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 Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. 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More information about this series at http://www.springer.com/series/8790 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 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authorsortheeditorsgiveawarranty,expressorimplied,withrespecttothematerialcontainedhereinor foranyerrorsoromissionsthatmayhavebeenmade. Printedonacid-freepaper ThisSpringerimprintispublishedbySpringerNature TheregisteredcompanyisSpringerInternationalPublishingAGSwitzerland 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.