Testing the electroweak sector and determining the absolute luminosity at LHCb using dimuon final

143 Pages·2009·7.59 MB·English

Testing the electroweak sector and determining
the absolute luminosity at LHCb using dimuon
final states
Jonathan Steven Anderson
UCD School of Physics
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09 A thesis submitted to University College Dublin
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Head of School: Prof. L. Hanlon
Supervisor: Dr. R. McNulty
October 2008
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Abstract
The LHCb[1] detector, which commenced data-taking at the Large Hadron Collider[2] in
September 2008, is a foward single arm spectrometer optimised for measurements of CP-
violatingandraredecaysinthebquarksector. ThisthesisdescribespreparatoryMonte-Carlo
basedstudiesoftwoproposedphysicsmeasurementsthatwillbemadeatLHCb.
Firstly, the trigger, reconstruction and selection efficiencies for recording Z → µ+µ− events
at the LHCb experiment are presented. From simulation we conclude that LHCb will be
capable of reconstructing Z bosons via this channel with rapidities in the range 1.7<y<4.9
with an overall efficiency of 0.358±0.002(stat.), yielding ∼172,500 events for an integrated
luminosityof1fb−1. Thebackgroundhasbeenstudiedatthefour-vectorlevelandisestimated
tobe(3.0±2.9)%ofthesignallevelwiththedominantcontributioncomingfromeventswhere
twohadronsaremisidentifiedasmuons,abackgroundsourcethatcanbestbeestimatedfrom
the data themselves. Systematic uncertainties in the efficiency and purity are discussed and
expected to be <0.5% for a cross-section measurement in the forward region (1.7<y<4.9).
Assuming the luminosity can be determined at a similar level, LHCb will rapidly be able
to make a unique measurement of σ ·Br(Z → µ+µ−) at high rapidities with a precision of
Z
about 1%. This measurement will provide an important cross-check for the corresponding
measurements at CMS and ATLAS, and, in conjunction with a measurement of the W cross
section, precisely test the electroweak sector at LHC energies. In addition measurements of
the differential distributions, dσ/dy and dσ/dP , will constrain the proton parton distribution
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functionsandprovideastringenttestofQCDathigh-Q2.
Secondly, the feasibility of using the elastic two photon process pp → p+µ+µ−+ p to
make luminosity measurements at LHCb is investigated. The overall efficiency at LHCb
for recording and selecting pp → p+µ+µ−+p events produced within the pseudorapidity
range1.6<η<5hasbeendeterminedusingMonte-Carlotobe0.059±0.001(stat.),yielding
∼5200 events for an integrated luminosity of 1fb−1. The main background processes where
dimuons are produced via inelastic two-photon fusion and double pomeron exchange have
been studied using the full LHCb detector simulation while the other background sources,
including backgrounds caused by K/π mis-identification, have been studied at the four-vector
level. Thebackgroundisestimatedtobe(4.1±0.5(stat.)±1.0(syst.))%ofthesignallevelwith
the dominant contribution due to K/π mis-identification. Systematic uncertainties on a lumi-
nositymeasurementatLHCbusingthischannelareestimatedtobe ∼1.3%andaredominated
by the uncertainty on the predicted cross-section for events containing dimuons produced via
doublepomeronexchange,anuncertaintythatisexpectedtobereducedinthenearfuture.
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Declaration
This thesis is the result of my own work, except where explicit reference is made to the work
ofothers,andhasnotbeensubmittedforanotherqualificationtothisoranyotheruniversity.
JonathanAnderson
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Acknowledgements
I am immensely grateful to Ronan McNulty for his excellent supervision during the last five
years. Almost every aspect of the work outlined in this thesis has benefited from his wisdom
and experience. I thank him for having faith in my abilities, dedicating so much of his time
to me, teaching me a great deal and for dissuading me from quitting when my enthusiam for
physicswasatitslowest. ThanksRonan!
My thanks go to the Head of the UCD School of Physics, Gerry O’Sullivan, for giving me
the opportunity to undertake this research, and to University College Dublin for providing
financialassistanceintheformofaResearchdemonstratorshipandaUCDpresidentsresearch
award. I also acknowledge the funding provided by the European Union via the Marie-Curie
grantMTKD-CT-2004-003134.
I would like to express my gratitude to the other members of the LHCb group at UCD, espe-
cially Aidan Smoker, Karol Hennessy, Tahar Kechadi and Zoltan Mathe, for their friendship
and for making the last five years so enjoyable. In particular I would like to thank Karol for
being such a great flat mate during our stay at CERN and for his patience when answering all
ofmycomputingrelatedquestions.
I thank the members of the LHCb group at the University of Liverpool, in particular Tara
ShearsandThemisBowcock,fortheirfriendshipandadvice,especiallyduringthelastmonths
ofmyPhDwhenIwasbasedatCERN.
I thank Alan Martin, James Stirling, Robert Thorne and Greame Watt for invaluable discus-
sions and suggestions regarding the Z cross-section measurement work outlined in chapter
4. I also thank Tara Shears for initially bringing the Z →µ+µ− channel to my attention and
subsequentlyforhermanyusefulsuggestionsandcomments.
I thank Mike Albrow, Valery Khoze, Alan Martin and Andrey Shamov for many useful com-
ments and suggestions regarding the work outlined in chapter 5. I am particularly grateful to
Andrey Shamov for providing me with a copy of his modified version of the LPAIR Monte-
Carlogenerator.
Aboveall,IwouldliketothankmyparentsSusanandSteven. Withouttheirloveandsupport
I would never have made it this far. Mum, Dad: I love you both very much, thank you for
everything! I would also like to express by love and gratitude to my sisters, Heidi and Lynn,
andtomyGrandmotherElla.
Finally, I would like to dedicate this work to the memory of my Grandfather Bob who passed
away as this thesis was nearing completion. It was his influence more than anything else that
has lead me towards a life in science. His friendship, wisdom, intelligence and advice are
sorelymissed.
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Contents
1 Introduction 1
2 Theoretical context 3
2.1 TheStandardModel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 TheFundamentalParticles . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.2 MathematicalFramework . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.3 QuantumElectrodynamics . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.4 QuantumChromodynamics . . . . . . . . . . . . . . . . . . . . . . 7
2.1.5 ElectroweakTheory . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.6 QuarkmixingandCPviolation . . . . . . . . . . . . . . . . . . . . 12
2.1.7 Feynmandiagramsandperturbativecalculations . . . . . . . . . . . 14
2.2 HardscatteringprocessesattheLHC . . . . . . . . . . . . . . . . . . . . . . 15
2.2.1 Hardscatteringandthefactorisationtheorem . . . . . . . . . . . . . 15
2.2.2 Partonshowering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.3 Hadronisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3 Experimental environment 21
3.1 TheLargeHadronCollider . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1.1 Designconsiderations . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1.2 TheLHCacceleratorsystem . . . . . . . . . . . . . . . . . . . . . . 23
3.1.3 Luminosityatcollidingbeamexperiments . . . . . . . . . . . . . . . 24
3.1.4 ThedesignluminosityoftheLHC . . . . . . . . . . . . . . . . . . . 25
3.1.5 OptimalrunningluminosityatLHCb . . . . . . . . . . . . . . . . . 27
3.1.6 ExpectedLHCrunningconditionsduringstart-up . . . . . . . . . . . 28
3.2 TheLHCbexperiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2.1 ProductionofBhadronsattheLHC . . . . . . . . . . . . . . . . . . 29
3.2.2 ThelayoutoftheLHCbdetector . . . . . . . . . . . . . . . . . . . . 31
3.2.3 Thetrackingsystem . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.2.4 Trackreconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 38
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3.2.5 Theparticleidentificationsystem . . . . . . . . . . . . . . . . . . . 43
3.2.6 Muonidentification . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.2.7 Thetriggersystem . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2.8 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4 Measuring σ ·Br(Z →µ+µ−) at LHCb 57
Z
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2 Theoreticalcontext . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.3 Signalevents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.3.1 Signaleventcharacteristics . . . . . . . . . . . . . . . . . . . . . . . 63
4.3.2 Geometricacceptance . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.3.3 Reconstructionandtriggerefficiencies . . . . . . . . . . . . . . . . . 67
4.4 Backgroundprocesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.4.1 Electroweakprocesses . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.4.2 QCDprocesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.4.3 Hadronmis-identification . . . . . . . . . . . . . . . . . . . . . . . 72
4.5 Signalselectionandbackgroundreduction . . . . . . . . . . . . . . . . . . . 77
4.6 Systematicandstatisticaluncertainties . . . . . . . . . . . . . . . . . . . . . 80
4.6.1 Signalcandidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.6.2 Expectedbackgrounds . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.6.3 Acceptances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.6.4 Reconstructionefficiency . . . . . . . . . . . . . . . . . . . . . . . . 84
4.6.5 Triggerefficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.6.6 Luminosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.7 Expectedmeasurementaccuracy . . . . . . . . . . . . . . . . . . . . . . . . 87
4.7.1 Totalcross-sectionmeasurement . . . . . . . . . . . . . . . . . . . 87
4.7.2 Forwardmeasurement . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.7.3 Differentialdistributions . . . . . . . . . . . . . . . . . . . . . . . . 89
4.8 PDFsensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
4.9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5 Luminosity measurements at LHCb using pp→ p+µ+µ−+p events 95
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
5.2 Accuracyofpredictedcross-section . . . . . . . . . . . . . . . . . . . . . . 97
+ −
5.2.1 Elasticµ µ productionviaphotonfusion . . . . . . . . . . . . . . 97
+ −
5.2.2 Inelasticµ µ productionviaphotonfusion . . . . . . . . . . . . . . 98
5.2.3 Rescatteringcorrections . . . . . . . . . . . . . . . . . . . . . . . . 99