PHOBOS at RHIC 2000

XIV Symposium of Nuclear PhysicsTaxco, MexicoJanuary 2001

Edmundo Garcia, University of Maryland

Outline

• Introduction

• The detector

• Performance and physics results for 2000

• Perspectives

• Final Notes

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

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

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

• 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)]

Acceptance

0 +3-3 +5.5-5.5

multiplicity detector

spectrometer

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

Multiplicity and Vertex Detector

Run 5374, Event 79495

vertex

octagon

rings

pid

Spectrometer

TOF

Trigger detectors functionality

Trigger counters: Paddle Countersone mip

time and energy spectra for all modules: run 56243

= 1 ns

Trigger Detectors: Cerenkov Counters

Zero Degree Calorimeters

ZDC

ADCZP +ADCZN (neutrons)

ZDC spectrum for data events at s1/2 = 130 AGeV

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

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

• 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

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

Centrality Measurement

Centrality. number of spectator neutrons in ZDCnumber of spectator neutrons in ZDC = f(Epaddles)Centrality Epaddles

Centrality Measurement peripheral

central

6%

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

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

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

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

• 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

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’

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 edmundo.garcia@bnl.gov 1/1/2001

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