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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