1. Peter Steinberg Systematics of Charged Particle Production in 4  with the PHOBOS Detector at RHIC Peter A. Steinberg Brookhaven National Laboratory George Washington University, 16 Nov 2001

2. Peter Steinberg Systematic Measurements Do Nucleus-Nucleus collisions show collective behavior Energy (or particle) density Scaling with centrality Hard and soft processes contribute Rapidity plateau Effect of initial geometry on final state Participants Spectators pp collisions pA collisions We study this with systematics of charged particle production: Energy, Rapidity, Centrality, Azimuthal angle

3. Peter Steinberg Centrality Nuclei are extended R Au ~ 6.4 fm (10 -15 m) Impact parameter (b) determines N part – 1 or more collisions N coll – binary collisions Proton-nucleus: N part = N coll + 1 (2 = 1+1 in pp) Nucleus-Nucleus N coll  N part 4/3 Participants Spectators pp collisions pA collisions b b N coll N part Useful quantities to compare Au+Au to N+N collisions!

4. Peter Steinberg Soft & Hard Particle Production Soft processes (p T < 1 GeV) Scales with number of participants Color exchange leads to excited nucleons that decay Create rapidity plateau Hard processes (p T > 1 GeV) pQCD can calculate jet cross sections Scales with number of binary collisions QCD evolution leads to narrower distribution around y=0 minijet

5. Peter Steinberg Rapidity Useful single-particle observable: Kinematics: Change of variables Dynamics: Particle distributions are expected to be “boost invariant”

6. Peter Steinberg Pseudorapidity Rapidity requires complete characterization of 4-vector Conceptually easy, but requires a spectrometer Experiments with high multiplicities and limited resources use “pseudorapidity” dN/d  ~ dN/dy for y<2. Easily seen from Jacobian (dy =  d  )  tanh(y) 1 -5 5 y where

7. Peter Steinberg UA5 Experiment

8. Peter Steinberg Energy Dependence in pp Feynman’s postulate of boost invariance dn/dy plateau is energy independent Requires F 2 ~ 1/x Pure parton model! No QCD evolution Violations of scaling at SppS energies No plateau! Models like HIJING can reproduce this behavior What about Au+Au

9. Peter Steinberg RHIC & Experiments Nucleus-Nucleus (Au+Au) collisions up to  s NN = 200 GeV Polarized proton-proton (pp) collisions up to  s NN = 450 GeV

10. Peter Steinberg PHOBOS Experiment @ RHIC Large acceptance to count charged particles Small acceptance, high- resolution spectrometer Focus is on simple silicon technology, timely results

11. Peter Steinberg PHOBOS Collaboration (Nov 2001) ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ROCHESTER UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND Birger Back, Alan Wuosmaa Mark Baker, Donald Barton, Alan Carroll, Joel Corbo, Nigel George, Stephen Gushue, Dale Hicks, Burt Holzman, Robert Pak, Marc Rafelski, Louis Remsberg, Peter Steinberg, Andrei Sukhanov Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki, Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak Wit Busza (Spokesperson), Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane, Judith Katzy, Piotr Kulinich, Johannes Muelmenstaedt, Heinz Pernegger, Michel Rbeiz, Corey Reed, Christof Roland, Gunther Roland, Leslie Rosenberg, Pradeep Sarin, Stephen Steadman, George Stephans, Gerrit van Nieuwenhuizen, Carla Vale, Robin Verdier, Bernard Wadsworth, Bolek Wyslouch Chia Ming Kuo, Willis Lin, Jaw-Luen Tang Joshua Hamblen, Erik Johnson, Nazim Khan, Steven Manly,Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs Russell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer Abigail Bickley, Richard Bindel, Alice Mignerey

12. Peter Steinberg The full PHOBOS Detector Mid-rapidity Spectrometer ~4  Multiplicity Array TOF 135,000 Silicon Pad channels: spectrometer + multiplicity Cerenkov Trigger Paddles

13. Peter Steinberg Multiplicity Measurements in 4    -5.4 +5.4 Single-event display Vertex “tracklets” – 3 point tracks 500 keV 60 keV dE/dx

14. Peter Steinberg Phobos acceptance (z vtx =0)

15. Peter Steinberg Measuring Centrality Cannot directly measure the impact parameter! but can we distinguish peripheral collisions from central collisions? “Spectators” Zero-degree Calorimeter “Spectators” Paddle Counter Can look at spectators with zero-degree calorimeters, and participants via monotonic relationship with produced particles

16. Peter Steinberg Centrality Selection HIJING predicts paddle signal (3<  <4.5) to be monotonic w/ N part Spectator matter measured in ZDC anti -correlates Expected if Cut on fractions of total cross section to estimate N part Central 6% N part ~341

17. Peter Steinberg Estimating 96% when really 90% overestimates N part We stop around N part ~100 Species scan might help Uncertainty on N part Error of fraction of total cross section determined by knowledge of trigger efficiency “Minimum-bias” still has bias Affects most peripheral events % Error on N part N part

18. Peter Steinberg Energy Dependence near  =0 Errors are dominated by systematics AGS/SPS points extracted by measured dN/dy and New data at 200 GeV shows a continuous near-logarithmic rise at mid-rapidity f pp (s) =

19. Peter Steinberg Ratio of dN/d  at 200 & 130 GeV 90% Confidence Level Hard scattering dominant contribution Limited role of hard scattering

20. Peter Steinberg Parton Saturation Gluon distribution rises rapidly at low-x Gluons of x~1/(2mR) overlap in transverse plane with size 1/Q At “saturation” scale Q s 2 gluon recombination occurs In RHIC Au+Au collisions, saturation occurs at a higher Q s 2 (thus higher x) Saturation describes HERA data! Scale depends on volume

21. Peter Steinberg Particle Density vs. Centrality Is this picture unique?… UA5 (pp) EKRT KN

22. Peter Steinberg UA5 KN 2C Two Component Model What if we move away from mid-rapidity?

23. Peter Steinberg Pseudo-rapidity Distributions Using Octagon and Ring subdetectors Measure out to |  |<5.4 Corrections Acceptance Occupancy Backgrounds (from MC) Systematic errors 10% near  =0 Higher near rings  Background Corr. HIJING Simulation 130 GeV: PRL 87 (2001) forthcoming

24. Peter Steinberg Consequences of Parton Saturation Saturated initial state gives predictions about final state.  N(hadrons) = c  N(gluons) (parton-hadron duality)  Describes energy, rapidity, centrality dependence of charged particle distributions Kharzeev & Levin, nucl-th/0108006 m 2 =2Q s m , p T =Q s ~.25 extracted from HERA F 2 data Kharzeev & Levin, nucl-th/0108006, input from Golec-Biernat & Wüsthoff (1999) Intriguing! Suggests simple path from initial to final state…

25. Peter Steinberg Comparison to pp and models Peripheral Central Scaled UA5 200 GeV data HIJING AMPT (rescattering) Y beam   (Y 130 /Y 200 ) dN/d  = f pp (s) PRL 87 (2001) forthcoming Systematic error not shown 130 GeV

26. Peter Steinberg Centrality Dependence vs.  N ch = 4200 ± 420 for central events HIJING good to 10% Above  3-4 decreases vs. N part “Crossover” not seen in HIJING, Models with rescattering do better job PRL 87 (2001) forthcoming

27. Peter Steinberg pA: Rapidity Distributions Several new features relative to pp 1.Peak of distribution shifts backwards 2.Depletion forward of beam rapidity 3.Cascading near target rapidity – rapid increase NA5 DeMarzo, et al (1984)

28. Peter Steinberg Centrality Dependence: pA NA5 showed ratio of multiplicites produced in rapidity regions in pA vs. pp vs, R = dN/dy| pA / dN/dy| pp vs. (n p ) Large enhancement in target rapidities At central rapidity, ratio seems to saturate to 3 (cf. AQM) At forward rapidity, energy degradation leads to less particle production than pp

29. Peter Steinberg Limiting Fragmentation UA5, Z.Phys.C33, 1 (1986) 130 GeV 200 GeV UA5 200 GeV True in central AA Difference to pp not surprising Depends on colliding system UA5 observation of ‘limiting fragmentation’  - Y beam = ln x F + ln (M N /p T )

30. Peter Steinberg Limiting Fragmentation, contd. Central A+A is 40% higher than pp at RHIC energies At 200 GeV, Simple linear scaling by 30% agrees (within systematics) over the whole distribution ! Higher p T in A+A vs. p+p should correct p+p by at least 5% Detailed balancing of jets and rescattering in A+A?? Complicates interpretation of central + fragmentation region in pp and central-AA

31. Peter Steinberg Conclusions Systematics of charged particle production have been explored by the PHOBOS experiment Energy, Centrality, Rapidity Broad features of particle production are consistent with our previous understanding of hadronic interactions pp and pA collisions are very instructive Limiting fragmentation Change in scaling behavior at high-  Some mysteries, however Same shape for pp and central Au+Au Theoretical models are assimilating new data Energy dependence (influence of hard processes) Parton saturation

32. Peter Steinberg Why Rapidity? Proton-proton cross section dominated by soft processes w/ limited p T Up to ISR energies, it was observed that The energy dependence becomes weak Transverse and Longitudinal dynamics factorize But if y = ½ ln(E+p z /E-p z ), dy = p L /E iff we assume F 1 (x) is constant at low x (NB, dy = x dx) which is true if structure functions go as 1/x

33. Peter Steinberg Proton-proton collisions Fits to Woods-Saxon dn/dy=C(1+exp(y-y o )/  ) -1  ~.59 High-multiplicity events at low-energy: shows narrowing effect of jets

34. Peter Steinberg Hit counting technique 1.Count hits binned in , centrality ( b ) 2.Calculate acceptance A ( Z VTX ) for that event 3.Find occupancy in hit pads O ( ,b ) by counting empty to hit channels assuming Poisson statistics 4.Fold in a background correction factor f B ( , b ) dN ch dd =  hits O( ,b) ×f B ( ,b) A(Z VTX )   RingsOctagonRings Low  E High  E Vertex Spec PoissonStatistics  E deposition In multiplicity detectors for one event.