Starting the Energy Scan - First Results from 62.4 GeV Au Au Collisions

1
Starting the Energy Scan - First Results from
62.4 GeV AuAu Collisions

• Introduction.
• High pT.
• Bulk matter observations.
• Collective motion.
• Summary

2
The story so far.

• We have discovered a strongly interacting medium
  with extremely high energy density which cannot
  be described in terms of simple hadronic degrees
  of freedom.

Phobos
3
Nuclear modification factor at 200 GeV
Charged Hadrons
d2N/dpTd? (AuAu)
RAA
NColld2N/dpTd? (pp)
High pT suppression and loss of back-to-back
signal in AuAu but not in dAu showed that
effect due to Jet Quenching in final state NOT
initial state parton saturation (GCG).
BRAHMS
Can we turn this effect off?
Observed by all 4 experiments
4
Need for an energy scan

• Provide constraints on jet quenching models.
• Study the excitation function of baryon
  transport.
• Constrain models for hadron production at
  intermediate pT.

Why 62.4 GeV?

• Located (on log scale) mid-way between SPS and
• 
  RHIC top energies.
• Many reference data from ISR.

5
Ncoll definition
Participants
Npart/2 A
LA1/3
Ncoll of NN collisions A4/3
Collisions
At 62.4 GeV Ncoll very different to 200 GeV
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First immediate result
Paddle Counter Signal top 50 of cross-section
200 GeV
62.4 GeV
PHOBOS Preliminary
Significant decrease in maximal multiplicity
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Charged hadron spectra and yields
Expect 62.4 GeV fit into vs systematics
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Jet quenching predictions at 62.4 GeV
I. Vitev nucl-th/0404052
Adil Gyulassy nucl-th/0405036
RAA (p0) 0.5 - 0.3 at pT 4 GeV
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RAA charged particles
same results from all 4 experiments
BRAHMS Preliminary
Maximum significantly higher than at 200 GeV.
There is a suppression for most central data
Similar shape evolution with centrality
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Universal centrality evolution?

• But, varying the beam energy changes
• Initial state sNN, Ncoll/Npart
• System ?, mB, Nch,
• Partonic xT, dE/dx

• Energy-independent
• Weak function of Npart, pT

.

• Use central AA as denominator
• Scale with 1/ltNpartgt
• Allows comparison between different experiments

nucl-ex/0405003
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PID spectra
BRAHMS
Phenix preliminary p0
W-
X-
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RAA for p0 for central data
Predictions 0.3 - 0.5 at 4 GeV/c Again maximum gt
200 GeV
STAR Preliminary
Charged ISR reference used for p0
Still discrepancy between charged and p0 Large
baryon contribution up to at least 4 GeV/c
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Rcp of baryons and mesons
Stat. Errors Only
STAR preliminary
STAR Preliminary
(stat. errors only)
The RCP(baryon) gt RCP(meson) at intermediate pT.
Problem with limited statistics.
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Baryon/meson ratios

• Again, large proton contribution at intermediate
  pT
• Small difference as function of centrality (not
  very peripheral 30-60)

STAR Preliminary
Less?p in central collisions at 62.4 GeV
Ratios factor 2-3 higher than in pp at pT 2-4
GeV
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Ratios at mid-rapidity
Clear systematic trend with collision energy
Minbias (0-80)
62.4 GeV
STAR Preliminary
Stat. Errors Only
STAR Preliminary
?L/L
?-/ ? 1.0170.002 K-/K 0.8350.006 ?p/p
0.4580.005
confirm varying y same effect as varying vs.
Ratios flat as function of pT
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Statistical model results
STAR preliminary AuAu at vsNN200GeV
and 62 GeV
TLQCD160-170MeV
TLQCD160-170MeV
Energy dependence but small Nch dependence
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(In)dependence of mid-rapidity yields
Preliminary
Preliminary

• T, µB, and V all vary with energy,

but in such a way as to ensure yields stay
constant
Preliminary
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Radial flow
Blastwave fit
Shape of the mT spectrum depends on mass
Tch
Radial flow!
p K p
STAR Preliminary
STAR Preliminary
STAR Preliminary
Same flow as at 200 GeV
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Kaon Slopes
Top 5 central collisions
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? interferometry

• HBT probes space-time evolution of system and
  system size at freeze-out.
• Studies at vs130, 200 GeV yielded similar HBT
  radii to SPS energies (HBT puzzle).
• Severe challenge to hydrodynamic calculations.
• At an intermediate energy, a larger expansion
  time might point to a long-lived mixed phase.

Systematics of central 0-5
Fully consistent Coulomb treatment in kT
dependence
same results from PHOBOS
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HBT from SPS to RHIC
No sign of qualitatively different expansion
dynamics at 62 GeV.

• Continues to be a severe
• challenge

submitted to Phys. Rev. C Rapid Communications
Same results from STAR
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Charged particle correlations
pT
Substantial signals attributable to elliptic flow
(v2 ltcos(2f)gt)
v2 apparently saturates and is the same as at 200
GeV
Jets are going to be a challenge
23
Longitudinal elliptic flow
hh-ybeam
Longitudinal scaling of v2
nucl-ex/0406021
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Identified Elliptic Flow
Although statistics not great again resembles 200
GeV
25
Quark coalescence?
Coalescence at intermediate pT leads to
v2 /nquark
The scaling works as for 200 GeV AuAu
Seems to be slightly lower than 200 GeV for
pT/nquarklt1 GeV/c
pT /nquark GeV/c
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Summary
Many of the results indicate environment similar
at 62.4 GeV to 200 GeV but its not identical

• Initial energy density lower
• Evidence of jet quenching
• Cronin effect is stronger
• HBT radii at 6 fm
• Chemical freeze-out conditions similar to 200 GeV
  (fn Nch)
• Baryon chemical potential much higher
• Radial flow as strong as at 200 GeV (fn Nch)
• Elliptic flow as strong as at 200 GeV (fn
  centrality)
• Consistent with Nquark scaling
• Elliptic flow appears universal at forward
  rapidities
• Jet contribution much weaker than at 200 GeV

As yet un-answered questions Are the gluon
densities the same in both systems? Do they spend
the same amount of time in each stage? i.e. do
they reach the same freeze-out conditions in

the same manner?
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• back up

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Why an energy scan?
Varying the geometry (A,b)

• jet quenching
• vary overall path length
• vary asymmetry
• Elliptic (and directed) flow
• Study A dependence at fixed eccentricity.

Npart
eccentricity
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pp references
/-25 uncertainty
p0 reference
charged reference
p0 reference