# Presentation: 295194

## Systematics of Soft Particle Production at RHIC:

- George S.F. Stephans
- Massachusetts Institute of Technology

- Lessons from

- (And some thoughts for the future)

##                                Detector

## Energy dependence of particle production:First RHIC physics result

**Theory 1999**

- Phys.Rev.Lett. 88 (2002) 22302
- Phys.Rev.Lett. 85 (2000) 3100

## Impact of first                measurement

- Invalidated many of the theoretical predictions in existence
- 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/d per participant pair in Au+Au is equal to dN/d in p+p at twice the center of mass energy (next slide).

## A+A, p+p, & e++e- energy dependence

- A+A, p+p, & e++e- energy dependence

- Phys. Rev. C74, 021902(R) (2006)

- 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.

## Pseudorapidity dependence  I

- Phys.Rev.Lett. 91 (2003) 052303

## Pseudorapidity dependence  II

- Energy and pseudorapidity dependencies  factorize

- Phys.Rev. C74 (2006) 021901
- Nucl.Phys. A774 (2006) 113

- Called “extended longitudinal scaling” and “limiting fragmentation”

## Pseudorapidity dependence  III

**dN****ch****/d********/<N****part****/2> **

- LHC
- prediction

- Acta Phys. Pol. 35 (2004) 2873

## Pseudorapidity dependence  IV

- Pseudorapidity dependence **** IV

***Au-Au data, h******±***

***0-40% centrality***

***Systematic errors only***

******'=|********|-y****beam**

- v2

- A similar effect is seen for elliptic flow

- However, v2 at =0 in 200 GeV Au+Au is claimed to approach the hydro limit

- What will happen at the LHC???

- Phys.Rev.Lett. 94 (2005) 122303

## 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

## Centrality dependence  II

- Energy and centrality dependencies factorize

- Effect of “harder” processes??

- Phys.Rev. C74 (2006) 021901
- arXiv:nucl-ex/0604017

## Centrality dependence  III

- Phys.Rev.Lett. 94 (2005) 082304
- Nucl.Phys. A774 (2006) 113-128

- Energy and centrality dependencies factorize even at pT~4 GeV/c

## Centrality dependence  IV

- 200 GeV p+p, Cu+Cu & Au+Au

- Midrapidity ratio of anti-particles over particles (“chemistry”) at most weakly dependent on system size

- 200, 62.4 GeV Cu+Cu

- arXiv:nucl-ex/0701064

## Particle production summary

- 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 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)

## 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 pT trigger particle

## Clusters in particle production

- Define Keff = effective cluster size
- i.e. number of particles/cluster

- Particles produced in clusters of 2-3.  Looking forward to doing systematics.

- arXiv:nucl-ex/0701055

## Conclusions

- Many properties of single particle production in A+A @ 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, & pT factorize to a surprising degree
- Onward to more complicated observables
- (And eventually to CMS@LHC!!)

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