Jet-like heavy-flavour particle correlations in proton-proton and lead-lead collisions in ALICE

165 Pages·2014·7.15 MB·English

Jet-like heavy-flavour particle
correlations in proton-proton and
lead-lead collisions in ALICE
ISBN:978-90-393-6152-8
AcataloguerecordisavailablefromtheUtrechtUniversityLibrary
This work is licensed under a Creative Commons Attribution-Non Commercial 3.0
UnportedLicence(CCBY-NC3.0)
Jet-like heavy-flavour particle
correlations in proton-proton and
lead-lead collisions in ALICE
Jet-achtige correlaties van zware quark
deeltjes in proton-proton en lood-lood
botsingen in ALICE
(met een samenvatting in het Nederlands)
Proefschrift
ter verkrijging van de graad van doctor aan de Universiteit
Utrecht op gezag van de rector magnificus, prof.dr. G.J. van der
Zwaan, ingevolge het besluit van het college voor promoties in
het openbaar te verdedigen op maandag 16 juni 2014 des
middags te 4.15 uur
door
Deepa Thomas
geboren op 14 februari 1987 te Mananthavadi, Kerala, India
Promotor: Prof. dr. T. Peitzmann
Co-promotor: Dr. A. Mischke
This thesis was accomplished with financial support from the European Re-
search Council (ERC) and the Nederlandse Organisatie voor Wetenschappelijk
Onderzoek(NWO).
Contents
1 Introduction 1
1.1 Heavyioncollisions . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Heavy-flavourproduction . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Experimentalobservables . . . . . . . . . . . . . . . . . . . . . . . 7
1.4 Azimuthalanisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5 Nuclearmodificationfactor . . . . . . . . . . . . . . . . . . . . . . 9
1.6 Modificationofjet-liketwoparticlecorrelationyield . . . . . . . . 11
1.7 Relativebeautyfractiontotheheavy-flavourdecayelectronyield 18
1.8 Theoreticalmodelcalculations . . . . . . . . . . . . . . . . . . . . . 20
2 Experimentalsetup 25
2.1 TheLargeHadronCollider . . . . . . . . . . . . . . . . . . . . . . . 25
2.2 TheALICEdetector . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3 Offlinecomputation 37
3.1 ClusterfindingintheEMCal . . . . . . . . . . . . . . . . . . . . . . 38
3.2 Primaryvertexdeterminationandtracking . . . . . . . . . . . . . 40
3.3 Centralitydetermination . . . . . . . . . . . . . . . . . . . . . . . . 42
4 Analysisstrategy 45
4.1 Datasetandeventselection . . . . . . . . . . . . . . . . . . . . . . 46
4.2 Correlationanalysisprocedure . . . . . . . . . . . . . . . . . . . . 47
4.3 Trackselection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.4 Electronidentification . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.5 Non-heavyflavourdecayelectronbackground . . . . . . . . . . . 60
4.6 Non-heavyflavourdecayelectronreconstructionefficiency . . . . 64
4.7 Azimuthalangulardistribution . . . . . . . . . . . . . . . . . . . . 69
iii
5 Relative beauty fraction to the heavy-flavour decay electrons in pp
collisions 75
√
5.1 ∆φdistributioninppcollisionsat s = 2.76and7TeV . . . . . . 76
5.2 Relativebeautyfractiontotheheavy-flavourdecayelectronyield 76
5.3 Beautydecayelectroncross-section . . . . . . . . . . . . . . . . . . 80
5.4 Estimationofsystematicuncertainty . . . . . . . . . . . . . . . . . 86
5.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
√
6 Near-sideparticlecorrelationyieldinppcollisionsat s = 7TeV 99
√
6.1 Validityofusingppcollisionsat s=7TeVasbaseline . . . . . . . 100
6.2 Trackingefficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
6.3 Near-sideyield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
6.4 Systematicuncertainties . . . . . . . . . . . . . . . . . . . . . . . . 103
6.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7 ModificationoftheparticlecorrelationyieldinPb-Pbcollisions 109
7.1 Azimuthalangulardistribution . . . . . . . . . . . . . . . . . . . . 110
7.2 Flowcontribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
7.3 Near-sideyield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
7.4 Systematicuncertainties . . . . . . . . . . . . . . . . . . . . . . . . 119
7.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
8 Conclusions 131
8.1 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
A PerugiaTune 135
Bibliography 137
Acknowledgements 145
Summary 149
Samenvatting 153
CurriculumVitae 157

Chapter 1
Introduction
Thefieldofparticlephysicsisabranchofphysicsthatstudiesthefundamental
constituents of matter at sub-atomic scale and their interactions. The current
understandingofthefundamentalparticlesandfieldsandtheirdynamicscomes
from the theory called Standard Model. It describes the electromagnetic, weak
and strong nuclear interactions. According to this model, matter is made up of
quarks and leptons and they interact via gauge bosons (such as photons and
gluons). The theory of electroweak interaction gives a unified description of
electromagnetism and weak interactions. The strong nuclear interaction is de-
scribedbytheQuantumChromodynamics(QCD).
Herewediscussthestrongnuclearinteractionsthatquarksandgluons(together
named as partons) undergo. According to the theory of QCD, all quarks and
gluonscarryasocalledcolourcharge. Quarkscarryoneofthethreeelementary
colourcharges,red,green,andblue,andgluonscarryamixtureofacolourand
an anticolour charge. Gluons are the gauge bosons, which mediate the strong
interactionsbetweenquarksbycouplingtothecolourcharge. Thisisanalogous
totheelectromagneticinteraction,inwhichphotonsareexchangedbetweenthe
interactingparticles. Sincegluonsalsocarrycolourcharge,gluonsinteractwith
other gluons unlike the electrically neutral photons in the electromagnetic in-
teractions.
Isolatedcolouredparticles(quarksandgluons)havenotyetbeenobserved. Ac-
cording to QCD, this is because of the phenomenon called confinement, where
quarks and gluons are confined inside colourless particles called hadrons. The
interactionstrengthbetweenquarksisgivenbythecouplingconstant,α ,which
s
1
2
FIGURE 1.1: Layout of the strongly-interacting matter phase diagram as it is
conceivedtoday. Image: BrookhavenNationalLaboratory.
increases with distance between the quarks, unlike electromagnetic interac-
tions. The energy required to separate two quarks is sufficient to produce new
quark-antiquarkpairstoformnewhadrons,whicharecolourless. Butatsmall
distances or at high energies, the strength of the strong interaction decreases.
Thisphenomenoniscalledasymptotic freedom ofQCD[1].
The behavior of matter, which undergoes the strong nuclear interactions, as a
functionofthebaryonicchemicalpotential(ameasureofthebaryonicdensity)
andofthetemperatureisdisplayedinthephasediagramshownin FIGURE 1.1.
Atlowtemperatureandchemicalpotential,quarksareconfinedinsidehadrons
to form a hadron gas. When the energy density of the system is increased, by
compression (towards the right) or by heating (upward), a phase transition is
predicted, where the density of partons becomes so high that the confinement
of coloured quarks in colourless hadrons vanishes and a deconfined state of
matter,theQuark-GluonPlasma(QGP)isformed[2,3]. Thisdeconfinedstateof
matter is analogous with the electromagnetic plasma, where ions and electrons
aredissociated.