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

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. Collaboration 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

3. DX Magnet Trigger Hardware: Z = 0 • Paddle Counters: • ±3.21m from Z=0 (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 < |η| < 4.5 for collision at Z=0 • 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

4. AuAu Triggering (2000-2002) Pdl N Pdl P • Basic Trigger: Coincidence of two Paddles with >1 of 16 Slats Hit Events 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)

5. 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 selection…Spectator neutrons not present for coincidence

6. Disadvantages: Potential for more background Possible Bias in Multiplicity and VTX Advantages: Sensitive to lower Mult events. Background and Bias Understood Change to 1-arm Trigger • 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. 400 300 (Negative side time – Crossing Clock) ns Green = Real collisions + reduced BG Blue andYellow= Background collisions 200 100 0 0 300 100 200 400 (Positive side time – Crossing Clock) ns Timing of Paddles: PdlP-CC vs PdlN-CC Negative side fires Analogous to AuAu picture shown earlier Both sides fire Positive side fires

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

9. 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 20 Ratio of Red/Black in above plot => 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

10. 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

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.