1. Charged Particle Multiplicity Measurement in 200 GeV pp Collisions with PHOBOS Joseph Sagerer University of Illinois at Chicago for the Collaboration DNP 2004: Chicago, IL October 29, 2004

2. Collaboration (April 2004) Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail Bickley, Richard Bindel, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Patrick Decowski, Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Adam Harrington, Michael Hauer, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Nazim Khan, Piotr Kulinich, Chia Ming Kuo, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer, Helen Seals, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch ARGONNE NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER

3.   0+3-3 +5.4 -5.4 Hardware: Z Paddle Triggers Octagon Detector (Silicon Pads) Covers |η|<3 (|z|<60cm) Unrolled Octagon Rings Cover 3 < |η| < 4.5 Ring Detectors (Silicon Pads)

4. Trigger Trigger is base on a particle in either of the Paddle Trigger Detectors (3<η<4.5) Sensitive to 95% of the Inelastic Cross- Section Sensitive to 95% of collisions Corrected Event Multiplicity (n)

5. Phobos Multiplicity Analysis Three independent methods –PHOBOS ‘hit counting’ (merged hits) –PHOBOS ‘analog’ (energy deposition) –Low multiplicity, wide range hit counting Unique to pp Allows us to study the PHOBOS detector response in greater detail This is what I will talk about

6. dE of Octagon hits in data before, after merging General Hit Merging For η ≠ 0 we have the possibility of a hit striking multiple pixels. Thus a merging algorithm is applied that sums adjacent pixels above threshold. A high η track passes through multiple pixels, the sharp angle results in a greater than normal track energy deposition A Secondary produced in the beam pipe, leaving the normal energy Unmerged hits Merged hits dE (MeV) Normalized to 300um of Silicon Red Line: dE cut to reduce secondary particles

7. Low Multiplicity η Overlap Region ‘Normal’ PHOBOS dN/dη use a narrow Z vertex range, creating simplicity in corrections. Low multiplicity method uses several bins of Z vertex position to make the same measurement in more than one detector element –Allows study of varying detector response

8. PHOBOS Multiplicity Array Acceptance vs Z Vertex Octagon -80cm to 80cm NSide Rings (PSide Symmetric) -80cm to 80cm

9. 0 Z = 0 cm, Nominal |Z| < 10 cm η Octagon Ring 1 Ring 2 Ring 3Ring 2Ring 3 |Z| < 60 cm 3.2 Octagon Ring 1 Ring 2 Ring 3Ring 2Ring 3 Octagon Ring 1 Ring 2 Ring 3Ring 2Ring 3

10. Monte-Carlo Based Corrections Corrections are made using MC + Geant simulations –Secondaries that remain after Normalized dE Cut –Effects of Decays –Geometrical Acceptance –Trigger Loss (vs Event Multiplicity) –Vertexing Algorithm Loss (vs Event Multiplicity) 10%-60% Trigger + Vertex Efficiency vs Mult and Z vertex Position Z Mult

11. PHOBOS 200 GeV pp dN/dη (preliminary) preliminary

12. PHOBOS 200 GeV pp dN/dη (preliminary) preliminary

13. Future Multiplicity Analysis Working on pp dN/dη for various event multiplicities –Have UA5 data to compare –Possibility of higher multiplicity bins P(N) with high statistics out to large multiplicities 400 GeV pp?

14. Event Multiplicity from UA5 Corrected Mult (n) Raw Mult UA5 had 4156 events @ 200GeV PHOBOS has ~3 million events => For the highest UA5 Mult bin (~5 events) PHOBOS has ~3000 events n P(n) Set Y Min to ZERO