CZECH TECHNICAL UNIVERSITY IN PRAGUE Faculty of Nuclear Sciences and Physical Engineering Department of Physics Bachelor thesis Study of jet production at RHIC Martin Kocm´anek Supervisor: RNDr. Jana Bielˇc´ıkov´a,Ph.D. Prague, 2015 CESKˇ E´ VYSOKEU´ CENˇ ´I TECHNICKE´ V PRAZE Fakulta Jadern´aa Fyzik´alnˇeInˇzen´yrsk´a Katedra Fyziky Bakal´aˇrsk´apr´ace Studium produkce jet˚una urychlovaˇciRHIC Martin Kocm´anek Supervisor: RNDr. Jana Bielˇc´ıkov´a,Ph.D. Praha, 2015 Prohl´aˇsen´ı: Prohlaˇsuji,ˇzejsem svou bakal´aˇrskou pr´acivypracoval samostatnˇea pouˇziljsem pouze podklady ( literaturu, software, atd. ) uveden´ev pˇriloˇzen´emseznamu. Nem´amz´avaˇzn´yd˚uvod proti uˇzit´ıtohoto ˇskoln´ıhod´ılave smyslu 60 Z´akona .121/2000 Sb., o pr´avuautorsk´em,o pr´avech souvisej´ıc´ıch s pr´avem autorsk´yma o zmˇenˇe nˇekter´ych z´akon˚u( autorsk´yz´akon ). V Praze dne Title: Study of jet production at RHIC Author: Martin Kocm´anek Specialization: Experimental nuclear physics Sort of project: Bachelor thesis Supervisor: RNDr. Jana Bielˇc´ıkov´a,Ph.D. Abstract: Many experiments of present particle physics focus on research of a new state of strongly interacting medium called quark-gluon plasma (QGP). It is an ex- tremely hot and dense nuclear matter consisting of free quarks and gluons, which are under normal condition confined in hadrons. QGP is created and studied in ultra-relativistic heavy-ion collisions which allow to reach extreme conditions. Dur- ing early stages of a collision, partons of incoming nuclei undergo fragmentation and hadronisation which results in jets, collimated sprays of particles. Jets are an important probe of the QGP and study of their modification in medium provides information about properties of the medium. Jet-finding algorithms are used for jet reconstruction and their main goal is to cluster a set of particles measured by detec- tors into jets. The main aim of this bachelor thesis is to give a brief introduction to jet physics and overview of various jet algorithms and techniques in jet analysis. The thesis summarizes the recent results of jet analysis measured mainly by the STAR experiment at RHIC. This work also serves as my preparation for further analysis of jets in nucleus-nucleus collisions at RHIC. Key words: quark-gluon plasma, nucleus-nucleus collisions, jet, jet algorithm, STAR N´azovpr´ace: Studium produkce jet˚una urychlovaˇciRHIC Autor: Martin Kocm´anek Abstrakt: Veˇla s´uˇcasn´ych experimentov ˇcasticovej fyziky sa zameriava na v´yskum novej formy silne interaguj´ucehom´edianazvan´ehokvark-glu´onov´aplazma (QGP). Kvark-glu´onov´aplazma je veˇlmi hor´ucaa hust´ajadrov´ahmota pozost´avaj´ucaz voˇln´ych kvarkov a glu´onov, ktor´es´uza norm´alnych podmienok viazan´ev hadr´onoch. QGP je vytvoren´aa ˇstudovan´av ultrarelativistick´ych ˇtaˇzko-iontov´ych zr´aˇzkach, ktor´en´amumoˇzˇnuj´uvytvoriˇt spom´ınan´eextr´emnepodmienky. Poˇcaspoˇciatoˇcn´ych ˇst´adi´ızr´aˇzkypart´ony nach´adzaj´ucesa v zr´aˇzan´ych jadr´ach podst´upiaproces frag- ment´aciea hadroniz´acie,ˇcovedie k vytvoreniu jetov, kolimovan´ych sp´rˇsokˇcast´ıc. Jety s´ud^oleˇzit´esondy QGP a ˇst´udiumich modifik´aci´ıv m´ediu n´amposkytuje in- form´acieo vlastnostiach vytvoren´ehom´edia. Jety s´urekonˇstruovan´epomocou je- tov´ych algoritmov, ktor´ych cieˇlom je spojiˇt skupinu ˇcast´ıcnameran´ych detektorom do jetov. Hlavn´ymcieˇlom tejto bakal´arskej pr´aceje poskytn´uˇt ´uvod do jetovej fyziky, prehˇlad r^oznych jetov´ych algoritmov a sp^osobov anal´yzyjetov. Zh´rˇnaniek- tor´ened´avnev´ysledkyanal´yzyjetov uskutoˇcnen´ych predovˇsetk´ymna experimente STAR na ur´ychˇlovaˇciRHIC. T´atopr´acataktieˇzsl´uˇziako moja pr´ıprava pre bud´ucu anal´yzujetov v jadro-jadrov´ych zr´aˇzkach na RHICu. Kˇl´uˇcov´eslov´a: kvark-gluonov´aplazma, jadro-jadrov´ezr´aˇzky, jet, jetov´yalgoritmus, STAR Acknowledgement I am very grateful to my supervisor RNDr. Jana Bielˇc´ıkov´a,Ph.D. for her willingness, patience, support, professional guidance, advices, language corrections and help with this bachelor thesis. Also I would like to thank to Jan Rusˇn´akfor his advices, suggestions and introducing me into jet analysis. And finally, I am grateful to all who supported me, especially my family and friends. Contents 1 Introduction to particle physics 1 1.1 Elementary particles . 1 1.2 Fundamental interactions . 3 1.3 QCD . 4 2 Physics of high energy nucleus collisions 7 2.1 Quark-gluon plasma . 7 2.2 Basic kinematic observables . 8 2.3 Geometry of nuclear collision . 10 2.3.1 Glauber model . 11 2.4 Space-time evolution of nuclear collision . 12 2.5 Probes of QGP . 14 2.5.1 Elliptic flow . 14 2.5.2 Nuclear Modification Factor . 15 2.5.3 Jet quenching . 17 3 STAR Experiment 19 3.1 RHIC . 19 3.2 STAR detector . 20 3.2.1 Time projection chamber . 20 3.2.2 Time of flight . 22 3.2.3 Barrel Electromagnetic calorimeter . 23 3.2.4 BEMC data . 24 3.2.5 Beam Beam Counter . 24 3.2.6 Vertex Position Detector . 26 3.2.7 Muon Telescope Detector . 26 3.2.8 Heavy Flavor Tracker . 27 3.2.9 Trigger system . 27 4 Jets 29 4.1 Definition of a jet . 29 4.2 Jet levels . 30 4.3 Jet background . 31 vii 5 Jet algorithms 33 5.1 Definition . 33 5.2 Attributes of ideal jet algorithm . 33 5.3 Classes of jet algorithms . 35 5.3.1 Cone Algorithms . 35 5.3.2 Clustering algorithms . 37 5.4 Jet areas . 39 5.5 Background subtraction . 40 5.6 FastJet . 42 6 Jet analysis at RHIC 43 6.1 Di-hadron and jet-hadron correlations . 43 6.2 Jet RAA, fragmentation and cross section measurement . 46 6.3 Results from Hard Probes 2015 . 49 6.3.1 Semi-inclusive recoil jet spectra . 49 6.3.2 Dijet imbalance . 50 Summary 53 Bibliography 55 List of Figures 1.1 The Standard model of elementary particles [1]. 2 1.2 The coupling constants of QED and QCD [3]. 5 1.3 A schematic view of the confinement mechanism. The separating qq¯ pair stretches a color string until the increasing potential energy is sufficient to create another qq¯ pair. More pairs may be produced in this way, which leads to the formation of final state hadrons [4]. 5 2.1 A schematic QCD phase diagram of nuclear matter [6]. 8 2.2 Relation between the pseudorapidity η and polar angle θ [7]. 9 2.3 Geometry of a nuclear collision according to the value of the impact parameter: (a) - distant collision, (b) - peripheral collision and (c) - central collision. [5] . 10 2.4 Dependence of cross section on number of produced charged parti- cles Nch. The graph also depicts the centrality definition from the final-state particle multiplicity and its correlation with the the im- pact parameter b, the number of participating nucleons Npart in the collision, centrality classes and the ratio of cross section σ in the most central collision to total cross section σtot of produced particles [10]. 11 2.5 A schematic representation of the Glauber model geometry with lon- gitudinal (a) and transverse (b) views [11]. 12 2.6 Space time evolution of a nuclear collision [6]. 13 2.7 Evolution of a heavy ion collision [12]. 14 2.8 A schematic view of the collision zone of two incoming nuclei [14]. 15 2.9 The nuclear modification factor RAB for minimum bias and central d+Au collisions, and central Au+Au collisions measured by the STAR experiment at RHIC [17]. 16 2.10 Jet quenching [22]. 17 3.1 Relativistic Heavy Ion Collider [25]. 20 3.2 STAR detector [27]. 21 3.3 A schematic view of a Time Projection Chamber [28]. 22 3.4 The result of energy loss dE=dx distribution measurement in TPC STAR as a function of the transverse momentum pT . Measurement is used for particle identification [28]. 23 3.5 A schematic view of the BEMC module [30]. 25 3.6 Distribution of total deposited energy in η−φ plane with cut Ecut > 2 GeV. 25 ix x LIST OF FIGURES 3.7 Total deposited energy in each BEMC tower in Au+Au collisions from Run 11 data. 26 4.1 (a) Theoretical interpretation of p+p collision, (b)schematic repre- sentation of real event [35]. 30 4.2 Evolution of a jet [38]. 31 5.1 Infrared safety of a jet algorithm [37]. 34 5.2 Collinear safety of a jet alorithm [37] . 35 5.3 Scheme of the SISCone algorithm [42]. 36 5.4 Scheme of kT algorithm [37] . 39 5.5 Timings for the clustering of a simulated 50 GeV di-jet event with Pythia) [40]. 40 5.6 Comparison of areas of jet algorithms [43] . 41 6.1 A sketch of various ways of observing a hard back-to-back event ex- perimentally [49]. 44 6.2 (a) Two-particle azimuthal distribution for minimum bias and cen- tral d+Au collisions and for p+p collisions. (b) Comparison of two- particle azimuthal distributions for central d+Au collisions to those seen in p+p and central Au+Au collisions [17]. 45 6.3 A schematic diagram demonstrating the structure of the near-side jet with the ridge [50]. 45 6.4 Jet-hadron correlations after background subtraction in Au+Au and p+p collisions [18]. 46 6.5 Cross section for inclusive jet production in p+p (left) and Au+Au (right) collisions [52]. 47 6.6 Ratios of inclusive jet cross sections in p+p and Au+Au collisions. Left: jet RAA. Right: ratio of cross sections R = 0:2=R = 0:4 [52]. 48 6.7 Left: pT spectra of recoil jets in p+p and Au+Au.