Alexander Milov Weizmann Institute, Israel

1. Charged Particle Multiplicity and Transverse Energy in Au+Au Collisions at ÖsNN = 130 GeV _ AlexanderMilovWeizmann Institute, Israel Alexander Bazilevsky RIKEN BNL Research Center , USA Klaus Reygers University of Münster, Germany • for the PHENIX Collaboration • Nucl-exp/0012008 (submitted to PRL) • QM2001 poster P179

2. Outline • Introduction: • Importance of Nch and Et measurements • The PHENIX detector. • Experimental results: • dNch/dh distribution at mid-rapidity • dEt /dh distribution at mid-rapidity • Centrality determination • Results and discussion: • Comparison to model predictions • Comparison to CERN and AGS data • Summary.

3. Global variables: Et and Nch • Initial conditions, energy density of the system. • Scaling with Ös. • Mechanism of particle production, soft vs hard? • Soft: NchNpart • Hard: Nch Ncoll • Constrain theoretical predictions.

4. Theoretical Predictions • Various models predict different trends for (dNch/dh)/Npart vs Npart: • HIJING: (Wang, Gyulassy, nucl-th/0008014)(dNch /dh)/Npart increases with Npart • Saturation model: (Eskola, Kajantie, and Tuominen hep-ph/0009246)(dNch /dh)/Npart constant vs Npart

5. PHENIX-Setup: Beam View • Pad Chambers: • RPC1 = 2.5 m RPC3 = 5.0 m • |h| < 0.35,Df= 90o • 8 x 4320 pads. • e > 99%, s ~ 2 mm • Lead Scintillator EMCal. • REMCal = 5.1 m. • |h| < 0.38, Df= 45o • 2 x 2592 PMT • 18X0, s ~ 8% for 1GeVg-quanta

6. PHENIX-Setup: Side View • Zero Degree Calorimeters • Spectator neutrons with |h| > 6 • |Z|=18.25 m • Beam Beam Counters • 64 Cherenkov quartz counters with PMT readout • 3.0 < |h| <3.9Df= 360o

7. Trigger and event selection • BBC trigger: • Coincidence of both BBC (at least two photomultipliers fired in each BBC). • Corresponds to (92 ± 2)% of the geometrical Au-Au cross section (sgeo. = 7.2 b). • ZDC trigger: • Coincidence of both ZDC (E > 10 GeV) • Includes mutual Coulomb dissociation processes • 97.8% of the BBC trigger events also satisfy ZDC trigger condition. • Event vertex restriction: • |Z| < 20 cm around centre of interaction region.

8. Charged Multiplicity Determination • Procedure: count tracks on a statistical basis, no explicit track reconstruction : • Combine all hits in PC3 with all hits in PC1. • Project resulting lines onto a plane through the beam line. • Count tracks within a given radius. • Determine combinatorial background by event mixing B=0

9. Track Vertex Distribution • 3 contributions: • Peak at low R: primary particles coming from the event vertex • Combinatorial background: dNB/dRR • Exponential tail: in-flight decays • Number of tracks per event: • Subtract average background on an event-by-event basis • Count all tracks within R=25 cm ( => 95.9% of all tracks)

10. Corrections • Tracks outside acceptance window: 4.3% • Pad Chamber inactive regions: 15.3% • Double hit resolution: • 13.6% for most central events • 3.6% to the subtracted background • Correction due to particle decays • Primary charged particles (p±) decays in-flight, • Neutral (K0, p0…) particles feed-down: . . Net correction: 2.8% based on HIJING

11. Minimum-bias multiplicity distribution at ÖsNN = 130 GeV 92% of sgeo(Missing events are all in the lowest bins) Shape at high multiplicities determined by fluctuations due to limited acceptance. Scaling factor (geometry) to one unit of rapidity 5.82 (lower axis). PHENIX Au-Au ÖsNN = 130 GeV

12. EMCal energy response • Energy scale: • EMCal measures full energy of g and e± • Slow hadrons are absorbed in EMCal • Relativistic hadrons produce MIP peak • EMCal energy response proportional to Et : Et = 1.17 ±0.05 EtEMCal • EMCal energy resolution not important for the Et measurement o peak at 136.7 MeV/c2 Red curve: Au-Au data. MIP peak at 270 MeV Blue curve is AGS test beam data with p+

13. Background from albedo • Background from decays • From simulations • From events with displaced vertex. Background sources MC Data

14. Minimum-bias transverse energy distribution at ÖsNN = 130 GeV • 92% of sgeo(Missing events are all in the lowest bins) • Shape at high multiplicities determined by fluctuations due to limited acceptance. • Scaling factors: • Transformation 1.17 • Dead areas 1.03 • Geometry 10.6 PHENIX preliminary Au-Au ÖsNN = 130 GeV

15. Minimum-bias multiplicity distribution at ÖsNN = 130 GeV 92% of sgeo(Missing events are all in the lowest bins) Shape at high multiplicities determined by fluctuations due to limited acceptance. Scaling factor (geometry) to one unit of rapidity 5.82 (lower axis). PHENIX Au-Au ÖsNN = 130 GeV

16. Centrality dependence Use BBC – ZDC response to define centrality cuts (in 5% bins of sgeo) Determine <dNch /dh > and <dEt /dh > vs centrality 15-20% 10-15% 5-10% 0-5% PHENIX preliminary

17. Calculation of Npart and Ncoll • Use simulated BBC – ZDC response to define centrality cuts. • Relate them to Npartand Ncollusing Glauber model. • Straight-line nucleon trajectories • Constant sNN=(40 ± 5)mb. • Woods-Saxon nuclear density:

18. Centrality dependence • Data shows clear increase of dNch/dh per participant vs Npart • In contrast with EKRT saturation model • Similar to HIJING (although data ~15% higher)

19. Particle production mechanism PHENIX preliminary Consistent results: Hard processes contribution increases with centrality: from ~30% mid-central to ~50% most central

20. Comparison to CERN results PHENIX preliminary a-value Multiplicity Transverse energy PHENIX WA98 WA97

21. dEt /dNch independent of Npart Transverse energy per charged particle dEt /dNch independent of ÖsNN

22. ÖsNN dependence • Assumptions: in Lab in C.M. • Energy density (Bjorken): • From SPS to RHIC • ~50% increase in dNch/dy • ~50% increase in dEt/dy • at least 50% increase in e

23. Summary • Centrality dependence of particle dNch /dh and dEt /dh have been measured in ÖsNN = 130 GeV Au+Au collisions. • Both dNch /dh and dEt /dh per participant increase with centrality: • in qualitative agreement with HIJING • in contrast to EKRT saturation model prediction • the increase is stronger than at SPS • dEt /dNchis independent of centrality and of ÖsNN.