Performance of the PHOBOS Trigger Detectors in 200 GeV pp Collisions at RHIC

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Performance of the PHOBOS Trigger Detectors in
200 GeV pp Collisions at RHIC

• Joseph Sagerer
• University of Illinois at Chicago
• for the Collaboration
• DNP 2002 Michigan State University
  October 12, 2002

• Trigger Hardware
• AuAu Trigger
• Concerns about pp

• pp Setup
• pp Data
• Performance of pp trigger

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Collaboration
ARGONNE NATIONAL LABORATORY Birger Back, Alan
Wuosmaa BROOKHAVEN NATIONAL LABORATORY Mark
Baker, Donald Barton, Alan Carroll, Nigel George,
Stephen Gushue, George Heintzelman, Burt
Holzman, Robert Pak, Louis Remsberg, Peter
Steinberg, Andrei Sukhanov INSTITUTE OF NUCLEAR
PHYSICS, KRAKOW Andrzej Budzanowski, Roman
Holynski, Jerzy Michalowski, Andrzej Olszewski,
Pawel Sawicki, Marek Stodulski, Adam Trzupek,
Barbara Wosiek, Krzysztof Wozniak MASSACHUSETTS
INSTITUTE OF TECHNOLOGY Maartin Ballintijn, Wit
Busza (Spokesperson), Patrick Decowski, Kristjan
Gulbrandsen, Conor Henderson, Jay Kane, Judith
Katzy, Piotr Kulinich, Jang Woo Lee, Heinz
Pernegger, Corey Reed, Christof Roland, Gunther
Roland, Leslie Rosenberg, Pradeep Sarin,
Stephen Steadman, George Stephans, Carla Vale,
Gerrit van Nieuwenhuizen, Gábor Veres, Robin
Verdier, Bernard Wadsworth, Bolek
Wyslouch NATIONAL CENTRAL UNIVERSITY,
TAIWAN Chia Ming Kuo, Willis Lin, Jaw-Luen
Tang UNIVERSITY OF ILLINOIS AT CHICAGO Russell
Betts, Edmundo García, Clive Halliwell, David
Hofman, Richard Hollis, Aneta Iordanova, Wojtek
Kucewicz, Don McLeod, Rachid Nouicer, Michael
Reuter, Joe Sagerer UNIVERSITY OF
MARYLAND Abigail Bickley, Richard Bindel, Alice
Mignerey, Marguerite Belt Tonjes UNIVERSITY
OF ROCHESTER Joshua Hamblen, Erik Johnson, Nazim
Khan, Steven Manly, Inkyu Park, Wojtek
Skulski, Ray Teng, Frank Wolfs
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Trigger Hardware
DX Magnet
Z 0

• Paddle Counters
• 3.21m from Z0 (Nominal Collision Point)

• ZDCs (0 Calorimeters)
• 18m
• Tungsten plates /w light guides to 3 PMTs
• Standard to RHIC experiments
• Observe neutrons (used in L1)
• Luminosity monitor

• Cover 3 lt ? lt 4.5 for collision at Z0
• 16 diamond shaped Scintillators with individual
  PMTs (Called Slats)
• Used for L0 trigger

Additional detectors used in L1 (AuAu runs) to
create different trigger mixes Cerenkovs T0s
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AuAu Triggering (2000-2002)

• Basic Trigger Coincidence of two Paddles with gt1
  of 16 Slats Hit

Pdl N
Pdl P
Events
Case 1 ?t 0ns. Nominal collision fraction
of case 2 between paddles (removed by requiring
ZDC coincidence or large Paddle Sum)
Case 2 ?t offset gt10ns Largely beam-gas
collisions
Paddle Time Difference (ns)
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200 GeV pp Environment, HiJing MC
Paddle Coverage Expect 2 particles per paddle
per event Multiplicity too low for AuAu trigger

• Additional concerns
• ZDCs not available for valid collision
  selectionSpectator neutrons not present for
  coincidence

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Change to 1-arm Trigger

• Implement single arm paddle trigger
  (Slats hit on one side
  gt0)
• Single particle may now trigger
• And with Synchronizing signal from RHIC
  (Crossing-Clock Gate)
• Crossing-Clock Gate reduces background by
  allowing triggers only when beam buckets overlap
  in the PHOBOS IR

• Disadvantages
• Potential for more background
• Possible Bias in Multiplicity and VTX

• Advantages
• Sensitive to lower Mult events.
• Background and Bias Understood

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Timing of Paddles PdlP-CC vs PdlN-CC
Negative side fires
Analogous to AuAu picture shown earlier
Both sides fire
Positive side fires
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Removal of Background Events Using Paddle Timing
Negative Side Paddle Time CC (ns)
(Negative-Positive) Side Paddle Time (ns)

• Smaller peaks are due to particles passing
  through the paddle range striking them in
  sequence
• This is analogous to AuAu trigger
• Green is the same cut we applied to the plot on
  left

• Smaller peak with shorter time indicates paddle
  was hit early relative to the crossing-clock gate
• Green lined data has been cut to remove this
  smaller peak

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Trigger vs charged particle Multiplicity in
200GeV HiJing MC
Red line events that have at least 1 hit in both
paddles Black line all MC events
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Ratio of Red/Black in above plot gt sensitive to
95 of collisions, better at higher multiplicity
Multiplicity of Event
Red line events that have at least 1 hit in both
paddles Black line events that have at least 1
hit in only 1 paddle
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Check of Possible Vertex Bias
200 GeV pp HiJing MC Vertex Range of /- 80cm
(Useful PHOBOS Range)
Black All Events Red Events
with Trigger
Ratio of two plots to left
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Effect of Trigger on dN/dEta
dN/d? vs ? for 200GeV pp MC
Black 100k events Red
Events with gt0 hits in Paddles
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Summary

• Implemented single-arm trigger for PHOBOS
• Trigger bias in new setup is minimal in vertex
  position and multiplicity
• The vertexing algorithm efficiency (Richards
  talk before mine) is dominant over the trigger
  efficiency.
• The combined efficiencies should allow physics
  studies in PHOBOS down to the lowest multiplicity
  200 GeV pp events.