# Presentation: 008370

## Centrality measurement andthe centrality dependence of  dNcharged/d at mid-rapidity 

- Judith Katzy (MIT)
- for
- the PHOBOS collaboration

## ARGONNE NATIONAL LABORATORY
Birger Back, Nigel George, Alan Wuosmaa
BROOKHAVEN NATIONAL LABORATORY
Mark Baker, Donald Barton, Alan Carroll, Stephen Gushue, George Heintzelman, Robert Pak, Louis Remsberg,  Peter Steinberg, Andrei Sukhanov
INSTITUTE OF NUCLEAR PHYSICS, KRAKOW
Andrzej Budzanowski, Roman Holynski, Wojtek Kucewicz, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Wit Busza* ,  Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane , Judith Katzy, Piotr Kulinich, Johannes Muelmenstaedt, Heinz Pernegger, 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
Willis Lin, JawLuen Tang
UNIVERSITY OF ROCHESTER
Erik Johnson, Josh Hamblen, Nazim Khan, Steven Manly, Robert Pak, Inkyu Park, Wojtech Skulski, R. Teng, Frank Wolfs
UNIVERSITY OF ILLINOIS AT CHICAGO
Russell Betts, Clive Halliwell, David Hofman, Burt Holzman, Don McLeod, Rachid Nouicer, Michael Reuter
UNIVERSITY OF MARYLAND
Richard Bindel, Edmundo Garcia-Solis, Alice Mignerey

* spokesperson

**Birger Back, Nigel George, Alan Wuosmaa**
_**BROOKHAVEN NATIONAL LABORATORY**_
**Mark Baker, Donald Barton, Alan Carroll, Stephen Gushue, George Heintzelman, Robert Pak, Louis Remsberg,  Peter Steinberg, Andrei Sukhanov**
_**INSTITUTE OF NUCLEAR PHYSICS, KRAKOW**_
**Andrzej Budzanowski, Roman Holynski, Wojtek Kucewicz, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak**
_**MASSACHUSETTS INSTITUTE OF TECHNOLOGY**_
**Wit Busza***** ****,  Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane , Judith Katzy, Piotr Kulinich, Johannes Muelmenstaedt, Heinz Pernegger, 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**_
**Willis Lin, JawLuen Tang**
_**UNIVERSITY OF ROCHESTER**_
**Erik Johnson, Josh Hamblen, Nazim Khan, Steven Manly, Robert Pak, Inkyu Park, Wojtech Skulski, R. Teng, Frank Wolfs**
_**UNIVERSITY OF ILLINOIS AT CHICAGO**_
**Russell Betts, Clive Halliwell, David Hofman, Burt Holzman, Don McLeod, Rachid Nouicer, Michael Reuter**
_**UNIVERSITY OF MARYLAND**_
**Richard Bindel, Edmundo Garcia-Solis, Alice Mignerey**
******* spokesperson**

- The PHOBOS Collaboration

## Global characterization of the Au-Au collision

_**Nuclear geometry **_
- Determination of impact parameter
- Determination of number of participants
-         measurement of  produced particles and spectator matter
_**Energy density**_
- Formation of entropy
- Process of particle production
-         measurement of particle density as a function of centrality

## The PHOBOS  Detector

- Paddle Trigger Counters

- Spectrometer

- Vertex Detector

## Trigger & Event Selection

- PN>0   t<10ns   PP>0

- <-2.5mrad
- >6

_**Offline analysis cuts:  **_
- tzdcn, tzdcp
-               background suppression
-  tpaddle < 4ns
- cm z < 60cm
-         register 97% of cross section

- Au

- Au

- ZDC N

- ZDC P

- <-3

- <3

- <2.5mrad
- <-6

- collisions

- tN

- tP

- single beam
- background

- ZDC time

## Collision Geometry

**Collision Geometry**

- <2.5mrad
- <-6

- Npart = A - Nn* 1.67

- Participants

- ZDC

- ZDC

**b**

- B

- B

- Spectators

- 2 independent methods
- with different systematic
- uncertainties

## Determination of Centrality

- In average both methods
- yield the same centrality bin

- No systematic variation between
- methods

## Determination of Npart

- Fragmentation
- pt broadening
- detector resolution

- Hadronic crosssection
- variation of event shape
- detector resolution

- Npart

- Glauber implementation
- Parametrization of  nucl. density (Wood-Saxon)
- Cross section measurement

## Result of Npart determination

- Npart

- (Npart)

**Total systematic error on Npart**

- Variation of Glauber implementation

-  determined the impact parameter and Npart with 2 independent
-    methods to exclude many systematic uncertainties
-  estimated the influence of the cross section measurement
-  estimated the influence of the Glauber implementation and
-    the parametrization of the nuclear density

- Variation of cross section

##  Measurement of the “unbiased” spectrum

-   Simulation:
-   (2.6 % +/-3) % lost due to trigger acceptance
-   Confirmation with data:
-  ** ****Measurement of relative cross sections**
-   loss >10%  excluded by comparison of event topologies in ZDCs

## Measurement of cross section ratios

- Measurement of cross section ratios

-                                           tot    =       hadron +     Coulomb
- theoretical predictions:   10.90 =       6.92    +      4.0 barn
- measurement (trigger):     all    =    paddles  +     ZDC

- hadron/ tottheory:     0.636 +/-  0.032  **(Nucl.Instr.Meth.A 417(1998)1)**
-                       data:      0.615 +/- 0.061

****

## Mutual Coulomb dissociation measured in ZDC

**Mutual Coulomb dissociation measured in ZDC**

- 0.449

- 1.364

- EP

- a.u.

- EN a.u.

- EN a.u.

- Background: 4%
- ZDC inefficiency <1%

- 1 neutron (Dipole resonance)

- 1n/ 1nX =  0.33
- data:             0..31 +/- 0.046
- 1nX/  tot    =  0.12
-   data:            0.13 +/- 0.018

## ZDC Simulation

## Measurement of charged particle density at mid-rapidity

- Ndataprim  =  (Ndatatrack - Ndataback) x NMCprim / (NMCtrack - NMCback)

- z

- x

- _Spectrometer_
- (P.Decowski Poster)

-   ( +  2) 1/2 < 0.015

**0 < ********<1**

## Vertex and Tracklet Reconstruction

_**Vertex reconstruction:**_
- Resolution z m
-                   x y  = m
-  selection for this analysis:
-               -4 cm < z < 12 cm

_**Tracklet reconstruction:**_

## Background

-  **2 - 13%** combinatorical background
**   ****0.5 %**  background from decaying
-                particles (Hijing)
-    **6.5 %**  secondaries originating in
-                dead material  (Hijing,Geant)

- background

- tracklets

- D

****

- primaries
- secondar.
- feeddown

## Npart

**N****par**t

- dN/d

## Result & error estimate

-  combinatorical background                                 1%
-  tracklet reconstruction and event selection         4%

- Npart

- dN/d5Npart

## Slide 19

## Comparison with theoretical models

- Npart

- Agrees with Glauber based model (KN)
- Agrees with gluon saturation model (KN)
- Disagrees with HIJING (Glauber, jet quenching, nuclear
-                                          shadowing)
- Disagrees with EKRT (gluon saturation model)

- Not distinguishable

- Npart

## Monotonicity

-  Proof of monotonicity for  signal in paddles and in ZDC
-  Anti-correlation confirms relation of signals  to spectators
-   and participants