The PHENIX High-p T Charged Particle Level-2 Trigger

1. The PHENIX High-pT Charged Particle Level-2 Trigger John Lajoie, Tom Hemmick, Jiangyong Jia, Paul Constantin Iowa State University and SUNY Stony Brook For The PHENIX Collaboration The PHENIX Experiment is one of the large detector projects at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory designed to study matter under extreme conditions. High transverse momentum particles created in the collision of heavy nuclei from the scattering of partons (quarks and gluons) are an excellent probe of the properties of the produced matter. The suppression of high transverse momentum particles through interaction with the surrounding matter may be an indication of a phase transition from ordinary hadronic matter to a deconfined state knows as the Quark Gluon Plasma (QGP). The PHENIX Level-2 trigger is a software based trigger tasked with selecting rare events and providing sufficient rejection to keep the data acquistion system within its allocated bandwidth for recording data. A high transverse momentum charged particle trigger has been developed by using a set of primitives that are generated online from the PHENIX tracking detector data. An overview of the design and performance of this Level-2 trigger and its physics capabilities will be presented. DNP 2001

2. The PHENIX Detector EM Calorimeter Beam-Beam Counter Time Expansion Chamber Muon Tracking Chambers Central Arms Muon ID Panels Pad Chambers Multiplicity/Vertex Detector North Muon Arm Drift Chambers South Muon Arm Time of Flight Panels Ring Imaging Cerenkov DNP 2001

3. High-pT Physics in PHENIX Interesting suppression of high-pT hadrons observed in Year-1 data (compared to pp). A high-statistics data sample is essential to extend the measurement in pT to further quantify this effect. leading particle hadrons q q hadrons QGP DNP 2001

4. Level-2 Trigger Requirements • RHIC operating at full luminosity • Collision (BBC Level-1) rate 1400Hz, average event size 0.15 Mb, required bandwidth 210MB/s • DAQ bandwidth is limited to 20MB/s - 60MB/s. • DAQ dedicate 20% bandwidth to minbias events, rest to Level-2. • High pT trigger allocated 5% of the total bandwidth. • High pT trigger requirements • Rejection power • Good efficiency • ~30ms decision time Assume 10 weeks run, 25% RHIC/PHENIX (combined) duty factor , 0.15MB minimum bias event size, 0.375MB high pT trigger event size, 30MB/sec bandwidth. DNP 2001

5. High-pT Charged Level-2 Trigger • A high-pT track essentially follows a straight line in the detector • Use the DC, PC and TEC tracking chambers independently to find candidates. • Reconcile the candidate lists to improve overall rejection. • East arm uses TEC in place of PC2 DNP 2001

6. y Drift Chamber  ~ 1/p hits  DC Charged Particle Level-2 • DC layers can be used for triggering. • Hit leading time and address which can be decoded to [f,layer] • Histogram f, cut on number of hits in any bin. • Bin width corresponds to momentum cut! • Correlate X1 and X2 (bend plane) layers to improve rejection. HIJING Events – Primary Particles High efficiency above momentum cutoff. x DNP 2001

7. PC/TEC Charged Particle Level-2 • (1) Start with a PC3 hit, draw a line between the PC3 hit and the BBC vertex and project to PC1. Look for a PC1 hit within a window of the PC3-vertex projection. • (The width of the search window determines the transverse momentum cutoff.) • (2) For every PC1 hit within the window project a PC1-PC3 line to PC2 or the TEC, and look for a hit cluster within ~2cm. • Achieved rejection factors of ~8 in East arm, ~15 in West arm, for a 5.0 GeV/c pT cut. Transverse plane XY Beam along Z Y (2) TEC or PC2 (1) X Drift Chamber Pad Chambers DNP 2001

8. False Candidates (West arm) False candidates due to random overlaps (~90%) and conversions (~10%). (GEANT momentum) DNP 2001

9. Trigger Rejection • The full trigger combines the candidates combines the candidates from the DC and PC+TEC, requiring a tight match in the phi angle of the track at PC1. The result is much better rejection than the product of the to individual triggers. • Trigger rejection is quoted by arm (East/West) and with/without TEC for a 5.0 GeV/c pT cut: Trigger: Event Rejection: West Arm (DC+PC) 190 East Arm (DC+PC) 50 East Arm (DC+PC+TEC) 96 • May need to prescale East arm trigger at moderate luminosity, West arm at high luminosity. DNP 2001

10. Trigger Efficiency Efficiency estimated using offline reconstruction and matching to candidates. Efficiency numbers are lower limits due to the fact that not all the background has been removed from the normalization sample. East Arm (DC+PC+TEC) West Arm (DC+PC) With TEC Without TEC Level-2 turn on Level-2 turn on e >70% No. of tracks e >50% e >60% No. of tracks pT (GeV/c) pT (GeV/c) DNP 2001

11. Expected Data Set • In 2000 run, we measured spectra up to 5 GeV/c • This year we have both arms and much better detector performance. Using 60 million minimum bias events spectra is not limited in statistics up to ~14GeV/c. • Using high pT trigger, the spectra can reach ~18 Gev/C. : 130Gev Au +Au minbias data : 130Gev P+P scaled by NCOLL 1/NCOLL 1/(2ppT) d2N/dpTdh (GeV/c)2 : 200Gev P+P scaled by NCOLL :Statistical limit for data sets We approximate 200GeV Au+Au spectra by taking UA1 p+p spectra fit scaled by number of binary collisions in minimum bias event. Also we scale RUN2000 spectra by the RUN2001 relative statistics. To be conservative, a scale down factor of two is assumed. 60 M x 4 2100 M x (4/2) pT (GeV/c) statistical limit DNP 2001

12. Summary • Good rejection and efficiency achieved by combining separate determinations of high-pT candidates in the DC and PC+TEC tracking systems. • Rejection factors and efficiencies adequate for start of PHENIX data run (with lower RHIC luminosity), still more work to improve efficiency. • The PHENIX high pT Level-2 trigger will provide a high statistics data set for studying high pT particle production in relativistic heavy ion collisions. DNP 2001