back correlations in
central AuAu collisions
34
Ultra-Peripheral Collisions

• b gt 2RA
• no hadronic interactions
• ltbgt 20-60 fermi at RHIC
• Ions are sources of fields
• Fields couple coherently to ions
• pt lt h/RA, 30 MeV/c for heavy ions
• p lt gh/RA 3 GeV/c at RHIC
• Photonuclear (Photon-Pomeron) Interactions
• gA r0, w, f, J/y, A
• Vector Meson Dominance
• gA qqA (elastic scattering) VA
• s(r) 350 mb at 130 GeV/nucleon
• 5 of sAuAu(had.)
• Electromagnetic particle production
• gg leptons, mesons
• Strong Field (nonperturbative?) QED

-
35
Exclusive r0
200GeV

• Trigger on low multiplicity events
• veto on cosmic rays
• 2 track vertex w/ charge 0
• reject (coplanar) cosmic rays
• peak for pt lt 150 MeV/c
• pp and p-p- give background shape
• scaled up by 2.1

Signal region ptlt0.15 GeV

• Preliminary

r0 pt
ptlt0.15 GeV
M(pp-)
36
Minimum Bias Data
200 GeV

• 800,000 triggers neutron signals in ZDCs
• Nuclear excitation tags small b interactions
• excitation and r0 are independent
• Analysis same as in peripheral

Signal region ptlt0.15 GeV
Preliminary
r0 pt
200 GeV
Preliminary

• Normalized to 7.2 b hadronic cross section
• Systematic uncertainties luminosity, overlapping
  events, vertex tracking simulations, single
  neutron selection, etc.

ds/dMpp (mb/GeV)
M(pp-)
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Summary of Analyses

• shown
• Particle yields / ratios / spectra
• Anisotropic Flow
• Jets at RHIC
• Ultra-Peripheral Collisions

• not shown
• Interferometry (HBT)
• Fluctuations
• Gluon density saturation
• Spin physics program

38
Conclusions About Nucl. 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
  greater than 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 not inconsistent with thermal
  equilibrium
• Excellent fits to particle ratio data with
  equilibrium thermal models
• Excellent fits to flow data with hydrodynamic
  models that assume equilibrated systems

Looks like we are observing the QGP!
39
RHIC Performance Goals for 2003

• 29 weeks of d-Au (including cooldown)
• 8 weeks of p?p?
• (We wont have Si-Si nor Au-Au next year.)

More interesting physics to come ...
40
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, Valparaiso, 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 The
Netherlands NIKHEF Poland Warsaw
University of Technology Russia MEPHI -
Moscow, IHEP - Protvino LPP LHE JINR - Dubna
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Encore Slides
42
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

43
RAA Comparison to pt 6 GeV/c
130 GeV nucl-ex/0206011
Preliminary ?sNN 200 GeV
Preliminary ?sNN 200 GeV
Similar Suppression at high pt in 130 and 200 GeV
data