New Results from the STAR Experiment

1
New Results from the STAR Experiment

• Markus Oldenburg
• Lawrence Berkeley National Laboratory
• High-Energy Physics Seminar
• University of Toronto

November 15, 2005
2
Overview

• The STAR Experiment at RHIC
• Physics
• I High-pT Measurements
• Modification of Jet Fragmentation
• II Low-pT Results
• Radial and Anisotropic Flow
• Anisotropic Flow of Strange Hadrons
• The Future of STAR
• III Detector Upgrades
• The Heavy-Flavor Tracker
• Summary

3
The Relativistic Heavy Ion Collider

• two independent accelerator rings
• 3.83 km in circumference
• accelerates everything from p to Au
• running conditions so far
• AuAu _at_ 20 GeV
• AuAu _at_ 62.4 GeV
• AuAu _at_ 130 GeV
• AuAu _at_ 200 GeV
• CuCu _at_ 62.4 GeV
• CuCu _at_ 200 GeV
• dAu _at_ 200 GeV
• p?p? _at_ 200 GeV
• p?p? _at_ 400 GeV

Long Island
4
The STAR Experiment at RHIC
5
One of the first Au on Au Events at CM Energy of
200 GeV?A
6
Timeline of a Heavy-Ion Collision
II
I
by S. Bass
7
Jets in Nucleus-Nucleus Collisions
I
AuAu ???? (STAR_at_RHIC)
pp ?jetjet (STAR_at_RHIC)
8
Evolution of ?? correlations at RHIC
?? correlations
Associated

• Trigger-associated technique valuable for
  tagging jets in high-multiplicity environment
  (vs. jet-cone algorithms)
• Probes the jets interaction with the QCD medium
• Provides stringent test of energy-loss models

Associated
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Probing fragmentation region with pT?

• How does ?? correlation evolve with increasing
  pT?
• Does away-side jet punch through?
• Is the fragmentation modified? If so, how?
• Does away-side suppression disappear at very high
  pT?

• Successful RHIC Run 4 (2004) ? 10x more
  central, minbias AuAu
• STAR event reconstruction ongoing
• Raising pT thresholds allows to
• Extend into fragmentation region
• Significantly reduce uncorrelated background,
  relative event shape (v2) contribution
• Look for leading away-side hadrons

10
Emergence of dijets with increasing pT(trig)

• ?? correlations (not background subtracted)

AuAu, 0-5 most central

• Hint of narrow back-to-back peak for higher
  pT(trig)
• Higher pT(trig) reflects higher-Q2 hard scattering

11
The beginnings of dijet tomography

• ?? correlations (not background subtracted)

• Narrow peak emerges cleanly above vanishing
  background.

• Near-side peak (??0) gets narrower for higher
  pT(assoc).
• Harder fragments are more closely aligned with
  jet direction.

preliminary
8 lt pT(trig) lt 15 GeV/c
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Characterization of recoiling jet
First differential measurement of energy loss
dN/dzT
by D. Magestro
dN/dzT hadron triggered fragmentation
function (X.-N. Wang) works without direct
knowledge of parton energy
10-1
0.54

• Recoil yield is suppressedIAA 0.25 RAA
  (central collisions)
• Fragmentation distribution essentially unchanged
  (zTgt0.4)

0.25
8 lt pT(trig) lt 15 GeV/c
STAR preliminary
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Comparison to a theoretical prediction
X-N Wang, PLB 595 (2004) 165

• X.-N. Wang Away-side fragmentation altered
  relative to pp due to partonic energy loss in
  the medium
• drops steeply until zT0.4
• roughly constant above zT0.5
• STAR data is lower than the prediction.

14
Hadron Spectra from RHICmid-rapidity, pp and
AuAu collisions at 200 GeV
II
5 10-20 20-40 40-60 60-80
centrality
Data from BRAHMS, PHENIX, and STAR.
15
Transverse Radial Flow
AuAu at 200 GeV
? -
STAR Preliminary
K -
?p

• Slopes decrease with mass.
• ?pT? and the effective temperature increase with
  mass.

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Radial Flow of Different Hadron Species

• freeze-out of multi-strange hadrons
• - at higher
• temperature Tfo
• - with lower
• collective
• velocity ??T?
• small cross section of strange hadrons with
  non-strange hadrons
• sensitivity to the early, partonic stage

Results from Blast-wave fits
STAR
Nucl. Phys. A715, 458c (2003) Phys. Rev. Lett.
92, 112301 (2004) Phys. Rev. Lett. 92, 182301
(2004)
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Anisotropic Flow vn

• non-central collisions azimuthal anisotropy in
  coordinate-space
• interactions ? asymmetry in momentum-space
• sensitive to early time in the systems evolution
• Measurement Fourier expansion of the azimuthal
  pT distribution

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v2 at low pT
min. bias
simulations P. Huovinen, private communications,
2004

• At low pT hydro model results fit mass hierarchy
  well!
• - Differences in the details

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v2 of identified hadrons up to pT 9 GeV/c
PHENIX (open symbols) Phys. Rev. Lett. 91,
182301 (2003)
200 GeV AuAu

• saturation and onset of decline at pT gt 3 GeV/c
• clear meson?baryon scaling at intermediate pT
• scaling extends out to high pT

20
Number-of-Constituent Quark-Scaling of v2

• scaled meson and baryon v2 agrees at intermediate
  pT
• high statistics measurements show deviation from
  ideal scaling

200 GeV AuAu
STAR preliminary
PHENIX (open symbols) Phys. Rev. Lett. 91,
182301 (2003)
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Importance of v2 of the ? meson
STAR Phys. Rev. C65, 041901(R) (02) Phys.
Lett. B612, 181 (2005)
Kaon coalescence effectively ruled out as a
dominant channel for ? production at these
energies.

• ? is a true signal from the early stage of the
  collision!
• anisotropic flow measurement of ? will conclude
• type of the system at an early stage
• partonic vs. hadronic
• origin of observed scaling at intermediate pT
• mass vs. particle type

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Particle identification and mass spectra

• ?
• identified via decay topology
• ? K K-
• background estimation by event mixing
• gt 10,000,000 particles identified

O- and (anti-O) identified via decay
topology ? ? ? K and ? ? p ? background
estimated by rotating the ? by 180 in the
transverse plane gt 13,000 particles identified
?- and (anti- ?) identified via decay
topology ? ? ? ? and ? ? p ? background
estimated by rotating the ? by 180 in the
transverse plane gt 450,000 particles identified
? results by Sarah Blyth
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v2 of multi-strange hadrons
_

• common fit to K and ?
• n number of constituent quarks
• X. Dong et al.,
• Phys. Lett. B597 (2004)

v2
?? OO ?
_
baryons
mesons
pT/n gt 0.75 GeV/c ?2/ndf for NCQ2 ?2/ndf for NCQ3
?? 82.7/6 27.4/6
OO 4.1/3 2.1/3
? 2.0/5 7.4/5
_
200 GeV AuAu
_
? results by Sarah Blyth
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Discussion of strange hadron v2 results
K0S and ? results by Yan Lu
STAR preliminary
? results by Sarah Blyth
Hydro P. Huovinen, priv. comm. (2004)

• ? flows as strongly as other mesons.
• Collective flow is generated during the partonic
  stage already.

• The strange baryons ? and O show strong elliptic
  flow.
• The ? v2 clearly follows the baryon v2(pT).

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Conclusions from the low-pT results

• Collectivity develops early among partons!
• Partonic Collectivity at RHIC

26
Timeline of a Heavy-Ion Collision
III
by S. Bass
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Heavy-Flavor Quarks
B. Mueller, nucl-th/0404015.

• Even in a QGP with chiral restoration, the c, b
  (and t) quarks stay heavy.

• If charm quarks show collective behavior ( flow)
  the must have suffered frequent interactions with
  the light quarks u, d, s.
• The light quarks u, d, s must be thermalized.

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Charm Elliptic Flow via the electron channel

• D ? e X
• Sizeable elliptic flow
• But large background ? ? ee- ?0 ? ee- ?
  ...? large stat. and syst. uncertainties
• ? Need direct open charm reconstruction!

M. Kaneta (PHENIX), J. Phys. G Nucl. Part. Phys.
30, S1217 (2004). F. Laue et al. (STAR), J. Phys.
G Nucl. Part. Phys. 31, S27 (2005).
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Open Charm Flow

• Two extreme scenarios
• (a) No charm quark flow (PYTHIA)
• (b) Charm quark flow (Hydro)
• ? Differences in D-meson spectra 30 at pT lt 2.0
  GeV/c
• D ? e X electron spectra indistinguishable!
• Electron spectrum contains no information on
  dynamics
• Need direct open charm reconstruction to low pT!

S. Batsouli et al., Phys. Lett. B 557 (2003) 26.
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A Heavy-Flavor Tracker for STAR

• D0 ? K p, c? 1.23 cm? need precise tracking
  device
• Two layers r 1.5, 5.0 cm
• 24 ladders
• 2 cm by 20 cm
• CMOS Sensors (monolithic)
• 100M pixels total , 100 k/cm2
• precise (lt10 µm)
• thin and low power
• 50 µm thick chip air cooling
• 0.36 radiation length
• Low power budget 100 mW/cm2

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The Heavy-Flavor Tracker in STAR
Heavy-Flavor Tracker
Magnet
Coils
E-M Calorimeter
Time Projection Chamber
Trigger Barrel
Electronics Platforms
Forward Time Projection Chamber
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Summary

• High-pT physics
• Jet fragmentation is modified by the medium.
• Higher statistics allows for quantitative
  measurements.
• Model comparisons involving partonic energy loss
  are in qualitative agreement with the data.
• Low and intermediate pT region
• Hydro models describe the data well at low pT.
• At intermediate pT NCQ scaling is observed
• (and deviations of the ideal scaling as well).
• ?, ?, and ? behave like other mesons and baryons.
• Flow develops during a partonic stage of the
  system!
• Detector upgrades
• STAR aims to build a new silicon tracker based on
  CMOS pixel technology for direct D reconstruction
  to answer the question of the systems
  thermalization.

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The Heavy-Flavor Tracker in STAR
Heavy-Flavor Tracker
Magnet
Coils
E-M Calorimeter
Time Projection Chamber
Trigger Barrel
Electronics Platforms
Forward Time Projection Chamber