# Presentation: 787075

## Mark D. Baker
for the                         collaboration
September 4, 2001

**for the                         collaboration**
**September 4, 2001**

**The Latest Results from PHOBOS****Systematics of Charged Particle Production****through ********s****NN**** = 200 GeV**** **

## The PHOBOS Collaboration

- Argonne National Laboratory
- Brookhaven National Laboratory
- Institute of Nuclear Physics Krakow
- Jagiellonian University Krakow
- Massachusetts Institute of Technology
- National Central University, Taiwan
- University of Rochester
- University of Illinois Chicago
- University of Maryland

- ~ 70 collaborators

## The French Connection

**The French Connection**

- Rachid Nouicer

## Why charged particle production?

- It’s a fundamental handle on the initial state
- Proportional to freezeout entropy density
- Upper bound on initial state entropy density
- Note: Entropy does NOT require equilibrium
- Proportional to initial gluon density
- It constrains the production of entropy in the initial state and in the subsequent evolution.

## The full PHOBOS Detector

**The full PHOBOS Detector**

**Mid-rapidity Spectrometer**

**~4******** Multiplicity Array**

**TOF**

**137,000 Silicon Pad channels:**
**   ****spectrometer + multiplicity**

**Cerenkov**

**Trigger Paddles**

## Multiplicity & Vertex Detector

## Collisions at 200 GeV!

## Min. Bias Trigger & Event Selection

**Min. Bias Trigger & Event Selection**

- _We attempt to trigger on ALL collisions._
- Good paddle timing.
- Good ZDC timing **OR **Large paddle signal.

- Negative Paddles

- Positive Paddles

- ZDC N

- ZDC P

- Au

- Au

- PP

- PN

- 3<||<4.5

- 

## Cut on fractions of  to estimate Npart

**Cut on fractions of ******** to estimate N****part**

- Peripheral

## Multiplicity Measurements in 4

**Multiplicity Measurements in 4******

- -5.4

- +5.4

- Single-event display

- Vertex “tracklets” – 3 point tracks

## Knowledge of dN/din 1999

**Theory 1999**

**Knowledge of dN/d********in 1999**

**SPS PbPb**

**pp**

**pp**

## Data from RHIC   (2000)

**PRL 85 (2000) 3100**

**Data from RHIC   (2000)**

**Theory 1999**

**SPS PbPb**

**AuAu**

**pp**

**pp**

**Theory 2000**

## Results for 200 GeV

**Results for 200 GeV**

**6% Most Central Events - Systematic Uncertainties Included**

- nucl-ex/0108009
- Submitted to PRL

## Full energy RHIC results!

**Theory 1999**

**PHOBOS 2001**

**Theory 2000**

**PHOBOS 2000**

**Full energy RHIC results!**

## Energy Systematics: 3-200 GeV

- nucl-ex/0108009
- Submitted to PRL

## Ratio of dN/d at 200 & 130 GeV 

- 90% C.L. systematic band

- nucl-ex/0108009

- KL: Saturation:

- soft+hard

- Kharzeev, Levin:
- nucl-th/0108006

- Jet quenching-> entropy

## dN/d distributions @ 130 GeV

- Measure out to ||<5.4
- Corrections
- Acceptance
- Occupancy
- Backgrounds (from MC)
- Systematic errors
- 10% near =0
- Higher near rings

- PRL 87 (2001) 102303

## Centrality Dependence at =0 

- Kharzeev/Nardi

- nucl-ex/0105011

**130 GeV**

##  ~4 shape vs. centrality (130 GeV) 

_**3% most central collisions**_
**<N****ch****> = 4200 ******** 470**
**                 **

** ****Total N****ch****(|********| ******** 5.4)**

** ****~4******** shape vs. centrality (130 GeV) **

**PRL 87 (2001) 102303**

## Comparison to pp and models

- Peripheral

- Central

- Scaled UA5 data

- HIJING

- AMPT(rescattering)

- Ybeam

-  (Y130/Y200)
- dN/d = fpp(s)

**PRL 87 (2001) 102303**

## Pseudo-Rapidity Dependence of Ratio

**Pseudo-Rapidity Dependence of Ratio**

**200 GeV**

- nucl-ex/0108009

- Systematic errors included

- 6% central

## Comparing 130 to 200 GeV

- Systematic errors not shown

- Data from nucl-ex/0108009

- 

*6*

- 6% central data samples

## Fragmentation Region Kinematics

- Fragmentation
- Region

- Fragmentation
- Region

**Fragmentation Region Kinematics**

- 0

- 1

- -1

- xF~0.1

- All energies

- 0

- ybeam

- -ybeam

- ybeam-2.3

- 0

- ybeam

- -ybeam

## Feynman Scaling in  space(limiting fragmentation)

**130 GeV      PHOBOS (6%)**

- UA5 200 GeV

- UA5, Z.Phys.C33, 1 (1986)

- 53 GeV

- 200 GeV

- 545 GeV

- 900 GeV

**200 GeV PHOBOS (6%)**

- Particle excess primarily at mid-rapidity

- Data from nucl-ex/0108009

## Elliptic Flow: A collective effect

**dN/d(********R ****) = N****0 ****(1 + 2V****1****cos (********R****) + 2****V****2****cos (2(********R****)) + ... ) **

- Elliptic flow

**Beam’s eye view of a**
**non-central collision: **

**Particles prefer to be “in plane”: **

****

## ~4 Elliptic Flow @ 130 GeV 

- Peripheral

**b**

- Central

**b**

**Normalized Paddle Signal**** **

-  (**STAR : Normalized N****ch**** )**** **

**PHOBOS ****Systematic**** error ~ 0.007**

**V****2**

**Centrality dependence of V****2**

**|********|<1.0**
****sNN=130GeV

**Preliminary**

**Preliminary**

## Particle Ratios (Chemistry)

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

- _Stat._       _Syst._

## Conclusions

- We’ve studied dN/d (entropy) up to sNN  = 200 GeV
- We see more entropy/Npart at RHIC than in pp
- Entropy increase with energy is modest (and logarithmic?)...
- The entropy increase is concentrated at mid-rapidity
**“****Feynman scaling” works over a broad range of ******
- Other measurements:
- Elliptic flow: Strong and -dependent
- Particle ratios are approaching small B
- Centrality dependence of dN/d.
- Theoretical models are assimilating new data
- None simultaneously describe *full* set of systematics!