# Presentation: 363167

## Performance of PHOBOS Vertex Finders in 200GeV pp Collisions at RHIC

- Richard S Hollis
- University of Illinois at Chicago
- For the PHOBOS Collaboration

## Collaboration

- 	_ARGONNE NATIONAL LABORATORY_	Birger Back, Alan Wuosmaa
- 	_BROOKHAVEN NATIONAL LABORATORY _	Mark Baker, Donald Barton, Alan Carroll, Nigel George,
- 		Stephen Gushue, George Heintzelman, Burt Holzman,
- 		Robert Pak, Louis Remsberg, Peter Steinberg, Andrei Sukhanov
- 	_INSTITUTE OF NUCLEAR PHYSICS, KRAKOW_	Andrzej Budzanowski, Roman Hołyński, Jerzy Michałowski, Andrzej Olszewski, Pawel Sawicki, Marek Stodulski,
- 		Adam Trzupek, Barbara Wosiek, Krzysztof Woźniak
- _	__MASSACHUSETTS INSTITUTE OF TECHNOLOGY_	Maartin Ballintijn, Wit Busza (Spokesperson), Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane, Judith Katzy, Piotr Kulinich, Jang Woo Lee, Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Leslie Rosenberg,
- 		Pradeep Sarin, Stephen Steadman, George Stephans,
- 		Carla Vale, Gerrit van Nieuwenhuizen, Gábor Veres,
- 		Robin Verdier, Bernard Wadsworth, Bolek Wysłouch
- 	_NATIONAL CENTRAL UNIVERSITY, TAIWAN_	Chia Ming Kuo, Willis Lin, Jaw-Luen Tang
- 	_UNIVERSITY OF ILLINOIS AT CHICAGO_	Russell Betts, Edmundo García, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer
- 	_UNIVERSITY OF MARYLAND_	Abigail Bickley, Richard Bindel, Alice Mignerey,
- 		Marguerite Belt Tonjes
- 	_UNIVERSITY OF ROCHESTER_	Joshua Hamblen, Erik Johnson, Nazim Khan, Steven Manly,
- 		Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs

## Detector

***Phobos Detector***

- 4 Multiplicity Array
- Octagon, Vertex and Ring Counters
- Mid-rapidity Spectrometer
- TOF wall for high momentum PID
- Triggering
- Scintillator Paddle Counters
- Zero Degree Calorimeter (ZDC)
- Čerenkov Counters

## Octagon

**High Multiplicity (AA) ****→ tracks through “Vertex Detector” and “Spectrometer”**
**Low Multiplicity (PP)→ too few tracks! – need a different method**

**Vertex from Monte-Carlo**

**Hits**

**Holes for Vertex Detector**
**and Spectrometer.**

## Step 2

- 

_***Step 1***_
- Particles traversing more material will deposit more energy
- – this is the principle behind the cosh(η) dependence (next slide).

- Z, beam axis

## Step 1

- Extract energy deposited in the octagon, as a function of pseudorapidity.

- Familiar cosh(η) dependence, lines for guidance.

## Step 3 and 4

- Divide out the cosh(η) dependence
- →  independence of energy with pseudorapidity at low pseudorapidity
- →  smaller energy deposits at high pseudorapidity.

_***Step 4***_
- Apply a simple merging technique
- → sum energies at high eta

## Step 5

- Make a cut on the data around the nominal MIP position
- → count the number of ‘Mip Hits’.

## Iterations

_***Scan over possible vertices***_
- The process is repeated for all possible vertex positions (-90 to 90 cm).
- → ‘best’ position is found for most Mip Hits (for a given trial Z).
*This is due to the correction applied being ‘vertex’ dependent.*

**Single Event, Monte-Carlo**

***True = -3.5 cm, Reconstructed = -3.9cm***

## Eff,Acc,Pur 1

_***Accuracy, Purity, Efficiency***_
- Accuracy – how close to the true vertex
- Purity – how many within 1 sigma (2cm) of truth.
- Efficiency – how often is a vertex found

## Eff,Acc,Pur 2

_***Accuracy***__***, Purity, Efficiency***_
- Accuracy – how close to the true vertex
- Purity – how many within 1 sigma (2cm) of truth.
- Efficiency – how often is a vertex found

## Eff,Acc,Pur 2

_***Accuracy, ***__***Purity***__***, Efficiency***_
- Accuracy – how close to the true vertex
- Purity – how many within 1 sigma (2cm) of truth.
- Efficiency – how often is a vertex found

## Eff,Acc,Pur 2

_***Accuracy, Purity, ***__***Efficiency***_
- Accuracy – how close to the true vertex
- Purity – how many within 1 sigma of truth.
- Efficiency – how often is a vertex found

## Effect on Multiplicity

***Red****** ***→ simulates the mip-based vertex finder (above)
***Green****** ***→ simulates the track-based vertex finder (left)

_***Effect of Vertex Accuracy on Multiplicity Distribution***_

- Uncertainty in Z 	  → shift in pseudorapidity
- ‘Peaks’ at η = ± 2 	  → more pronounced
- ‘Dip’ at midrapidity → deeper

- Monte-Carlo

## Conclusions

- Vertex Finding Procedure Works.
- Accuracy	→ Strong Function of Hits
- 		→ Width ≈ 2cm
- 		→ Known Effect on Pseudorapidity Density