Modification of K0s and Lambda(AntiLambda) Transverse Momentum Spectra in Pb-Pb Collisions at √sNN = 2.76 TeV with ALICE

216 Pages·2016·12.35 MB·English

by  Simone Schuchmann (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Simone Schuchmann

0
Modification of K and
s
–
Λ(Λ) Transverse Momentum
Spectra in Pb–Pb Collisions
—
at √s = 2.76 TeV with
NN
ALICE
Springer Theses
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Simone Schuchmann
fi 0 ΛðΛÞ
Modi cation of K and
s
Transverse Momentum
–
Spectra in Pb Pb Collisions
pffiffiffiffiffiffiffi
¼ :
at s 2 76 TeV
NN
with ALICE
Doctoral Thesis accepted by
Goethe University Frankfurt, Frankfurt, Germany
123
Author Supervisor
Dr. Simone Schuchmann Prof. HaraldAppelshäuser
Institut für Kernphysik Frankfurt Department ofPhysics
Goethe-University Frankfurt Institut fürKernphysik Frankfurt
Frankfurt Goethe-University Frankfurt
Germany Frankfurt
Germany
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-43457-5 ISBN978-3-319-43458-2 (eBook)
DOI 10.1007/978-3-319-43458-2
LibraryofCongressControlNumber:2016947028
©SpringerInternationalPublishingSwitzerland2016
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In den Experimenten über Atomvorgänge
haben wir mit Dingen und Tatsachen zu tun,
mit Erscheinungen, die ebenso wirklich sind
wie irgendwelche Erscheinungen im
täglichen Leben. Aber die Atome oder die
Elementarteilchen sind nicht ebenso wirklich.
Sie bilden eher eine Welt von Tendenzen und
Möglichkeiten als eine von Dingen und
Tatsachen.
Werner Heisenberg: Physik
und Philosophie 1959
’
Supervisor s Foreword
Quark-gluon plasma (QGP) is the most elementary type of matter we know. Its
relevantdegreesoffreedomarequarksandgluons,thefundamentalbuildingblocks
in the Standard Model of particle physics. Creation of QGP occurs when the
temperatureofmatterexceeds1012K,andthedeconfinementofquarksandgluons
inside hadrons is lifted. This continuum state of quantum chromodynamics
(QCD) filled the entire universe in the first microseconds after the Big Bang and
may still exist in the interior of dense massive stellar objects like neutron stars.
Intherecentdecades,thestudyofQGPbecamepossibleatparticleaccelerators
wherelargenucleiarecollidedathighenergies.Mostrecently,attheLargeHadron
Collider (LHC) at CERN, lead ions collide at centre-of-mass energies in the PeV
range. Under such conditions, nuclear matter is heated and compressed, and hot
QGP is created. A detailed characterization of the QGP properties is the ultimate
goal of the CERN heavy-ion programme.
ALICE is a dedicated detector at the LHC for the study of QGP. The ALICE
detector is designed to measure the final-state products of lead–lead collisions,
which allow to infer the properties of the matter created in the collision. A key
measurement isthatofhadrons with large transverse momenta(p ),whichemerge
T
asfinal-stateproductsofhardparton–partonscatteringsthatoccurintheearlystage
of the lead–lead collisions. In vacuum, this process is well understood and
describedintheframeworkofperturbativeQCD.Inlead–leadcollisions,however,
the scattered partons interact with the dense and colour-charged QGP, leading to a
significant energy loss by elastic collisions and gluon radiation, and finally to a
dramatic suppression of high-p hadrons in the final state. A quantitative assess-
T
ment of the emerging suppression patterns is instrumental in determining the
colour-charged transport coefficient of QGP.
The energy-loss formalism itself is not yet fully established. A significant
dependenceonthemassandcolourchargeofthepartons,implyingdifferentenergy
loss of light (up, down and strange) and heavy quarks (charm and bottom) is
predicted by models. In particular, the energy loss of gluons is expected to exceed
that of quarks due to their larger effective colour charge. A detailed experimental
vii
viii Supervisor’sForeword
surveyofthesuppressionpatternofdifferenthadronspecies,includingbaryonsand
mesons with different quark content, may unravel the underlying mechanism and
shedlightonthepropertiesofQGP.Inparticular,theroleofstrangenessathighp
T
has not yet been thoroughly explored experimentally.
In this thesis, SimoneSchuchmann presents transverse momentum distributions
of Λ, Λ(cid:1) and K0 in lead–lead collisions, measured with the ALICE detector at the
s
LHC. In order to quantify the suppression of these hadrons with strange quarks at
high p in lead–lead collisions, she also constructed a reference baseline from
T
measurements in proton–proton interactions. The results presented in this thesis
significantly expand existing measurements of baryons and mesons with strange
quarks, in particular with respect to their statistical and systematic quality. Simone
describes the details of an impressive analysis work that contains several original
andinnovativeapproaches.Simonedemonstratesthathadronswithstrangenessare
just as suppressed at high p as hadrons with up and down quarks only. She also
T
confirmsthatthesuppressionpatternofbaryonsiscompatiblewiththatofmesons.
This result comes as something of a surprise, since baryons and mesons were
expectedtoshedlightonpossiblydifferentmagnitudeofenergylossofquarksand
gluons, emerging from their different colour coupling strength. In a careful com-
parison of her data to recent energy-loss calculations, she demonstrates that this
similarityappearsasaconsequenceofaninterplaybetweenthespecificenergyloss
of quarks and gluons, and their characteristic fragmentation pattern into baryons
andmesons.Theseresultsclearlysupporttheperturbativenatureoftheunderlying
energy-loss mechanism.
Simone’s thesis is extremely rich in technical and physics content, reaching far
beyondamerestudyofhigh-p suppressionofstrangehadrons.Itsformalqualityis
T
impressive,anditsscientificachievementshavealargeimpactonourresearchfield.
Simone’s thesis has received the ALICE thesis award and will remain a reference
work for many studies to follow.
Frankfurt am Main Prof. Harald Appelshäuser
May 2016
Abstract
Measurements of the transverse momentum (p ) spectra of K0andΛðΛÞ in Pb–Pb
pffiffiffiffiffiffiffi T s
and pp collisions at sNN ¼2:76TeV with the ALICE detector at the LHC at
CERNuptop =20GeV/candp =16GeV/c,respectively,arepresentedinthis
T T
thesis. In addition, the particle rapidity densities at mid-rapidity and nuclear
modification factors of K0andΛðΛÞ are shown and discussed.
s
TheanalysiswasperformedusingthePb–Pbdatasetfrom2010andtheppdata
set from 2011. For the identification of K0andΛðΛÞ, the on-the-fly V0 finder was
s
employed on tracking information from the TPC and ITS detectors. The Λ and Λ
spectra were feed-down corrected using the measured published Ξ(cid:2) spectra as
input.
Regarding the rapidity density at mid-rapidity, a suppression of the strange
particle production in pp as compared to Pb–Pb collisions is observed at all cen-
tralities, whereas the production per pion rapidity density stays constant as a
function of dNch=dη including both systems. Furthermore, the relative increase
oftheindividualparticlespeciesinppandAAcollisionsiscompatiblefornon-and
pffiffiffiffiffiffiffi
single-strange particles when going from RHIC ( sNN ¼0:2TeV) to LHC ener-
gies.Ontheotherhand,incaseofmulti-strangebaryons,astrongerincreaseinthe
particle production in pp is seen. The Λ and Λ production in Pb–Pb and pp
collisions was found to be equal. Concerning the nuclear modification factors, at
lowerp (p <5GeV),anenhancementoftheR ofΛwithrespecttothatofK0
T T AA s
andchargedhadronsisobserved.Thisbaryon-to-mesonenhancementappearingin
central Pb–Pb collisions at RHIC and LHC is currently explained by the interplay
of the radial flow and recombination as the dominant particle production mecha-
nism in this p sector. The effect of radial flow is thus also seen in the low and
T
intermediate p region of R , where a mass hierarchy is discovered among the
T AA
baryonsandmesons,respectively,withtheheaviestparticlebeingleastsuppressed.
WhencomparingtheresultsfromRHICandLHC,theR isfoundtobesimilarat
CP
low-to-intermediate p , while a significantly smaller R of K0 and Λ in central
T AA s
and peripheral events at the LHC is observed in this p region as compared to the
T
RHICresults.ThiscanbeattributedtothelargerradialflowinAAcollisionsandto
ix
x Abstract
the harder spectra at the LHC. At high p (p > 8 GeV/c), a strong suppression in
T T
central Pb–Pb collisions with respect to pp collisions is found for K0andΛðΛÞ.
s
A significant high-p suppression of these hadrons is also observed in the ratio of
T
central-to-peripheral collisions. The nuclear modification of K0andΛðΛÞ is com-
s
patible with the modification of charged hadrons at high p . The calculations with
T
the transport model BAMPS agree with these results suggesting a similar energy
loss for all light quarks, i.e. u, d and s. Moreover, a compatible suppression for
c-quarksappearsintheALICEmeasurementsviatheDmesonR aswellasinthe
AA
BAMPScalculations,whichhintstoaflavour-independentsuppressioniflight-and
c-quarks are regarded. Within this consideration, no indication for a
medium-modified fragmentation is found yet.
To summarize, for the particle production in Pb–Pb collisions at the LHC rel-
ative to pp neither at lower p (rapidity density) nor at higher p (nuclear modi-
T T
fication factor), a significant difference of K0andΛðΛÞ carrying strangeness to
s
hadrons made of u- and d-quarks was found.