Performance of PHOBOS Vertex Finders in 200GeV pp Collisions

at RHIC

Richard S Hollis

University of Illinois at Chicago

For the PHOBOS Collaboration

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC

Collaboration

ARGONNE NATIONAL LABORATORYBirger 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, KRAKOWAndrzej Budzanowski, Roman Hołyński, Jerzy Michałowski, Andrzej Olszewski, Pawel Sawicki, Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Woźniak

MASSACHUSETTS INSTITUTE OF TECHNOLOGYMaartin 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, TAIWANChia Ming Kuo, Willis Lin, Jaw-Luen Tang

UNIVERSITY OF ILLINOIS AT CHICAGORussell Betts, Edmundo García, Clive Halliwell, David Hofman, Richard Hollis, Aneta Iordanova, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter, Joe Sagerer

UNIVERSITY OF MARYLANDAbigail Bickley, Richard Bindel, Alice Mignerey,

Marguerite Belt Tonjes

UNIVERSITY OF ROCHESTERJoshua Hamblen, Erik Johnson, Nazim Khan, Steven Manly, Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UICCollaboration

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

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC Detector

Octagon: View of “unrolled” detector

High Multiplicity (AA) → tracks through “Vertex Detector” and “Spectrometer”

Low Multiplicity (PP)→ too few tracks! – need a different method

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC Octagon

Vertex from Monte-CarloHits

Holes for Vertex Detectorand Spectrometer.

Particle along y, = 0

D

D cm of silicon

Particle in y-z plane, > 0

D/sin() cm of silicon

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

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC Step 2

))2/ln(tan(

Z, beam axis

Step 2Extract energy deposited in the octagon, as a function of pseudorapidity.

Familiar cosh(η) dependence, lines for guidance.

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC Step 1

Single Event, Monte-Carlo 1000 Events, Monte-Carlo

Step 3Divide out the cosh(η) dependence → independence of energy with pseudorapidity at low pseudorapidity→ smaller energy deposits at high pseudorapidity.

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

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UICStep 3 and

4

Single Event, Monte-Carlo 1000 Events, Monte-Carlo

Step 5Make a cut on the data around the nominal MIP position→ count the number of ‘Mip Hits’.

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC Step 5

Single Event, Monte-Carlo 1000 Events, Monte-Carlo

Optimised Mip Hits – best vertex

Scan over possible verticesThe 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.

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UICIterations

Single Event, Monte-Carlo

True = -3.5 cm, Reconstructed = -3.9cm

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC

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

Truth – Found Z position

Monte-Carlo

Eff,Acc,Pur 1

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC

Eff,Acc,Pur 2Accuracy, Purity, Efficiency

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

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC

Eff,Acc,Pur 2Accuracy, Purity, EfficiencyAccuracy – how close to the true vertex

Purity – how many within 1 sigma (2cm) of truth. Efficiency – how often is a vertex found

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC

Eff,Acc,Pur 2Accuracy, Purity, EfficiencyAccuracy – how close to the true vertexPurity – how many within 1 sigma of truth.

Efficiency – how often is a vertex found

Black → original (unsmeared) dN/d distributionRed → simulates the mip-based vertex finder (above)Green → simulates the track-based vertex finder (left)

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UIC

Effect of Vertex Accuracy on Multiplicity Distribution

Effect on Multiplicity

Change in pseudorapidity vs pseudorapidity Change in pseudorapidity density shape

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

Monte-Carlo

Conclusions

Fall 2002 DNP meeting: Performance of PHOBOS Vertex Finders in 200GeV PP Collisions at RHIC: Richard S Hollis, UICConclusion

s

Vertex Finding Procedure Works.

Accuracy → Strong Function of Hits→ Width ≈ 2cm→ Known Effect on Pseudorapidity Density