Jet Quenching in Relativistic Heavy Ion Collisions at the LHC

180 Pages·2014·8.805 MB·English

by  Aaron Angerami (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Aaron Angerami

Jet Quenching in
Relativistic Heavy
Ion Collisions at
the LHC
Springer Theses
Recognizing Outstanding Ph.D. Research
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Aaron Angerami
Jet Quenching in Relativistic
Heavy Ion Collisions
at the LHC
Doctoral Thesis accepted by
Columbia University, New York, USA
123
Author Supervisor
Dr. Aaron Angerami Prof.BrianCole
PhysicsDepartment PhysicsDepartment
Columbia University Columbia University
New York, NY New York, NY
USA USA
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-01218-6 ISBN 978-3-319-01219-3 (eBook)
DOI 10.1007/978-3-319-01219-3
SpringerChamHeidelbergNewYorkDordrechtLondon
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For Vincent Angerami, my grandfather
and my biggest fan
Supervisor’s Foreword
In2003theUSNationalResearchCouncilinitsreport,‘‘ConnectingQuarksWith
The Cosmos: Eleven Science Questions for the New Century’’ posed what it
considered the most important outstanding questions in physics and astronomy.
One of those questions is ‘‘Are there new states of matter at ultra-high tempera-
tures and densities?’’ Implicit in the question was the understanding that if the
answer is ‘‘yes,’’ then studying and understanding the properties of the new state
matter was of equal importance.
Sincethe above questionwas posed,experimentsat theRelativisticHeavy Ion
Collider (RHIC) and subsequently, the Large Hadron Collider (LHC) have
definitively answered it in the affirmative by demonstrating the creation of a new
state of matter, ‘‘quark-gluon plasma’’ in very high energy nuclear collisions.
Ordinarily, quarks andgluons,the fundamentalparticles ofthe strong interaction,
are confined inside the protons and neutrons that make up ordinary matter.
However, at temperatures in excess of 1012 K (roughly 100,000 times the core
temperature of the sun) that are briefly attained in the nuclear collisions, ordinary
mattermeltsproducingasystemofinteractingquarksandgluons.Experimentsat
RHIC and the LHC have not only provided first convincing evidence that the
quark-gluon plasma is created in high-energy nuclear collisions, they also have
shown that the plasma has unexpectedly strong coupling between its quark and
gluon constituents causing the plasma to behave like nearly perfect fluid.
To better understand the quark-gluon plasma, experiments have exploited a
method commonly used in particle physics, namely probing its properties using
high-energy particles. In this case the high-energy particles are also quarks and
gluons, produced in large-momentum-transfer scattering processes in the initial
stages of nuclear collisions. Normally, such high-energy quarks will produce a
collimated spray of particles referred to as a ‘‘jet.’’ But the quarks and gluons
produced in nuclear collisions can lose a substantial amount of energy while
propagating through the quark-gluon plasma, causing the jet to become ‘‘quen-
ched.’’ Since the quenching results from the direct interaction of the high-energy
quarks with the plasma, experimental studies of the quenched jets can provide
insight on the plasma properties. Measuring full jets in nuclear collisions is dif-
ficult, because the jets can be obscured by the many thousands of unassociated
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viii Supervisor’sForeword
particlescreatedinthecollisions.However,thehighenergiesofnuclearcollisions
at the LHC produces jets with sufficient energy that they stand out from the
background.
This thesis provides two early, seminal measurements of jet quenching using
fullyreconstructedjetsinnuclearcollisionsattheLHC.Theobservationofevents
withhighlyasymmetricjetpairswasthefirstjetquenchingmeasurementmadeat
theLHC.ItwaspublishedwithinthreeweeksofthestartofthefirstLHClead-lead
run and was both the topic of a Physical Review Letters Viewpoint article and
selected forthecoverofPhysicalReviewLetters. Thesecondanalysisinvolveda
moredetailedstudyofjetquenchingbymeasuringtheimpactofthejetquenching
on jet production rates. By providing crucial measurements that will ultimately
providedeeperinsightintothepropertiesofthequark-gluonplasma,thisthesishas
directly helped answer one of the ‘‘Eleven Science Questions for the New
Century.’’
New York, September 2013 Prof. Brian Cole
Abstract
Jet production in relativistic heavy ion collisions is studied using Pb ? Pb colli-
sions at a center of mass energy of 2.76 TeV per nucleon. The measurements
reportedhereutilizedatacollectedwiththeATLASdetectorattheLHCfromthe
2010 Pb ion run corresponding to a total integrated luminosity of 7 lb-1. The
resultsareobtainedusingfullyreconstructedjetsusingtheanti-k algorithmwitha
t
per-event background subtraction procedure. A centrality-dependent modification
of the dijet asymmetry distribution is observed, which indicates a higher rate of
asymmetricdijetpairsincentralcollisionsrelativetoperipheralandppcollisions.
Simultaneously, the dijet angular correlations show almost no centrality depen-
dence. These results provide the first direct observation of jet quenching. Mea-
surementsofthesingleinclusivejetspectrum,measuredwithjetradiusparameters
R = 0.2, 0.3, 0.4, and 0.5, are also presented. The spectra are unfolded to correct
for the finite energy resolution introduced by both detector effects and underlying
event fluctuations. Single jet production, through the central-to-peripheral ratio
R , is found to be suppressed in central collisions by approximately a factor of
CP
two, nearly independent of the jet p . The R is found to have a small but
T CP
significant increase with increasing R, which may relate directly to the aspects of
radiative energy loss.
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Acknowledgments
I would like to thank CERN and my collaborators in ATLAS for successful
operation of the LHC and data taking with our detector. I am indebted my thesis
committee—GustaafBrooijmans,JianweiQiu,JamieNagle,MiklosGyulassy,and
BrianCole—fortheirparticipationinmymostsignificanteducationalexperience.
I would also like to thank the various faculty of the Physics Department at
Columbiawhohavetaughtmeandwhohavebeenextremelyaccessibletocurious
students. I would like to thank my high school physics teacher, Mr. Arnold, for
introducingmetophysics.Ifirmlybelievethatyoulearnthemostfromyourpeers,
and I would like to thank the many physics students from whom I have had a
chancetolearnsomething.Specialthanksaredueformyclassmatesandfriends—
Anshuman Sahoo, Rob Spinella, David Tam, and Fabio Dominguez.
The story of the heavy ion program in ATLAS is very much a story of an
underdog. I am grateful to Professors Zajc and Hughes for their unwavering
support of this effort, especially when things looked bleak. I would like to thank
Martin Spousta and Martin Rybar, my teammates working on jets through this
wholeexperience.IoweaspecialthankstoPeterSteinbergforprovidingmewith
guidance during my time as a graduate student and for his commitment to the
ATLAS heavy ion effort; any success from this effort would not have been pos-
siblewithouthisextraordinarydetermination.Ioweagreatdebtofgratitudetomy
thesis adviser Brian Cole, who took a chance on mentoring a precocious student
many years ago. He has continually inspired me with his extraordinary commit-
ment to science. From him, not only I have learned a great deal of physics, but
more importantly I have learned what it means to be a physicist.
I have the benefit of having a large family that has provided me with uncon-
ditional support and encouragement for as long as I can remember. My grand-
parents have always shown so much pride in my accomplishments; seeing this
pride has brought me great joy. I would like to thank my dog Tito for being
unconditional and uncomplicated. My brother Matteo has supported me by
showing a fierce sense of loyalty and pride for his big brother. My parents have
always made sure I had what I needed to pursue my dreams, from indulging my
obsessions andinterestswhenI was young,tobuilding mysense ofself-belief by
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