Abstract Jet measurements in pp and Pb{Pb collisions in ALICE Rongrong Ma 2014 Lattice-QCD predicts the existence of a new form of hot, dense matter called the Quark Gluon Plasma (QGP) above a critical energy density. Such matter is believed to be created in relativistic heavy-ion collisions, where sufficient energy is expected to be deposited by colliding ions in a limited volume. To study the properties of the QGP, high transverse momentum (pT) partons produced at the early stage of the collisions are used as probes. Since partons are not directly measurable, jet reconstruction, which assembles the hadron fragments of a parton into a \jet", provides an experimental tool to reconstruct the parton kinematics. When traversing the colored medium, partons lose energy via both induced gluon radiation and elastic scatterings. Consequently jet structure is modified relative to p jets generated in vacuum. The inclusive differential jet cross section in pp collisions at s = 2.76 TeV is measured, which serves as a reference for jet measurements in Pb{Pb collisions p at the same sNN. Two jet cone radii are used to reconstruct jets, and the ratio of the cross sections is formed. The good agreement between these results and perturbative QCD calculations at Next-to-Leading Order confirms the validity of the theoretical calculations in a new energy regime. Performing the same analysis in Pb{Pb collisions is challenging because of the large background. Therefore, a novel approach using the difference of the jet distributions recoiling from trigger hadrons in two disjoint pT intervals is developed to remove the contribution of background jets. In this thesis, azimuthal correlations between trigger hadrons and recoil jets are studied, and seen to remain essentially the same in pp and Pb{Pb collisions, implying that jets are not further deflected in the medium. These results are consistent with radiating multiple relatively soft (low pT) gluons instead of a single hard (high pT) one as the preferred way for partons to lose energy in the medium. 1 Jet measurements in pp and Pb{Pb collisions in ALICE A Dissertation Presented to the Faculty of the Graduate School of Yale University in Candidacy for the Degree of Doctor of Philosophy By Rongrong Ma Dissertation Director: John Harris May 2014 2 c 2014 by Rongrong Ma All Rights Reserved 3 Acknowledgements First to my parents, who have been there with me all along the way. You have been supportive of every choice I have made, and forgiving of every mistake I have made. I really appreciate the freedom you have given me to choose the life I want to live. I would never be able to achieve what I have today if you were not there. Most thanks to my wife Fanli, who enlightened up my life. Marrying you was the best luck I ever had, and since then my heart has a home. Thanks very much for making a colorful life in the family. There will be ups and downs in the future, but your company is my source of strength and determination. My daughter Dorothy was born during the preparation of this thesis. You are the most precious gift I have even received. You have brought so much joy and laughter to the family, and made me the happiest father in the world. I will always miss the time when you sleep like an angel in my arms. I want to express my very special thanks to the RHI group at Yale. To this day, I have felt very lucky to have chosen to join this productive, creative, and extremely friendly group when I first came to Yale. It has been the best five and half years of my life being part of the group. Many thanks to John and Helen, from whom I have learnt so much both academically and socially. You are always a model for my career, and it was my great honor to be your student. Last but not least, to Peter with whom I have worked very closely during my Ph.D. study. Your enthusiasm in research has inspired me and your adventurous, yet rigorous, attitude towards science is the quality I will insist on throughout my career. 4 Contents 1 Introduction 14 2 QCD and hard probes 24 2.1 QCD . 24 2.2 Hard probes . 26 2.3 Parton energy loss in the QGP . 28 3 Jet finding algorithm 32 3.1 Cone algorithms . 32 3.2 Sequential recombination algorithms . 35 3.3 Recombination scheme . 36 4 Experimental setup 37 4.1 LHC . 37 4.1.1 Magnetic dipole . 37 4.1.2 Magnetic quadrupole . 39 4.1.3 Radio frequency system . 43 4.1.4 Injection chain . 45 4.2 ALICE . 46 4.2.1 Inner Tracking System . 48 4.2.2 Time Projection Chamber . 53 4.2.3 Electromagnetic Calorimeter . 58 5 Jet cross section in pp collisions 64 6 Jet raw yield 65 6.1 Event selection . 65 6.2 Charged track selection . 66 6.3 EMCal cluster selection . 72 6.3.1 Cluster reconstruction and calibration . 73 6.3.2 Exotic clusters . 76 5 6.3.3 Hadronic correction . 77 6.3.4 Detailed features of EMCal clusters . 81 6.4 Jet reconstruction . 82 6.4.1 zleading cut................................. 83 6.4.2 Uncorrected jet distributions . 86 7 Trigger bias on jets 88 7.1 MB trigger . 88 7.2 EMCal SSh trigger . 89 7.2.1 Trigger bias . 89 7.2.2 Trigger acceptance . 93 8 Correction to particle level 95 8.1 Validate simulation . 95 8.2 Detector effects . 97 0 8.2.1 Unmeasured neutron and KL energy . 101 8.2.2 Tracking efficiency . 101 8.2.3 Missing secondary particles from weak decays . 104 8.2.4 Residual charged-energy double counting . 106 8.2.5 Intrinsic detector resolution . 107 8.2.6 Non-linearity due to minimum pT cuts . 108 8.2.7 zleading cut................................. 109 8.3 Underlying event subtraction . 109 8.4 Acceptance Correction . 111 8.5 Cross check via unfolding . 112 9 Integrated luminosity 114 9.1 MB data . 114 9.2 EMCal-triggered data . 115 10 Systematic uncertainties 116 10.1 Systematic uncertainty of the JES . 116 6 10.1.1 Tracking efficiency . 117 10.1.2 Hadronic correction . 117 10.1.3 Sensitivity to clustering algorithm . 118 10.1.4 EMCal energy scale . 119 10.1.5 EMCal non-linearity . 120 10.1.6 Model dependence of fragmentation . 121 0 10.1.7 Correction for unmeasured neutrons and KL's . 124 10.1.8 UE subtraction . 124 10.2 Systematic uncertainty of the jet yield . 125 10.2.1 Input spectrum shape . 125 10.2.2 Trigger efficiency . 126 10.2.3 Track pT resolution . 128 10.2.4 EMCal energy resolution . 128 10.2.5 Cross section normalization . 128 10.3 Total uncertainty . 129 11 Corrected pp jet cross section 131 11.1 Comparison to pQCD calculations . 131 11.2 Jet cross section ratio . 133 12 Hadron-jet coincidence measurements in Pb{Pb collisions 136 12.1 Motivation . 136 12.2 Azimuthal correlation from data . 139 12.3 PYTHIA reference . 142 12.4 Systematic uncertainties . 142 12.5 Comparison between data and PYTHIA reference . 145 13 Summary & discussion 146 Appendices 149 A Analysis setup for measurement of the jet cross section in pp collisions 149 7 B Analysis setup for hadron+jet coincidence measurement in Pb{Pb colli- sions 151 C Acronyms 153 8 List of Figures 1 QCD phase diagram . 14 2 Lattice-QCD calculation . 15 3 Dynamic evolution of a heavy-ion collision . 16 4 Charged particle pseudo-rapidity density . 18 5 Spectrum of direct photons . 19 6 Elliptic flow . 19 7 Thermal fit to particle yields . 21 8 Tfo vs hβTi .................................... 22 9 HBT correlations . 22 10 QCD running coupling . 24 11 Charged hadron RAA measured by ALICE . 28 12 Energy loss mechanism . 29 13 Collinear unsafe algorithms . 33 14 Infrared unsafe algorithms . 34 15 LHC layout . 38 16 Coordinate system for particles in a synchrotron . 38 17 Electromagnetic field in a quadrupole . 40 18 A FODO cell . 40 19 Variables to describe transverse motion in a synchrotron . 41 20 Effects of quadrupoles analogous to optical lenses . 42 21 Motion of particles in a RF system . 44 22 LHC injection chain . 46 23 ALICE layout . 47 24 ITS layout . ..