PHOBOS at RHIC 2000 XIV Symposium of Nuclear Physics Taxco, Mexico January 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 particles nucleus qgp energy/density

RELATIVISTIC HEAVY ION COLLIDER RHIC:  s = GeV AGS:  s = 4.8 GeV SPS:  s = 17 GeV RHIC: pp, pA, AA Energies: 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 11 o in  (azimuthal) Covers about 1% of particles Capable to reconstruct low momentum particles ( 55 MeV/c  ) pseudorapidity  ln (tan  )) rapidity y = 1/2 * ln [( E + p) L / (E - p L )]

Acceptance   multiplicity detector spectrometer

PHOBOS Silicon

Multiplicity and Vertex Detector Run 5374, Event  vertex octagon rings

pid Spectrometer

TOF

Trigger detectors functionality

Trigger counters: Paddle Counters one mip time and energy spectra for all modules: run  = 1 ns

Trigger Detectors: Cerenkov Counters

Zero Degree Calorimeters

ZDC ADC ZP +ADC ZN (neutrons) ZDC spectrum for data events at s 1/2 = 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: 1 st PHOBOS Au + Au  s = 56 A GeV 24 June: 1 st PHOBOS Au + Au  s = 130 A GeV Run 5332 Event /31/00 Not to scaleNot all sub-detectors shown Au-Au Beam Momentum = GeV/c

 s NN = 130 GeV  s NN = 56 GeV 1.31 ±0.04±0.05 Ratio (density per participant pair) 3.24 ±0.10± ±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  s NN = 130 GeV/56 GeV Elements for measurement: Triggering Centrality, vertex Silicon Counting Phys. Rev. Lett (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 CommissioningRun Setup Commissioning Run Setup

Au x z PP PN ZDC PZDC N Paddles time difference (run 3551) 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 ZDC number of spectator neutrons in ZDC = f(  E paddles ) Centrality  E paddles

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

Vertex 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: 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 ( d  2 + d  2 ) < Counting in VTX and SPEC was done independently

Corrections,systematic errors  (z vtx ) Calculated from MC studies 90% contribution from known g geometrical acceptance generator: HIJING 1.35 simulations: 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  GeV  GeV

dN/d  obtained at RHIC is 70 % higher then at SPS increase of energy density by 70% dN/d  per participating nucleon obtained in AuAu significantly higher then in pp collisions Au 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 y Reaction Plane Particle Flow P y’ P x’

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 1/1/2001

Systematic 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%  N part   Loss of trigger efficiency at low-multiplicity <10% Uncertainty on  N part  <1%  Uncertainty in modeling paddle fluctuations Uncertainty on  N part  <6% ( dN/d  /  N part  ) 130 / ( dN/d  /  N part  ) 56  Many uncertainties cancel in the ratio