UCT Seminar VI: Scaling of Particle Yields Counting on QCD

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UCT Seminar VIScaling of Particle
Yields(Counting on QCD)

• Peter Steinberg
• Brookhaven National Laboratory
• Fulbright Scholar Program

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SppS Collisions
UA1, 900 GeV
proton
anti-proton
?s 200, 546, 900 GeV
10s of particles
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RHIC Collisions
?sNN 130, 200 GeV
Gold
Gold
(center-of-mass energy per nucleon-nucleon
collision)
1000s of particles
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Heavy-Ion Collisions
VNI Simulations Geiger, Longacre, Srivastava,
nucl-th/9806102
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2
3
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Colliding Nuclei
Parton Cascade
Hadron Gas Freeze-out
HardCollisions
No transverseenergy!
E2-5 GeV?
E0-2 GeV
E5-10 GeV?

• Entropy produced as system evolves
• Where does most of it come from?
• Initial, partonic or hadronic stage?

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Learning by Counting

• Larger systems produce more particles
• But can carefully counting
• the number of final state particles
• help us understand
• the nature of the entire history of the
  collision?
• Many things to consider
• Geometry how many elementary collisions?
• Interactions how are particles produced?
• Time evolution how do the interactions change?

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Geometry of AA Collisions

• Binary Collisions
• Jet Production
• Heavy Flavor

b
Glauber model of AA
Binary Collisions
Npart, Ncoll

• Color Exchange
• Soft Hadron Production
• Transverse Energy

Participants
b (fm)
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How are particles produced?
p
p
p
p
e
q
p
g
g
K
q
e-
g
K
Hard partons radiate brehmsstrahlung gluons ?
which themselves radiate (_at_ lower scale) ? and
eventually hadronize (_at_ hadronic scales)
p
p
If hadronization is soft, multiplicity counts
soft gluons!
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Rapidity

• Hadronic collisions are characterized by limited
  transfer of transverse momentum
• Most particles we observe carry only small
  fraction of (anti)proton longitudinal momentum (x
  pz/pz,max)
• Rapidity variable increases dynamic range
  (xlt.1)
• Lorentz boost changes y by a constant

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Pseudorapidity

• Rapidity requires complete characterization of
  4-vector
• Conceptually easy, but requires a spectrometer
• Experiments with high multiplicities and limited
  resources use pseudorapidity
• dN/dh related to dN/dy, but not the same,
  especially for slower particles

?
beam axis
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Pseudorapidity Distributions in pp
UA5, ZPC 33 (1986) / CDF, PRD 41 (1990)
All charged particles
dN/dh
Anti-proton fragmentation region
Protonfragmentation region
mid-rapidity plateau
h
h
1
-1
2
-2
0
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Particle density near midrapidity
Scale AA multiplicity by Npart/2to compare to pp
data
50 increaseat all energies
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How should dN/dh vary with Npart?

• Two-component models

• Saturation Models

(EKRT, KN, McLV,)
(HIJING, KN,)
At low-x, gluons recombine at a critical density
characterized by saturation scale Qs2 1-2 GeV
(RHIC)
Soft(color exchange) wounded nucleons
Hardbinary collisions
RHIC nuclear collisions are like eA collisions at
HERA!
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Two-component vs. Saturation

• Two PHOBOS measurements at midrapidity vs. energy
• 2-component fits use Glauber picture to
  interpolate between pp and central AA
• x goes from .09 to .11
• Predicted by minijet approach
• Saturation model also describes basic trend and
  energy dependence for Npartgt60

pp
Central AuAu
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Is Mid-rapidity All You Need?

• The big RHIC experiments only measure near h0
• This is where hard processes are predicted to
  appear
• Per participant, 50 difference between
• Proton-proton (Ncoll1, Npart2)
• Nucleus-Nucleus (Ncoll1000, Npart350)
• In the 2-component picture, this means that
  40-50 of particles derive from hard processes
  (minijets)
• This interpretation is fine for many people,
  since it means that there are a lot of hard
  processes to play with at RHIC!
• However, what if our detectors were not so
  limited to measurements at 90 degrees?

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PHOBOS Multiplicity Centrality
Events
Paddle Signal
(monotonic w/ Npart)
4p Multiplicity Detector
500
0 keV
5.4
-5.4
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PHOBOS Data on dN/dh
130 GeV
200 GeV
19.6 GeV
PHOBOS Preliminary
dN/dh
Most Central
Npart
h
h
h

• AuAu collisions at ?s19.6, 130, 200 GeV
• dN/dh for hlt5.4 over full azimuth
• Centrality from paddles (130/200) Nhits (19.6)
• Top 50 of total cross section (Npart65-360)

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Limiting Fragmentation
Plotting dN/dh vs. shows scaling behavior
inthe forward region?
PHOBOS AuAu
200 GeV
19.6 GeV Preliminary
130 GeV
dN/dh?/?Npart/2?
19.6 GeV
h? h - ybeam
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Centrality Dependence of dN/dh?
CentralityDependence
Interpretation
Location

• Are these effects related?
• Long-range correlations?
• Energy conservation?
• Stopping?
• Other collision systems?

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Rapidity Distributions at 200 GeV
q
q
200 GeV Central AuAu
ee- measures dN/dyT(rapidity relative
tothrust axis)
yT
h
AA/pp 1.4-1.5
Surprising agreement in shape between AA/ee- /pp
Correspondence between perturbative and
non-perturbative approaches?
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Particle density near midrapidity
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Total Multiplicity vs. Beam Energy
Central AA
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Leading Particle Effect
?Nch?
(pp)
Basile et al (1980-84)
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Approach to Universality
pp?pX
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How can AA scale like ee-?
E910
pA collisions
NA49
n counts collisions Npartn1

• With increasing n
• Proton stops (i.e. deposits energy)
• Pion yield saturates

• Above n3, pion yields constant
• Central AA has n5-6 per participant
• Scaling with Npart
• Reduces leading particle effect
• Scaling with ?s

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Centrality Dependence of ?Nch?
Error band due to high-h extrapolation
sinel42 mb (RHIC)
AuAu
sinel33 mb (SPS)
sinel21 mb (AGS)
Glauber Monte Carlo
19.6 GeV Preliminary
The Return of the Wounded Nucleon Model
Bialas Czyz 1976, Elias et al 1978
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Any Lessons Learned?

• We understand ee- in a perturbative language
  (can write down diagrams)
• Radiative processes lead to particles
• Initial hard scale makes problem tractable (since
  coupling constant is small at first)
• We dont know precisely the initial scale in pp
  or AA collisions
• Particle production probably also radiative
• Saturation models suggest typical scale of 1-2
  GeV
• Particle production not sensitive to the hard
  scale
• Rather, available energy seems to dominate
• Moreover, particle production scales simply with
  number of participating baryons
• Fewer degrees of freedom than expected!
• But what about real hard processes? Friday