X Mexican School of Particles and Fields

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Recent advances from the STAR Experiment

• Highlights from
• Inclusive hadron spectra
• Azimuthal correlations

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Outline

• Heavy Ion Physics and QCD
• STAR experiment at RHIC
• Measurement highlights of interest to High Energy
• Case I Inclusive charged hadron spectra
• Case II Azimuthal anisotropy
• Case III Two-particle correlations
• Conclusions

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Heavy Ions How does nuclear matter look at high
temperature?

• High Density QCD Matter in Laboratory
• Determine its properties
• QCD Prediction Phase Transitions
• Deconfinement to Q-G Plasma
• Chiral symmetry restoration
• Relevance to other research areas?
• Quark-hadron phase transition in early Universe
• Cores of dense stars
• High density QCD

e 1-3 GeV/fm3
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The Relativistic Heavy Ion Collider
Two Superconducting Rings
Design Performance Au Au p p Max ?snn
200 GeV 500 GeV L cm-2 s -1 2 x 1026
1.4 x 1031 Interaction rates 1.4 x 103 s -1 6
x 105 s -1
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The STAR Experiment
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Detector components in STAR
1st year detectors (2000) 2nd year detectors
3rd year detectors
Coils
TPC Endcap MWPC
Zero Degree Calorimeter
Central Trigger Barrel
RICH
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Focus on high pt

• We know very little about early time
• AuAu collisions to study strongly interacting
  matter under extreme conditions
• Large momentum transfers ? early time scales
• Use high pt jet phenomena as probe of medium
• Hard scattering has been done but not in hot
  medium
• Measurement of fragmentation products ? insight
  into gluon density1

1 R. Baier, D. Schiff, and B. G. Zakharov,
Annu. Rev Part. Sci. 50, 37 (2000).
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Centrality and Participants in HI
Npart (Wounded Nucleons) soft production Nbin
hard processes
peripheral (grazing shot)
Centrality classes based on mid-rapidity
multiplicity
central (head-on) collision
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Case I Leading hadron suppression
Wang and Gyulassy DE ? softening of
fragmentation ? suppression of leading hadron
yield
Ivan Vitev, QM02

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High pT hadrons in AuAu
Preliminary
(nucl-ex/0206011, PRL in press)
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Inclusive charged hadron suppression
130 and 200 GeV, Central/peripheral
130 GeV normalized to NN centrality dependence
Clear evidence for high pT hadron suppression in
central collisions ?significant nuclear
interactions to very high pT Now seen by all 4
RHIC collaborations (BRAHMS, PHENIX, PHOBOS, STAR)
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Case II Azimuthal Anisotropy, or Elliptic Flow
Fourier analysis ? 12v2cos2(?lab-?plane)
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Case II Azimuthal Anisotropy, or Elliptic Flow
Finite v2 at high pt pT gt 2GeV v2 constant
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Method I Direct Jet Identification

• jet-jet correlations in pp?

• jet-jet correlations in AuAu?

• Comparison ?statistical method

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Method II High pT Correlations

• Statistical leading particle analysis
• Histogram in 2-d
• N ?? vs. ??
• project

Ntrigger Total number of trigger particles
(4ltpTlt6)
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Result AuAu Distribution

• Harmonic structure
• Peaks at 0, ?
• Non-zero mean value
• How do we extract jet signal from background?

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Background Subtraction

• di-jets
• Flow
• Combinatorial background
• Resonance decays
• jets

Subtract large ?? correlations Isolate intra-jet
correlations Removes di-jet signal
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First Results STAR 130 GeV
Significant peak remains after subtraction Jets?!
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Jets at 200 GeV
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Jets at 200 GeV

• Shape
• Clear near away side signal
• Same sign correlation
• Unlikely due to resonance decays

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Jets at 200 GeV

• Shape
• Clear near away side signal
• Same sign correlation
• Unlikely due to resonance decays

di-jets in AuAu?
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Jet Charge
Measured by DELPHI Well described by LUND string
model
Expect opposite charge sign between leading,
next-to-leading charged particles
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Jets at 200 GeV
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Jets at 200 GeV
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What Have we Shown?

• First direct evidence of jets at RHIC

• What about di-jets at RHIC?
• Study away side in AuAu
• But large ?? subtraction removes away side
• Need different method to deal with background

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Reference Model

• Incorporate known sources of signal and dominant
  background

• AuAu correlations
• Jets
• di-jets
• elliptic flow
• multiple hard-scatterings per event

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Reference Model

• Algorithm

AuAu measurement Background term
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Data Comparison to Ref. Model

• Absolute scale
• Background contribution increases with centrality

• 4/7 centrality bins
• Other bins qualitatively, quantitatively similar

• Near side well matched for all centralities

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Data Comparison to Ref. Model

• Away-side suppression

• Suppression increases with increasing centrality

• Quantify with centrality

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Quantify with Ratio
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Dissappearance of the Jets from the Far Side
Centrality dependent numerator Common denominator

• Sys. errors v2 5/-20

Away-side suppression in central AuAu

• HIJING model constant ratio1

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Suppression of away-side jet consistent with
strong absorption in bulk, emission dominantly
from surface
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?s dependence (200/130) at high pT

• Inclusive spectra growth with ?s follows pQCD
  prediction (XN Wang)
• (systematic uncertainties are correlated better
  estimates in progress)

• v2 independent of ?s for pTgt2 GeV/c
• Geometric origin of v2 at high pT?

Rates change but shape does not.
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Away side suppression open issues

• Why not 1 for peripheral?
• evidently not due to experimental error or
  uncertainty
• not due to mismeasured v2 even v20 has little
  effect for most peripheral and central

• Initial state effects
• Shadowing in AuAu?
• Nuclear kT Initial state multiple scattering
  ?????
• Hijing estimate Maximum 20 effect
• Resolution Need to measure in dAu

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Summary of STAR high pT measurements

• hadrons at pTgt3 GeV/c are jet fragments
• central AuAu
• strong suppression of inclusive yield at pTgt5
  GeV/c
• suppression factor constant for 5ltpTlt12 GeV/c
• large elliptic flow, finite for non-central to
  pT6 GeV/c
• strong suppression of back-to-back hadron pairs
• Possible interpretation
• Hard scattered partons (or their fragments)
  interact strongly with medium
• Observed fragments are emitted from the surface
  of the hot dense zone created in the collision

?
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And back to our original question

• If partons absorbed large DE ? large ?gluon
• But have not yet proven partonic DE where does
  absorption occur?
• Is it an initial state, partonic effect, or late
  hadronic effect?
• theory input what are experimental handles to
  distinguish hadronic from partonic absorption?
  (e.g. correlation function widths)

JETS
JETS
?
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Extra Slides
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Look for partonic energy loss in dense matter
Thick plasma (Baier et al.)
Gluon bremsstrahlung
Thin plasma (Gyulassy et al.)

• Linear dependence on gluon density ?glue
• measure DE ? gluon density at early hot, dense
  phase
• High gluon density requires deconfined matter
  (indirect QGP signature)

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Future

• Coming run 50 of full barrel Electromagnetic
  Calorimeter
• triggers high tower, ET, jet
• jets, p0, g, electrons
• dAu
• Cronin effect/nuclear ltkTgt
• enhancement of inclusive yield
• suppression of back-to-back pairs
• gluon shadowing
• Long term
• g-jet coincidences (ultimate jet energy loss
  experiment)
• heavy quark jets (dead cone effect)
• surprises.

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Soft Physics

• Chemical Freezeout 170 MeV
• Lattice 160 - 180 MeV
• Collective motion
• Large Elliptic flow
• Large pressure gradients in the system
• System seems to approach thermodynamic
  equilibrium
• Kinetic freezeout 110 MeV
• Freezeout seems to be very fast, almost explosive

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Energy loss in cold matter
Wang and Wang, hep-ph/0202105
F. Arleo, hep-ph/0201066
Modification of fragmentation fn in e-A dE/dx
0.5 GeV/fm for 10 GeV quark
x1
Drell-Yan production in p-A dE/dx lt0.2 GeV/fm
for 50 GeV quark
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Inclusive hadron suppression at RHIC
Phenix p0 peripheral and central over measured
pp
STAR charged hadrons central/peripheral
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v2 comparison to parton cascade
Parton cascade (D. Molnar)

• Detailed agreement if
• 5x minijet multiplicity from HIJING or
• 13x pQCD gg?gg cross section

? extreme initial densities or very large cross
sections
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v2 centrality and pT dependence

• broad plateau, v2 finite at pT10 GeV/c except
  for most central collisions
• significant in-medium interactions to very high
  pT
• Shuryak (nucl-th/0112042) plateau exhausts
  initial spatial anisotropy

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Near-angle correlations at high pT

• Jet core Df x Dh 0.5 x 0.5
• look at near-side correlations (Df0) of high pT
  hadron pairs
• Complication elliptic flow
• high pT hadrons that are correlated with
  reaction plane orientation are also correlated
  with each other (v22)
• but elliptic flow has long range correlation (Dh
  gt 0.5)
• Solution compare azimuthal correlation
  functions for Dhlt0.5 and Dhgt0.5

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Non-flow effects?

• Non-flow few particle correlations not related
  to reaction plane
• jets, resonances, momentum conservation,
• ? contrast v2 from reaction plane and
  higher-order cumulants (Borghini et al.)

• Non-flow effects are significant
• 4th order cumulants consistent with other
  non-flow estimates
• But large finite v2 and saturation persist at
  high pT

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Single Particle Selection
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Away side suppression and nuclear kT

• same thresholds for AuAu and pp
• nuclear ltkTgt
• enhances near-side in AuAu
• suppress away-side in AuAu
• similar centrality dependence

Stronger near-side correlation for pTtriggt3 GeV/c
than pTtriggt4 GeV/c
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Full dataset 4ltpt(trig)lt6 GeV/c
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Central AuAu 6ltpT(trigger)lt8 GeV/c
Stronger signal but limited statistics in
non-central bins
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Combinatorial Background

• pp 1 hard scattering per event
• Expect peak at 0, ?
• zero background

• AuAu many hard scatterings per event
• Expect peak at 0, ?
• Flat, non-zero background