THE PHOBOS EXPERIMENT AT RHIC Judith Katzy for the PHOBOS Collaboration

ARGONNE NATIONAL LABORATORY Birger Back, Russell Betts, Alan Wuosmaa, Nigel George BROOKHAVEN NATIONAL LABORATORY Mark D.Baker, Donald Barton, Alan Carroll, Stephen Gushue, George Heintzelman, Louis Remsberg, Andrei Sukhanov INSTITUTE OF NUCLEAR PHYSICS, KRAKOW Wojciech Bogucki, Andrzej Budzanowski, Tomir Coghen, Kazimierz Galuszka, Jan Godlewski,Roman Holynski, Jerzy Kotula, Marian Lemler, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak, Pawel Zychowski JAGELLONIAN UNIVERSITY, KRAKOW Andrzej Bialas, Wieslaw Czyz, Kacper Zalewski MASSACHUSETTS INSTITUTE OF TECHNOLOGY Wit Busza*, Patrick Decowski, Kristjan Gulbrandsen, P. Haridas, Piotr Kulinich, Heinz Pernegger, Miro Plesko, Gunther Roland†, Leslie Rosenberg, Pradeep Sarin, Stephen Steadman, George Stephans, Gerrit van Nieuwenhuizen, Carla Vale, Robin Verdier, Bernard Wadsworth, Bolek Wyslouch‡ NATIONAL CENTRAL UNIVERSITY, TAIWAN Yuan-Hann Chang, Augustine Chen, Willis Lin UNIVERSITY OF ROCHESTER Adam Hayes, Erik Johnson, Steven Manly, Robert Pak, Inkyu Park, Frank Wolfs UNIVERSITY OF ILLINOIS AT CHICAGO Russell Betts, Clive Halliwell, Burt Holzman, Judith Katzy, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter UNIVERSITY OF MARYLAND Edmundo Garcia-Solis, Peter Stanskas, Alice Mignerey PHOBOS Collaboration

Physics Goals of PHOBOS: Study QCD Vacuum and Confinement Look for Excess Entropy Multiplicity Distribution and Fluctuations Source Size and Lifetime Look for Collective Phenomena over large V Low Transverse Momentum Look for Unusual Medium Effects Phi Mass and Width

Time-of-Flight Walls Ring Detectors Spectrometer Octagon and Vertex Detector Magnet first year : full multiplicity and vertex detector 1 spectrometer arm 2 TOF elements The Phobos detector multiplicity array:  spectrometer:  deg vertex detector tof trigger detectors magnet

Triggering and data rates Simple, unbiased trigger: paddle scintillators 3.7  |  |  4.5 ring cerenkov 4.9   Rates at nominal Luminosity L = cm 2 s 1 R(Au+Au) = 600 Hz (b   fm) R(Au+Au, central) = 60 Hz R(BeamGas) = 100 Hz R(e+e+) = 10 Hz data collection volume 18 kb/ event => 60 Tb raw Tb total / year Reduced by vertex timing cut

octagon ring counter -1.1 m 1.1 m 2.3 m -2.3 m 5.0 m - 5.0m Multiplicity array and Vertex detector Multiplicity measurement: octagon and 6 ring counters acceptance: |  | <  bin width  vertex interaction point

z 11 cm 5.5 cm Vertex reconstruction  z = 60  m Vertex Resolution : Vertex Reconstruction in x,y: with tracks reconstructed in spectrometer z beam Vertex acceptance: -20cm  z  20 cm

Multiplicity array Part of octagon 1 ring counter

Silicon Detector modules used in the Multiplicity array Octagon module 8.5cm 6.5 cm 120 pads / sensor  ~  =  /16 64 pads / sensor  ~0.05  =  /32 Ring module

Multiplicity Reconstruction from Energy Loss Reconstructed multiplicity in Pad i: M i = E dep i / (E i single N i tot /N i prim ) E dep i : deposited Energy (angle corrected) E i single : mean energy loss for one particle traversing pad N i tot /N i prim : ratio of total/primary tracks 1.5 * 10 7 distributions in 1 month at 1% nominal luminosity Error on total Multiplicity: 2/3

Event by Event Multiplicity Average over 50 Events

1 GeV/c  MeV/c   + Low field region for pattern recognition particle id via dE/dx High B-field Spectrometer and Tracking

conventional magnet maximal B-field: 2.1 Tesla The Magnet

Silicon Detectors mounted on Spectrometer Frame

Response of Silicon Detector to 285MeV/c Pions - Data! without gaussian addition in collaboration with hephy in Vienna

Spectrometer Acceptance

Momentum Resolution

Track Finding - very low P  deg   deg  7 Planes, E>800 keV 7 Planes, all Hits Stopping Kaon

HBT Acceptance for  Truth Resolution smeared Source Parameterization: C 2 = 1 +  * exp(-Q i 2 R i 2 ) i = L, TO,TS

HBT Capabilities “Standard” Scenario –Extrapolate from AGS/Cern Energies Exotic Scenario I –Very Large Source

Phi measurement  results after 1 nominal week, 15% central cut, Si pid:  Resolution:  (M inv ) = 1.6 MeV –Dominated by momentum resolution Error on Width: 1.0 MeV  K + K -

Summary - PHOBOS Highlights Coverage for Charged Multiplicity in almost 4  with the Multiplicity Array Momentum Measurement and Particle Identification at Mid-rapidity with Low p t Threshold with the Spectrometer Unbiased Taking Data Due to High Data Rate Capability READY FOR RHIC START-UP IN NOVEMBER