Dijet Angular Distributions in Proton-Proton Collisions: At √s = 7 TeV and √s = 14 TeV

175 Pages·2012·4.26 MB·English

by  Nele Boelaert (auth.)

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Nele Boelaert
Dijet Angular Distributions
in Proton–Proton Collisions
p p
ffiffi ffiffi
At s = 7 TeV and s = 14 TeV
Doctoral Thesis accepted by
Lund University, Sweden
123
Author Supervisor
Dr. NeleBoelaert Prof.Torsten Åkesson
Department of Physics Department of Physics
LundUniversity LundUniversity
22100Lund 22100Lund
Sweden Sweden
e-mail: [email protected] e-mail: [email protected]
ISSN 2190-5053 e-ISSN2190-5061
ISBN 978-3-642-24596-1 e-ISBN978-3-642-24597-8
DOI 10.1007/978-3-642-24597-8
SpringerHeidelbergDordrechtLondonNewYork
LibraryofCongressControlNumber:2011940787
(cid:2)Springer-VerlagBerlinHeidelberg2012
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Supervisor’s Foreword
OneofthemostcommonexperimentalmethodsinPhysicsistoilluminateatarget,
observethepatternofthescattering,andlearnsomethingaboutthestructureofthe
target.AfamousexampleisofcoursetheexperimentofRutherfordin1911,when
he illuminated a gold foil with alpha particles and discovered the atomic nucleus.
A later example is the electron scattering experiment using the Stanford Linear
Accelerator which led to the discovery of the quarks inside the protons and
neutrons. The highest resolution is obtained with the highest energy particles,
and today, those are to be found at the Large Hadron Collider (LHC) at CERN.
Inthehistoricexamplesabove,thetargetswereatrestinthelaboratory,whilethe
scatteringisinadifferentrest-frameatacollider.Nevertheless,theprincipleisthe
same. By studying the angular distribution of the scattered quarks and gluons, at
theLHC,theirpossiblestructureandtheforcesactingbetweenthem,areexplored.
This was the thesis topic of Dr Boelaert.
In 2006 the Physics Department at Lund University got the opportunity to
launch a graduate school in High Energy Physics, thanks to a Marie Curie grant
from the 6th framework programme of the European Union. It allowed seven
students to start as graduate students. The students came from Belgium, China,
Denmark, Italy, Romania and UK, and studied and did their research together in
Lund during four years. I believe that this has given them and us, the permanent
staff, an experience for life. Nele Boelaert came to this group with a degree in
EngineeringPhysicsfromtheTechnicalUniversityinGent/BelgiumandIhadthe
privilege to become her supervisor.
Myinteractionswithherwereasequenceofpositivesurprisesinlearninghow
muchshehaddone,thequalityofwhatshehaddone,andherinnovativeapproach.
N. Boelaert started before the LHC became operational, and the original plan
wasthatafterayearofcoursesandayearofresearchpreparation,shewouldhave
a few years of data from the LHC as basis for her thesis. However, like many
frontier projects, the LHC encountered some delays. This gave her time to enter
deeperintothephenomenologicalaspectsofthemeasurementstobedone,butalso
reduced the prospects of the scientific potential of the data that would be at hand
forthethesis.Nevertheless,theLHCstartedduringthelastyearofherpositionin
v
vi Supervisor’sForeword
Lund allowing her to get results from the first, but limited, data from this new
facility.Thisdatawasjustatinyfirststepintothevastspacethatthecomingyears
of LHC operation will allow to be explored.
Her analysis has shown how well these measurements can be done, how well
ourcurrenttheory,theStandardModel,describestheenteringpointintothespace
openeduptheLHC,andhasshowntheprecisionoftheunderstandingofATLAS
at the LHC that has been built up by the scientific collaboration that constructed
this facility. Nele Boelaert’s thesis has set the best possible stage to pursue this
explorationintheyearstocomeastheLHCintegratesluminosityandincreasesits
energy. Her thesis is very well structured and written, and provides an excellent
introduction for new graduate students to come.
Lund, August 2011 Torsten Åkesson
Acknowledgements
I would not have been able to complete this thesis without the help and influence
of many people.
Firstofall,IwishtothankmymainsupervisorTorsten,becausehissuggestion
to work on dijet angular distributions has resulted into a very fascinating and
versatile research project.
Else became my co-supervisor nearly two years ago. Her advice concerning
my research and work in the ATLAS collaboration has been extremely helpful.
Else has given me the necessary confidence to grow and stand up inside the
collaboration.
MywarmestgratitudegoestoTorbjörn.Asmytheoreticalco-supervisor,hehas
ledmesuccessfullythroughmyfirstphenomenologystudy.Buthishelphasgone
farbeyondthat;hehasalwaysbeeneagertosharehisverybroadexperiencewith
me, explaining me everything I wanted to know, and giving me insight in how to
proceed with my work. I have taken up so many hours of his time without him
showing the least bit of impatience, and for all this I am very grateful.
I also wish to thank everybody who has been involved in setting up and
organizing the Lund-HEP EST graduate school, which has given me the oppor-
tunity to start a PhD in high energy physics.
PartofmyworkconsistedofusingaMonteCarloparticlegeneratorwhichwas
createdbyLeif.Iappreciateverymuchthetimehetookforhelpingmeuseitand
interpret the results.
Furthermore, I want to thank the ATLAS collaboration for integrating me and
givingmescopetodevelopandlearn.Inparticular,IwishtothankthosepeopleI
havecloselyworkedwithonthedijetanalysis:PO,Frederik,Lukas,Georgiosand
Sing.Ourgrouphasbeenverysuccessful,withresultsofthemeasurementsbeing
published only a few months after the data had been recorded.
My work on ATLAS data quality was under the supervision of Michael, and I
wish toexpressmy gratitude tohim because he has taughtme well inthe field of
software development and the ATLAS offline framework.
HereinLund,IwishtothankAnders,theheadofthedivisionofExperimental
High-Energy Physics, because he has given me excellent feedback on my thesis.
vii
viii Acknowledgements
Notonlywasheaverygoodlistenertoanykindofproblembuthealsoalwaysdid
whatever he could to help solving them.
Nowadays computers cannot be left out when studying high energy physics.
Eventhoughtheyaresuchagreataid,theyoftendon’tdowhatyouwantthemto
do.Fortunately,wheneverIhadaproblemthatseemedunsolvable,Richardwould
helpmeout.Butmoreimportantly,asmysignificantother,hehascaredformein
a much broader context.
FinallyIwishtothankmyparents.Theyhavebeenaconstantandgreatsupport
throughout my whole life. Without them, I wouldn’t even have started this PhD
study.
ThisthesisworkwaspartlyfinancedfromtheMarieCurieMobility-2actionof
theEuropeanUnion6thframeworkprogramme.SwedishparticipationinATLAS
was granted by the Swedish Research Council (VR) and the Knut and Alice
Wallenberg foundation (KAW).
Abstract
Dijetangulardistributionsprovideanexcellenttoolforlookingathightransverse
momentum parton interactions in order to study both QCD and new physics
processes. With the Large Hadron Collider (LHC) recently brought into use, an
unprecedentedenergyregimehasopenedup.ATLASisoneoftheexperimentsat
the LHC. Its high performance calorimeter system providing near hermetic cov-
erageinthepseudorapidityrange|g|\4.9,enablesATLAStoperformreliablejet
p
measurements. Detailed Monte Carlo studies at ffisffi = 14 TeV, the LHC design
p
collision energy, and at ffisffi = 7 TeV, the collision energy foreseen for the initial
years of the LHC operation, clearly show that dijet angular distributions can be
used to discriminate the Standard Model from a new physics model describing
gravitationalscatteringandblackholeformationinlargeextra dimensions.When
considering only the shape of the distributions, both the theoretical and the
experimental uncertainties are predicted to be small in those regions where new
p
physicsisexpectedtoshowup.Thestudyat ffisffi=7TeVindicatesthatATLASis
already sensitive to large extra-dimensional gravity mediated effects with 1 pb-1
p
of data. The first measurement of dijet angular distributions at ffisffi = 7 TeV with
ATLAS was carried out in two mass bins, using data that were recorded early
2010, corresponding to an integrated luminosity of about 15 nb-1. The measure-
mentshowsgoodagreementwithQCDpredictionsanddemonstratesthatATLAS
isreadytosearchfornewphysicsinthedijetangulardistributionswithmoredata.
ix
List of Publications Related to this Thesis
p
1. Dijet angular distributions at ffisffi = 14 TeV
By N. Boelaert
Published in Proceedings of Science (EPS-HEP 2009) 298
2. Softwaredesignforpromptassessmentoftime-varyingdataquality
ByN.Boelaert,M.D’Onofrio,A.Dotti,C.Guyot,M.Hauschild,R.Hawkings,
B. Heinemann, A. H¨ocker, V. Kazazakis, E. Lytken, M. Mar´tınez-Perez,
R.McPherson,P.U.E.Onyisi,A.Schaetzel,R.SeusterandM.G.Wilson
PublishedasATLASinternalnote,ATL-COM-GEN-2010-002-Geneva:CERN,
2010
3. Implementation of the GravADD generator in Athena
By N. Boelaert
Published as ATLAS internal note, ATL-PHYS-INT-2010-012-Geneva:
CERN, 2010
p
4. Dijet angular distributions at ffisffi = 14 TeV
By N. Boelaert and T. Åkesson
arXiv:0905.3961 [hep-ph]
Published in The European Physical Journal C, 33, (2010) 343–357
5. ATLAS sensitivity to contact interactions and large extra dimensions using
p
dijet events a ffisffi = 7 TeV
By N. Boelaert, G. Choudalakis, P. O. Deviveiros, E. Feng, H. Li, J. Poveda,
L. Pribyl, F. Ruehr and S. L. Wu
Published as ATLAS internal note, ATL-COM-PHYS-2010-136-Geneva:
CERN, 2010
p
6. High-pT dijet angular distributions in pp interactions at ffisffi = 7 TeV measured
with the ATLAS detector at the LHC
By N. Boelaert, R. Buckingham, S. L. Cheung, G. Choudalakis, T. Davidek,
P. O. De-Viveiros, E. Feng, J. Frost, M. Kaneda, H. Li, H. Peng, L. Pribyl,
M. Shupe, K. Terashi, S. L. Wu
Published as ATLAS internal note, ATL-COM-PHYS-2010-359-Geneva:
xi