Studies into tau reconstruction, missing transverse energy and photon induced processes with the ATLAS detector at the LHC [Elektronische Ressource] / Robindra P. Prabhu
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Studies into tau reconstruction, missing transverse energy and photon induced processes with the ATLAS detector at the LHC [Elektronische Ressource] / Robindra P. Prabhu

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Studies into tau reconstruction, missing transverse energyand photon induced processes with the ATLAS detector atthe LHCDissertationzurErlangung des Doktorgrades (Dr. rer. nat.)derMathematisch-Naturwissenschaftlichen Fakult atderRheinischen Friedrich-Wilhelms-Universit at Bonnvorgelegt vonRobindra P. PrabhuausOslo, NorwegenBonn 2010Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakult at derRheinisch Friedrich-Wilhelms-Universit at Bonn1. Gutachter: Prof. Dr. Klaus K. Desch2. Gutachter: Prof. Dr. Jochen DingfelderTag der Promotion: 27. Oktober 2010Erscheinungsjahr: 2011AbstractThe ATLAS experiment is currently recording data from proton-proton collisions de-livered by CERN’s Large Hadron Collider. As more data is amassed, studies of bothStandard Model processes and searches for new physics beyond will intensify. Thisdissertation presents a three-part study providing new methods to help facilitate thesee orts.The rst part presents a novel -reconstruction algorithm for ATLAS inspired by theideas of particle ow calorimetry. The algorithm is distinguished from traditional -reconstruction approaches in ATLAS, insofar that it seeks to recognize decay topologiesconsistent with a (hadronically) decaying -lepton using resolved energy ow objectsin the calorimeters. This procedure allows for an early classi cation of -candidatesaccording to their decay mode and the use of decay mode speci c discrimination againstfakes.

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Published 01 January 2011
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Studies into tau reconstruction, missing transverse energy
and photon induced processes with the ATLAS detector at
the LHC
Dissertation
zur
Erlangung des Doktorgrades (Dr. rer. nat.)
der
Mathematisch-Naturwissenschaftlichen Fakult at
der
Rheinischen Friedrich-Wilhelms-Universit at Bonn
vorgelegt von
Robindra P. Prabhu
aus
Oslo, Norwegen
Bonn 2010Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakult at der
Rheinisch Friedrich-Wilhelms-Universit at Bonn
1. Gutachter: Prof. Dr. Klaus K. Desch
2. Gutachter: Prof. Dr. Jochen Dingfelder
Tag der Promotion: 27. Oktober 2010
Erscheinungsjahr: 2011Abstract
The ATLAS experiment is currently recording data from proton-proton collisions de-
livered by CERN’s Large Hadron Collider. As more data is amassed, studies of both
Standard Model processes and searches for new physics beyond will intensify. This
dissertation presents a three-part study providing new methods to help facilitate these
e orts.
The rst part presents a novel -reconstruction algorithm for ATLAS inspired by the
ideas of particle ow calorimetry. The algorithm is distinguished from traditional -
reconstruction approaches in ATLAS, insofar that it seeks to recognize decay topologies
consistent with a (hadronically) decaying -lepton using resolved energy ow objects
in the calorimeters. This procedure allows for an early classi cation of -candidates
according to their decay mode and the use of decay mode speci c discrimination against
fakes. A detailed discussion of the algorithm is provided along with early performance
results derived from simulated data.
The second part presents a Monte Carlo simulation tool which by way of a pseudorapidity-
dependent parametrization of the jet energy resolution, provides a probabilistic estimate
for the magnitude of instrumental contributions to missing transverse energy arising
from jet uctuations. The principles of the method are outlined and it is shown how the
method can be used to populate tails of simulated missing transverse energy distributions
su ering from low statistics.
The third part explores the prospect of detecting photon-induced leptonic nal states in
early data. Such processes are distinguished from the more copious hadronic interactions
at the LHC by cleaner nal states void of hadronic debris, however the soft character of
the nal state leptons poses challenges to both trigger and o ine selections. New trigger
items enabling the online selection of such nal states are presented, along with a study
into the feasibility of detecting the two-photon exchange process pp(!)p p with
early data.
iiiAcknowledgements
Over the last few years, I have had the great fortune to interact with a number of
remarkable people without whose guidance, insight, perspectives and sharing of ideas
this work would likely never have seen the light of day.
I warmly thank Prof. Dr. Klaus Desch for "taking me under his wings", for the op-
portunity to work on a range of di erent topics and for providing me with the space to
develop and mature and bring the ideas presented herein to fruition. His encouragement,
patience and support has been greatly appreciated.
I am equally grateful to Peter Wienemann, for the countless discussions we have had
on physics and equally often on topics far beyond. His impressive insight and clarity
of thought has been an invaluable resource to me throughout and a constant source of
wonder and amazement.
I must also thank Sebastian Fleischmann and Mark Hodgkinson for the fruitful collabo-
ration on the development of PanTau. Their ideas and contributions were instrumental
to parts of the work presented in Chapter 2 and I am thankful for all that they have
taught me along the way.
I am grateful for the encouragement, constructive critizisms and many helpful comments
provided by both Wolfgang Mader and Yann Coadou in their capacity as conveners for
the ATLAS Tau Working Group during my stay at CERN. Frank Paige, Shoji Asai
and Dan Tovey also deserve mention for their friendly and helpful input to the studies
presented in Chapter 3.
I am hugely indebted to Andrew Hamilton for enthusiastically sharing his expertise on
two-photon physics at hadron colliders and to Andrew Pilkington, Olya Igonkina, Anna
Sfyrla and Stefania Xella for all their help and assistance with the ATLAS trigger. A
big word of thanks also to Christian Limbach for enthusiastically carrying the torch
onwards.
I would go amiss if I failed to acknowledge the e orts of all the people who have pro-
vided invaluable technical support during the course of this work: Michael Heldmann
and Michael Duhrssen for introducing me to the ATLAS software and for patiently an-
swering all my tedious questions during my early months in Freiburg. The "Bonner"
administrators have always been equally friendly, accommodating and quick to answer
queries. Last but not least, I am indebted to Wolfgang Ehrenfeld and the NAF support
team for accommodating all my computing needs over the last few years.
I wish to thank all members of the HEP groups in Freiburg and Bonn for the good
company and friendly atmosphere, especially all the people with whom I have at one
point or another shared an o ce. The "Kochgruppe" at CERN should not be forgotten,
thank you all for making my stay at CERN all the more enjoyable. I am indebted to Peter
Wienemann and Carolin Zendler for lending their keen eyes to the proof-reading of this
manuscript and for the many helpful comments they both provided. Frau Furstenb erg
ivand Frau Streich also both deserve a special mention for the countless occasions on which
they have extended a helpful hand.
Warm and heartfelt thanks to Carolin, Duc, Giacinto, Gisela, Henrik, Michael, Natalia,
Olav and Zong for all the good times we have shared and for making the past few years
all the more enjoyable and worthwhile.
Finally, I wish to thank my dearest parents for their unwavering support and encour-
agement through thick and thin.To my parents.
viContents
Abstract iii
Acknowledgements iv
Symbols and abbreviations xi
1 Introduction 1
1.1 Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 The Large Hadron Collider and its experimental environment . . . . . . . 4
1.2.1 The LHC machine . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2 Early data prospects . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 The phenomenology of proton-proton collisions . . . . . . . . . . . . . . . 9
1.3.1 Anatomy of hadronic interactions at the LHC . . . . . . . . . . . . 10
1.3.2 The hard interaction . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.3.3 From hard scattering to experimental observables . . . . . . . . . . 14
1.3.3.1 Parton showers . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3.3.2 Hadronization . . . . . . . . . . . . . . . . . . . . . . . . 15
1.3.4 Photon interactions in proton-proton collisions . . . . . . . . . . . 17
1.3.4.1 Theoretical description of photon interactions at hadron
colliders . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.3.5 The underlying event . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.3.6 Pile-up and Minimum Bias . . . . . . . . . . . . . . . . . . . . . . 22
1.4 The ATLAS experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.4.1 The central detector systems . . . . . . . . . . . . . . . . . . . . . 25
1.4.1.1 The innermost tracking detectors . . . . . . . . . . . . . 26
1.4.1.2 The calorimetry . . . . . . . . . . . . . . . . . . . . . . . 29
1.4.1.3 The Muon Spectrometer . . . . . . . . . . . . . . . . . . 33
1.4.2 The forward detector systems . . . . . . . . . . . . . . . . . . . . . 35
1.4.2.1 MBTS (z=3.6 m, 2.12<jj< 3.85) . . . . . . . . . . . 35
1.4.2.2 LUCID (z=17 m, 5.4<jj<6.1) . . . . . . . . . . . . . 37
1.4.2.3 ZDC (z=140 m,jj>8.3) . . . . . . . . . . . . . . . . . 38
1.4.2.4 ALFA (z=240 m, 10.6<jj<13.5) . . . . . . . . . . . . 38
1.4.2.5 AFP - ATLAS Forward Proton Project . . . . . . . . . . 38
1.5 Trigger and Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . 39
viiContents
1.6 A note on Monte Carlo simulations . . . . . . . . . . . . . . . . . . . . . . 41
2 Tau Reconstruction and Identi cation 43
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.1.1 The decay phenomenology of the -lepton . . . . . . . . . . . . . . 43
2.1.2 The r^ole of tau leptons in the LHC physics programme . . . . . . . 45
2.1.3 Kinematics of tau decays at the LHC and challenges to tau recon-
struction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.2 Conventional approaches to tau reconstruction and identi cation in ATLAS 50
2.2.1 Tau energy determination . . . . . . . . . . . . . . . . . . . . . . . 51
2.2.2 Tau identi cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.3 The case for energy ow in ATLAS . . . . . . . . . . . . . . . . . . . . . . 52
2.3.1 Clustering of calorimeter cells in ATLAS . . . . . . . . . . . . . . . 54
2.3.1.1 Principle of topological clustering . . . . . . . . . . . . . 54
2.3.1.2 The application of topological clustering to -decays . . . 56
2.3.2 Energy ow in ATLAS . . . . . . . . . . . . . . . . . . . . . . . . . 58
2.3.2.1 Determining expected energy deposits . . . . . . . . . . . 59
2.3.2.2 Subtraction of energy deposits . . . . . . . . . . . . . . . 59
2.3.2.3 Recovering split showers . . . . . . . . . . . . . . . . . . . 62
2.3.2.4 Applications to tau reconstruction . . . . . . . . . . . . . 63
2.4 PanTau - particle ow inspired -reconstruction . . . . . . . . . . . . . . . 63
2.4.1 The philosophy of PanTau . . . . . . . . . . . . . . . . . . . . . . . 64
2.4.2 General overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
2.4.3 Seeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
2.4.3.1 Seed classi cation performance . . . . . . . . . . . . . . . 69
2.4.3.2 A note on the choice of jet algorithm . . . . . . . . . . . 71
2.4.4 Discrimination against QCD jets . . . . . . . . . . . . . . . . . . . 72
2.4.4.1 The relative composition of the signal within categories . 72
2.4.4.2 The p dependence of the classi cation of fakes . . . . . 73T
2.4.4.3 Feature classes and feature de nitions . . . . . . . . . . . 74
2.4.5 Prong-dependent feature selection . . . . . . . . . . . . . . . . . . 77
2.4.5.1 Category: 1-prong . . . . . . . . . . . . . . . . . . . . . 78
2.4.5.2 1-prong+neutral . . . . . . . . . . . . . . . . 78
2.4.5.3 Category: 3-prong . . . . . . . . . . . . . . . . . . . . . 80
2.4.5.4 3-prong+neutral . . . . . . . . . . . . . . . . 80
2.4.5.5 Category: 2-prong . . . . . . . . . . . . . . . . . . . . . 80
2.4.5.6 2-prong+neutral . . . . . . . . . . . . . . . . 81
2.4.6 Likelihood discriminants and performance evaluation . . . . . . . . 81
2.4.6.1 Transverse energy resolutions . . . . . . . . . . . . . . . . 88
2.4.6.2 Resolving decay resonances . . . . . . . . . . . . . . . . . 91
2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
3 A method to improve Monte Carlo statistics of QCD induced instru-
mental E= 107T
3.1 Calculating E= in ATLAS . . . . . . . . . . . . . . . . . . . . . . . . . . . 108T
3.2 Fake contributions to E= . . . . . . . . . . . . . . . . . . . . . . . . . . . 109T
fake3.3 Accounting for QCD induced E= from jet uctuations . . . . . . . . . . 110T