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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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Two nuclei approach relativistically contracted
Hard collisions take place during first stages of reaction
Interactions of produced particles act at soft and hard scales
Final particles freeze out towards the detectors
atoms
particlesnucleus
qgp
energy/density
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RELATIVISTIC HEAVY ION COLLIDER
RHIC: s = 53-200 GeV
AGS: s = 4.8 GeV
SPS: s = 17 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
• < 5.3 over 4 coverage event by event
Particle spectra• 0.5 < < 1.5 and 2 x 11o in (azimuthal)
• Covers about 1% of particles
• Capable to reconstruct low momentum particles ( 55 MeV/c )
pseudorapidityln (tan)) rapidity y = 1/2 * ln [( E + p)L/ (E - pL)]
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Acceptance
0 +3-3 +5.5-5.5
multiplicity detector
spectrometer
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PHOBOS Silicon
Sensor Type Pad Size(width x height mm2)
Pad( rows x columns)
Spectrometer
Type 1 1 x 1 22 x 3Type 2 0.43 x 6 22 x 70
Type 3 0.67 x 7.5 8 x 64Type 4 0.67 x 15 8 x 64
Type 5 0.67 x 19 4 x 64Multiplicity
Octagon 2.6 x 9.5 30 x 40
Ring 50 64 ρφpadVerte 1x 0.3 3.3x 56xVerte x 0.3 46.6x 56x
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Multiplicity and Vertex Detector
Run 5374, Event 79495
vertex
octagon
rings
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pid
Spectrometer
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TOF
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Trigger detectors functionality
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Trigger counters: Paddle Countersone mip
time and energy spectra for all modules: run 56243
= 1 ns
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Trigger Detectors: Cerenkov Counters
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Zero Degree Calorimeters
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ZDC
ADCZP +ADCZN (neutrons)
ZDC spectrum for data events at s1/2 = 130 AGeV
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Physics in year one
Published:
• Multiplicity measurement
for | | < 1
Work in process for QM:
• Multiplicity vs.
• Multiplicity vs. centrality
• Particle spectra
• HBT
• Flow
13 June: 1st PHOBOS Au + Au Collisions @ s = 56 A GeV24 June: 1st PHOBOS Au + Au Collisions @ s = 130 A GeV
Run 5332 Event 35225 08/31/00Run 5332 Event 35225 08/31/00Not to scale Not all sub-detectors shown
Au-A
u B
eam
Mom
entu
m =
65
.12
GeV
/c
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sNN = 130 GeVsNN = 56 GeV
1.31±0.04±0.05
Ratio(density per
participant pair)
3.24±0.10±0.25
2.47±0.10±0.25
dN/d | <1
per participant pair
555±12(stat) ±35(syst)
408±12(stat)±30(syst)
dN/d | <1
Measurable
Energy
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
Phys. Rev. Lett. 85 3100(2000)
Results
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• 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
CommissioningCommissioning Run SetupRun SetupCommissioningCommissioning Run SetupRun Setup
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Au Aux
z
PPPN
ZDC P
ZDC N
Paddles time difference (run 3551)
time (ns) time (ns)
Paddles time difference (run 3555)
White background 76 ns coincidence window, light gray 9.5 ns window, gray mult. PP and mult. PN > 3. Events selected with ZDC time difference < 20 ns.
Triggering
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Centrality Measurement
Centrality. number of spectator neutrons in ZDCnumber of spectator neutrons in ZDC = f(Epaddles)Centrality Epaddles
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Centrality Measurement peripheral
central
6%
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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
Charged multiplicity measurement
( )1<
↔=η
ηα
η d
dNZ
d
dN primarytracklets
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Vertex DistributionsVertex DistributionsVertex DistributionsVertex Distributions
X Y
Z
• Beam Orbit can be calculated for each fill, it was found to be very stable
• For the 130 AGeV data X = -.17 cm, X = .17 cm
Y = .14 cm, Y = .08 cm
• Make a cut in Z to define a fiducial volume:
cmZ 1525 <<−
3 mm in transverse direction
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Tracklets
VTX
SPEC
Vertex tracklets:
• Formed by 1st layer hits and second layer hits within:
| d | < 0.1
Spectrometer tracklets:• Formed by 1st layer hits and second
layer hits within:
sqrt ( d2 + d2 ) < 0.015
Counting in VTX and SPEC was done independently
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Corrections,systematic errors
(zvtx)
• Calculated from MC studies
• 90% contribution from known g
• geometrical acceptance
generator: HIJING 1.35simulations: Geant 3.21
Sources of systematic errors•Background subtraction•Uncertainty on due to model differences •feed-down from strange decays •stopping particles Total uncertainty on dN/d is ±8%
good understanding of detector geometry and tracking efficiency
spec vtx
130GeV56 GeV
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• dN/d obtained at RHIC is 70 % higher then at SPSincrease of energy density by 70%•dN/d per participating nucleon obtained in AuAu significantly higher then in pp collisionsAu Au collisions differ from simple superposition of pp
Comparison of Results
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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
<2• Currently extending analysis
to use full coverage < 5 Look for directed flow at
large
x
yReaction Plane
Particle Flow
Py’
Px’
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Final Notes
For QM:
• Multiplicity vs.
• Multiplicity vs. centrality
• Particle spectra
• HBT
• Flow
For 2001 run
• Detector fully operational and ready for new physics
Edmundo Garcia, University of Maryland [email protected] 1/1/2001
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Systematic UncertaintiesSystematic UncertaintiesSystematic UncertaintiesSystematic Uncertainties• dN/d
Background subtraction on tracklets < ±5% Uncertainty on due to model differences < 5%
• Total contribution due to feed-down correction < 4% (typically 1%)
• Total fraction lost due to stopping particles < 5%• Both are corrected via MC normalization
Total uncertainty on dN/d is ±8% Npart
Loss of trigger efficiency at low-multiplicity <10%• Uncertainty on Npart <1%
Uncertainty in modeling paddle fluctuations • Uncertainty on Npart <6%
• ( dN/d / Npart )130 / ( dN/d / Npart )56
Many uncertainties cancel in the ratio