PHOBOS at RHIC 2000

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PHOBOS at RHIC 2000
XIV Symposium of Nuclear PhysicsTaxco,
MexicoJanuary 2001
Edmundo Garcia, University of Maryland
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• Outline
• Introduction
• The detector
• Performance and physics results for 2000
• Perspectives
• Final Notes

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energy/density
nucleus
particles
atoms
qgp
Interactions of produced particles act at soft
and hard scales
Two nuclei approach relativistically contracted
Hard collisions take place during first stages of
reaction
Final particles freeze out towards the detectors
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RELATIVISTIC HEAVY ION COLLIDER
AGS ?s 4.8 GeV
SPS ?s 17 GeV
RHIC ?s 53-200 GeV
RHIC pp, pA, AA Energies 30 - 200 GeV
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RHIC Physics

• Study of matter at the highest energy density
• Look for signatures of QGP (evidence of
  existence at CERN)
• Deconfinement of phase transition
• Chirial symmetry restoration

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• One of the small RHIC experiments, size (6 x
  6 x 3 m), and people (50 scientist)
• Designed to be able to examine and analyze a very
  large amount of minimum bias interactions (high
  trigger rate capability)
• Measurements
• Multiplicity and angular distribution of charged
  particles
• h lt 5.3 over 4 p coverage event by event
• Particle spectra
• 0.5 lt h lt 1.5 and 2 x 11o in f (azimuthal)
• Covers about 1 of particles
• Capable to reconstruct low momentum particles (
  55 MeV/c p)

pseudorapidity h - ln (tan(q/2))
rapidity y 1/2 ln ( E p)L/ (E - pL)
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Acceptance
spectrometer
multiplicity detector
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PHOBOS Silicon
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Multiplicity and Vertex Detector
vertex
octagon
Run 5374, Event 79495
rings
h
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Spectrometer
pid
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TOF
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Trigger detectors functionality
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Trigger counters Paddle Counters
one mip
time and energy spectra for all modules run
56243
s 1 ns
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Trigger Detectors Cerenkov Counters
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Zero Degree Calorimeters
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ZDC
ZDC spectrum for data events at s1/2 130 AGeV
ADCZP ADCZN (neutrons)
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Physics in year one
13 June 1st PHOBOS Au Au Collisions _at_ ?s 56
A GeV 24 June 1st PHOBOS Au Au Collisions _at_
?s 130 A GeV

• Published
• Multiplicity measurement for h lt 1
• Work in process for QM
• Multiplicity vs. h
• Multiplicity vs. centrality
• Particle spectra
• HBT
• Flow

Au-Au Beam Momentum 65.12 GeV/c
Not to scale
Not all sub-detectors shown
Run 5332 Event 35225 08/31/00
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• Measurement
• Charged Particle Multiplicity Near Mid-Rapidity
• for the 6 most central events
• at two collision energies
• ratio of ?sNN 130 GeV/56 GeV
• Elements for measurement
• Triggering
• Centrality, vertex
• Silicon Counting

Results
?sNN 130 GeV
?sNN 56 GeV
Energy
Measurable
555 12(stat) 35(syst)
408 12(stat)30(syst)
dN/dh hlt1
3.24 0.100.25
2.47 0.100.25
dN/dh hlt1 per participant pair
1.31 0.040.05
Ratio (density per participant pair)
Phys. Rev. Lett. 85 3100(2000)
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Commissioning Run Setup

• Configuration used for first data
• SPEC 6 planes of a single spectrometer arm
• VTX Half of the Top Vertex Detector
• Paddles 2 sets of 16 scintillators paddles

Acceptance of SPEC and VTX
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Triggering
PP
PN
ZDC P
ZDC N
Au
Au
White background 76 ns coincidence window, light
gray 9.5 ns window, gray mult. PP and mult. PN gt
3. Events selected with ZDC time difference lt 20
ns.
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Centrality Measurement
Centrality. a number of spectator neutrons in
ZDC number of spectator neutrons in ZDC
f(DEpaddles) Centrality a DEpaddles
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Centrality Measurement
peripheral
central
6
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Charged multiplicity measurement

• Counting
• Restrict the location of collisions vertex to the
  region in which the silicon detectors had good
  acceptance
• Tracklets 3 point tracks passing through firs
  four layers of spectrometer (SPEC) or from vertex
  detector (VTX)
• Determination of number of primary particles from
  tracklets
• Primaries are all charged hadrons produced in
  collision, including products from strong
  interactions and electromagnetic decays but
  excluding products from weak decays and hadrons
  produced in secondary interactions
• Determination of systematic errors

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Vertex Distributions
Y
X

• Beam Orbit can be calculated for each fill, it
  was found to be very stable
• For the 130 AGeV data
• X -.17 cm, sX .17 cm
• Y .14 cm, sY .08 cm

Z

• Make a cut in Z to define a fiducial volume

3 mm in transverse direction
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Tracklets
SPEC
Counting in VTX and SPEC was done independently
VTX

• Spectrometer tracklets
• Formed by 1st layer hits and second layer hits
  within sqrt ( dh2
  df2 ) lt 0.015

• Vertex tracklets
• Formed by 1st layer hits and second layer hits
  within dh lt
  0.1

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Corrections,systematic errors
generator HIJING 1.35 simulations Geant 3.21
good understanding of detector geometry and
tracking efficiency
spec
vtx
130 GeV 56 GeV

• Sources of systematic errors
• Background subtraction
• Uncertainty on a due to model differences
• feed-down from strange decays
• stopping particles
• Total uncertainty on dN/dh is 8

• a(zvtx)
• Calculated from MC studies
• 90 contribution from known g
• geometrical acceptance

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Comparison of Results

• dN/dh obtained at RHIC is 70 higher then at
  SPS
• increase of energy density by 70
• dN/dh per participating nucleon obtained in AuAu
  significantly higher then in pp collisions
• Au Au collisions differ from simple superposition
  of pp

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Flow measurement

• Expectation
• Asymmetry in initial-state collision geometry
• ? ellipsoidal distribution in final state
  momentum distribution
• Estimate reaction plane
• Clear signal observed in hlt2
• Currently extending analysis to use full coverage
  h lt 5
• Look for directed flow at large h

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Final Notes

• For QM
• Multiplicity vs. h
• Multiplicity vs. centrality
• Particle spectra
• HBT
• Flow
• For 2001 run
• Detector fully operational and ready for new
  physics

Edmundo Garcia, University of Maryland
edmundo.garcia_at_bnl.gov 1/1/2001
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Systematic Uncertainties

• dN/dh
• Background subtraction on tracklets lt 5
• Uncertainty on a due to model differences lt 5
• Total contribution due to feed-down correction lt
  4 (typically 1)
• Total fraction lost due to stopping particles lt
  5
• Both are corrected via MC normalization
• Total uncertainty on dN/dh is 8
• ?Npart?
• Loss of trigger efficiency at low-multiplicity
  lt10
• Uncertainty on ?Npart ? lt1
• Uncertainty in modeling paddle fluctuations
• Uncertainty on ?Npart ? lt6
• ( dN/dh / ?Npart? )130 / ( dN/dh / ?Npart? )56
• Many uncertainties cancel in the ratio