1. 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

2. ARGONNE NATIONAL LABORATORYBirger 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, KRAKOWAndrzej Budzanowski, Roman Hołyński, Jerzy Michałowski, Andrzej Olszewski, Pawel Sawicki, Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Woźniak MASSACHUSETTS INSTITUTE OF TECHNOLOGYMaartin 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 Wysłouch NATIONAL CENTRAL UNIVERSITY, TAIWANChia Ming Kuo, Willis Lin, Jaw-Luen Tang UNIVERSITY OF ILLINOIS AT CHICAGORussell Betts, Edmundo García, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer UNIVERSITY OF MARYLANDAbigail Bickley, Richard Bindel, Alice Mignerey, Marguerite Belt Tonjes UNIVERSITY OF ROCHESTERJoshua Hamblen, Erik Johnson, Nazim Khan, Steven Manly, Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs Collaboration

3. Trigger Hardware: Paddle Counters: ±3.21m from Z=0 (Nominal Collision Point) Cover 3 < |η| < 4.5 for collision at Z=0 16 diamond shaped Scintillators with individual PMTs (Called ‘Slats’) Used for L0 trigger Z = 0 ZDCs (0° Calorimeters): ± 18m Tungsten plates /w light guides to 3 PMTs Standard to RHIC experiments Observe neutrons (used in L1) Luminosity monitor DX Magnet Additional detectors used in L1 (AuAu runs) to create different trigger mixes: Cerenkovs & T0s

4. AuAu Triggering (2000-2002) Basic Trigger: Coincidence of two Paddles with >1 of 16 Slats Hit Pdl NPdl P Case 1: |Δt| ~0ns. Nominal collision + fraction of case 2 between paddles (removed by requiring ZDC coincidence or large Paddle Sum) Case 2: |Δt| offset >10ns Largely beam-gas collisions Paddle Time Difference (ns) Events

5. 200 GeV pp Environment, HiJing MC Additional concerns: ZDCs not available for valid collision selection…Spectator neutrons not present for coincidence Paddle Coverage: Expect ~2 particles per paddle per event Multiplicity too low for AuAu trigger

6. Change to 1-arm Trigger Disadvantages: Potential for more background Possible Bias in Multiplicity and VTX Advantages: Sensitive to lower Mult events. Background and Bias Understood Implement single arm paddle trigger (Slats hit on one side >0) 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

7. (Negative side time – Crossing Clock) ns 0 200100 300 400 100 200 300 400 Blue and Yellow = Background collisions Green = Real collisions + reduced BG 0 (Positive side time – Crossing Clock) ns Timing of Paddles: PdlP-CC vs PdlN-CC Negative side fires Positive side fires Analogous to AuAu picture shown earlier Both sides fire

8. Removal of Background Events Using Paddle Timing 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 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 Negative Side Paddle Time – CC (ns)(Negative-Positive) Side Paddle Time (ns)

9. Trigger vs charged particle Multiplicity in 200GeV HiJing MC: 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 Red line: events that have at least 1 hit in both paddles Black line: all MC events Ratio of Red/Black in above plot => sensitive to 95% of collisions, better at higher multiplicity 20

10. Check of Possible Vertex Bias Black: All Events Red: Events with Trigger Ratio of two plots to left 200 GeV pp HiJing MC Vertex Range of +/- 80cm (Useful PHOBOS Range)

11. Effect of Trigger on dN/dEta: dN/dη vs η for 200GeV pp MC Black: 100k events Red: Events with >0 hits in Paddles

12. Summary Implemented single-arm trigger for PHOBOS Trigger bias in new setup is minimal in vertex position and multiplicity The vertexing algorithm efficiency (Richard’s 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.