Czech Technical University in PragueFaculty of Nuclear Sciences and Physical

Engineering

Diploma Thesis

Study of di-hadron correlations in d+Aucollisions in the STAR experiment

Prague, 2011 Author: Bc. Michal Sloboda

Supervisor: RNDr. Jana Bielckov, Ph.D.

Prehlsenie

Prehlasujem, e som svoju diplomov prcu vypracoval samostatne a pouil somlen podklady (literatru, projekty, SW atd.) uveden v priloenom zozname.

Nemm zvan dvod proti pouitiu tohto kolskho diela v zmysle 60 Zkonac.121/2000 Sb., o prve autorskom, o prvach svisiacich s prvom autorskm a ozmene niektorch zkonov (autorsk zkon).

V Prahe dna

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Acknowledgements

First of all, I am very grateful to my supervisor Jana Bielckov for her invaluablehelp, movation, patience, and support during creation of this work. I am also verythankful to Jan Kapitn for his precious advices and to Jan Rusnk for language

corrections.

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Nzov prce: tdium di-hadronovch korelci v d+Au zrkach v experi-mente STAR

Autor: Bc. Michal Sloboda

Obor: Jadrov ininierstvo

Zameranie: Experimentlna jadrov fyzika

Druh prce: Diplomov prca

Vedci prce: RNDr. Jana Bielckov, Ph.D., stav jadern fyziky Akademie vedCesk republiky, v. v. i.

Konzultant: Mgr. Jan Kapitn, stav jadern fyziky Akademie ved Cesk repub-liky, v. v. i.

Abstrakt: Azimutlne korelcie s jednou z metd pouvanch pri tdiu pro-dukcie jetov na tatistickej bzi v ultra-relativistickch zrkach takch iontov.Cielom diplomovej prce je aplikcia dvoj-casticovch korelci na dta z d+Auzrok pri energii

sNN= 200 GeV nameranch v experimente STAR na urchlo-

vaci RHIC v roku 2008. Korekcia korelacnch funkci, charakteristika korelacnchvrcholov a urcenie velicn jT a kT , ktor charakterizuj vlastnosti jetov.

Klcov slov: dvoj-casticov korelcie, STAR, jadro-jadrov zrky, hadrny, jety

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Title: Study of di-hadron correlations in d+Au collisions in the STAR experi-ment

Author: Bc. Michal Sloboda

Field of study: Nuclear Engineering

Specialization: Experimental Nuclear Physics

Sort of project: Diploma thesis

Supervisor: RNDr. Jana Bielckov, Ph.D., Nuclear Physics Institute of the ASCR,p. r. i.

Consultant: Mgr. Jan Kapitn, Nuclear Physics Institute of the ASCR, p. r. i.

Abstract: Azimuthal correlations are one of the method used for statistical obser-vation of jet production in ultra-relativistic heavy ion collisions. The aim of thisdiploma thesis is application of two-particle correlations to data from d+Au colli-sions at energy

sNN= 200 GeV measured in the STAR experiment at RHIC accel-

erator in 2008. Correction of correlation functions, characterization of correlationpeaks and evaluation of values of jT and kT , which characterize the jet properties.

Keywords: two-particle correlations, STAR, nucleus-nucleus collisions, hadrons,jets

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Contents

1 Preface 9

2 Introduction 102.1 Quark Gluon Plasma . . . . . . . . . . . . . . . . . . . . . . . . . 102.2 Heavy ion collisions . . . . . . . . . . . . . . . . . . . . . . . . . 122.3 Jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3.1 Measurements of single-particle inclusive spectra . . . . . . 152.3.2 Azimuthal correlations . . . . . . . . . . . . . . . . . . . . 162.3.3 Jet reconstruction . . . . . . . . . . . . . . . . . . . . . . . 19

2.4 Jet properties from di-hadron correlations . . . . . . . . . . . . . . 20

3 RHIC 253.1 The STAR experiment . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1.1 Time Projection Chamber (TPC) . . . . . . . . . . . . . . . 283.1.2 Barrel Electromagnetic Calorimeter (BEMC) . . . . . . . . 293.1.3 Time of Flight (TOF) . . . . . . . . . . . . . . . . . . . . . 293.1.4 Beam Beam Counter (BBC) . . . . . . . . . . . . . . . . . 303.1.5 Vertex Position Detector (VPD) . . . . . . . . . . . . . . . 303.1.6 Zero Degree Calorimeter (ZDC) . . . . . . . . . . . . . . . 303.1.7 Heavy Flavor Tracker (HFT) . . . . . . . . . . . . . . . . . 31

4 Data Analysis 324.1 Data sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324.2 Azimuthal correlation method . . . . . . . . . . . . . . . . . . . . 334.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.3.1 Properties of correlation functions . . . . . . . . . . . . . . 344.3.2 Width of near- and away-side peaks . . . . . . . . . . . . . 384.3.3 The near- and away-side yields . . . . . . . . . . . . . . . . 404.3.4 jT measurements . . . . . . . . . . . . . . . . . . . . . . . 464.3.5 kT measurements . . . . . . . . . . . . . . . . . . . . . . . 484.3.6 Comparison to other measurements . . . . . . . . . . . . . 50

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CONTENTS CONTENTS

5 Summary and Conclusion 51Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

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Chapter 1

Preface

One of the most dynamic branches of nuclear physics is heavy-ion physics. Stud-ies of hot and dense nuclear matter allow us to experimentally explore and identifythe processes that took place in the first few moments of existence of the Universe.In the last decade, the experiments at the Relativistic Heavy Ion Collider (RHIC) atBNL1 have investigated properties of this hot and dense matter.

In central Au+Au collisions at

sNN2 = 200 GeV, the experiments at RHIC ob-serve a large suppression of production of high transverse momentum (pT ) particles,in comparison with the p+p and d+Au data [15]. The tool to study this suppressionon a statistical base are azimuthal correlations between high-pT trigger particle andlower pT associated particles. In this diploma thesis di-hadron correlations in d+Aucollisions at

sNN = 200 GeV are investigated using data measured by the STAR

experiment. Lighter systems, such as p+p and d+A collisions, do not reach theenergy densities required for QGP3 formation, and provide means to quantify thecold nuclear matter effects. A quantitative understanding of the cold nuclear mattereffects is essential when investigating hot nuclear matter effects and trying to under-stand the QGP. Characterization of correlation peaks and measurements of jT , kTvalues and study of some of their properties in the d+Au collisions is the main goalof this thesis.

1Brookhaven National Laboratory2energy of reaction per nucleon-nucleon pair3Quark-Gluon Plasma

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Chapter 2

Introduction

2.1 Quark Gluon PlasmaThe Quantum Chromodynamics (QCD) is a fundamental theory describing stronginteractions among quarks and gluons. QCD has two special features: confinementand asymptotic freedom.

Confinement refers to the fact that the force between quarks does not allowthem to be free. Quarks have another quantum number - color. Quarks color maybe one of three states: "red", "blue" and "green". In agreement with the QCDquarks can not be separated singularly, they form only colorless objects - baryons(3 quarks: RGB) and mesons (quark-antiquark: RR, GG, BB). The force, that bindsquarks increases with distance, if the distance between quarks reaches 1 fm, thenthe stored potential energy is sufficient to create a new quark-antiquark pair (me-son). It follows that it is not possible to release the color quark, because it wouldrequire an infinite amount of energy.

Asymptotic freedom means that in very high-energy reactions, quarks and glu-ons interact very weakly. This means that in close proximity they behave almost asfree particles. This behavior can be observed in high energy collisions, where largemomentum is transferred.

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2 Introduction 2.1 Quark Gluon Plasma

Figure 2.1: A schematic phase diagram of strongly interacting matter.

The nuclear matter exists under normal conditions in the form of protons andneutrons, each containing three valence quarks, virtual quark-antiquark pairs andgluons. Finite radius of nucleons is about 0.87 fm and an average density is about0.35 GeV/ f m3. The value of nuclear density in the nuclei is generally smaller thanthe density of a single neutron and amounts to about 0.15 GeV/ f m3, which indicatesthat the nucleons are well separated and do not overlap.

The nuclear matter diagram is shown in Figure 2.1. Quark-gluon plasma (QGP)is a new phase of matter, in which quarks and gluons are not restricted by the phe-nomenon of color confinement and they are free. Calculations of the lattice QCDexpect the phase transition from nuclear matter to quark-gluon plasma at the criticaltemperature T 170 MeV which corresponds to the energy density of 1 GeV/ f m3.The revelation of the new state of matter was announced by a press release on the10th of February, 2000 [11].

Under certain conditions of high temperature T and/or high density the QGPcan be found in three places:

(i) in the early Universe

(ii) at the center of compact stars

(iii) in the initial stage of colliding heavy nuclei at high energies

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2 Introduction 2.2 Heavy ion collisions

(i) QGP existed in the early Universe until about 105 s after the Big Bang. Atthe time 105 104 s after the Big Bang the QGP in space underwent the phasetransition.

(ii) At the cores of superdense stars such as quark stars and neutron stars. Inthe center of neutron stars density is sufficient to generate a cold quark matter, thismatter mostly consists of u, d, and s quarks.

(iii) In the initial stage of the "Little Bang" at relativistic nucleus-nucleus colli-sions. When the center of mass energy per nucleon exceeds 100 GeV, the collidingnuclei pass through each other and produce matter with high energy density andhigh temperature [2].

2.2 Heavy ion collisionsHeavy ion collisions are very useful experimental instrument to e