# Presentation: 564375

## Charged-Particle Multiplicity and Particle Ratiosin Au + Au Collisions at RHIC EnergiesUsing the PHOBOS Detector 

- The Latest Results from PHOBOS

- Rachid Nouicer
- University of Illinois at Chicago
- for the                      Collaboration

## The PHOBOS Collaboration

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

- ~ 70 collaborators

## PHOBOS Apparatus

**Completed Spring 2001 **
**4******** Multiplicity Array**
**- Octagon, Vertex & Ring Counters**
** ****Two Mid-rapidity Spectrometers**
** ****TOF wall for High-Momentum PID**
** ****Triggering**
**Scintillator Paddles + Cerenkov Counters**
** ****Zero Degree Calorimeter**

**135000 Silicon Pad channels**

## Single Event in PHOBOS

- Collisions at 200 GeV!

## PHOBOS Silicon Detector

- _Sample Silicon pad sizes_
**Octagon Detector: 2.7 x 8.8 mm****2**
**Vertex Detector:  0.4 x 12 mm****2**
**Ring Counter:    20 -106mm****2**
**Spectrometer:       1 x 1 mm****2**

- Ring Counter

**Octagon Detector**

**Vertex Detector**

- Spectrometer

## Silicon Detector Performance 

- Silicon Detector Performance

**dE/dx resolution = 6.9%**

- 

- K

- p

- d

-  Signal is well reproduced by MC
-  Signal/Noise ~ 14 to 20
-  Particles are well identified by
-   spectrometer within 2T magnetic field

## Triggering on Collisions & Centrality 

- Triggering on Collisions & Centrality

- Negative Paddles

- Positive Paddles

- Au

- Au

- PP

- PN

- Positive ZDC

- Negative ZDC

- NegativeCerenkov

- PositiveCerenkov

**Paddle signal (a.u.)**

**ZDC Signal (a.u.)**

- Npart

-  Nspectator=2A-Npart

- Central

- Peripheral

**200 GeV Data**

- Coincidence between Paddle counters at t = 0 defines a valid collision
- Paddle + ZDC timing reject background
- Trigger sensitive to 97% of inelastic cross section

- Measurement

## Multiplicity Measurements in 4

- Single-event display

- Vertex “tracklets” – 3 point tracks

- 1

- -5.4<<5.4

- Multiplicity Measurements in 4

##                Charged Particle Multiplicity                 near Mid-rapidity  vs Energy 

**  **  56 & 130 GeV : PRL 85, 3100(2000)
-             200 GeV : PRL 88, 22302(2002)

-                Charged Particle Multiplicity                 near Mid-rapidity  vs Energy

-    Results :

**Central**

**Extrapolation to  LHC :**** **E(LHC)=30 E(RHIC)

**- KN Saturation Model    :**

**-  A + A Fit****                   : **

**Model prediction  :**

-  no unexpected increase
-  200  GeV AuAu 55% higher than pp
-  dN/d**** = 650 +/- 35
-   for AuAu at 200 GeV (6% central)

**hep-ph/0111315**

- PHOBOS : AuAu at RHIC

**Notes:**
Add discussion of model comparisons here

## Centrality Dependence  & Comparison to Models

**PHOBOS : 130 GeV : PRC  65, 031901(2002)**
**PHOBOS : 200 GeV submitted to PRC **
**Saturation Model : nucl-th/0108006**

- Central

- Peripheral

-    Results

**Two-component Fit : **
**dN/d********=  (1-x)n****pp****N****part****/2   +   xn****pp****N****coll**
**	****	****   “****soft”                  “hard”**

**|********| < 1**** **

- PHOBOS : AuAu at RHIC

**Notes:**
HIJING: Wang & Gyulassi, PRL 86, 3496 (2001)

EKRT:

Kharzeev: 

##   Results   :   Charged Particle Production vs 

- dN/d

- dN/d

- dN/d

- dN/d

- dN/d

- dN/d

- 

- 

- 

- 

- 

- 

-   Results   :   Charged Particle Production vs 

**Au+Au at 200 GeV : latest results ****   **

**AuAu at 130 GeV  :  PHOBOS PRL 87, 102303 (2001) **** **

**200 GeV**

**130 GeV**

**Central **

**Peripheral**

- Systematic Errors

**Notes:**
So now lets look at the data.  First of all let me show some distributions for a selected set of centrality bins, here described by the fraction of the cross section they correspond to. The colors are there just to show you that the data from these disparate pieces of the experiment actually match up together quite nicely.  And just looking at these distributions you can start to get a sense of  what’s happening as the data evolve with centrality.  For instance, its clear that for the most central collisions the data in the fragmentation region are relatively lower to those at mid rapidity than for more peripheral collisions.  But lets look at these things in a bit more detail.

## Energy Dependence & Comparison to pp

**Most central**** **

-    Width Increases with Ecm

**Au+Au & pp at 200 GeV**** **

**A+A**** **

-    Results :

**   ****HI Particle Production is Increased at Mid-Rapidity**

**RHIC : PHOBOS AuAu ********s = 200 GeV **

**SPS   : EMU-13   PbPb ********s = 17 GeV **

**RHIC : PHOBOS AuAu ********s = 130 GeV **

**Systematic errors not included**

## Results : Limiting Fragmentation  

- Results : Limiting Fragmentation

**Fragmentation**

**A+A**** **

**p + p **** **

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

**AuAu and pp show the same behavior, ****the extent of the fragmentation region grows with collision energy **

**Fragmentation**

**Systematic errors not included**

## Results : Limiting Fragmentation vs Centrality   

- Results : Limiting Fragmentation vs Centrality

-  **The extent of the limiting fragmentation region is independent of centrality**
** ****but the shape in this region depends on centrality ! **

**200 GeV**

**200 GeV**

**200 GeV Data**

**130 GeV Data**

**130 GeV**

**130 GeV**

**Fragmentation**

**Fragmentation**

## Anti-particle / particle ratios

-  Tracking in the spectrometer
-  Alternate 2T magnetic fields
-  Energy loss and momentum

- K-

- p

- -

- p

- K+

- +

**Positive **
**Charge **

**Negative**
**Charge **

## Anti-particle/particle ratios

** ****AuAu130 GeV**

- Central region
- not Baryon-Free

**Centrality =12 %**
**<Npart> ~ 310**

**PHOBOS : Phys.Rev.Lett. 87, 102301(2001) **

- _Stat._       _Syst._

- K. Redlich, Quark Matter 2001

-    Results :

- T= 170 MeV

**Notes:**
Centrality 

## Compare to Lower Energy

**PHOBOS : 130 GeV **
**Phys. Rev. Lett. 87, 102301 (2001)**

-    Results :

-  Antiproton/proton ratio
- Increasing towards unity
- (baryon free)

**PHOBOS**

## dNch/d @ | per participant
AuAu 55% higher yield  than pp at maximum RHIC energy
Npart >100  the R200/130  approximately constant at 1.14 +/- 0.01+/- 0.06
AuAu data at 200 & 130 GeV are in good agreement with parton saturation model predictions and the two-component fits to the data (hard + soft) 
Using the two-component fits the hard processes increase by 34 % and the soft by 7 %  from 130 to 200 GeV in AuAu
dNch/d in 4
200 GeV : <Nch> = 4960 +/- 250  (||< 5.4) for 6% central
Additional particle production near | for central events
Central AuAu has very similar shape to pp 
Extent of fragmentation region grows with collision energy
Particle ratios
p/p ratio 0.60 +/- 0.04(stat) +/- 0.06(syst)   
Central region not yet baryon free 

**AuAu 55% higher yield  than pp at maximum RHIC energy**
**N****part ****>100  the R****200/130  ****approximately constant at 1.14 +/- 0.01+/- 0.06**
**AuAu data at 200 & 130 GeV are in good agreement with parton saturation model predictions and the two-component fits to the data (hard + soft) **
**Using the two-component fits the hard processes increase by 34 % and the soft by 7 %  from 130 to 200 GeV in AuAu**
**dN****ch****/d******** in 4******
**200 GeV : <N****ch****> = 4960 +/- 250  (|********|< 5.4) for 6% central**
**Additional particle production near |******** for central events**
**Central AuAu has very similar shape to pp **
**Extent of fragmentation region grows with collision energy**
**Particle ratios**
**p/p ratio 0.60 +/- 0.04(stat) +/- 0.06(syst)****   **
**Central region not yet baryon free **

- Summary

## Some references

**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 near mid-rapidity in central Au+Au collisions Phys. Rev. Lett 88, 22302(2002)
- Centrality Dependence of Charged Particle Multiplicity at | in Au+Au Collisions at 130 GeV Phys. Rev. C65, 031901(2002)
- Centrality Dependence of Charged Particle Multiplicity at |<1  in Au+Au Collisions at  130 and 200 GeV Submitted to Phys. Rev. C (2002)
**Technical**
- Silicon Pad Detectors for the PHOBOS Experiment at RHICNucl. Instr. Meth. A461, 143-149 (2001)
- Array of Scintillator Counters for PHOBOS at RHICNucl. Instr. Meth. A474, 38-45 (2001)