Measurement of the D0 Meson Production in Pb–Pb and p–Pb Collisions: A Study Performed with the ALICE Experiment at the LHC

184 Pages·2016·8.67 MB·English

by  Andrea Festanti (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Andrea Festanti

Measurement
of the D0 Meson
Production in
Pb–Pb and p–Pb
Collisions
A Study Performed with the ALICE
Experiment at the LHC
Springer Theses
Recognizing Outstanding Ph.D. Research
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Andrea Festanti
0
Measurement of the D
–
Meson Production in Pb Pb
–
and p Pb Collisions
A Study Performed with the ALICE
Experiment at the LHC
Doctoral Thesis accepted by
the University of Padua, Italy
123
Author Supervisor
Dr. Andrea Festanti Dr. Andrea Dainese
Department ofPhysics INFN-Sezione di Padova
INFN-Sezione diPadova Padua
Universitàdegli Studi diPadova Italy
Padua
Italy
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-43454-4 ISBN978-3-319-43455-1 (eBook)
DOI 10.1007/978-3-319-43455-1
LibraryofCongressControlNumber:2016946963
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’
Supervisor s Foreword
The aim of experimental high-energy heavy-ion physics is to study strongly
interacting matter, quarks and gluons, in conditions of high density and tempera-
ture; high with respect to the conditions characterizing the ordinary nuclear matter
that constitutes the known Universe. Theoretical calculations predict that at a
critical temperature of about 160 MeV, corresponding to an energy density εc ’ 1
GeV/fm3, nuclear matter undergoes a phase transition to a deconfined state of
quarks and gluons, the quark-gluon plasma (QGP). The phase transition can be
produced in the laboratory in ultra-relativistic heavy-ion collisions, where one
attains energy densities that reach and exceed the critical value εc.
The ALICE experiment is devoted to the study of heavy-ion collisions at the
CERN Large Hadron Collider (LHC). Its goal is to use nucleus–nucleus, proton–
proton and proton–nucleus collisions at unprecedented high energy to characterize
the properties of the quark-gluon plasma. Andrea Festanti obtained his Ph.D. with
measurements of charmed-hadron production in Pb–Pb and p–Pb collisions using
data from the first LHC run.
Charm quarks are regarded as effective probes of the conditions of the QGP,
because they traverse this “medium” and interact with its constituents (quarks and
gluons), via both gluon radiation and elastic scatterings. The study of the
medium-induced suppression (nuclear modification factor R ) and of the azi-
AA
muthal anisotropy of the production yields in non-central collisions (elliptic flow
v ),providesinsightintothetransportpropertiesofheavyquarksintheQGP.This
2
includes the role of the different energy loss processes, the degree to which heavy
quarks are thermalizedand participateinthe collectiveexpansionof theQGP, and
the heavy-quark hadronization mechanisms in the presence of a high-density par-
tonic system.
Andrea carried out the first measurement of the charmed D0 meson production
relative to the reaction plane in Pb–Pb collisions and the measurement of the D0
production in p–Pb collisions. The measurement of the D0 azimuthal anisotropy
with respect to the reaction plane indicates that low-momentum charm quarks
participateinthecollectiveexpansionoftheQGP,despitetheirlargemassofabout
v
vi Supervisor’sForeword
1.5 GeV/c2. This observation can be explained by charm hadronization via
recombination with light quarks from the QGP and elastic interactions with the
expanding QGP constituents. The measurement of the D0 production in p–Pb
collisions is crucial to disentangle the effects induced by cold nuclear matter, like
thereductionofthepartondensitiesinnuclei,fromthefinal-stateeffectsinducedby
the hot QGP formed in Pb–Pb collisions. The D0 production in p–Pb collisions is
consistent with the binary collision scaling of the production in pp collisions,
demonstratingthatthemodificationofthemomentumdistributionobservedinPb–
Pb collisions with respect to pp is induced by final-state effects such as the energy
loss of charm quarks in the QGP.
In sum, the measurements described in the thesis, which are in many aspects
unique at the LHC, provide a clear indication that heavy quarks undergo strong
energy loss via interactions within the quark-gluon plasma and take part in its
collective expansion.
Padua Dr. Andrea Dainese
April 2016
Abstract
ThisthesispresentsthemeasurementofthecharmedD0mesonproductionrelative
tothereactionplaneinPb–Pbcollisionsatthecentre-of-massenergypernucleon–
pffiffiffiffiffiffiffi
nucleoncollisionof psNffiNffiffiffiffi¼ffiffi 2:76TeV,andthemeasurementoftheD0production
in p–Pb collisions at sNN ¼5:02 TeV with the ALICE detector at the CERN
Large Hadron Collider.
TheD0azimuthalanisotropywithrespecttothereactionplaneissensitivetothe
interaction of the charm quarks with the high-density strongly interacting medium
formed in ultra-relativistic heavy-ion collisions and, thus, to the properties of this
state of matter. In particular, this observable allows to establish whether
low-momentum charm quarks participatein thecollective expansion ofthe system
andwhethertheycanreachthermalequilibriumwiththemediumconstituents.The
azimuthalanisotropyisquantifiedintermsofthesecondcoefficientv inaFourier
2
expansionoftheD0azimuthaldistributionandintermsofthenuclearmodification
factorRAA,measuredinthedirectionofthereactionplaneandorthogonaltoit.The
measurement of the D0 production in p–Pb collisions is crucial to disentangle the
effectsinducedbycoldnuclearmatterfromthefinal-stateeffectsinducedbythehot
medium formed in Pb–Pb collisions.
TheD0productionismeasuredinbothsystemsbyreconstructingthetwo-prong
hadronicdecayD0!K−π+inthecentralrapidityregion,exploitingtheseparation
of the decay vertex from the primary vertex. The raw signal is obtained with an
invariant mass analysis, and corrected for selection and reconstruction efficiency.
Apositive elliptic flow v isobservedinPb–Pbcollisionsinthecentrality class
2
30–50%,withameanvalueof0:204þ0:099 intheinterval2<p <6GeV/c,which
(cid:2)0:036 T
decreases towards more central collisions. Consequently, the nuclear modification
factor shows a stronger suppression in the direction orthogonal to the reaction
plane. The v2 and the RAA measured in two azimuthal regions with respect to the
reactionplanearecomparedtotheoreticalcalculationsofcharmquarktransportand
energy loss in high-density strongly-interacting matter. The models that include
substantial elastic interactions with an expanding medium provide a good
description of the observed anisotropy.
vii
viii Abstract
The D0 nuclear modification factor RpPb in p–Pb collisions is compatible with
unity within uncertainties. The measured RpPb is compared to theoretical models
includinginitial-stateeffects,aswellastothenuclearmodificationfactormeasured
incentralPb–Pbcollisions.TheD0RpPb resultsareconsistentwiththemodification
of the nucleon parton distribution functions induced by the nuclear environment,
and provide experimental evidence that the modification of the D meson momen-
tum spectrum observed in Pb–Pb with respect to pp collisions is due to strong
final-state effects induced by the hot medium.
Acknowledgements
I would like to thank my supervisor, Andrea Dainese, for his constant support
during my Ph.D. and for being always available for discussion in case of any
questions or doubts I had in these 3 years, his suggestions were fundamental to
improve this write-up. Iamalso verygrateful toRosario Turrisi for hisexperience
and the stimulating discussions I had with him, and to all the ALICE group in
Padova,thatgavemethepossibilitytoattendschoolsandconferencesandallowed
metospendanamazing andfruitfulyearatCERN.Itwasapleasuretoworkwith
Davide on the analysis and I am very grateful to him for his help and suggestions
andforthetimespentlearninganddiscussingaboutphysics,hebecamealsoagood
friend.ThanksalsotoZaida,FrancescoandRalf,theyhavebeengreatcoordinators
andIlearntalotaboutheavy-flavourphysicsfromthediscussionswiththemduring
themeetingsoftheworkinggroup.IthankAndrea,Riccardo,CristinaandGrazia,it
was a pleasure to cooperate with them in the D meson analysis.
Iamverythankfultomyparentsfortheirloveandforallowingmetopursuemy
studies and researches, to my sister for being always with me, and to my grand-
parents for being always proud of my work even if so far from their experiences.
Last,IwishtothankmydearfriendsAndrea, Luca,Emanuela,Carlo,Emiland
Federica, they accompanied me during these years and we shared many enjoying
and relaxing moments.
ix