Search for Supersymmetry in Trilepton Final States with the ATLAS Detector and the Alignment of the ATLAS Silicon Tracker

10.1 MB·English

by  Oleg Brandt

UNIVERSITY OF OXFORD
Sub-Department of Particle Physics
Search for Supersymmetry in Trilepton Final States
with the ATLAS Detector and
the Alignment of the ATLAS Silicon Tracker
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Oleg Brandt

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Corpus Christi College
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Thesis submitted in fulfilment of the requirements
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X for the degree of Doctor of Philosophy
r
a at the University of Oxford
Trinity Term, 2009
Postal address:
University of Oxford
Denys Wilkinson Building
Sub-Dept. of Particle Physics
Keble Road, Oxford
January 8, 2010
OX1 3RH, UK
UNIVERSITY OF OXFORD
Sub-Department of Particle Physics
Search for Supersymmetry in Trilepton Final States
with the ATLAS Detector and
the Alignment of the ATLAS Silicon Tracker
Oleg Brandt
Corpus Christi College
Abstract
One of the main goals of the ATLAS detector at the Large Hadron Collider of CERN, a proton-
proton collider with a nominal centre-of-mass energy of √s = 14 TeV, is to search for New
Physics beyond the Standard Model. A widely favoured Beyond the Standard Model candidate
is Supersymmetry (SUSY), which postulates a superpartner with the same quantum numbers,
butaspinchangedby1/ foreachStandardModelparticle. Thefirst partofthisthesisdescribes
2
a strategy for an early discovery of SUSY using the trilepton signature, with a focus on gravity-
mediated SUSY breaking, mSUGRA. The discovery potential for SUSY at the LHC for the case
where strongly interacting supersymmetric particles are very massive is critically investigated.
A possible choice of triggers for L = 1031cm−2s−1 is suggested by optimising the event yield
at intermediate and final selection stages. A novel method to measure the rate of leptons from
heavy flavour decays passing isolation requirements by isolating tt¯events in data is outlined.
The task of the ATLAS silicon tracker is to track particles produced in proton-proton collisions
in its centre, measuring their momenta and production vertices. The precise knowledge of the
silicontrackermodulepositionsandtheirorientationinspace(alignment)downtosomemicrons
andfractionsofamiliradianinthecriticalcoordinatesisofvitalimportanceforlargepartsofthe
ambitiousATLASphysicsprogram. Inthesecond partofthethesis,thealignmentoftheATLAS
silicon tracker using the Robust Alignment algorithm and particle tracks is described. The
algorithm is applied to align end-cap A of the pixel detector using cosmic ray particle tracks
recorded during its on-surface commissioning in 2006. Finally, about 2M cosmic ray tracks
collected by ATLAS in situ in autumn 2008 are utilised to provide a coherent alignment of the
entire silicon tracker with the Robust Alignment algorithm.
Thesis submitted in fulfilment of the requirements
for the degree of Doctor of Philosophy
at the University of Oxford
Trinity Term, 2009
Der Wissenschaftler muss durch sein Handeln immer wieder kund tun,
dass er zum humanen Teil der Menschheit geh¨ort.
The scientist is to prove by his deeds
that he belongs to the human fraction of Mankind indeed.
J. W. von Go¨the, “Zur Farbenlehre”.
This thesis was typeset with LATEX2ε.
c Oleg Brandt, 2008.
(cid:13)
All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system, or transmitted, in any form or by any means, electronic, mechanical, photocopy-
ing, recording or otherwise, without express permission of the author.
Published at the University of Oxford, Oxford, United Kingdom.
iii
Acknowledgements
The last three years of research towards a D.Phil. degree certainly are among the most
intense and interesting periods of my life. They shaped me in both scientific as well as
in personal aspects. I would like to thank everyone who made these last three years so
precious and worthwhile!
Firstly, I would like thank the Sub-Department of Particle Physics for accepting me
into their excellent D. Phil. in Particle Physics program, and their financial support
throughout this period for attending innumerous meetings, workshops, conferences, and
schools which is a highly important ingredient to scientific development. I would also like
to express my gratitude to the Deutscher Akademischer AustauschDienst (DAAD) for
their generous financial and ideological support, and the Studienstiftung des Deutschen
Volkes for their ideological support over the last 2.5 years. Last but not least I would like
mention the Joint graduate Fellowship and the Senior Scholarship from Corpus Christi
College, for which I am very grateful.
There are too many names to mention on the scientific side, and writing this just a
couple of hours before submission I am sure I will forget to mention some – please excuse
my oversight.
In the first place, I would like to express my deep appreciation for the help and advice,
innumerous highly interesting discussions, but also directness and personal support from
my supervisors: Dr. Pawel Bru¨ckman de Rentstrom and Dr. Alan James Barr. The same
appreciation goes to Dr. Anthony Weidberg. They always were open for discussions and
had an open ear for my questions. I would also like to thank Prof. Arnulf Quadt for
his support during the initial orientation phase at Oxford, and Prof. Hans Kraus, my
college advisor, for his steady encouragement. I highly value the engagement of Dr. Todd
Huffman in student matters.
I owe a special word of gratitute to the 2006 ATLAS group students: Maria Fiascaris,
Guillaume Kirsch, and Kristin Lohwasser. The tight collaboration with them, especially
in our first year, was hightly fruitful and enjoyable at the same time. The same is true
for Florian Heinemann, who helped me enormously to find a good starting point with the
Robust Alignment algorithm.
Mentioning the Oxford ATLAS group, I cannot close this paragraph before mentioning
very interesting discussions with Pierre-Hughes Beauchemin, Sinead Farrington, James
Ferrando, Stephen Gibson, Chris Hays, Cigdem Issever, Richard Nickerson, and Mu¨ge
Karago¨z Unel.
I would like to thank my collaborators in the ATLAS Supersymmetry group lead by
Giacomo Polesello, Davide Constanzo, and Paul de Jong. In particular, I would like to
mention the names of people mainly involved in the trilepton analysis: Christina Potter,
Katarina Peichel, Antonella de Santo. I appreciate the discussions with Tobias Golling,
Chris Lester, Dan Tovey, and Giacomo Polesello.
I would also like to express gratitute to my collaborators in the Inner Detector alignment
iv
group lead by Salvador Marti i Garcia and Jochen Schieck. Particular thanks go to the
Valencia group: Carlos Escobar, Sergio Gonzalez-Sevilla, Vicente Lacuesta, Regina Moles
Valls; the Munich group: Giorgio Cortiana, Tobias Go¨ttfert, and Roland Ha¨rtel; John
Alison and Andrea Bocci from the TRT group; Ben Cooper and Tobias Golling from the
alignment monitoring group; Stephen Haywood from the Tracking Performance group;
Anthony Morley for the tight collaboration on the TRT curvature constraint; and Daniel
Froidevaux for keeping a watchful eye on the alignment activities.
My thanks go also to Sarah Allwood-Spiers, Ellie Dobson, Mark Owen for the highly
interesting collaboration in organising the Young Experimentalist and Theorist Institute
2009 (YETI), and to Nigel Glover and the IPPP for giving us this unique opportunity.
Lastbut notleast, Iwouldalsoliketo thankoursecretariat: Sue Geddes, KimProudfood,
Laura Nevay and Faheem Khan, who were always extremely helpful.
My deepest gratitude goes to my family, who always supported me in my scientific and
non-scientific endeavours. Same goes to my friends whom I don’t want to mention here
explicitly – you know who you are!
v
Contents
1. Introduction and Motivation 2
I. Theoretical Aspects and Experimental Setup 6
2. Theoretical Aspects 7
2.1. The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1. Brief Overview of the Standard Model . . . . . . . . . . . . . . . . 7
2.1.2. The Shortcomings of the Standard Model . . . . . . . . . . . . . . . 9
2.2. The Minimal Supersymmetric Extension of the Standard Model . . . . . . 10
3. Experimental Setup 13
3.1. The Large Hadron Collider Accelerator Complex . . . . . . . . . . . . . . . 13
3.2. The ATLAS Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2.1. Inner Detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2.2. Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2.3. Muon Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.2.4. Trigger System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2.5. ATLAS Coordinate Frames . . . . . . . . . . . . . . . . . . . . . . 24
3.2.6. Athena: the ATLAS Software Framework . . . . . . . . . . . . . . 25
II. Search for Supersymmetry in Trilepton Final States 27
4. Search for Supersymmetry in Trilepton Final States: Introduction 28
5. Signal Signature and Backgrounds 30
5.1. Supersymmetric Production of Trilepton Final States . . . . . . . . . . . . 30
5.2. Trilepton Final States
in the Massive Sparton Scenario . . . . . . . . . . . . . . . . . . . . . . . . 34
5.3. Backgrounds to the Trilepton Final State . . . . . . . . . . . . . . . . . . . 35
6. Monte Carlo Samples Used 37
6.1. Software Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
7. Preselection and Overlap Removal 39
7.1. Muon Preselection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
vi
Contents
7.2. Electron Preselection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
7.3. Jet Preselection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
7.4. Overlap Removal Between
Electrons or Photons and Jets . . . . . . . . . . . . . . . . . . . . . . . . . 42
8. Signal Selection 45
8.1. Opposite Sign Same Flavour Lepton Pair Selection . . . . . . . . . . . . . 45
8.2. Selection of Three Isolated Leptons . . . . . . . . . . . . . . . . . . . . . . 46
8.3. Final Cuts Against Standard Model Backgrounds . . . . . . . . . . . . . . 48
9. Results 50
10.Lepton Trigger Study 53
11.Background Estimation Techniques 57
11.1.Classification of Systematic Uncertainty Sources . . . . . . . . . . . . . . . 57
11.2.Assessing Instrumental Uncertainties . . . . . . . . . . . . . . . . . . . . . 58
11.3.Assessing Physics Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . 59
11.4.Secondary Leptons from b-Decays
Passing Isolation Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
11.4.1. Estimation of the Rate of Secondary Leptons
Passing Isolation Criteria Using tt¯Events . . . . . . . . . . . . . . . 61
11.4.2. Backgrounds to the Estimation of the Rate of Secondary Leptons
Passing Isolation Criteria Using tt¯Events . . . . . . . . . . . . . . . 63
11.4.3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
11.5.Estimation of Systematic Uncertainties . . . . . . . . . . . . . . . . . . . . 64
12.Trilepton SUSY Search: Conclusion 65
III. The Alignment of the ATLAS Silicon Tracker 67
13.Introduction: Alignment at ATLAS 68
13.1.Hardware-Based Alignment: an Overview . . . . . . . . . . . . . . . . . . . 69
13.2.Track-Based Alignment: an Overview . . . . . . . . . . . . . . . . . . . . . 72
13.2.1. Important Track-Based Alignment Structures . . . . . . . . . . . . 75
13.2.2. ATLAS Tracking Event Data Model . . . . . . . . . . . . . . . . . 76
14.The Robust Alignment Algorithm 78
14.1.Input to Robust Alignment: (Overlap) Residuals . . . . . . . . . . . . . 78
14.1.1. Residuals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
14.1.2. Overlap Residuals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
14.2.Definition of the Robust Alignment Procedure . . . . . . . . . . . . . . 82
14.2.1. Level 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
14.2.2. Pixel Stave Bow Alignment . . . . . . . . . . . . . . . . . . . . . . 86
vii
Contents
14.2.3. Level 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
14.2.4. Level 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
14.2.5. Robust Alignment as an Iterative Procedure . . . . . . . . . . . 98
14.2.6. Running of the Robust Alignment Algorithm . . . . . . . . . . . 101
14.2.7. Steering Options to Robust Alignment . . . . . . . . . . . . . . 101
15.Pixel End-Cap A Alignment with SR1 Cosmic Ray Data 105
15.1.SR1 Pixel End-Cap A Experimental Setup . . . . . . . . . . . . . . . . . . 105
15.2.Robust Alignment procedure in SR1 Pixel End-Cap A Setup . . . . . . 106
15.2.1. Input to the Robust Alignment procedure . . . . . . . . . . . . 106
15.2.2. Convergence of the Robust Alignment procedure . . . . . . . . . 107
15.3.Determination of the Layer Thickness . . . . . . . . . . . . . . . . . . . . . 109
15.4.Derivation of c , c Alignment Constants . . . . . . . . . . . . . . . . . . . 110
x y
15.4.1. Comparison of the Alignment Constants Derived using Robust
Alignment with the Survey . . . . . . . . . . . . . . . . . . . . . 112
15.5.Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
16.Silicon Tracker Alignment with M8+ Cosmic Ray Data 114
16.1.The Analysed Dataset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
16.2.Selection of Residuals for Alignment . . . . . . . . . . . . . . . . . . . . . 120
16.2.1. Track Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
16.2.2. Hit Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
16.2.3. Residual Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
16.3.The Robust Alignment Procedure for M8+ . . . . . . . . . . . . . . . . 134
16.3.1. Level 1 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
16.3.2. Level 2 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
16.3.3. Pixel Stave Bow Alignment . . . . . . . . . . . . . . . . . . . . . . 160
16.3.4. Level 3 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
16.4.Alignment Results with Robust Alignment in M8+ . . . . . . . . . . . 175
16.5.Validation of Robust Alignment Alignment Results in M8+ . . . . . . . 185
16.6.Differences in Residual and Overlap Residual Means
for B-field Off and On in M8+ . . . . . . . . . . . . . . . . . . . . . . . . . 189
17.Alignment: Conclusion and Outlook 192
Glossary 195
Bibliography 196
A. Minimal Supergravity Sparticle Mass Spectra 211
B. Complete Set of Pixel Stave Bow Fits 215
viii
List of Figures
3.1. The LHC Accelerator Complex at CERN and its major components . . . . 13
3.2. Cut-away view of the ATLAS detector . . . . . . . . . . . . . . . . . . . . 15
3.3. The ATLAS Inner Detector and its major components. . . . . . . . . . . 16
3.4. A technical drawing of a quadrant of the ATLAS ID in the R-Z plane . . . 16
3.5. Schematic view of the pixel subdetector . . . . . . . . . . . . . . . . . . . . 17
3.6. Schematic view of a pixel and SCT barrel module . . . . . . . . . . . . . . 18
3.7. Schematic view of the SCT subdetector . . . . . . . . . . . . . . . . . . . . 19
3.8. Schematic view of the ATLAS calorimeters . . . . . . . . . . . . . . . . . . 21
3.9. Schematic view of the Muon Spectrometer . . . . . . . . . . . . . . . . . . 23
5.1. Apossibleproductiondiagramofthetrileptonfinalstateviadirectgaugino
pair-production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
7.1. Leading muon pµ1 and multiplicity of muons per event for SU2 . . . . . . . 40
T
7.2. Leading electron pe1 and multiplicity of electrons per event for SU2 . . . . 42
T
7.3. Transverse energy of the leading jet Ej1 and multiplicity of jets per event
T
for SU2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
7.4. Distance in ∆R between jets and electrons before overlap removal as found
in tt¯events and Ee/Ej for ∆R(e,j) < 0.2 . . . . . . . . . . . . . . . . . . 43
T T
8.1. Track Itrk and calorimeter isolation Ical distribution for muons and electrons 47
0.2 0.2
8.2. Track versus calorimeter isolation for muons and electrons . . . . . . . . . 48
9.1. OSSF dilepton mass distribution for 10fb−1 for inclusive SU2 signal with
and without jet veto, as well as direct gaugino production in SU2 and SU3
without any jet veto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
9.2. OSSF dilepton mass distribution for 10fb−1 after all cuts and without any
jet veto for SU1, SU3, SU4, and SU8 . . . . . . . . . . . . . . . . . . . . . 52
10.1.TriggerefficienciesforAND-combinationsofvarioustriggersforthemassive
sparton scenario and SU3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
11.1.Two representative tt¯ decay scenarios, where the secondary lepton is of
opposite sign or of like sign to the prompt lepton . . . . . . . . . . . . . . 62
12.1.OSSF dilepton mass distribution for SU2 for 100fb−1 from an Atlfast
study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
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List of Figures
13.1.Schematic illustration for the alignment of the Silicon Tracker. . . . . . . . 69
13.2.The support structure of the Inner Detector . . . . . . . . . . . . . . . . . 70
13.3.The geodetic grid of the FSI system . . . . . . . . . . . . . . . . . . . . . . 71
13.4.Schematic illustration for the principle of track-based alignment of the Sil-
icon Tracker. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
13.5.Thecanonicaltrackrepresentationswithrespecttotheperigeeandaplanar
surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
14.1.The definition of a track-hit residual r in x and y direction . . . . . . . . . 78
14.2.Residuals: reality vs. situation as “seen” by reconstruction . . . . . . . . . 82
14.3.Overlap residuals: reality vs. situation as “seen” by reconstruction . . . . . 84
14.4.Two fully equipped pixel staves with modules . . . . . . . . . . . . . . . . 86
14.5.Residual mean versus sector number distribution for two pixel staves . . . 87
14.6.L2 misalignments: reality vs. situation as “seen” by reconstruction . . . . 89
14.7.The distribution r (Φ) for a typical barrel layer of the pixels and SCT
x stave
(cid:104) (cid:105)
each . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
14.8.The distribution r (Φ) for an EC disk of the pixels and SCT each . . 95
x mod
(cid:104) (cid:105)
14.9.The distribution r (Φ) for the barrel of pixels and SCT . . . . . . . . 96
x stave
(cid:104) (cid:105)
14.10.The distribution r (Φ) for the pixel end-caps . . . . . . . . . . . . . . 97
x mod
(cid:104) (cid:105)
15.1.SR1 pixel end-cap A experimental setup . . . . . . . . . . . . . . . . . . . 106
15.2.Differential shifts applied to modules per iteration . . . . . . . . . . . . . . 107
15.3.Number of modules shifted per iteration . . . . . . . . . . . . . . . . . . . 107
15.4.Residual distributions in local x direction for nominal alignment and after
20 iterations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
15.5.Overlap residual distributions o for nominal alignment and after 20 iter-
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ations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
15.6.The σ-parameter of a Gaussian fit to the o residual distribution . . . . . 110
xx
15.7.Alignmentconstantsinlocalx,y after20iterationswithrespecttonominal
geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
15.8.Alignment constants in local x,y for layer 0 of the pixel end-cap A subde-
tector as obtained with the survey . . . . . . . . . . . . . . . . . . . . . . . 111
15.9.Alignment constants in local x,y for layer 1 of the pixel end-cap A subde-
tector as obtained with the survey . . . . . . . . . . . . . . . . . . . . . . . 111
15.10.Alignment constants in local x,y for layer 2 of the pixel end-cap A subde-
tector as obtained with the survey . . . . . . . . . . . . . . . . . . . . . . . 112
16.1.Number of cosmic ray tracks vs. run number collected by the ATLAS ID
in M8+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
16.2.ATLAS detector in the pit and topological incidence of ID tracks in M8+ . 117
16.3.Number of hits-on-track for B-field off in the barrel of the pixel detector
in M8+ before any alignment . . . . . . . . . . . . . . . . . . . . . . . . . 118
16.4.Number of hits-on-track for B-field off in the barrel of the SCT detector in
M8+ before any alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
x