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PHOBOS at RHIC 2000

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. energy/density. nucleus. particles. atoms. qgp.

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PHOBOS at RHIC 2000

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  1. PHOBOS at RHIC 2000 XIV Symposium of Nuclear PhysicsTaxco, MexicoJanuary 2001 Edmundo Garcia, University of Maryland

  2. Outline Introduction The detector Performance and physics results for 2000 Perspectives Final Notes

  3. 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

  4. 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

  5. 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

  6. 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 < 5.3 over 4 p coverage event by event • Particle spectra • 0.5 < h < 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)]

  7. f -5.5 -3 0 +3 +5.5 h Acceptance spectrometer multiplicity detector

  8. PHOBOS Silicon

  9. Multiplicity and Vertex Detector vertex octagon Run 5374, Event 79495 rings h

  10. Spectrometer pid

  11. TOF

  12. Trigger detectors functionality

  13. Trigger counters: Paddle Counters one mip time and energy spectra for all modules: run 56243 s = 1 ns

  14. Trigger Detectors: Cerenkov Counters

  15. Zero Degree Calorimeters

  16. ZDC ZDCspectrum for data events at s1/2 = 130 AGeV ADCZP +ADCZN (neutrons)

  17. Published: Multiplicity measurement for | h | < 1 Work in process for QM: Multiplicity vs. h Multiplicity vs. centrality Particle spectra HBT Flow Physics in year one 13 June: 1st PHOBOS Au + Au Collisions @ s = 56 A GeV 24 June: 1st PHOBOS Au + Au Collisions @ s = 130 A GeV Au-Au Beam Momentum = 65.12 GeV/c Not to scale Not all sub-detectors shown Run 5332 Event 35225 08/31/00

  18. 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 Energy sNN = 56 GeV sNN = 130 GeV Measurable dN/dh | |h|<1 408 ±12(stat)±30(syst) 555 ±12(stat) ±35(syst) dN/dh | |h|<1 per participant pair 2.47 ±0.10±0.25 3.24 ±0.10±0.25 Ratio (density per participant pair) 1.31 ±0.04±0.05 Phys. Rev. Lett. 85 3100(2000)

  19. CommissioningRun 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

  20. Paddles time difference (run 3555) Paddles time difference (run 3551) x z time (ns) time (ns) Triggering PP PN ZDC N ZDC P Au Au 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.

  21. Centrality Measurement Centrality. a number of spectator neutrons in ZDC number of spectator neutrons in ZDC = f(DEpaddles) Centrality a DEpaddles

  22. Centrality Measurement peripheral central 6%

  23. 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

  24. 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

  25. SPEC Tracklets Counting in VTX and SPEC was done independently VTX Spectrometer tracklets: • Formed by 1st layer hits and second layer hits within: sqrt ( dh2 + df2 ) < 0.015 Vertex tracklets: • Formed by 1st layer hits and second layer hits within: | dh | < 0.1

  26. generator: HIJING 1.35 simulations: Geant 3.21 Corrections,systematic errors 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

  27. 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 AuAusignificantly higher then in pp collisions • Au Au collisions differ from simple superposition of pp

  28. y Reaction Plane Py’ x Particle Flow Px’ Flow measurement • Expectation: • Asymmetry in initial-state collision geometry  ellipsoidal distribution in final state momentum distribution • Estimate reaction plane • Clear signal observed in |h|<2 • Currently extending analysis to use full coverage |h| < 5 • Look for directed flow at large h

  29. 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@bnl.gov 1/1/2001

  30. Systematic Uncertainties • dN/dh • Background subtraction on tracklets < ±5% • Uncertainty on a 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/dh 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/dh / Npart )130 / ( dN/dh / Npart )56 • Many uncertainties cancel in the ratio

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