Charged particle multiplicity studies with PHOBOS Birger Back Argonne National Laboratory for the PHOBOS Collaboration

2 Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Joshua Hamblen, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, 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 PHOBOS experiment: June 2000 – June <  < o <  < o Main emphasis: 4  multiplicity and low p T particles

4 PHOBOS multiplicity measurements Systems 1/2 (GeV) Au+Au200, 130, 62.4, 19.6 Cu+Cu200, 62.4, 22.4 d+Au200 p+p200, 410

5 dN/d  basics

6 What would an isotropic source look like? Isotropic emission: 1/cosh 2  Only 22% emitted with p T > p L These particles carry information about the densest region formed in the collisions

7 What happens to the original protons? AGS SPS RHIC 62 RHIC 200 LHC 5500 (BRAHMS preliminary) dN/dy I.Bearden (BRAHMS), QM2006 Net (original) protons move away from mid- rapidity region with increasing collision energy Mid-rapidity region begins to look like a pure energy-density region reminiscent of the early universe

8 dN/d  =0

9 Pre-RHIC theoretical predictions PHOBOS, Nucl. Phys. A747, 28 (2005) First RHIC results New data PRC 74, (2006)

10 Factorization of centrality and energy dependence |  |<1 Data: PHOBOS, PRL 97, (2006); PRC70, (R) (2004); PRC65, (R) (2002)

11 Energy density

12 Energy density estimates? Absolute maximum: Total available energy: Volume at instant of overlap: Instantaneous energy density: Not equilibrated matter Birger’s estimate: Isotropic energy: Volume after  0 =1 fm/c: Equilibrated energy density:

13 Bjorken estimate of energy density Phenix: E T measurement at 130 GeV  0 = 4.6 [GeV/fm 3 ] PRL 87, (2001) NA49: E T measurement at 17 GeV  0 = 3 [GeV/fm 3 ] PRL 75, 3814 (1995) Brahms Conclusion: All reasonable estimates are substantially larger than the predicted transition density of  0 = GeV/fm 3

14 Complete dN/d  distributions

15 PHOBOS Au+Au Data * PHOBOS PRL 91,52303 (2003) PHOBOS, PRL 91, (2003); PRC 74, (2006)

16 Cu+Cu data

17 Centrality (%) N part (total) N part (Au) N part (d) d+Au centrality dependence PHOBOS, Phys. Rev. C72, (R) (2005) d Au Central: asymmetric Peripheral: symmetric Momentum conservation? Peripheral Central > ~

18 System size: Au+Au vs. Cu+Cu

19 Unscaled dN/d  very similar for Au+Au and Cu+Cu at same N part Scaling Laws Cu+Cu Preliminary 3-6%, N part = 100 PHOBOS 62.4 GeV200 GeV Au+Au 35-40%,N part = 98 Cu+Cu Preliminary 3-6%, N part = 96 Au+Au 35-40%, N part = 99 See poster by Richard Hollis dN/d  in Cu+Cu vs Au+Au for N part ~ 100

20 Unscaled dN/d  very similar for Au+Au and Cu+Cu at same N part Scaling Laws Cu+Cu Preliminary 15-25%, N part = 61 PHOBOS 62.4 GeV200 GeV Au+Au 45-50%,N part = 62 Cu+Cu Preliminary 15-25%, N part = 60 Au+Au 45-55%, N part = 56 Also true for mid-central Cu+Cu vs peripheral Au+Au dN/d  in Cu+Cu vs Au+Au for N part ~ 60

21 Extended longitudinal scaling

22 Extended longitudinal scaling – Au+Au PHOBOS Phys. Rev. Lett. 91, (2003) / Nucl. Phys. A757, 28 (2005) independent of energy Works also for dN/d  because:

23 Scaling Laws 19.6 GeV62.4 GeV130 GeV200 GeV PHOBOS preliminary “Extended Longitudinal Scaling” of all longitudinal distributions  - y beam preliminary PHOBOS Au+Au 0-6% Au+Au 0-40% Au+Au 0-40% 200GeV 130GeV 62.4 GeV (prel) 19.6 GeV Extended longitudinal scaling – Au+Au

24 Scaling Laws Same for Cu+Cu preliminary PHOBOS 62.4 GeV200 GeV ‘Extended Longitudinal Scaling’ also seen in Cu+Cu Persists from p+p to Au+Au over large range in  ’ preliminary PHOBOS  - y beam Cu+Cu 0-6% 200GeV 62.4GeV Cu+Cu 0-40%

25 Extended longitudinal scaling in p-A and d-A PHOBOS, Phys. Rev. C72, (R) (2005)

26 Total charged particle multiplicity

27 Total charged particle multiplicities in Au+Au N ch Q N part Width x height = const

28 Total multiplicity as a function of energy N ch = dN/d  x  (2y beam +0.3-dN/d  /195) height x width width height PHOBOS PRL 91,52303 (2003)

29 Energy dependence of N ch 0-6% Central AuAu collisions N part =344 PHOBOS, PRL 91,52303 (2003)

30 Universality – comparing AA to pp and e + e - PHOBOS, PRC 74, (R) (2006) Shapes: Au+Au and e + e - “similar” Total N ch : Au+Au same as e + e - p+p: leading hadron removes 50% of energy

31 Evolution of N ch /N pp ratio vs N part N ch /(N part /2) constant with centrality d+Au also lower than Au+Au d+Au data similar to low-energy p+A N ch dAu =0.5 N part N ch pp (1 Deuteron = 2 protons) d+Au: Centrality dependence of total N ch PHOBOS, Phys. Rev. C72, (R) (2005)

32 Other new PHOBOS results (QM2006) New data on antiparticle/particle ratios Identified particle spectra for 62.4 GeV Au+Au Event-by-event v 2 measurement and flow fluctuations Two-particle correlations and cluster size

33 New Data: antiparticle/particle ratios PHOBOS, QM , 62.4 GeV Cu+Cu 200 GeV Cu+Cu and Au+Au Energy dependence for Cu+Cu System dependence at 200 GeV

34 Identified particle spectra for 62.4 GeV Au+Au First published identified spectra for 62.4 GeV Au+Au at RHIC (down to very low p T, a unique PHOBOS measurement) blast-wave fits PHOBOS, nucl-ex/ Accepted for PRC

35 New Analysis: event-by-event v 2 measurement Measure v 2 on an event-by-event basis Average and compare to our standard analysis Agreement with both hit and track based PHOBOS results 200 GeV Au+Au PHOBOS, QM2006; arXiv:nucl-ex/ Submit to PRL this week

36 Event-by-event flow: fluctuations PHOBOS, QM2006 Submit to PRL this week v 2 fluctuations mirror  part fluctuations  (v 2 )/ and  (  part )/ in 200 GeV Au+Au Collisions PHOBOS  part prediction PHOBOS v 2 result 90% CL MC with no fluctuations

37 Effective cluster size analysis On average, particles produced in clusters with a size of 2-3. Interesting centrality dependence – compare to other systems p+p scale error 2 K eff = effective cluster size PHOBOS, QM2006

38 Summary and conclusion Multiplicity –PHOBOS have performed complete charged particle multiplicity measurements for Au+Au, Cu+Cu, d+Au, and p+p collisions –Systen size dependence –‘Complete’ pseudorapidity distributions Midrapidity multiplicity –Factorization of centrality and energy dependencies Limiting fragmentation – extended longitudinal scaling –Seen for Au+Au, Cu+Cu, and d+Au –Also observed in flow observables Total charged particle multiplicity –N ch /N part constant with centrality –‘Universality’ – compared to elementary e+e- collisions Future: Finish up many analysis and reviews

39 Why mid-rapidity? Emphasize produced or scattered particles Triple Gaussian fit function 200 GeV Au+Au 0-6% central

40 System size dependence - Au+Au vs. Cu+Cu

41 Centrality dependence 200 GeV

42 Centrality dependence 62.4 GeV

43 Cu+Cu elliptical flow – eccentricity scaling PHOBOS, QM2006