# Presentation: 572479

## Measurement of the Centrality Dependence of Charged Particle Pseudorapidity Density with the PHOBOS Detector

- Michael Reuter
- University of Illinois at Chicago
- for the                         Collaboration
- APS Spring Meeting:  Washington, DC    April 29, 2001

## The PHOBOS Detector

- Ring Counters

- Paddle Counter

- Spectrometer

- TOF

- Octagon+Vertex

- : ANL, BNL, IFJ,
-   MIT, NCU, UIC,
-   UMD, UR

- Au+Au
- sNN=130 GeV

- APS Meeting: Washington, DC

- April 29, 2001

## The BIG Picture

**Q:**
**Entropy Production**
**Thermal Equilibration**
**Hadro-Chemistry**

**Charged Particle Density**
***    ******C. Vale J12.004***
**Event Anisotropy  - Flow**
**Particle Ratios**
**    *****C. Henderson Q12.003***

**Context:**

- APS Meeting: Washington, DC

- April 29, 2001

## Selecting Collisions

-  Coincidence between Paddle counters
-  Paddle + ZDC timing reject background
- Sensitive to 97% of inelastic cross-section for Au+Au at sNN = 130 GeV

- APS Meeting: Washington, DC

- April 29, 2001

## Determining Centrality

**N****part**

** ****HIJING + GEANT**
** ****Glauber Calculation**
** ****Model of Paddle Response**

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

- Counts

- Counts

- APS Meeting: Washington, DC

- April 29, 2001

## Uncertainty on Npart

- Estimating 97% when really 94% overestimates Npart

- Uncertainty on Npart

- % Error on Npart

- Npart

- Measurement sensitive to trigger bias
- “Minimum-bias” still has bias
- Affects most peripheral events

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

- Counts

- APS Meeting: Washington, DC

- April 29, 2001

## Detector Geometry

- APS Meeting: Washington, DC

- April 29, 2001

## Vertex Finding

- cm

- Vertex @
- Z = -0.12 cm

- +z

- cm

- _Vertex Resolution:_
- x ~ 450 m
- y ~ z ~ 200 m**    **

- Counts

- APS Meeting: Washington, DC

- April 29, 2001

## Tracklet Analysis

- Tracklets are two point tracks that are constrained by the event vertex.

- || < 0.04   || < 0.3

**Vertex**

**Spectrometer**

- D = 2 + 2

- D < 0.015

-  = 1 – 2
-  = 1 – 2

**D**

- APS Meeting: Washington, DC

- April 29, 2001

## Tracklet Systematic Error

-  Reconstruction: Vertex selection, Tracklet algorithm etc.
- 	VTX: 1.8%       SPEC: 3%

-  Weak Decays:   Mostly Ks and 
-            VTX: 2%          SPEC: <1%

-  Background: Combinatorial, -electrons
-            VTX: 1.5%       SPEC: 1%

-  MC Generators: Different particle production, background etc.
-            VTX: 5%          SPEC: <<1%

** ****Total:****    VTX: ****7.5%****        SPEC: ****4.5%**

- APS Meeting: Washington, DC

- April 29, 2001

## Measured dNch/d

**PHOBOS Preliminary**

**Central**

**Peripheral**

- APS Meeting: Washington, DC

- April 29, 2001

## Centrality Dependence of dNch/d

- dNch/d/(0.5 Npart)

- Npart

- Yellow
- Band:
- Systematic
- Error

- APS Meeting: Washington, DC

- April 29, 2001

## Comparison to SPS

** ****General features (rapid rise/flat top) similar**
** ****Note that WA98 dN****ch****/d******** measured in lab frame**

- Au+Au

- Pb+Pb

- APS Meeting: Washington, DC

- April 29, 2001

## Summary

-  Measured centrality dependence of charge particle
-    pseudorapidity density at || < 1 for Au+Au
-    at sNN = 130 GeV
-  Charged particle density 40% larger than pp
-    and 70% larger than at SPS
-  Similar centrality dependent features as SPS
-  Eagerly awaiting 200 GeV data!
-  Species scan helps understanding of Npart

- APS Meeting: Washington, DC

- April 29, 2001