Z Boson Transverse Momentum Distribution, and ZZ and WZ Production: Measurements Using 7.3 – 8.6 fb–1 of p¯p Collisions at √s = 1.96 TeV

225 Pages·2012·10.171 MB·English

by  Mika Vesterinen (auth.)

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Mika Vesterinen
Z Boson Transverse
Momentum Distribution,
and ZZ and WZ Production
–1
Measurements Using 7.3–8.6 fb of
p
ffiffiffi
pp(cid:2) Collisions at s = 1.96 TeV
Doctoral Thesis accepted by
the University of Manchester, UK
123
Author Supervisor
Dr. MikaVesterinen Prof.TerryWyatt
Universityof Manchester Universityof Manchester
UnitedKingdom UnitedKingdom
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-642-30787-4 ISBN 978-3-642-30788-1 (eBook)
DOI 10.1007/978-3-642-30788-1
SpringerHeidelbergNewYorkDordrechtLondon
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Supervisor’s Foreword
This Ph.D. thesis presents three major pieces of new work:
• thedevelopmentofnoveltechniquesformeasuringthetransversemomentumof
Z bosons;
• theapplicationofthesenoveltechniquestodatafromtheDØexperimentatthe
Tevatron and a comparison of the measured distributions with state-of-the-art
QCD calculations;
• precise measurements of the cross sections for WZ and ZZ vector boson pair
production [1].
A novel analysis variable, /(cid:2), is proposed [2]. This is sensitive to the same
g
physics asthe p‘‘ distribution,butissignificantly lesssusceptibletothe effects of
T
detector resolution and efficiency. In particular, since /(cid:2) depends exclusively on
g
the directions of the two leptons, which are measured with a precision of a mil-
liradian or better, /(cid:2) is experimentally very well measured compared to any
g
quantities that rely on the momenta of the leptons.
Using 7.3 fb(cid:3)1 of p(cid:2)p collisions collected by the DØ detector at the Fermilab
Tevatron,thenormalizedcrosssectionismeasuredasafunctionofthevariable/(cid:2)in
g
bins of boson rapidity and in both dielectron and dimuon final states [3]. These
measurementsareofunprecedentedprecisionandexposedeficienciesinthecurrent
state-of-the-artQCDpredictionsforvectorbosonproductionathadroncolliders.In
addition, these data exclude in hadron–hadron collisions the hypothesis of ‘low-x
broadening’(Thisistheideathattheintrinsictransversemomentumofpartonsinside
theprotonbecomesbroaderasthemomentumfractionxbecomesverysmall).
These new experimental techniques and measurements have stimulated con-
siderableinterestinthetheoreticalandexperimentalparticlephysicscommunities.
For example, resummed QCD calculations of the distribution of /(cid:2) have been
g
made and compared with the above DØ measurements [4]. Measurements of /(cid:2)
g
are now being extended at the Tevatron to lepton pairs with masses outside the Z
v
vi Supervisor’sForeword
resonance region. These techniques have now been picked up by the LHC
experiments, where measurements of /(cid:2) distributions are now being performed.
g
The measurements of the cross sections for WZ and ZZ vector boson pair
production build on earlier measurements by DØ, but use improved analysis
techniques and almost the full Tevatron Run II data set. At the time of their
publication [1]theyrepresenttheworld’smostprecisemeasurementsofthecross
sections for WZ and ZZ production. These cross sections are sensitive to anom-
alous triple-gauge-couplings, and thus probe the electroweak component of the
standard model. WZ and ZZ production are two of the smallest cross section
processes currently accessible at a hadron collider and represent major sources of
backgroundinsearchchannelsforHiggsbosons.Understandingtheseprocessesis
thereforecrucialfordemonstratingsensitivitytothepresenceofastandardmodel
Higgs boson at the Tevatron.
Manchester, April 2012 Prof. Terry Wyatt
References
1.DØ Collaboration, V. M. Abazov et al., arXiv:1201.5652v1 [hep-ex] (2012) [submitted to
Phys.Rev.D]
2.A.Banfi,S.Redford,M.Vesterinen,P.Waller,T.R.Wyatt,Eur.Phys.J.C71,1600(2011)
3.DØCollaboration,V.M.Abazovetal.,Phys.Rev.Lett.106,122001(2011).
4.A.Banfi,M.Dasgupta,S.Marzani,L.Tomlinson,HighEnergyPhys.1201,044(2011)
Acknowledgments
First of all I would like to thank my Supervisor, Terry Wyatt, for guiding me
through the past 4 years with endless enthusiasm and ingenuity.
ThankstoFredLoebingerforinspiringmetoapplytothisprogram(andalsoto
the MPhys program 4 years earlier), and to Stefan Söldner-Rembold for encour-
aging me to join the D0 experiment. I am grateful for the tireless IT support(and
entertainment)fromSabahSalihwhilebasedinManchester.Iwouldliketothank
Mark Owen for helping me to get started with the various D0 frameworks and
generalcodingduringmyfirstyear.ThankstoallmembersoftheD0Manchester
group and other friends who helped to make my 2 years at Fermilab enjoyable.
OfcourseIamthankfulthatmyfiancéeJoannaBurgesshasbeensopatientand
understanding during the last 4 years; particularly the 2 years that I was based at
Fermilab. I also thank my parents for always supporting my apparent interest in
science from an early age.
vii
Contents
1 Introduction and Theoretical Background. . . . . . . . . . . . . . . . . . . 1
1.1 The Standard Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Scattering Amplitudes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Electroweak Interactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Strong Interactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 The Drell-Yan Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5.1 Factorisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5.2 Higher Order Corrections . . . . . . . . . . . . . . . . . . . . . 9
1.5.3 Monte Carlo Event Generators. . . . . . . . . . . . . . . . . . 11
1.5.4 Drell-Yan Transverse Momentum Distribution. . . . . . . 13
1.6 Electroweak Diboson Production. . . . . . . . . . . . . . . . . . . . . . 13
1.6.1 Previous Measurements . . . . . . . . . . . . . . . . . . . . . . 14
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2 Experimental Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1 The Accelerator Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.1 The Initial Accelerator Chain . . . . . . . . . . . . . . . . . . 17
2.1.2 The Main Injector and Antiproton Source. . . . . . . . . . 18
2.1.3 The Tevatron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2 The D0 Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.1 Inner Tracker. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.2 Calorimeter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.3 Muon System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.4 Trigger. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3 Experimental Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.1 Detector Alignment and Calibration. . . . . . . . . . . . . . . . . . . . 29
3.1.1 Calorimeter Calibration. . . . . . . . . . . . . . . . . . . . . . . 29
ix
x Contents
3.2 Particle Reconstruction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2.1 Charged Tracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2.2 Electrons and Photons . . . . . . . . . . . . . . . . . . . . . . . 30
3.2.3 Muons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.2.4 Hadronic Taus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.5 IC Electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.6 Hadronic Jets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.7 Missing Transverse Energy. . . . . . . . . . . . . . . . . . . . 34
3.3 Monte Carlo Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.4 Unfolding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.5 Multivariate Classifiers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4 Electron and Photon Energy Calibration . . . . . . . . . . . . . . . . . . . 37
4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.2 Dataset. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.3 Correction of the /-Bias. . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.4 Energy Calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.4.1 Additional Calibrations. . . . . . . . . . . . . . . . . . . . . . . 40
4.4.2 A / Dependence Energy Calibration . . . . . . . . . . . 41
mod
4.4.3 Shower Shape Dependence. . . . . . . . . . . . . . . . . . . . 49
4.4.4 Pseudorapidity Dependence. . . . . . . . . . . . . . . . . . . . 49
4.4.5 Fits for the / Dependence . . . . . . . . . . . . . . . . . . 49
mod
4.4.6 Iteration of the Corrections. . . . . . . . . . . . . . . . . . . . 49
4.4.7 Resolution Improvements . . . . . . . . . . . . . . . . . . . . . 50
4.4.8 Energy Dependence . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.5 Monte Carlo Over-Smearing . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.5.1 The Crystal Ball Function. . . . . . . . . . . . . . . . . . . . . 56
4.5.2 Method to Fit for the Crystal Ball Parameters. . . . . . . 59
4.5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.5.4 Data Versus MC Comparisons. . . . . . . . . . . . . . . . . . 63
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5 Novel Variables for Studying the Drell-Yan
Transverse Momentum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.1 Previous Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.2 First Idea: the a Variable . . . . . . . . . . . . . . . . . . . . . . . . . . 65
T
5.3 Second Idea: Mass Ratios of a and p‘‘ . . . . . . . . . . . . . . . . . 67
T T
5.4 Third Idea: The /(cid:2) Variable . . . . . . . . . . . . . . . . . . . . . . . . . 68
g
5.5 Simple Parameterised Detector Simulation . . . . . . . . . . . . . . . 69
5.6 Scaling Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.7 Experimental Resolution for Dilepton Scattering Angle . . . . . . 70
5.8 Experimental Resolution for Variables Related to p‘‘. . . . . . . . 70
T
5.9 Acceptance and Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 72
Contents xi
5.10 Sensitivity to the Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.11 Discussion on the Different Variables. . . . . . . . . . . . . . . . . . . 76
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
6 Measurement of the Drell-Yan /(cid:2) Distribution. . . . . . . . . . . . . . . 81
g
6.1 The Observables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
6.2 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
6.2.1 Event Selection Strategy. . . . . . . . . . . . . . . . . . . . . . 82
6.2.2 Data Sample and Skims . . . . . . . . . . . . . . . . . . . . . . 84
6.2.3 Monte Carlo Samples. . . . . . . . . . . . . . . . . . . . . . . . 85
6.2.4 Common Dielectron and Dimuon Requirements . . . . . 85
6.2.5 Dielectron Event Selection . . . . . . . . . . . . . . . . . . . . 85
6.2.6 Dimuon Event Selection. . . . . . . . . . . . . . . . . . . . . . 86
6.3 Corrections to the Fully Simulated Monte Carlo Events . . . . . . 89
6.3.1 Instantaneous Luminosity Profile. . . . . . . . . . . . . . . . 91
6.3.2 Generator Level Physics Re-Weightings. . . . . . . . . . . 91
6.3.3 Electron Energy and Muon Momentum Smearing . . . . 92
6.3.4 Track / and g Smearing. . . . . . . . . . . . . . . . . . . . . . 92
6.3.5 Electron Track p Smearing . . . . . . . . . . . . . . . . . . . 92
T
6.3.6 Local Muon p Smearing . . . . . . . . . . . . . . . . . . . . . 94
T
6.3.7 Trigger and Offline Efficiencies. . . . . . . . . . . . . . . . . 94
6.4 Backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.5 Comparison of Data with Simulation . . . . . . . . . . . . . . . . . . . 110
6.6 Unfolding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6.6.1 Binning in /(cid:2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
g
6.6.2 Bin-by-Bin Unfolding. . . . . . . . . . . . . . . . . . . . . . . . 133
6.7 Systematic Uncertainties. . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
6.8 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.8.1 Theoretical Predictions. . . . . . . . . . . . . . . . . . . . . . . 135
6.8.2 Comparison of Data and RESBOS . . . . . . . . . . . . . . . . 137
6.8.3 Fitting for g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
2
6.9 Cross Checks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
6.9.1 Dielectron Versus Dimuon Comparison . . . . . . . . . . . 137
6.9.2 /-Gap Checks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
6.9.3 Unfolding Closure Test Using g . . . . . . . . . . . . . . . . 138
2
6.9.4 Data Subset Checks Using g . . . . . . . . . . . . . . . . . . 139
2
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
7 Measurement of the ZZ and WZ Production Cross Sections. . . . . . 141
7.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
7.2 Dataset and MC Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
7.2.1 Dataset. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
7.2.2 MC Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141