Soft Hadron Production at RHIC

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Soft Hadron Production at RHIC
Masahiro Konno (University of Tsukuba)
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Outline

• A Picture of Relativistic Heavy Ion Collisions
• Motivation (this talk)
• RHIC-PHENIX
• Single spectra at low pT ( 2 GeV/c)
• Single spectra at Intermediate pT (25 GeV/c)
• Elliptic flow
• Jet modification in the medium
• Summary

Results
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Picture of Relativistic Heavy Ion Collisions
Space-time evolution

• the created system lasts for only 10 fm/c.

time
4. Cooling, Freeze-out
3. Hadronization
2. Thermalization QGP
1. Hard scattering
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Motivation
Why soft particle production important to heavy
ion collisions? gt Soft particle production
dominates the bulk yield. Through this
study, we can know bulk properties of the
system including collective flow. Hadrons
interact strongly so it is a probe to know
the evolution of the system and medium
effect to hadronization.
Useful Observables - Single particle spectra
- Azimuthal anisotropy of particle emission -
Modification of jet shape/yield in the medium
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RHIC-PHENIX
EM Calorimeter (PID)
TOF (PID)
- Central Arm Detectors (magnetic
spectrometer) - Event Characterization detectors

• Centrality and Reaction Plane
• determined on an Event-by-Event basis.

Aerogel Cherenkov (PID)
Drift Chamber (momentum meas.)
Tracking detectors (PC1,PC2,PC3)
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• PID is a powerful tool
• to study hadron production!

PID detectors
(PID Particle Identification)
Aerogel Cherenkov
No Cut
p
K
Time of Flight (TOF)
p
Aerogel Veto
Clear Proton Line!
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Single spectra (low pT)
pT spectra for ?/K/p (mid rapidity)
PRC 69, 034909 (2004)

• ltpTgt ?ltKltp (mass dependence)
• consistent with radial flow picture.

Spectra for heavier particles has a convex shape
due to radial flow.
Tkin 100 MeV ltvT/cgt 0.5
particle spectra (blast wave fit) gt kinetic
freeze-out properties.
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Single spectra (low pT)
Statistical mode fit
particle ratios (statistical model) gt chemical
freeze-out properties. Tch 160 MeV, gs
1.0

• Hadron yields and spectra are consistent with
• thermal emission from a strongly expanding
• source (radial flow driven by pressure
  gradient).
• The observed strangeness production is
• consistent with complete chemical equilibrium.

nucl-th/0405068
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Single spectra (intermediate pT)
Nuclear Modification Factor RAA
AuAu 200 GeV
nucl-ex/0603010

• High-pT suppression due to parton energy loss
• in the medium (jet quenching).
• The suppression patterns depend on particle
  type.
• Protons are enhanced, while pions and kaons are
  suppressed.

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Baryon Enhancement
p/? ratio
PRL 91, 172301 (2003)
Elliptic flow (v2)

• - p/? ratio 1 for central AuAu at intermediate
  pT (2-4 GeV/c).
• Larger than expected from jet fragmentation
  (measured in pp, ee-).
• Baryon / Meson difference at intermediate pT.
• (on RAA (nuclear modification factor) , v2
  (elliptic flow) etc.)

What is the origin of (anti-)proton enhancement
at intermediate pT?
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Quark Recombination
p/?
Fries, R et al PRC 68 (2003) 044902 Greco, V et
al PRL 90 (2003) 202302 Hwa, R et al PRC 70(2004)
024905
At intermediate pT, recombination of partons may
be a more efficient mechanism of hadron
production than fragmentation.
A number of models predicted a turnover in the
B/M ratio at pT just above where the available
data finished
gt pT spectra and particle ratio (Baryon/Meson)
at higher pT provide most basic tool to
study the hadronization mechanism.
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pT spectra
for (anti-)protons
Using Aerogel Cherenkov detector
pT reach extended for (anti-)protons with fine
centrality bins.
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p/? vs. pT
p/?
No feed-down correction.
- p/? (pbar/?) ratios seem to turn over at
intermediate pT, and close to the value of
fragmentation at higher pT. - Indicating
transition from soft to hard at intermediate pT.
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p/? vs. pT (centrality dependence)
p/?
No feed-down correction. pp data
(nucl-ex/0603010)
- p/? ratios look to have a peak at intermediate
pT (2-4 GeV/c). - Clear peak in central events
than that in peripheral.
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Beam energy dependence in CuCu

• p/? ratio decreasing as a function of ?sNN.
• ?p/?- ratio increasing as a function ?sNN.
• CuCu 22.5 GeV central data reaches the pp
  values.
• CuCu 62.4 GeV central data is higher than that
  in 22.5 GeV.

Suggesting a significant contribution of incoming
protons (not produced protons) in lower energies
CuCu.
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Beam energy dependence of net protons
nucl-ex/0313023
nucl-ex/0410003
SPS
AGS
- The shape of net proton distribution change
dramatically with beam energy.
- pbar/? ratio could be a good indicator of
thermalization.
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RdAu vs. RAuAu
preliminary
preliminary
Pions suppressed by a factor of 5 with respect
to protons
Proton Cronin effect larger by 30

• - In dAu Particle type dependence ?, K lt p
• (recombination in dAu?)
• Cronin effect (in dAu) cannot account for the
  huge gap between
• protons and pions in central AuAu collisions.

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Elliptic Flow

• Overlap region is like ellipsoid
• at the beginning of collision.
• Spatial anisotropy of the system
• followed by multiple scattering of
• particles (pressure gradient) in the
• evolving system
• - Spatial anisotropy gt momentum anisotropy

Z
Reaction plane
Y
X
Py
Pz
v2 2nd harmonic Fourier coefficient in
azimuthal distribution of particles with respect
to the reaction plane
Px
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Elliptic Flow
M. Issah, A. Taranenko, nucl-ex/0604011

• KET mTm0
• at y 0

PHENIX preliminary

• Large elliptic flow observed.
• - Mass ordering seen at low pT (lt1.5 GeV/c).
• KET scaling (for hadronic flow) vanish this mass
  
• dependence but give clear splitting of
  meson/baryon v2.

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Elliptic Flow
M. Issah, A. Taranenko, nucl-ex/0604011
PHENIX preliminary

• Species dependence of v2 well accounted for
• by scaling v2 and pT (KET) with of quarks.
• Evidence of partonic flow! v2 is developed
  before
• hadrons form.

v2q(pT) v2h(pT/n)/n, v2q(KET) v2h(KET/n)/n
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? meson -- A test of recombination
(? -gt KK)
H.Masui CIPANP06

• ? RAA looks like ?0 rather than
• proton even if mass(?) mass(p).
• Suggest that its not mass effect
• (radial flow).

• Mass ordering at pT lt2 GeV/c.
• For pT gt2 GeV/c, v2 favors quark
• composition rather than mass

centrality 20-60
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Jet modification in the medium

• In heavy ion collisions, we expect
• (1) Scattered partons travel through the
  medium.
• (2) Partons loose their energy because of a
  large gluon density.
• (3) Suppression of high-pT leading particles.

Observables - Single spectra RAA -- to look at
yield suppression - Angular correlation -- to
look at jet modification
Jet partons interact with the medium via the
strong force. So jet can be a probe of the medium.
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Method of Jet study

• Using two-particle correlation function.
• Should treat large combinatorial back
• ground (flow, decay) in heavy ion collisions.
• Use mixed events for correction of pair
  acceptance.

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Jets at intermediate pT
Meson vs. Baryon associated partner (for fixed
Hadron trigger)
W.Holzmann HardProbes06

• Particle species dependence of jet modification
• BTW, how can hadrons at intermediate pT show
• jet-like structure? gt pickup of soft quarks
  by jets?

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Jet Associated Identified Conditional Yield
Meson vs. Baryon associated partner (for fixed
Hadron trigger)

• Different pT trends of associated meson and
  baryon yields.

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Jet Associated Baryon to Meson Ratio
Meson vs. Baryon associated partner (for fixed
Hadron trigger)
- Near-side like pp, Away-side Larger B/M
ratio. - Baryon/meson ratio can be an indicator
for the degree of thermalization (density of
comoving constituents) in a jet.
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Summary
Systematic study (especially for colliding
species, vs) with enhanced PID capabilities.