Systematics of Charged Particle Production in 4p with the PHOBOS Detector at RHIC

1
Systematics of Charged Particle Production in 4p
with the PHOBOS Detector at RHIC

• Peter A. Steinberg
• Brookhaven National Laboratory
• George Washington University, 16 Nov 2001

2
Systematic Measurements

• Do Nucleus-Nucleus collisions show collective
  behavior
• Energy (or particle) density
• Scaling with centrality
• Hard and soft processes contribute
• Rapidity plateau
• Effect of initial geometry on final state

pp collisions
pA collisions
We study this with systematics of charged
particle production Energy, Rapidity,
Centrality, Azimuthal angle
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Centrality

• Nuclei are extended
• RAu 6.4 fm (10-15 m)
• Impact parameter (b) determines
• Npart 1 or more collisions
• Ncoll binary collisions
• Proton-nucleus
• Npart Ncoll 1 (2 11 in pp)
• Nucleus-Nucleus
• Ncoll ? Npart4/3

b
Ncoll
Npart
Useful quantities to compare AuAu to NN
collisions!
b
4
Soft Hard Particle Production

• Soft processes (pT lt 1 GeV)
• Scales with number of participants
• Color exchange leads to excited nucleons that
  decay
• Create rapidity plateau
• Hard processes (pT gt 1 GeV)
• pQCD can calculate jet cross sections
• Scales with number of binary collisions
• QCD evolution leads to narrower distribution
  around y0

minijet
minijet
5
Rapidity

• Useful single-particle observable

Kinematics Change of variables
Dynamics Particle distributions are expected to
beboost invariant
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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 dN/dy for ylt2. Easily seen from Jacobian
  (dy b dh)

btanh(y)
1
-1
5
-5
y
where
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UA5 Experiment
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Energy Dependence in pp

• Feynmans postulate of boost invariance
• dn/dy plateau is energy independent
• Requires F2 1/x
• Pure parton model!
• No QCD evolution
• Violations of scaling at SppS energies
• No plateau!
• Models like HIJING can reproduce this behavior
• What about AuAu

9
RHIC Experiments

• Nucleus-Nucleus (AuAu) collisions up to ?sNN
  200 GeV
• Polarized proton-proton (pp) collisions up to
  ?sNN 450 GeV

10
PHOBOS Experiment _at_ RHIC

• Large acceptance to count charged particles
• Small acceptance, high-resolution spectrometer
• Focus is on simple silicon technology, timely
  results

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PHOBOS Collaboration (Nov 2001)

• ARGONNE NATIONAL LABORATORY
• BROOKHAVEN NATIONAL LABORATORY
• INSTITUTE OF NUCLEAR PHYSICS, KRAKOW
• MASSACHUSETTS INSTITUTE OF TECHNOLOGY
• NATIONAL CENTRAL UNIVERSITY, TAIWAN
• UNIVERSITY OF ROCHESTER
• UNIVERSITY OF ILLINOIS AT CHICAGO

• Birger Back, Alan Wuosmaa
• Mark Baker, Donald Barton, Alan Carroll,
  Joel Corbo, Nigel George, Stephen Gushue, Dale
  Hicks, Burt Holzman, Robert Pak, Marc Rafelski,
  Louis Remsberg, Peter Steinberg, Andrei Sukhanov
• Andrzej Budzanowski, Roman Holynski,
  Jerzy Michalowski, Andrzej Olszewski, Pawel
  Sawicki , Marek Stodulski, Adam Trzupek, Barbara
  Wosiek, Krzysztof Wozniak
• Wit Busza (Spokesperson), Patrick
  Decowski, Kristjan Gulbrandsen, Conor Henderson,
  Jay Kane , Judith Katzy, Piotr Kulinich, Johannes
  Muelmenstaedt, Heinz Pernegger, Michel Rbeiz,
  Corey Reed, Christof Roland, Gunther Roland,
  Leslie Rosenberg, Pradeep Sarin, Stephen
  Steadman, George Stephans, Gerrit van
  Nieuwenhuizen, Carla Vale, Robin Verdier, Bernard
  Wadsworth, Bolek Wyslouch
• Chia Ming Kuo, Willis Lin, Jaw-Luen Tang
• Joshua Hamblen , Erik Johnson, Nazim
  Khan, Steven Manly,Inkyu Park, Wojtek Skulski,
  Ray Teng, Frank Wolfs
• Russell Betts, Edmundo Garcia, Clive
  Halliwell, David Hofman, Richard Hollis, Aneta
  Iordanova, Wojtek Kucewicz, Don McLeod, Rachid
  Nouicer, Michael Reuter, Joe Sagerer
• Abigail Bickley, Richard Bindel, Alice Mignerey

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The full PHOBOS Detector
Trigger Paddles
Mid-rapidity Spectrometer
TOF
Cerenkov
4p Multiplicity Array
135,000 Silicon Pad channels spectrometer
multiplicity
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Multiplicity Measurements in 4p
dE/dx
-5.4
5.4
500 keV
60 keV
Single-event display
Vertex tracklets 3 point tracks
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Phobos acceptance (zvtx0)
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Measuring Centrality

• Cannot directly measure the impact parameter!
• but can we distinguish
• peripheral collisions from
• central collisions?

Spectators
Zero-degreeCalorimeter
Paddle Counter
Spectators
Can look at spectators with zero-degree
calorimeters, and participants via monotonic
relationship with produced particles
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Centrality Selection
Npart341
Central 6

• HIJING predicts paddle signal (3lthlt4.5) to be
  monotonic w/ Npart

• Spectator matter measured in ZDC anti-correlates
• Expected if

Cut on fractions of total cross section to
estimate Npart
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Uncertainty on Npart

• Error of fraction of total cross section
  determined by knowledge of trigger efficiency
• Minimum-bias still has bias
• Affects most peripheral events

Error on Npart
Npart

• Estimating 96 when really 90 overestimates
  Npart
• We stop around Npart100
• Species scan might help

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Energy Dependence near h0
Errors are dominated by systematics
fpp(s)
AGS/SPS points extracted by measured dN/dy and
ltmTgt
New data at 200 GeV shows a continuous
near-logarithmic rise at mid-rapidity
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Ratio of dN/dh at 200 130 GeV
90 Confidence Level
Hard scattering dominant contribution
Limited role of hard scattering
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Parton Saturation

• Gluon distribution rises rapidly at low-x
• Gluons of x1/(2mR) overlap in transverse plane
  with size 1/Q
• At saturation scale Qs2 gluon recombination
  occurs
• In RHIC AuAu collisions, saturation occurs at a
  higher Qs2 (thus higher x)

Saturation describes HERA data!
Scale depends on volume
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Particle Density vs. Centrality
EKRT
KN
UA5 (pp)
Is this picture unique?
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Two Component Model
2C
KN
UA5
What if we move away from mid-rapidity?
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Pseudo-rapidity Distributions
130 GeV PRL 87 (2001) forthcoming

• Using Octagon and Ring subdetectors
• Measure out to hlt5.4
• Corrections
• Acceptance
• Occupancy
• Backgrounds (from MC)
• Systematic errors
• 10 near h0
• Higher near rings

Background Corr.
h
HIJING Simulation
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Consequences of Parton Saturation
Kharzeev Levin, nucl-th/0108006, input from
Golec-Biernat Wüsthoff (1999)
Kharzeev Levin, nucl-th/0108006

• Saturated initial state gives predictions about
  final state.
• N(hadrons) c ? N(gluons) (parton-hadron
  duality)
• Describes energy, rapidity, centrality dependence
  of charged particle distributions

m22Qsmr, pTQs l.25 extracted from HERA
F2 data
Intriguing! Suggests simple path from initial to
final state
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Comparison to pp and models
PRL 87 (2001) forthcoming
Systematic error not shown
Central
AMPT(rescattering)
HIJING
Peripheral
Scaled UA5 200 GeV data
h ? h (Y130/Y200) dN/dh fpp(s)
130 GeV
Ybeam
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Centrality Dependence vs. h
PRL 87 (2001) forthcoming

• Nch 4200 420 for central events
• HIJING good to 10
• Above h 3-4 decreases vs. Npart
• Crossover not seen in HIJING,
• Models with rescattering do better job

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pA Rapidity Distributions

• Several new features relative to pp
• Peak of distribution shifts backwards
• Depletion forward of beam rapidity
• Cascading near target rapidity rapid increase

NA5 DeMarzo, et al (1984)
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Centrality Dependence pA

• NA5 showed ratio of multiplicites produced in
  rapidity regions in pA vs. pp vs, R dN/dypA /
  dN/dypp vs. n(np)
• Large enhancement in target rapidities
• At central rapidity, ratio seems to saturate to 3
  (cf. AQM)
• At forward rapidity, energy degradation leads to
  less particle production than pp

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Limiting Fragmentation
200 GeV
130 GeV
UA5 200 GeV
UA5, Z.Phys.C33, 1 (1986)

• True in central AA
• Difference to pp not surprising
• Depends on colliding system

• UA5 observation of limiting fragmentation
• ?h - Ybeam ln xF ln (MN/pT)

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Limiting Fragmentation, contd.

• Central AA is 40 higher than pp at RHIC
  energies
• At 200 GeV, Simple linear scaling by 30 agrees
  (within systematics) over the whole distribution!
• Higher pT in AA vs. pp should correct pp by at
  least 5
• Detailed balancing of jets and rescattering in
  AA??
• Complicates interpretation of central
  fragmentation region in pp and central-AA

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Conclusions

• Systematics of charged particle production have
  been explored by the PHOBOS experiment
• Energy, Centrality, Rapidity
• Broad features of particle production are
  consistent with our previous understanding of
  hadronic interactions
• pp and pA collisions are very instructive
• Limiting fragmentation
• Change in scaling behavior at high-h
• Some mysteries, however
• Same shape for pp and central AuAu
• Theoretical models are assimilating new data
• Energy dependence (influence of hard processes)
• Parton saturation

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Why Rapidity?

• Proton-proton cross section dominated by soft
  processes w/ limited pT
• Up to ISR energies, it was observed that
• The energy dependence becomes weak
• Transverse and Longitudinal dynamics factorize
• But if y ½ ln(Epz/E-pz) , dy pL/E
• iff we assume F1(x) is constant at low x (NB, dy
  x dx)
• which is true if structure functions go as 1/x

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Proton-proton collisions

• Fits to Woods-Saxon
• dn/dyC(1exp(y-yo)/D)-1 , D.59
• High-multiplicity events at low-energy
• shows narrowing effect of jets

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Hit counting technique
DE deposition In multiplicity detectors for one
event.
h

1. Count hits binned in h, centrality (b)
2. Calculate acceptance A(ZVTX) for that event
3. Find occupancy in hit pads O(h,b) by counting
   empty to hit channels assuming Poisson statistics
   
4. Fold in a background correction factor fB(h,b)

O(h,b) fB(h,b)
dNch
Shits

dh
A(ZVTX)