# Presentation: 524007

## Multiplicity Measurements

**Russell Betts (UIC)**
**for the **
- PHOBOS **Collaboration**

- withThe PHOBOS Detector

- 18th Winter Workshop on Nuclear Dynamics
- Nassau, Jan 20th-27th,2002

## The   PHOBOS   Collaboration

**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**

**       ****Birger Back, Nigel George, Alan Wuosmaa**
**        ****Mark Baker, Donald Barton, Alan Carroll, ****Joel Corbo****,**** Stephen Gushue, George Heintzelman, ****Dale Hicks****, Burt Holzman,Robert Pak, ****Marc Rafelski****, Louis Remsberg, Peter Steinberg, Andrei Sukhanov**
**        ****Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak**
**        ****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**
**	**** ****Chia Ming Kuo****, Willis Lin, Jaw-Luen Tang**
**        ****Joshua Hamblen , Erik Johnson, Nazim Khan****, Steven Manly,Inkyu Park, Wojtek Skulski, ****Ray Teng****, Frank Wolfs**
**        ****Russell Betts, Edmundo Garcia, Clive Halliwell, David Hofman, ****Richard Hollis, Aneta Iordanova****, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, ****Michael Reuter, Joe Sagerer**
**        ****Richard Bindel****, Alice Mignerey**

## 137000 Silicon Pad channels

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

**137000 Silicon Pad channels**

## Outline of Talk

- Centrality Determination        Nparticipant and Ncollision
- Techniques for Multiplicity Measurements
- Tracklets
- Hit Counting
- Energy Deposition
- Results
- Energy Dependence for  1
- Centrality Dependence
- dN/d Shapes
- Summary and Taster of Future Delights

## Triggering on Collisions

**Coincidence between Paddle counters at ********t = 0 defines a valid collision.**
**Paddle + ZDC timing reject background.**
**Sensitive to 97****±3 ****% of inelastic cross section for Au+Au.**

- Negative Paddles

- Positive Paddles

- ZDC N

- ZDC P

- Au

- Au

- PP

- PN

- Paddle Counter

**Valid**
**Collision**

- ZDC Counter

## Trigger Selection - ZDC vs Paddles

- Peripheral

## Determining Centrality

**N****part**

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

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

- Counts

- Counts

## Uncertainty on Npart

- Estimating 97% when really 94% overestimates Npart

**Uncertainty on N****part**

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

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

- Counts

## Multiplicity Distributions

**Octagon**

**Rings**

- Hits in One Layer of Silicon

**Vertex**

**Multiplicity Distributions**

## Au+Au Collision Event Display

## Event Vertex Finding

- +z

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

## Vertex Tracklet Reconstruction

-  = 1 – 2
-  = 1 – 2

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

- || < 0.04      || < 0.3

## Combinatorial Background

- Outer Hit Bin 10 (Data)

- All Pairs of Hits

- “Background Flip”

## Backgrounds

- Weak Decays

-  Electrons

## Vertex Tracklet Systematic Error

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

-  Weak Decays: Mostly Ks and 2%

-  Background: Combinatorial, -electrons -  1.5%

-  MC Generators: Different particle production, background etc.
-         		       - 5%

** ****Total:****   ****7.5%**

## Analog and Digital Hit-Counting

**Octagon, Ring and Vertex Detectors (unrolled)**
**Count Hits or Deposited Energy**

****

****

## Discriminating Background with dE 

- E (“MIP”)

- Data

- Monte Carlo

- Si

- E vs. ** in the Octagon

- From vertex

- Not from vertex

**

**

## Count hits binned in , centrality (b)  

- Calculate acceptance ***A***(*Z**VTX*)* *for that event
- Find the occupancy per hit pad ***O*****(***********,b*****)**
- Fold in a background correction factor ***f******B***(**,*b*)

- 

- 

***E deposition*
*in multiplicity*
*detectors for 1 event.*

## “Measuring” the Occupancy

- “Measuring” the Occupancy

*N*=number of tracks/pad
**=mean number of tracks/pad

- The numbers of empty, and occupied, pads
- determine the occupancy as a function of **,*b*

- Method: Assume Poisson
- statistics

- Ntracks/hit pad

- 

- 0-3%

- 50-55%

- Octagon

- Rings

- (central)

- (peripheral)

## Estimating remaining backgrounds

- MC, Occupancy Corrected

- MC “truth”

- 

- 

- Compare PHOBOS Monte Carlo “data” analyzed using
- occupancy corrections to “truth” -  the difference gives corrections  for remaining background.

*f*B(,*b*)

*f*B=MCTruth/MCOcc

- dNch/d**

- 200

- 400

- 600

## Energy Loss  Multiplicity

- 300 m Si

- Measured S/N = 10 - 20  <<  Landau Width
- Use Non-Hit pads - for
- 	Common-Mode Noise Suppression
- M = 240 ± 15 ± 5 ± CMN for one sensor (120 channels) at  = 0

- 0.30 - 0.40

**Energy deposited in i****th**** pad (truncated)**
**corrected for angle of incidence**

**Mean energy loss for one**
**particle traversing pad**

**RATIO OF TOTAL TRACKS**
**TO PRIMARY TRACKS**

## Uncertainty in Theoretical Predictions

## Constraining the Models

## Ratio 200/130 GeV

**Phobos Measurement**

**Ratio 200/130**
**averaged for four **
**PHOBOS methods**

**R****200/130**** = 1.14 +/- 0.05 Moderate Increase in Energy Density?**

**Systematic Uncertainty**

## Hard and Soft Processes

- Soft processes (pT < 1 GeV)
- Color exchange excites baryons
- Baryons decay to soft particles
- Varies with number of struck nucleons
- “Wounded Nucleon Model”
- Hard processes (pT > 1 GeV)
- Gluon exchange in a binary collision creates jets
- Jets fragment into hadrons, dominantly at mid-rapidity

- (mini)jet

- (mini)jet

## Multiple Collisions with Nuclei

- Nuclei are extended
- RAu ~ 6.4 fm (10-15 m)
- cf. Rp ~ .8 fm
- Geometrical model
- Binary collisions (Ncoll)
- Participants (Npart)
- Nucleons that interact inelastically
- Spectators (2A – Npart)
- p+A: Npart = Ncoll + 1
- (Npart ~ 6 for Au)
- A+A: Ncoll  Npart4/3

- b(fm)

- b

- Ncoll

- Npart

- 0

- 18

- 9

- 1200

- 400

## Hard & Soft

- What about non-central events?

- We already expect that charged particle
- production can have two components:

- We can tune the relative contribution by
- varying the collision centrality

- proton-proton multiplicity

- Fraction from hard processes

- Is this Description unique ?

## Parton Saturation

- Below this scale, the nucleus looks “black” to a probe

- Gluons below x~1/(2mR) overlap in transverse plane with size 1/Q

- Scale depends on **volume**(controlled by centrality!)

- t

***“******Colored Glass Condensate”***

- McLerran, Venugopalan,
- Kharzeev, Dumitru, Schaffner-Bielich...

## Data and Models for 130 GeV

- Yellow band:
- Systematic
- Error

## Data and Models for 200 GeV

- Yellow band:
- Systematic
- Error

## Shapes of dN/d Distributions at 130 GeV - Hit Counting

- Shapes of dN/d Distributions at 130 GeV - Hit Counting

-  Shapes only weakly dependent    on centrality
-  Differ in details

## 130 GeV

## Energy Dependence and Comparison to pp

- Width increases with Ecm
- Increase **** = ****ybeam
- Scaling in fragmentation region

- HI part. Production is increased at mid-rapidity

- 7-10% syst error

- 7-10% syst error

## Scaling in the Fragmentation Region

- Fragmentation

- Fragmentation

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

- PHOBOS 2000/2001

- 7-10% syst error

## Summary

- Energy and Centrality Dependence of Mid-Rapidity Multiplicity has Constrained Models and given Insight into Interplay of Different Processes
- Shapes of Multiplicity Distributions show Scaling in Fragmentation Region illustrating Common Mechanism for Particle Production which Evolves to Features Unique to HI Situation at Mid-Rapidity
- To Come:
- 	Shapes versus Centrality at 200 GeV
- 	Multiplicity at 20 GeV
- 	pp Data with PHOBOS at 200 GeV