# Presentation: 928236

## Measuring Mid-Rapidity Multiplicity in PHOBOS

- Aneta Iordanova
- University of Illinois at Chicago
- For the                           collaboration

## Outline

- Multiplicity Analysis Technique
- Vertex Tracklet reconstruction method
- Results
- Mid-rapidity charged-particle multiplicity and its centrality dependence for 19.6 and 200GeV
- Compare the results with model predictions
- Conclusions

##                 Collaboration

- Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail Bickley,
- Richard Bindel, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Patrick Decowski,
- Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Clive Halliwell,
- Joshua Hamblen, Adam Harrington, Michael Hauer, Conor Henderson, David Hofman,
- Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Nazim Khan,
- Piotr Kulinich, Chia Ming Kuo, Willis Lin, Steven Manly, Alice Mignerey,
- Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park,
- Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer,
- Helen Seals, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Maciej Stankiewicz, Peter Steinberg,
- George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale,
- Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger,
- Frank Wolfs,  Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch
- 	ARGONNE NATIONAL LABORATORY	BROOKHAVEN NATIONAL LABORATORY
- 	INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW	MASSACHUSETTS INSTITUTE OF TECHNOLOGY
- 	NATIONAL CENTRAL UNIVERSITY, TAIWAN	UNIVERSITY OF ILLINOIS AT CHICAGO
- 	UNIVERSITY OF MARYLAND	UNIVERSITY OF ROCHESTER

## Slide 4

## Multiplicity measurement at mid-rapidity (||<1)

## Vertex Detector

- 8192  silicon channels
*  **Outer Layer:* 2 × 2048 channels, 0.47mm × 24.1mm
*  **Inner Layer:* 2 × 2048 channels, 0.47mm × 12.0mm

## Tracklet Reconstruction

**Tracklet Reconstruction**

- Tracklet
- 	Two-hit combinations from Outer and Inner Vertex (Top or Bottom), pointing to the reconstructed vertex.
- Reconstructed vertex
- from Spectrometer Detector
- 	19.6GeV
- x,y,z=0.3,0.3,0.4 mm (central)
- x,y,z=0.6,0.5,0.8 mm (mid-central)

## Tracklet Reconstruction

- Second Pass

*Seed Layer*

*Search Layer*

*Reconstructed *
*Vertex*

- hit

- hit

- Search ,Search

- Extrapolate  Seed ,Seed

-  || = |Search – Seed| <  0.3
-  || =|Search – Seed| <  0.1
-  *smallest*  combination.

- Tracklets with a common hit
- in the “Search Layer”
- smallest combination.

- Top Vertex

**Tracklet Reconstruction**

## Multiplicity Determination

**Multiplicity Determination**

## Acceptance and Efficiency Correction Factor 

-  depends on:
- Z-vertex position
- multiplicity in detector (hits)

- 

- Hits in Outer Vertex Layer / 20

**19.6 GeV**
**200 GeV**

- corrects for:
- azimuthal acceptance of the detector
- tracklet reconstruction efficiency
- secondary decays

## Combinatorial Background Correction Factor 

- Combinatorial background:
- formed by rotating Inner Vertex Detector layers 1800 about the beam pipe

- Z, 

*Beam pipe*

- 

## Combinatorial Background Correction Factor 

- Tracklets/Background  for 80 to 100 Hits in Outer Vertex Layer, 19.6 GeV
-   = Nbg_tracklets/Nreconstructed
-   =0.76

- Data

- MC

- Counts

- Counts

- 

- 

## Results

## Centrality Determination

**Centrality Determination**

- Select the “same” regions at 200 and 19.6 GeV
- Have two centrality methods at each energy
- One at mid-rapidity
- One away from mid-rapidity
- Mechanism for comparing ‘like’ regions to see systematic effects
- Results presented here
- for a) and c)

- Regions are ‘matched’ according to the ratio of beam rapidities
- (a) with (c)
- (b) with (d)

## Measured pseudorapidity density per participant pair as a function of <Npart>

**Measured pseudorapidity density per participant pair as a function of <N****part****>**

- ‘Geometry-normalized’ multiplicity in Au-Au collisions higher than corresponding values for inelastic
- Percentile cross-section
- 0-50% for 200 GeV
- 0-40% for 19.6 GeV

***90 % C.L.***

## Models predictions
Hijing
does not follow data trend
Saturation Model (KLN)
	Phys.Lett.B523 79 (2001)
	arXiv:hep-ph/0111315
better agreement 

- Models predictions
- Hijing
- does not follow data trend
- Saturation Model (KLN)
- 	***Phys.Lett.B523 79 (2001)***
***	******arXiv:hep-ph/0111315***
- better agreement

***90 % C.L.***

**Measured pseudorapidity density per participant pair as a function of <N****part****>**

## Ratio of the two data sets – systematic errors

- Most of the systematic errors on the individual measurements at the two energies will cancel in the ratio
- Analyses performed with the same method
- Detector
- Centrality determination
- Percentile cross-section used in ratio
-  top 40%
- Errors are estimated as 1-.

## Ratio of the two data sets – systematic errors

**Ratio of the two data sets – ****systematic errors**

- R
- Most of geometry/efficiency effects cancel in the ratio
- Contribution from secondary decays
- R
-   is found to be the same for Data/MC for the two data sets
- Uncertainty from measured y-beam position
- RNpart
- Nucleon-nucleon inelastic cross-section
- MC simulations of the detector response
- Glauber model calculations

| R | R19.6 | R200 | RNpart |
| --- | --- | --- | --- |
| 2% | 0.4% |  | 0.4% |

- 

- 

- 

## Ratio of the two data sets  –systematic and statistical errors

**Ratio of the two data sets  –****systematic and statistical errors**

- RNrec
- Counting statistics
- Uncertainty in trigger efficiency (centrality bin position)
- central events 0%
- mid-central events 6%
- Final 1- systematic and statistical error
- Centrality dependent
- central events 3%
- mid-central events 7%

| R | R19.6 | R200 | RNpart | RNrec | Rcentral |
| --- | --- | --- | --- | --- | --- |
| 2% | 0.4% |  | 0.4% | 2.2% | 3.0% |

- 

- 

## Ratio for the data sets

- Data ratio
- Au+Au1 (fixed fraction of cross-section)

***1-************ errors***

## Ratio for the data sets

- Data ratio
- Au+Au1 (fixed fraction of cross-section)
- No centrality (geometry) dependence
- R = 2.03 ± 0.02 ± 0.05 (simple scale-factor between 19.6 and 200GeV)

***1-************ errors***

## Ratio for the data sets

- Data ratio
- Au+Au1 (fixed fraction of cross-section)
- No centrality dependence
- R = 2.03 ± 0.02 ± 0.05
- Au+Au2 (fixed <Npart>)
- No centrality dependence

***1-************ errors***

## Ratio for the data sets

- Models
- Hijing
- increase in mid-rapidity with centrality
- Saturation Model (KLN)
- flat centrality dependence as in data

***1-************ errors***

- _[http://xxx.lanl.gov/nucl-ex/0405027](http://xxx.lanl.gov/nucl-ex/0405027)_

## Other ‘Geometry Scaling’ observations in			

- Multiplicity
- 200/130 GeV mid-rapidity ratio
***Phys.Rev.C65 061901(R) (2002)***
- 19.6-200GeV Nch/<Npart/2>
- Plot from QM 2002 talks

## Charged hadron pT spectra
Ratio of yield for 200 and 62.4 GeV is centrality independent for all measured pT bins

- Ratio of yield for 200 and 62.4 GeV is centrality independent for all measured pT bins

**Other ‘Geometry Scaling’ ****observations in**

## Conclusions

- We measured charged-particle pseudorapidity density at mid-rapidity for Au-Au collisions at 200 and 19.6GeV
- Centrality, derived from different -regions for each of the two Au-Au collision energies, yield consistent results
- An increase in particle production per participant pair for Au-Au compared to the corresponding values for    	 collisions
- The ratio of the measured yields for the top 40% in the cross section gives a simple scaling factor between the two energies