diffraction in alice and trigger efficiencies.

148 Pages·2011·11.39 MB·English

DIFFRACTION IN ALICE AND
TRIGGER EFFICIENCIES.
by
SPARSH NAVIN
A thesissubmittedto
TheUniversityofBirmingham
forthedegreeof
DOCTOR OFPHILOSOPHY
Supervisors: ParticlePhysicsGroup
Cristina Lazzeroni School ofPhysicsand Astronomy
Roman Lietava TheUniversityofBirmingham
July,2011
Abstract
ALICE is built to measure the properties of strongly interacting matter created in heavy-ion col-
lisions. In addition, taking advantage of the low p acceptance in the central barrel, ALICE is
T
playinganimportantroleinunderstandingppcollisionswithminimumbiastriggersatLHCener-
gies. The work presented in thisthesis isbased on pp datasimulatedby theALICE collaboration
and early datacollected at acenter-of-mass energy of7TeV.
A procedure to calculate trigger efficiencies and an estimate of the systematic uncertainty due to
the limited acceptance of the detector are shown. A kinematiccomparison between Monte Carlo
eventgenerators, PYTHIA 6, PYTHIA 8 and PHOJET isalsopresented. To improvethedescrip-
tion of diffraction in PYTHIA, a hard diffractive component was added to PYTHIA 8 in 2009,
whichisdescribed. Finallyatriggerwithahighefficiencyforpickingdiffractiveeventsisusedto
select a sample with an enhanced diffractive component from pp data. These data are compared
to MonteCarlo models, and the results are summarized with an estimateof the systematicuncer-
tainty.
Thisthesisis dedicatedto themostimportantpeopleinmy life
-Momma,Poppa, Pattyand Mano.
Acknowledgments
I would like to acknowledge that the work presented in this thesis was funded by the STFC (Sci-
ence and Technology Facilities Council, UK) and MCnet (European Union funded Marie Curie
Research TrainingNetwork).
The first two people I should thank are my supervisors, Roman Lietava and Cristina Lazzeroni,
who have helped me with work and otherwise over the last 4 years. Without the two of you, I
wouldnothavecomethisfarand finishedmythesis. Then, I shouldthankmysupervisorinLund
for 4 months, Torbjo¨rn Sjo¨strand, who spent many hours teaching me everything I know about
PYTHIA.
I havelearnt alotaboutphysicsandwork from thethreeofyou.
I would like to thank David, Lee, Peter, Gron, Anju, Anton, Pedja and Marian for help with vari-
ous thingslikecoding, proof-reading and understandingthe ALICE trigger. I would liketo make
a special mention of Orlando who has read many versions of my thesis at very short notice, and
Marek who isalways willingtohelp.
Others at Birminghamwho havehelped meare Pete, bothPauls, Nigel, Chris, Norma, Maria and
Anna. Paul Newman has answered my many questions on diffraction and provided a PDF set
includedinPYTHIA8. IdefinitelycannotforgetLawrie,whohasalwayshelpedsolvecomputing
issues with a smile. Forkeeping me sane and some good humour, thanks to everyoneI’ve shared
office space with especially Plamen, Arvinder, Ravjeet, Zoe, Angela, Chris, Tim, Dave, Owen,
Patrick and Dan. Plamen, thanks for the many installations of AliROOT. I also thank everyone
who danced and playedbadmintonwithme.
ForhelpwiththeALICEsoftwareandphysics,IwouldliketothankJanFiete,Michele,Andreas,
Peter, Jurgen, Karel, Jean-Pierre, Cvetan, Martin Poghosyan and Martin Vala. Karel, thanks also
for introducing me to the Prevessin badminton club where I found many friends and a husband.
ThankstoMichelle,Carnita,Ulla,Lucy,MaryElizabethandCeline,formakingmystayatCERN
moreenjoyable.
MCnetandLundmemberswhomademystaytherefunandfruitfulareBosse,Leif,Lisa,Richard,
Christoffer, Ilaria, Stefan, Nele, Alex, Philippe, Weina, Caroline, Lu Jie and Hendrik. Thanks to
all ofyou. Thanksto Andyforhelp withrivet.
Thanks to lecturers and tutors from the summer schools I’ve been to. I would also like to thank
York, who firstintroducedmeto particlephysics.
Finally I would like to express my gratitude to all my family and friends - old and new, for sup-
porting me in my work and life. I want to specially thank my parents (Navin and Aradhana), and
mybrother(Mano)forinspiringandsupportingmethroughoutmylife. Iwouldalsoliketothank
my husband (Patrice) from the bottom of my heart for always being there. Thank you for the
lovelypicnicbasketswhenIwasonshiftandtheweekendadventuresinLund. Ilovedmybike! I
wanttothankthefourofyouforyourencouragementandbeliefinme. Youmeantheworldtome.
Contents
Acronyms i
1 Introduction 1
1.1 First LHCPhysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 MinimumBias triggerand Efficiency . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Importanceofdiffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 ThesisOrganisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 ALICEatthe LHC 8
2.1 TheLHC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 ALICE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.1 TrackingDetectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.2 TriggeringDetectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3 TheALICE Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4 ALICE offlineand theAlirootframework . . . . . . . . . . . . . . . . . . . . . 20
2.4.1 Track reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.4.2 VertexReconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3 Hadroninteractions andMonte Carlogenerators 24
3.1 Strong interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1.1 Deep InelasticScattering . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.1.2 ReggeTheory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.1.3 Classificationofhadron-hadron interactions . . . . . . . . . . . . . . . . 30
3.2 MonteCarlo EventGenerators . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.2.1 PHOJET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2.2 PYTHIA 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.2.3 PYTHIA 6.214vs PHOJET1.12 . . . . . . . . . . . . . . . . . . . . . . 42
3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4 DiffractioninPYTHIA 8 48
4.1 Hard DiffractionKinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.1.1 Pomeron fluxparameterisations . . . . . . . . . . . . . . . . . . . . . . 51
4.1.2 DiffractivePDFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2 Eventgenerationand particleproduction . . . . . . . . . . . . . . . . . . . . . . 55
4.3 PYTHIA 8.130vs PYTHIA 6.214and PHOJET1.12 . . . . . . . . . . . . . . . 57
4.4 Diffractiveparameters vsobservables . . . . . . . . . . . . . . . . . . . . . . . 59
4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5 TriggerEfficiencies and Systematics 68
5.1 MinimumBias Triggers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.2 TriggerEfficiencies and Fractions . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.3 Systematicson multiplicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5.4 Estimate of trigger efficiency and systematics on multiplicity measurement for
ALICE publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.4.1 TriggerSelection Efficiency . . . . . . . . . . . . . . . . . . . . . . . . 79
5.4.2 Systematicuncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6 Diffractivedata inALICE 88
6.1 Selection ofadiffractivesample . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.2 Dataselection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
6.3 Track and vertexCuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.4 Systematicuncertaintyon themeasurements . . . . . . . . . . . . . . . . . . . . 97
6.4.1 Track Cuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.4.2 VertexCuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.4.3 Track-to-vertexcuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6.4.4 Final systematicuncertaintyestimates . . . . . . . . . . . . . . . . . . . 105
6.5 Kinematicdistributions-comparisonofdatawithMC . . . . . . . . . . . . . . 108
6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
7 Summary 119
Appendix I
A CollisionKinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
B DerivationoftheJacobianto convertfrom h to y . . . . . . . . . . . . . . . . . III
C c 2 testforcomparingtwounweightedhistograms . . . . . . . . . . . . . . . . . V
Bibliography VII
List of Figures
1.1 dN /dh distributionat ALICE, CMSand UA5at 900GeV. . . . . . . . . . . . 3
ch
1.2 Multiplicitydistributionat ALICE compared to different tunes of MC generators
at 900GeV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 p distribution of inelastic events in ALICE compared to different tunes of MC
T
generators at 900GeV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 ExperimentalresultsofmeanmultiplicityasafunctionofCMenergyforppandpp¯. 6
2.1 TheLHCringwithits sectorsand fourexperiments. . . . . . . . . . . . . . . . . 9
2.2 Thevariousringsusedin thepreparation ofbeamsforthe LHC. . . . . . . . . . 10
2.3 TheALICE detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4 PseudorapiditycoverageoftheALICE detector. . . . . . . . . . . . . . . . . . . 13
2.5 LayoutoftheITSdetector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.6 LayoutoftheTPCdetector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.7 Dataprocessingframework inALICE. . . . . . . . . . . . . . . . . . . . . . . . 21
3.1 Strong couplingconstant(a ) asa functionofenergy scaleQ. . . . . . . . . . . 26
s
3.2 Thespinversus m2 plotforvariousmesonsandresonances. . . . . . . . . . . . . 29
3.3 Dataand modelpredictionsforthetotalcross-sectionin ppand pp¯ interactions. . 30
3.4 Diagram forelasticscattering. . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5 Diagram foraSD process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.6 Diagram foraDD process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.7 Diagram foraCD process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.8 Diagram foraND process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.9 String beingstretchedin diffractiveprocesses. . . . . . . . . . . . . . . . . . . . 42
3.10 h distributionsforND and SD eventsat 7TeV comparingPYTHIA 6 andPHOJET. 43
3.11 Multiplicity distributions for ND and SD events at 7TeV comparing PYTHIA 6
and PHOJET. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.12 p distributionsforNDandSDeventsat7TeVcomparingPYTHIA6andPHOJET. 45
T
4.1 Exchangediagramforsinglediffraction. . . . . . . . . . . . . . . . . . . . . . . 49
4.2 DiffactivemassdistributionsinPHOJETandPYTHIAfordifferentPomeronfluxes. 53
4.3 h distributionforonesideSDeventsat7TeVinPHOJET,PYTHIA6andPYTHIA
8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.4 p distributionforSD eventsat7TeV inPHOJET,PYTHIA 6and PYTHIA 8. . 59
T
4.5 MultiplicitydistributionforSDeventsat7TeVinPHOJET,PYTHIA6andPYTHIA
8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.6 Averagenumberofcharged particles in h <0.5at differentenergies in SD events. 60
| |
4.7 Averagenumberofchargedparticlesin h <0.5atdifferentenergiesfordifferent
| |
Pomeron fluxesin SD events. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.8 Averagenumberofchargedparticlesin h <0.5atdifferentenergiesfordifferent
| |
Pomeron-protontotalcross-sectionsinSD events. . . . . . . . . . . . . . . . . . 63
4.9 Averagenumberofchargedparticlesin h <0.5atdifferentenergiesfordifferent
| |
Pomeron PDFs in SD events. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.10 Averagenumberofchargedparticlesin h <0.5atdifferentenergiesfordifferent
| |
diffractivemass thresholdsinSD events. . . . . . . . . . . . . . . . . . . . . . . 64
4.11 Averagenumberofchargedparticlesin h <0.5atdifferentenergiesfordifferent
| |
distributionsofthe p cut-offinSD events. . . . . . . . . . . . . . . . . . . . . 66
T
5.1 SystematicsplotswithMB1triggerat aCM energy of900GeV. . . . . . . . . . 79
5.2 SystematicsplotswithMB1triggerat aCM energy of7TeV. . . . . . . . . . . . 80
5.3 SystematicsplotswithMB1 triggerat aCM energy of900GeV forinelasticevents. 83
5.4 SystematicsplotswithV0AND triggerat aCM energy of900GeV forNSD events. 84
5.5 SystematicsplotswithPIX1triggerataCM energyof2.36TeVforinelasticevents. 84
5.6 SystematicsplotswithPIX1 triggerat aCM energy of2.36TeV forNSD events. 85