Measurement of pions, kaons and protons with the ALICE detector in pp collisions at the LHC Meting van pions, kaons en protons met de ALICE detector in pp botsingen in de LHC (met een samenvatting in het Nederlands) Proefschrift ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rector magnificus, G.J. van der Zwaan, ingevolge het besluit van het college voor promoties in het openbaar te verdedigen op donderdag 3 mei 2012 des ochtends te 12.45 uur door Marek Chojnacki geboren op 23 januari 1983 te Radom, Polen Promotor: Prof. dr. R.J.M. Snellings Prof. dr. R. Kamermans ( ) † Co-promotor: Dr. ir. G.J.L. Nooren Dr. ir. M. van Leeuwen Dit werk maakt deel uit van het onderzoekprogramma van de Stichting voor Fundamenteel Onderzoek der Materie (FOM), die financieel wordt gesteund door de Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). Contents 1 Theory 9 1.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2 Quantum Chromodynamics (QCD) . . . . . . . . . . . . . . . . . 9 1.3 Modelling of pp collisions . . . . . . . . . . . . . . . . . . . . . . 11 1.3.1 Classification of collisions and their mechanism . . . . . . 11 1.3.2 Lund string fragmentation model . . . . . . . . . . . . . . 14 1.3.3 Event generators . . . . . . . . . . . . . . . . . . . . . . . 15 1.4 Phenomenologicalmodels . . . . . . . . . . . . . . . . . . . . . . 16 1.4.1 Thermal model . . . . . . . . . . . . . . . . . . . . . . . . 16 1.4.2 Spectra in non-extensive thermodynamics . . . . . . . . . 16 1.4.3 Collective behaviour in proton-protoncollisions . . . . . . 20 1.4.4 The Blast Wave model . . . . . . . . . . . . . . . . . . . . 20 1.5 Increasing the energy of pp collisions . . . . . . . . . . . . . . . . 21 2 Experiment set-up and offline framework 23 2.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.2 ALICE detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.2.2 ITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.2.3 TPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.2.4 TOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.2.5 VZERO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.3 DAQ system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.4 Software tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.4.1 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.4.2 Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 29 2.4.3 Offline pile-up tagging procedure . . . . . . . . . . . . . . 30 3 The energy loss signal in the SSD 31 3.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.2 The SSD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.2.1 Principle of operation . . . . . . . . . . . . . . . . . . . . 31 3.2.2 The SSD set-up . . . . . . . . . . . . . . . . . . . . . . . . 31 3.2.3 SSD operation . . . . . . . . . . . . . . . . . . . . . . . . 33 3.2.4 SSD reconstruction . . . . . . . . . . . . . . . . . . . . . . 33 3.2.5 SSD simulation . . . . . . . . . . . . . . . . . . . . . . . . 34 3.3 Calibration method . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.3.1 Charge matching calibration . . . . . . . . . . . . . . . . 34 3 4 CONTENTS 3.3.2 Calibration of the energy loss signal . . . . . . . . . . . . 35 3.4 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.4.1 First calibration using the test set-up . . . . . . . . . . . 36 3.4.2 Calibration based on data from the 2009 run . . . . . . . 36 3.4.3 Calibration based on data from the 2010 run . . . . . . . 37 3.4.4 MPV as a function of the track length . . . . . . . . . . . 38 3.5 Performance studies . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.5.1 Calibration at the level of a single HAL25 chip . . . . . . 38 3.5.2 Charge matching vs. charge . . . . . . . . . . . . . . . . . 41 3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4 Method 45 4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 4.2 Data sample and event selection . . . . . . . . . . . . . . . . . . 45 4.2.1 Data sample . . . . . . . . . . . . . . . . . . . . . . . . . 45 4.2.2 Event selection . . . . . . . . . . . . . . . . . . . . . . . . 46 4.3 Measurement of the p spectra . . . . . . . . . . . . . . . . . . . 46 t 4.3.1 Definition of the energy loss signal in the ITS . . . . . . . 47 4.3.2 Track selection . . . . . . . . . . . . . . . . . . . . . . . . 48 4.3.3 Determination of the dE/dx parametrization . . . . . . . 49 4.3.4 Extraction of the raw yields in p bins . . . . . . . . . . . 50 t 4.3.5 Tracking and PID efficiency . . . . . . . . . . . . . . . . . 53 4.3.6 Contamination . . . . . . . . . . . . . . . . . . . . . . . . 56 4.3.7 TPC-ITS matching correction . . . . . . . . . . . . . . . . 59 4.3.8 Interactions with the detector material . . . . . . . . . . . 61 4.3.9 Event efficiency . . . . . . . . . . . . . . . . . . . . . . . . 64 4.3.10 Pile-up correction . . . . . . . . . . . . . . . . . . . . . . 65 4.4 Evaluation of systematic uncertainties . . . . . . . . . . . . . . . 65 4.4.1 Fitting procedure . . . . . . . . . . . . . . . . . . . . . . . 65 4.4.2 Contamination correction . . . . . . . . . . . . . . . . . . 67 4.4.3 Track cut variations . . . . . . . . . . . . . . . . . . . . . 71 4.4.4 Material budget . . . . . . . . . . . . . . . . . . . . . . . 72 4.4.5 Total uncertainty . . . . . . . . . . . . . . . . . . . . . . . 73 4.5 Final spectra and uncertainties . . . . . . . . . . . . . . . . . . . 73 4.6 Negative to positive ratios . . . . . . . . . . . . . . . . . . . . . . 78 4.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 5 Combined Spectra 81 5.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 5.2 Other methods of measurement . . . . . . . . . . . . . . . . . . . 81 5.2.1 ITS standalone (ITSsa) . . . . . . . . . . . . . . . . . . . 81 5.2.2 TPC-TOF combined measurement (TPCTOF) . . . . . . 82 5.2.3 TOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 5.3 Combined spectra in pp collisions at √s=7 TeV . . . . . . . . . 82 5.3.1 Combining spectra . . . . . . . . . . . . . . . . . . . . . . 82 5.3.2 L´evy-Tsallis fits. . . . . . . . . . . . . . . . . . . . . . . . 84 5.4 Calculation of p and integrated yields . . . . . . . . . . . . . . 87 t h i 5.5 Comparison with data at lower energies . . . . . . . . . . . . . . 87 5.6 Comparison with the event generators . . . . . . . . . . . . . . . 91 5.7 Blast Wave fits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 CONTENTS 5 5.7.1 Interpretation of the Blast Wave fits . . . . . . . . . . . . 97 5.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 6 Multiplicity studies 99 6.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 6.2 Two multiplicity estimators . . . . . . . . . . . . . . . . . . . . . 99 6.3 Selection bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 6.4 Correction for the selection bias . . . . . . . . . . . . . . . . . . . 103 6.5 Spectra in multiplicity bins . . . . . . . . . . . . . . . . . . . . . 105 6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 7 Conclusions 111 A Spectra at √s=900 GeV using the 2010 data sample 113 B Collision kinematics 115 6 CONTENTS Outline The research reported in this thesis was performed using data collected by A Large Ion Collider Experiment (ALICE) during the first year of the Large Hadron Collider (LHC) operation. The main subject is a measurementof pion, kaon, proton and antiprotonproduction as a function of transverse momentum (p spectra) in proton-proton collisions. The particle identification is based on t the energy loss in the Inner Tracking System (ITS), a silicon detector. Chap- ter 1 is an introductory chapter, which presents the theoretical background of the particle production in proton-proton collisions. Chapter 2 contains the de- scription of the ALICE detector. In Chapter 3 the calibration procedure of the Silicon Strip Detector (part of the ITS) is described. In Chapter 4 the method of the particle production measurement is described, which was used for two energies of the colliding protons (√s=7 TeV and √s=900GeV). In Chapter 5 the7TeVresultsfromChapter4arecomparedandcombinedwithresultsfrom other methods of particle identification. The combined result is comparedwith theoretical models, predictions of event generators and previous experimental results. In Chapter 6 the method from Chapter 4 is used to measure the spec- tra as a function of the event multiplicity. Chapter 7 is the concluding chapter which also contains a brief discussion of possible new measurements. Theworkinthis thesiswaspresentedattwointernationalconferences[1,2]. 7 8 CONTENTS Chapter 1 Theoretical background and previous experimental results 1.1 Introduction In this chapter our theoretical understanding of some of the basic properties of proton-protoncollisions is discussed, in particular the productionof low energy pions, kaons, protons and antiprotons. Some models which describe heavy– ion collisions are presented because they can also be applied to proton-proton collisions. This chapter is organized as follows. Section 1.2 introduces basics properties of Quantum Chromodynamics and the proton structure. In Section 1.3 a short description of the proton-proton collision modelling is presented. In Section 1.4 some of phenomenological models used to describe the particle production in proton-proton and heavy–ion collisions are introduced. It also contains a brief discussion of the possible collective behaviour of the system produced in proton-proton collisions. Section 1.5 presents open issues for the LHC data concerning the particle productions. 1.2 Quantum Chromodynamics (QCD) Quantum Chromodynamics (QCD) is the part of the StandardModel, the cur- rently accepted theory of particle physics, describing the strong interaction be- tween quarks and gluons (partons). Gluons are bosons which carry the strong interaction between quarks. There are six known types of quarks, listed in Ta- ble 1.1. Each quark has one out of three possible QCD charges. The QCD chargeislabelledusingcolour: red(r),green(g)andblue (b). Antiquarkshave an anticolour. The interaction in QCD is invariant under a SU(3) transforma- tion in colour space (SU(3) colour symmetry). To each gluon a combination of colourandanticolourisassigned. Thereexistsnineofsuchcombinations. They are divided into a singlet and an octet state according to the properties of the SU(3) group symmetry. Experimental results show that gluons belong to the octet state. 9 10 CHAPTER 1. THEORY Table 1.1: Classification of quarks name mass electric charge in e units down (d) 4.1-5.7 MeV/c2 -1/3 up (u) 1.7-3.1 MeV/c2 2/3 strange (s) 100 MeV/c2 -1/3 ∼ charm (c) 1.29 GeV/c2 2/3 ∼ bottom (b) 4.2 GeV/c2 -1/3 ∼ top (t) 172.9 GeV/c2 2/3 ∼ The strengthofthe interactionbetweenquarksandgluonsis determinedby the strong coupling constant (α ), which is the equivalent of the fine structure s constantinQuantumElectrodynamics. Asaconsequenceoftherenormalization procedurethevalueofα dependsontheamountofexchangedfour-momentum s between the interacting partons [3] (Equation 1.1): 12π α (Q2 )= , (1.1) s | | (11n 2f)ln(Q2 /Λ2) − | | where Q2 is the squareofthe exchangedfour-momentum(energyscale), n the | | number of colours in QCD (equal to 3), f the number of quark flavours (equal to 6)andΛ 300 MeV/c is aconstantcalculatedfromexperimentaldata. QCD ∼ A key property of QCD is that 11n 2f > 0. As a consequence, α decreases s − with increasing energy scale (decreasing distance). This is called asymptotic freedom. When a cross-section in QCD is calculated based on Feynman diagrams, contributions from higher order diagramscome with higher powers of α . Such s series can only converge if α is less than one. This is the case for large values s of Q2 . In this regime, QCD processes can be calculated using perturbative | | techniques. If Q2 is low then α is large, meaning that the perturbative approach s | | cannot be used. In such cases QCD properties can still be calculated using Lattice QCD. This regime of QCD is often called the soft regime. An important property of QCD is colour confinement, which means that a colour charged particle cannot be isolated. As a result all hadrons (stable particles consisting quarks)are colourless. Hadronsare built from three quarks (antiquarks) with different colours or from a quark-antiquark pair in a colour singlet state. The first type of hadron is called a baryon while the second type is called a meson. Examples of hadrons can be found in Table 1.2. In the full theory, the picture is more complicated. Quarks and antiquarks insideahadroninteractallthetimeproducingquark-antiquarkpairsandgluons. The parton content of a hadron is described using the parton density functions (PDF) f (x ,Q2) which are the probabilities of finding a parton a with x a a a fraction of the total longitudinal momentum of the hadron during a collision with four-momentum transfer Q2. At any moment inside a baryon there are three more quarks than antiquarks and inside a meson the number of quarks is equal to the number of antiquarks.