The Razor Boost analysis-Another step in the hunt for new physics at CMS

223 Pages·2015·21.73 MB·English

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FACULTEIT WETENSCHAPPEN
The Razor Boost analysis
Another step in the hunt for new
physics at CMS
Nadja Strobbe
Promotoren: Prof. Dr. Dirk Ryckbosch, Dr. Michael Tytgat
Proefschrift ingediend tot het verkrijgen van de academische graad van
Doctor in de Wetenschappen: Fysica
Vakgroep Fysica en Sterrenkunde, academiejaar 2014-2015

The important thing is not to stop questioning.
Curiosity has its own reason for existing.
Albert Einstein

Acknowledgements
NowthatmytimeasaPhDstudentiscomingtoanend,itistimetoreflectfor
a moment on the past 3.5 years. So much happened, so many people crossed
my path. It all contributed to shaping the person I am today. I like to think
it made me a better person. It definitely made me a more knowledgeable
person. I learned so many new things, from physics and organization to just
remembering to take the time to have fun every once in a while. Before the
bravest among you embark on the effort to read through this thesis, I would
liketogivemythankstoeveryonethathelpedmeinanywaythroughoutthese
past years.
First of all, I would like to thank my promotors Dirk and Michael, for
allowingmetoplungeintotheadventurethatisaPhD.Yourdoorswerealways
open to answer my questions, or to give me advice. And above all, thank you
for sending me off to CERN! That year-and-a-half was an experience I will
never forget.
Sezen,withoutyouthisthesiscouldnothavehappened. Icannotthankyou
enough for adopting me as your student, and convincing me to join you in the
razor boost effort. Working with you is always a pleasure, and I hope we can
continue working on some joint projects in the future. Since Lukas introduced
us way back in 2010, we have become great friends, and that is a gift to be
treasured.
Harrison, every time we chat I feel like I have learned something. Your
colourful way of expressing yourself never fails to bring a smile to my face. I
am very honoured to have worked with you on this analysis, pioneering the
way towards a proper statistical treatment.
I would also like to thank Maurizio, and the rest of the razor team, for
providing help and insight whenever we were stuck.
Ofcourse,abigthankyougoestoallmyfriends. TothepeopleoftheINW,
thank you for making it a fun workplace. To my friends at CERN, thank you
forthecrazyeveningsatMeyrinoise,andthealwaysentertainingPenndinners.
Bedankt aan al mijn krachtbalvrienden, de trainingen en wedstrijden die we
samenmeegemaakthebbenwarendeidealeontspanningtussendedruktedoor.
Karen en Mieke, met al onze drukke schema’s hebben we elkaar niet zo vaak
kunnen zien, maar ik wil dat jullie weten dat ik onze vriendschap koester. Ik
hoop dat we in de toekomst ook in contact kunnen blijven!
Een dikke merci ook aan mijn ouders en broer. Jullie zorgden ervoor dat
ik het contact met de ‘echte wereld’ niet verloor. Dankjewel voor alle steun
5
6
doorheen de jaren. Zonder jullie had ik dit niet gekund!
Brett, since I met you, my life has never been the same. We have seen and
done so many fun things over the past couple of years. Thank you for being
therethroughallthemadtimes,rushingtomeetoneortheotherdeadline. The
last couple months were not easy, with us living 800km apart. The evenings
spent talking until past midnight about all our future plans really kept me
going. Now,weareembarkingonabrandnewandsuperexcitingadventurein
Chicago. We cannot know what the future will hold, but I know that we can
conquer anything together!
Contents
1 Introduction 11
2 The Standard Model of particle physics 15
2.1 Particles and interactions . . . . . . . . . . . . . . . . . . . . . 15
2.2 QCD and asymptotic freedom . . . . . . . . . . . . . . . . . . 17
2.3 Brout-Englert-Higgs mechanism . . . . . . . . . . . . . . . . . 18
2.4 A success story . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4.1 Higgs boson discovery . . . . . . . . . . . . . . . . . . . 23
2.4.2 Precision electroweak fits . . . . . . . . . . . . . . . . . 24
3 The need for physics beyond the Standard Model 29
3.1 Open questions . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2 The hierarchy problem . . . . . . . . . . . . . . . . . . . . . . 31
3.3 Extensions of the Standard Model . . . . . . . . . . . . . . . . 32
3.3.1 Supersymmetry . . . . . . . . . . . . . . . . . . . . . . 33
3.3.2 Little Higgs scenarios . . . . . . . . . . . . . . . . . . . 33
3.3.3 Extra dimensions . . . . . . . . . . . . . . . . . . . . . 34
4 Supersymmetry 35
4.1 Supermultiplets . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.2 Supersymmetry breaking mechanisms . . . . . . . . . . . . . . 37
4.3 Minimal Supersymmetric Standard Model . . . . . . . . . . . 38
4.3.1 Particle content . . . . . . . . . . . . . . . . . . . . . . 38
4.3.2 Supersymmetric Lagrangian . . . . . . . . . . . . . . . 39
4.3.3 R-parity . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.3.4 Soft SUSY breaking in the MSSM . . . . . . . . . . . . 41
4.4 Electroweak symmetry breaking and natural SUSY . . . . . . 42
4.5 Sparticle production and decay . . . . . . . . . . . . . . . . . . 45
4.5.1 Mass eigenstates . . . . . . . . . . . . . . . . . . . . . . 45
4.5.2 Sparticle decays . . . . . . . . . . . . . . . . . . . . . . 46
4.5.3 Signature at hadron colliders . . . . . . . . . . . . . . . 47
4.6 Natural SUSY . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.7 Simplified model spectra . . . . . . . . . . . . . . . . . . . . . 49
7
8 CONTENTS
5 The Large Hadron Collider and the Compact Muon Solenoid
experiment 51
5.1 The Large Hadron Collider . . . . . . . . . . . . . . . . . . . . 51
5.1.1 A proton machine . . . . . . . . . . . . . . . . . . . . . 51
5.1.2 LHC accelerator complex and experiments . . . . . . . . 52
5.1.3 Superconducting magnets . . . . . . . . . . . . . . . . . 55
5.1.4 Accelerating cavities . . . . . . . . . . . . . . . . . . . . 56
5.1.5 Past and future running periods . . . . . . . . . . . . . 57
5.2 The Compact Muon Solenoid experiment . . . . . . . . . . . . 58
5.2.1 Coordinate system and basic variables . . . . . . . . . 59
5.2.2 Magnet system . . . . . . . . . . . . . . . . . . . . . . . 60
5.2.3 Tracker . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.2.4 Electromagnetic calorimeter . . . . . . . . . . . . . . . 61
5.2.5 Hadron calorimeter . . . . . . . . . . . . . . . . . . . . 63
5.2.6 Muon system . . . . . . . . . . . . . . . . . . . . . . . . 64
5.2.7 Trigger and data acquisition . . . . . . . . . . . . . . . 65
6 Event generation, simulation and reconstruction 69
6.1 What is an “event”? . . . . . . . . . . . . . . . . . . . . . . . 70
6.1.1 Factorization theorems . . . . . . . . . . . . . . . . . . . 73
6.1.2 Parton distribution functions . . . . . . . . . . . . . . . 74
6.1.3 Hard interaction . . . . . . . . . . . . . . . . . . . . . . 76
6.2 Event generation . . . . . . . . . . . . . . . . . . . . . . . . . . 78
6.2.1 Matrix element generators . . . . . . . . . . . . . . . . 78
6.2.2 Parton shower . . . . . . . . . . . . . . . . . . . . . . . 82
6.2.3 Matching the matrix element to the parton shower . . 84
6.2.4 Hadronization . . . . . . . . . . . . . . . . . . . . . . . 87
6.2.5 Generator tuning . . . . . . . . . . . . . . . . . . . . . . 88
6.3 Event simulation . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.3.1 CMS Full Simulation using Geant4 . . . . . . . . . . . 89
6.3.2 CMS Fast Simulation . . . . . . . . . . . . . . . . . . . 90
6.4 Event reconstruction . . . . . . . . . . . . . . . . . . . . . . . 91
6.4.1 Particle flow . . . . . . . . . . . . . . . . . . . . . . . . 92
6.4.2 Physics object identification . . . . . . . . . . . . . . . 96
6.4.3 Event cleaning . . . . . . . . . . . . . . . . . . . . . . . 107
6.4.4 Event reweighting . . . . . . . . . . . . . . . . . . . . . 109
7 The razor boost analysis 113
7.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
7.2 General strategy . . . . . . . . . . . . . . . . . . . . . . . . . . 116
7.3 Razor variables . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
7.3.1 Kinematical configuration and notation . . . . . . . . . 119
7.3.2 Derivation of M . . . . . . . . . . . . . . . . . . . . . . 122
R
7.3.3 Derivation of R2 . . . . . . . . . . . . . . . . . . . . . . 125
7.3.4 Improved M and R2 definitions . . . . . . . . . . . . . 126
R
7.3.5 Generalization to longer decay chains . . . . . . . . . . 127
7.4 Boosted W boson tagging . . . . . . . . . . . . . . . . . . . . . 128
CONTENTS 9
7.4.1 Jet algorithm . . . . . . . . . . . . . . . . . . . . . . . . 128
7.4.2 Jet pruning . . . . . . . . . . . . . . . . . . . . . . . . . 130
7.4.3 N-subjettiness . . . . . . . . . . . . . . . . . . . . . . . 132
7.4.4 W boson tagging definitions . . . . . . . . . . . . . . . . 132
7.4.5 W boson tagging scale factors . . . . . . . . . . . . . . 134
7.5 Trigger and datasets . . . . . . . . . . . . . . . . . . . . . . . . 145
7.5.1 Data and trigger . . . . . . . . . . . . . . . . . . . . . . 145
7.5.2 Simulated samples . . . . . . . . . . . . . . . . . . . . . 147
7.6 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
7.6.1 The razor variables in the razor boost analysis . . . . . 149
7.6.2 Baseline selection . . . . . . . . . . . . . . . . . . . . . 150
7.6.3 Signal region selection . . . . . . . . . . . . . . . . . . . 153
7.6.4 Control region selection . . . . . . . . . . . . . . . . . . 154
7.6.5 Closure tests . . . . . . . . . . . . . . . . . . . . . . . . 167
7.7 Statistical modelling . . . . . . . . . . . . . . . . . . . . . . . . 169
7.8 Systematic uncertainties . . . . . . . . . . . . . . . . . . . . . 174
7.8.1 Jet energy scale corrections . . . . . . . . . . . . . . . . 174
7.8.2 Parton distribution functions . . . . . . . . . . . . . . . 175
7.8.3 Trigger efficiency . . . . . . . . . . . . . . . . . . . . . . 176
7.8.4 b tagging . . . . . . . . . . . . . . . . . . . . . . . . . . 176
7.8.5 W tagging. . . . . . . . . . . . . . . . . . . . . . . . . . 179
7.8.6 Lepton identification . . . . . . . . . . . . . . . . . . . 180
7.8.7 Initial State Radiation . . . . . . . . . . . . . . . . . . . 180
7.8.8 Top quark p spectrum . . . . . . . . . . . . . . . . . . 180
T
7.8.9 Pileup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
7.8.10 QCD spectrum . . . . . . . . . . . . . . . . . . . . . . . 181
7.8.11 Z( νν¯)+jets in association with heavy flavour . . . . . 181
→
7.8.12 Summary of separate systematic effects . . . . . . . . . 183
7.9 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
7.10 Interpretation in terms of simplified model spectra . . . . . . . 184
8 Summary and discussion 193
A Data sets 197
B Systematic uncertainties for signal 201
Nederlandstalige samenvatting 205
Bibliography 209