Prompt D production in proton-proton and lead-lead collisions, measured with the ALICE ...

150 Pages·2013·27.95 MB·English

Prompt D∗+ production in proton-proton
and lead-lead collisions, measured with the ALICE experiment
at the CERN Large Hadron Collider
DirecteD∗+ productieinproton-proton
enlood-loodbotsingen,gemetenmethetALICEexperiment
aandeCERNLargeHadronCollider
(meteensamenvattinginhetNederlands)
Proefschrift
terverkrijgingvandegraadvandoctoraandeUniversiteitUtrechtopgezagvanderector
magnificus,G.J.vanderZwaan,ingevolgehetbesluitvanhetcollegevoorpromotiesin
hetopenbaarteverdedigenopdinsdag14mei2013om18.00uur
door
RaoulStefandeRooij
geborenop16januari1981
teUtrecht
Promotores: Prof. dr.R.J.M.Snellings
Prof. dr. R.Kamermans
(cid:134)
Co-promotores: Dr. A.Mischke
Dr. A.Grelli
Dit proefschrift werd (mede) mogelijk gemaakt met financile steun van de Neder-
landseOrganisatievoorWetenschappelijkOnderzoek(NWO).
Iffirstyoudon’tsucceed,useabiggergun.

Outline
Inthisthesistheresultsarepresentedoftheworld’sfirstmeasurementsoftheD∗+me-
sonnuclearmodificationfactorR inheavyioncollisionsattheLargeHadronCol-
AA
lider(LHC)usingtheALICE(ALargeIonColliderExperiment)detectoratCERN,
usingdataaccumulatedover2010and2011forbothlead-leadandproton-protoncol-
lisions.
Chapter1givesanqualitativeoverviewofthehistoryandtheoreticalbackground
relevanttothisthesis,aswellastheresultsfrompreviousexperiments.Italsocontains
asummaryofvarioustheoreticalmodelsusedtodescribethedata.Thesecondchapter
focusses on the design and construction of the ALICE detector, and also includes a
description of the analysis framework used for data analysis. The analysis strategy
used for D∗+ reconstruction via the D∗+ →D0p+ →K−p+p+ decay channel is
soft soft
discussed in detail in chapter three. Here an important aspect is the application of
selectioncutsandtheiroptimisationtoimprovesignalsignificance.Theimportanceof
theproton-protonresultsasareferenceisfurtherhighlighted. Inchapterfourthe p -
T
differentialinclusiv√eproductioncrosssectionofpromptD∗+ mesons,intherapidity
range|y|<0:5,in s=7TeVproton-protoncollisionsispresented. Inchapterfive
the p -differential inclusive production yield of prompt D∗+ mesons, in the rapidity
T √
range|y|<0:5andthep range2-36GeV/c,in s =2:76TeVlead-leadcollisions
T NN
isshown. Thenuclearmodificationfactor,withrespecttotheproton-protonreference
obtainedandscaleddowntoacentre-of-massenergyof2.76TeV,isthendetermined
and compared with several model based theoretical predictions. The conclusions of
thisworkwillbediscussedinchaptersix.
The results have been published in various papers [72, 73, 74] and presented at
international conferences (e.g. [115]). A colour version of this thesis will become
availableonline.

Contents
1 Heavyionphysics 7
1.1 Abriefhistory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 QCDandtheStandardModel. . . . . . . . . . . . . . . . . . . . . . 13
1.3 HeavyioncollisionsandtheQGP . . . . . . . . . . . . . . . . . . . 17
1.4 SignaturesoftheQGP . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.5 Heavyflavourproduction . . . . . . . . . . . . . . . . . . . . . . . . 26
1.6 Opencharmproduction . . . . . . . . . . . . . . . . . . . . . . . . . 28
1.7 Theoreticalmodels . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2 TheALICEdetector 39
2.1 Generaloverview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.1.1 InnerTrackingSystem . . . . . . . . . . . . . . . . . . . . . 41
2.1.2 TimeProjectionChamber . . . . . . . . . . . . . . . . . . . 44
2.1.3 Time-Of-Flight . . . . . . . . . . . . . . . . . . . . . . . . . 45
2.1.4 Triggering. . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.2 Theanalysisframework . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.3 Trackingandvertexing . . . . . . . . . . . . . . . . . . . . . . . . . 50
3 D∗+analysis 53
3.1 D∗+reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.1.1 D0reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 54
3.2 Selectioncuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.2.1 Eventselection . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.2 Trackselection . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.3 Topologicalcuts . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.4 Particleidentification . . . . . . . . . . . . . . . . . . . . . . 60
3.3 Cutoptimisation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.3.1 Themultidimensionalmethod . . . . . . . . . . . . . . . . . 61
3.4 D∗+yieldextraction . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.4.1 Invariantmassanalysis . . . . . . . . . . . . . . . . . . . . . 63
3.4.2 Signalextraction . . . . . . . . . . . . . . . . . . . . . . . . 64
3.5 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.6 Proton-protonreference . . . . . . . . . . . . . . . . . . . . . . . . . 67
5
CONTENTS CONTENTS
4 Proton-protonanalysis 69
4.1 D∗+rawyield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.2 D∗+reconstructionefficiency . . . . . . . . . . . . . . . . . . . . . . 75
4.3 Systematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.4 D∗+productioninproton-proton . . . . . . . . . . . . . . . . . . . . 80
4.5 Comparison7and2.76TeVdata . . . . . . . . . . . . . . . . . . . . 82
5 Pb-Pbanalysis 87
5.1 D∗+analysisusing2010datasample . . . . . . . . . . . . . . . . . . 87
5.1.1 Rawyieldextraction . . . . . . . . . . . . . . . . . . . . . . 89
5.1.2 Efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
5.1.3 Systematicuncertainties . . . . . . . . . . . . . . . . . . . . 91
5.1.4 PromptD∗+yielddistribution . . . . . . . . . . . . . . . . . 93
5.1.5 Nuclearmodificationfactor . . . . . . . . . . . . . . . . . . 94
5.2 D∗+analysisusing2011datasample . . . . . . . . . . . . . . . . . . 95
5.2.1 Topologicalcuts . . . . . . . . . . . . . . . . . . . . . . . . 95
5.2.2 Rawyieldextraction . . . . . . . . . . . . . . . . . . . . . . 97
5.2.3 Efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.2.4 Systematicuncertainties . . . . . . . . . . . . . . . . . . . . 100
5.2.5 Nuclearmodificationfactor . . . . . . . . . . . . . . . . . . 104
6 Conclusions 109
6.1 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
A Backgroundreconstruction 113
A.0.1 Datasetandanalysisprocedure . . . . . . . . . . . . . . . . . 114
A.1 D0reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
A.2 Like-signpairmethod . . . . . . . . . . . . . . . . . . . . . . . . . . 115
A.2.1 Resultsfromthelike-signpairmethod . . . . . . . . . . . . . 116
A.2.2 Resultsfromthelike-signpairmethodusingD0selectioncuts 117
A.3 Multiple-rotationsmethod . . . . . . . . . . . . . . . . . . . . . . . 121
A.3.1 Descriptionofthemethod . . . . . . . . . . . . . . . . . . . 121
A.3.2 Detectorinefficienciesandeventtopologybias . . . . . . . . 122
A.3.3 Consideringdetectorinefficienciesformultiple-rotationsback-
ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
A.3.4 Resultsfromthemultiple-rotationsmethod . . . . . . . . . . 125
A.3.5 Results from the multiple-rotations method after D0 recon-
structioncuts . . . . . . . . . . . . . . . . . . . . . . . . . . 127
A.3.6 Comparisonbetweenmultiple-rotationsandlike-signpairmeth-
ods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
A.3.7 Resultsfordifferentrotationparameters . . . . . . . . . . . . 131
A.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6
Chapter 1
Heavy ion physics
An important aspect of man’s culture and history has been his need to contemplate
his place in the universe. One of those many enigmas that has interested mankind
is the question of ’what is matter’? Throughout the ages many great thinkers have
ponderedthisquestion. Asatributetotheeffortsofthesemenandwomen, thefirst
sectionofthischaptercontainsabriefsummaryofhumanity’svoyageofdiscoveryto
understandthefundamentalsofmatter,uptothepresentday.
ThenexttwosectionsdescribeQuantumChromodynamicsanditsroleintheStan-
dard Model. Then the principle of heavy ion collisions is discussed as well as the
Quark Gluon Plasma (QGP). The signatures for the existence of the QGP are de-
tailed, as well as the actual measurements performed by the Relativistic Heavy Ion
Collider at Brookhaven National Laboratory (USA). The physical properties of az-
imuthalanisotrophyandjetquenchingintheQGParediscussed. Finally, aspectsof
the main topic of this work will be introduced; the study of D meson production in
proton-proton and lead-lead collisions, which can be used to probe the properties in
theQuarkGluonPlasma.
1.1 Towards Elementary Particle Physics, a brief his-
tory
For centuries mankind has wondered about the nature of matter. This has led to in-
spired,yetflawedinterpretationsliketheclassicalelementsoffire,water,earthandair
duringtheGreekclassicalperiod,thatstayedpopularwellintotheRenaissance. Yet
evenintheClassicalperiodthephilosophersLeucippus,DemocritusandEpicurusar-
guedthatthepropertiesofamaterialcouldbetracedbacktotheirsmallestundividable
components.
Thelackofempiricalevidenceonewayortheotherensuredthatfortwomillennia
thequestionofthestructureofmatterwaspurelyphilosophical.1 Eventheworksof
Descartes, Gassendi, andNewtoninthethe17thcenturyfocussedontheinterpreta-
tion of the resurgent theory of small indivisible components, or atoms. Fortunately,
1Ofcourse,upuntilthenineteenthcenturyallscientificthinkingwasconsideredtobephilosophy.The
currentsegregationofscienceandphilosophyisarathermoderndevelopment.
7
1.1.Abriefhistory CHAPTER1.Heavyionphysics
mankindbecameinterestedandexperiencedinchemistryinthe18thcentury. Using
measurements and experiments those early chemists catalogued the properties of a
greatmanymaterialsandprocesses,pavingthewayfortheworksofDaltonandAvo-
gadrowhowouldstartofhumanity’scurrentunderstandingofatomsandmolecules.
As the knowledge of atoms and molecules grew, so too grew the realisation that
atoms are not the smallest possible components and even smaller elements existed.
Thestudyofthese’elementaryparticles’beganwhenThomsondiscoveredtheelec-
tronin1897withitsnegativeelectricchargeanditsextremelysmallmass,forwhich
he correctly surmised that these particles were part of the atom. Though Thomson
was proven incorrect about the origin of the positive charge to compensate for the
electrons’negativemassintheneutralatom,Rutherfordproveddecisivelythatatoms
haveinternalstructurewithhisfamousscatteringexperimentwherehediscoveredthe
presence of the atomic nucleus. This led to the discovery of the positively charged
and heavy proton and the heavy but neutral neutron. With these three particles in
place and a satisfactory model to describe them the period sometimes known as the
’classicalperiod’ofelementaryparticlephysicswasconcluded[1].
However, startingwiththe1930sthe’middleperiod’began, drawingknowledge
not only from the discoveries of the ’classical period’ of particle physics, but also
on the revolutionary changes wrought by the discoveries of quantum mechanics and
general relativity. The discovery of the photoelectric effect brought turmoil to the
age-old question whether light was a wave or particle phenomena, introducing the
photonandtheconceptofenergyquanta. Thestudyofblack-bodyradiationshowed
conclusively that the ’classical’ picture of electrons orbiting nuclei was flawed and
quantisationofclassicalsystemswasrequired.
Naturally,therealisationthatanatomicnucleusconsistlargelyofpositiveparticles
gave rise to the question what ’strong force’ was keeping the nuclei together, which
apparently only interacted at very short distances. It was Yukawa who linked this
’strongforce’withamediatingparticlewhichhecalledthemeson,whichwasmore-
or-lessconfirmedbyPowelletal. in1947whenhiscollaborationdiscoveredboththe
pionandmuonparticles[2,3,4,5].
Theoriginalversionofquantummechanicswasbasedonaformalismofclassical
timeandspace,butevolvingitintoarelativisticversiongaverisetosomefar-reaching
realisations.ThisstartedwhenDiracdiscoveredtheexistenceofnegativeenergystates
andhis’fix’ofproposingtheDiracsea,whereeverynegativeenergystateisalready
filledbyaparticle.Thisofcourseledtotheideathatanelectroncouldbeexcitedfrom
this’sea’creatinga’hole’withsimilarpropertiesbutoppositechargeoftheelectron.
ThisideainturnwasmigatedbytheinterpretationofFeynmanandStuckelbergofthe
existenceofanactualantiparticle. Atanyrate,thediscoveryofthepositronin1931
canberegardedasatriumphofelementaryparticlephysics.
Thethirdpillarthatdefinedthe’middleperiodofelementaryparticlephysics’is
the realisation (and discovery) of the existence of another particle, which would ac-
countforthemissingkinematicenergyoftheelectroninbetadecays. Fermiincorpo-
ratedthisparticleproposedbyPauliandnamedittheneutrino.Thisverylightparticle
couldalsobeusedtoexplaintheawkward’kink’visibleinthebubblechamberswhen
a pion or muon decayed. Despite this indirect evidence, one had to wait until the
mid-1950swhenCowanandReinesconfirmedtheneutrino’sexistence. Thisdidnot
answer yet whether the neutrino and its anti-neutrino were different particles or one
8