Systematics%20of%20Charged%20Particle%20Production%20with%20the%20PHOBOS%20detector

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Systematics of Charged Particle Production with
the PHOBOS detector

• Peter Steinberg
• Brookhaven National Laboratory
• ISMD2001, China

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PHOBOS Detector _at_ RHIC
(not to scale!)
(2 out of 6 shown)
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Multiplicity Measurements in 4p
-5.4
5.4
Single-event display
Vertex tracklets 3 point tracks
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Systematic Measurements

• Wish to understand if AA collisions show
  collective behavior
• How does the energy density scale with energy?
• Do particle production scale simply with
  participants or collisions?
• Is there a central plateau?
• Does the final state carry memory of the initial
  geometry?

pp collisions
pA collisions
We study this with systematics of charged
particle production Energy, Rapidity,
Centrality, Azimuthal angle
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Centrality Selection
Central 6

• HIJING shows particle production in paddles is
  monotonically related to Npart

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

Cut on fractions of total cross section to
estimate Npart
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Energy Dependence at 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
logarithmic rise at midrapidity
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Ratio of dN/dh at 200 130 GeV
90 Confidence Level
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Centrality Dependence at h0
EKRT saturation model approx. constant in Npart
Kharzeev/NardiSaturation and modifiedwounded
nucleon model give same results.
HIJING Soft/hard scaling
PHOBOS 130 GeV

• Measurement using vertex and spectrometer
  tracklets
• Systematic errors, 4.5 in spectrometer, 7 in
  vertex detector
• Participants estimated using Glauber model in
  HIJING

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Pseudo-rapidity Distributions
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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Centrality Dependence vs. h
PRL 87 (2001) forthcoming

• Total charged multiplicity is about 4200 420
  for central events
• At high h 3-4 multiplicity starts to decrease
  as a function of Npart
• Similar feature seen in pA collisions

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Comparison to pp and models
PRL 87 (2001) forthcoming
Systematic error not shown
Central
AMPT(rescattering)
HIJING
Peripheral
Scaled UA5 data
DeMarzo et al, 1984
h ? h (Y130/Y200) dN/dh fpp(s)
Ybeam
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Change in dN/dh with energy
200 GeV
130 GeV
UA5 200 GeV (NSD)
UA5, Z.Phys.C33, 1 (1986)

• UA5 looked for limiting fragmentation by
  plotting dN/dh with h ?h - Ybeam
• We can do the same thing with the PHOBOS data
• agreement for AA in the fragmentation region

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Saturation model fits to 130 Data
Kharzeev Levin, nucl-th/0108006, input from
Golec-Biernat Wüsthoff (1999)
m22Qsmr, pTQs, l.3 extracted from HERA F2 data
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Azimuthal Asymmetry at h0

• X Scos(nfi), Y Ssin(nfi)
• ?n atan(Y/X)
• v2 ltcos 2(f- ?2)gt
• At midrapidity, using SymOct
• Account for detector response using weighting
  matrix.

• 130 GeV data from PHOBOS
• Correct for occupancy, resolution of reaction
  plane estimate
• Good agreement with hydro calcuations for central
  events (and STAR)

Padsin f
Pads in z
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Asymmetry vs. Multiplicity
PRELIMINARY

• Compare
• Flow for mid-central events
• Multiplicity per participant pair for central
  events (only 10 variation down to Npart100)
• Flow (for more peripheral events) seems to scale
  with particle density
• 200 GeV data should be interesting!
• Saturation or scaling?

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Asymmetry vs. Rapidity
P. Kolb, Utah proc.
Systematic Error .007
Preliminary Results final results coming soon!
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Conclusions

• Systematics of charged particle production have
  been explored by the PHOBOS experiment
• Energy multiplicity rises approximately
  logarithmically
• Centrality data shows simple interpolation
  between pp and central AA
• Rapidity scaling with Npart changes in
  fragmentation region
• Azimuthal elliptic flow appears to scale with
  multiplicity
• Particle ratios are reaching zero net baryon
  number, small mB
• Broad features of particle production are
  consistent with soft nature of strong
  interactions
• pp and pA collisions are very instructive
• Theoretical models are assimilating new data
• None simultaneously describe full set of
  systematics!

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Results on particle ratios

• Measured near midrapidity
• 0 lt y lt1 (species-dependent)
• Final results submitted to PRL (Apr. 2001)
• Consistent within systematic errors with results
  presented at QM2001 (Jan. 2001)
• Smaller systematic errors (10 ? 6)

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Glauber Calculations
Optical Approximation
Full Calculation
Correct Npart(b), Ncoll(b) incorrect extraction!
We use full calculation by means of Monte Carlo
approach in HIJING
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Energy dependence of v2
Logarithmic rise