Systematics of Soft Particle Production at RHIC: Lessons from

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1 Systematics of Soft Particle Production at RHIC: Lessons from (And some thoughts for the future) George S.F. Stephans Massachusetts Institute of Technology

2 Detector

3 Energy dependence of particle production: First RHIC physics result PHOBOS 2000 Theory 1999 Theory 1999 Theory 2000 PHOBOS 2001 Theory 2000 PHOBOS 2000 PHOBOS 2001 Phys.Rev.Lett. 88 (2002) Phys.Rev.Lett. 85 (2000) 3100

4 Impact of first measurements Invalidated many of the theoretical predictions in existence at the time Provided support for further development of theories of parton saturation If theory is validated, this process may dominate low-x physics at high enough energy, even in p+p Implied that tracking and other measurements in heavy ion collisions at the LHC may not be as horrendous as previously thought Furthermore, found that dn/dη per participant pair in Au+Au is equal to dn/dη in p+p at twice the center of mass energy (next slide).

5 A+A, p+p, & e + +e energy dependence What will happen at the LHC (p+p@14 TeV, A+A@5.5 TeV)? Correspondence between e + +e and p+p@( s)/2 known for some time. Phys. Rev. C74, (R) (2006)

6 Pseudorapidity dependence I Scale by N part /2 Shift to η = η - y beam dn ch /dη PHOBOS Au+Au dn ch /dη /<N part /2> PHOBOS Au+Au 6% central! $# " $! y beam Phys.Rev.Lett. 91 (2003) Effectively viewed in rest frame of one nucleus

7 Pseudorapidity dependence II Called extended longitudinal scaling and limiting fragmentation Phys.Rev. C74 (2006) Nucl.Phys. A774 (2006) 113 Energy and pseudorapidity dependencies factorize

8 Pseudorapidity dependence III LHC prediction dn ch /dη /<N part /2> central data Acta Phys. Pol. 35 (2004) 2873

9 Pseudorapidity dependence IV A similar effect is seen for elliptic flow v 2 Au-Au data, h ± 0-40% centrality Systematic errors only However, v 2 at η=0 in 200 GeV Au+Au is claimed to approach the hydro limit What will happen at the LHC??? Phys.Rev.Lett. 94 (2005) η'= η -y beam

10 Centrality dependence I Impact parameter varies in heavy ion collisions: Not just more of the same Large impact parameter Few nucleons participate Few collisions Smaller average number of collisions per participant Small impact parameter Many nucleons participate Many, many collisions Larger average number of collisions per participant Common claim is that harder ( softer ) processes scale with collisions (participants). Relative abundance of harder processes should rise with beam energy. Centrality dependence should change with energy

11 Centrality dependence II dn ch d!!=0 = f (s) " g(n part ) Energy and centrality dependencies factorize Effect of harder processes?? Phys.Rev. C74 (2006) arxiv:nucl-ex/

12 Centrality dependence III Phys.Rev.Lett. 94 (2005) Nucl.Phys. A774 (2006) Energy and centrality dependencies factorize even at p T ~4 GeV/c

13 Centrality dependence IV 200, 62.4 GeV Cu+Cu 200 GeV p+p, Cu+Cu & Au+Au Midrapidity ratio of anti-particles over particles ( chemistry ) at most weakly dependent on system size arxiv:nucl-ex/

14 Particle production summary Particle production in heavy ion collisions Star formation in galaxies follows a quite simple pattern and simple patterns often mean that there are only [a] few basic physical mechanisms at work.... We can now find out what these mechanisms are by measuring how particle production energy, pseudorapidity, centrality, and p T star formation behaves with time and mass of the galaxy and compare that behavior to models. Kai Noeske UC Santa Cruz quoted in Science News 171, 121 (2007)

15 What s next? Fully incorporate all data, including p+p, d+au, Cu+Cu & Au+Au (over expanded range of centrality) into the systematic studies Continue expanding analysis into new area of fluctuations, including flow (last talk) and search for rare events of various types Continue expanding analysis into new area of correlations, including both inclusive (next slide) and relative to a high p T trigger particle

16 Clusters in particle production Define K eff = effective cluster size i.e. number of particles/cluster Cu+Cu PHOBOS Preliminary h ± :clusters 200 GeV 0-10% central p+p scale error arxiv:nucl-ex/ Particles produced in clusters of 2-3. Looking forward to doing systematics.

17 Conclusions Many properties of single particle production in RHIC energies are describable with a small number of surprisingly simple scalings Range from very basic such as total multiplicity to more complicated features such as elliptic flow Dependencies on incoming energy, pseudorapidity, centrality, & p T factorize to a surprising degree Onward to more complicated observables (And eventually to CMS@LHC!!)

18 Collaboration 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 LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF MARYLAND 9 Current Ph.D. Students BROOKHAVEN NATIONAL LABORATORY MASSACHUSETTS INSTITUTE OF TECHNOLOGY UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF ROCHESTER