# Presentation: 050079

##            , the Early Years

-            , the Early Years

- The Ghost of              Past
- What was built and why
- The Ghost of              Present
- What has been accomplished
- The Ghost of              Future
- What is still to come

- Presented in the spirit(s)
- of Charles Dickens:

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

##               Collaboration (Jan 2002)

- 	_ARGONNE NATIONAL LABORATORY_	Birger Back, Alan Wuosmaa
- 	_BROOKHAVEN NATIONAL LABORATORY _	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
- 	_INSTITUTE OF NUCLEAR PHYSICS, KRAKOW_	Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak
- _	__MASSACHUSETTS INSTITUTE OF TECHNOLOGY_	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
- 	_NATIONAL CENTRAL UNIVERSITY, TAIWAN_	Chia Ming Kuo, Willis Lin, Jaw-Luen Tang
- 	_UNIVERSITY OF ROCHESTER_	Joshua Hamblen , Erik Johnson, Nazim Khan, Steven Manly,Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs
- 	_UNIVERSITY OF ILLINOIS AT CHICAGO_	Russell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer
- 	_UNIVERSITY OF MARYLAND_	Abigail Bickley, Richard Bindel, Alice Mignerey

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Goals of                        , 

- Goals of                        ,

- Measure numerous observables quickly & accurately
- Perform several unique measurements
- Large multiplicity phase space coverage
- Particle measurements extended to low p
- Large event sample
- Eliminate the word “preliminary” from relativistic heavy     	ion vocabulary
- Good (to be)published data: Thanks to the collaboration
- Wild physics speculation: Blame GSFS

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Detectors used by                 ,

- Multiplicity array (Si sensors)
- Almost 4 coverage and high granularity
- 2 Arm Spectrometer (Si sensors)
- 2 Tesla magnetic field
- PID using  dE/dx
- Time-of-Flight wall for extended PID
- Trigger counters (Scintillator Paddles & Cherenkov T0)
- ZDCs common to all RHIC experiments

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Frodo (to scale)

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## All held together by 

- ~4 Multiplicity array

- Trigger paddles

- All held together by
- excellent engineering!

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Detector Performance I

- Detectors signals stable and well understood

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Detector Performance II

- Excellent signal/noise

- Very few dead channels
- (even after RHIC assault)

- Before RHIC blasts

- 10%

- 15

- ~137,000 total Si channels

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Triggering on Interactions

- Negative Paddles

- Positive Paddles

- ZDC N

- ZDC P

- Au

- Au

- PP

- PN

- 3<||<4.5

**Valid**
**Collision**

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Determining Centrality of Interaction

- For more discussion,
- see later talk by
- Andrzej Olszewski...

**Data**

**Paddle signal**

**N****participants**

**Data+MC**

** ****HIJING +GEANT**
** ****Glauber calculation**
** ****Model of paddle trigger**

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Multiplicity Measurements

- Unrivaled phase space coverage
- High granularity in  and 
- Low mass detectors situated very close to the 	beam pipe
- Multiple detectors and/or independent analysis 	methods for the same observable
- MC and data combined for a very detailed 	understanding of systematics

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Single-event display

- -5.4

- +5.4

- Octagon&Ring hits

- Vertex
- tracklets

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Energy Dependence of Central Multiplicity 

- Energy Dependence of Central Multiplicity

**|****|**1  6% most central AA collisions

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

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Centrality Dependence I

- nucl-ex/0105011
- Accepted: Phys Rev C

**Kharzeev/Nardi     **

- HIJING: PRL 86, 3496 (2001)
- EKRT: hep-ph/0106330
- KN scaling PLB 507,121 (2001)

**|****|**1  AuAu

- dNch/d /<Npart>/2

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Centrality Dependence II

- To be submitted to PRL

- See: Kharzeev and
- Levin, Phys. Lett.
- B523, 79 (2001)

**|****|**1  AuAu

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Two Component Parameterization

- Hard Scattering

*x* is the fraction of particles produced by hard scattering

- At RHIC:* **n**pp*~2.3, *x*~10%

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Two Component Parameterization

- In old language:

- From geometry, the number of collisions:

- For Symmetric systems, colliding pairs:

- where  is the average
- number of binary collisions

- Ratio to pp is a mix
- 	of nuclear geometry and the fraction of
- 	hard scattering

- Note: Asymmetric systems (pAu,
- SiAu, etc.) will have different ratios
- between *N**Coll* and *N**Part*

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Model Comments

- Fit of AuAu centrality data to two component 	parameterization extrapolates very close to pp data.
- Underlying physics of two-component model and 	saturation model are very different!
- Hopefully, further study (as well as other systems 	including pA) will help to differentiate the two.
- Note: Several different ‘saturation’ calculations agree 	that gluon densities are “large” on the QCD scale.

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## 2-Component Energy Prediction

- ~20 at LHC
- dN/d~3500

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Shapes I

- Distributions get
- narrower for more
- central collisions.

- 130 GeV AuAu Data

- Full dN/d distribution yields the total number of charged particles.
- For 3% most central
- <Nch> = 4200  470

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Shapes II

- PRL 87,102303
- (2001)

- 130 GeV

- Note
- crossover

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Shapes III

- Fragmentation

- Fragmentation

- UA5: Alner *et al.*, Z. Phys. C33,1 (1986)

- PHOBOS 2000/2001

- 7-10% syst error

- 200 GeV shape from Phys
- Rev Lett 88, 22302 (2002)

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Multiplicity Conclusions (so far)

- Whatever measure or model is used, systems 	being created are dense & denser.
- Extensive results on energy, centrality, and	rapidity dependence
- Data have significant impact on theory
- Initial conditions and subsequent evolution
- Global properties and fundamental interactions
- Rules out or severely restricts many proposed 	exotic processes
- Much more to come...

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Charged Multiplicity Future

- Soon: 20 GeV AuAu (RHIC injection energy, run 	in November specifically for Phobos) and
- 		200 GeV pp (currently running)
- Later: More details, fluctuations, event shape...
- Next Run: More species and energies

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## ||1  AuAu

- Observation: Centrality data at both beam energies rise for
- the most central events (systematics?  physics? mean p?)

- ||1  AuAu

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

##            Spectrometer

-            Spectrometer

- Magnetic Field

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Spectrometer Characteristics

- dE/dx resolution 7%

- Particle ID using dE/dx

- Momentum resolution

- 2%

- 10 GeV

- 

- K

- p

- d

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Spectrometer I: Chemistry

- 130 GeV Data    Phys.Rev.Lett. 87, 102301 (2001)

- _Stat._   _Syst._

- Using model of Redlich (QM ’01)  T~165  implies B=45

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Particle Ratios @ 130 GeV

**K****-****/K****+**** vs Energy**

**Phys Rev Lett 87, 102301 (2001) **

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Spectrometer II: Stopping ParticlesThe Ultimate in Low p

- For tracks stopping in the 5th Si layer:
- p 	50	MeV/c
- pK  	140	MeV/c
- pp  	200	MeV/c
- Note: At low p, particles are at y~0 for any angle

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## dE/dx from  in Individual Layers

- High signals from
- nuclear fragmentation
- can identify **-**

- dE/dx from  in Individual Layers

- 0

- 2

- 0

- 20

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## MC Results

- Eloss = (ΣdEi )/nhits ,  i=A-E

- Mi = (dE/dx)i *   Ei
-               (~1/2)   (m2)

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Spectrometer Future

- Soon: Particle ratios from 200 GeV AuAu
- Later:
- Spectra (with and without PID)
- Extended PID
- Low and high p
- HBT
- Beyond: Reaction plane, resonances (especially 	 at low p), and much more...

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Future for              ,

- Eagerly awaiting more beam energies and beam 	species (including pA) for systematic study
- Continue the program discussed as well as 	many additional physics topics (flow...)
- Far Future: Considering addition of electron 	identification to study charm production

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## One Possible Charming Future...

**ALICE prototype TRD Electron-ID**
**EM-Calorimeter**
**Micro-Vertex Detector**

**Use existing spectrometer**

**Discussing upgrade to focus on charm production at RHIC.**
**Measure single electrons from displaced vertices.**

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## Conclusion

- Early results have proven more robust and 	more interesting than (I) expected.
- Detector (and analysis teams) have performed 	spectacularly.
- Bright prospects for productive years ahead.
- An even Happier 2002!

- G.S.F.Stephans                                                   Epiphany 2002

- 4-Jan-2002

## For More               Information...

- <date/time>

- PHOBOS web-site: _www.phobos.bnl.__gov_
- Physics Results
- Charged particle multiplicity near mid-rapidity in central Au+Au collisions at 56 and 130 GeV Phys. Rev. Lett. 85, 3100 (2000)
- Ratios of charged antiparticles-to-particles near mid-rapidity in Au+Au collisions at 130 GeV Phys. Rev. Lett. 87, 102301 (2001)
- Charged-particle pseudorapidity density distributions from Au+Au collisions at 130 GeV Phys. Rev. Lett. 87, 102303 (2001)
- Energy dependence of particle multiplicities in central Au+Au collisions Phys. Rev. Lett. 88, 22302 (2002)
- Centrality Dependence of Charged Particle Multiplicity at h=0  in Au+Au Collisions at 130 GeV Accepted to Phys. Rev. C (December 2001); nucl-ex/0105011
- Technical
- Array of Scintillator Counters for PHOBOS at RHICNucl. Instr. Meth. A474, 38-45 (2001)
- Silicon Pad Detectors for the PHOBOS Experiment at RHICNucl. Instr. Meth. A461, 143-149 (2001)
- Development of a double metal, AC-coupled silicon pad detectorThe silicon detector for the PHOBOS experiment at RHICNucl. Instr. Meth. A389, 415 (1997)