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  • University of Turin DEPARTMENT OF PHYSICS

    Master’s Degree in Nuclear and Subnuclear Physics

    Measurement of the relative phase between EM and strong amplitudes in ψ(2s)→ pp̄

    Candidate:

    Barbara Passalacqua

    Supervisor:

    Prof. Marco Maggiora

    Co-Supervisor:

    Prof. Marco Giovanni Maria Destefanis

    Reviewer:

    Prof. Margherita Obertino

    Academic Year 2018-2019

  • Alle notti insonni superate grazie al

    sostegno dei miei professori, della mia

    famiglia e dei miei amici.

  • Enthusiasm is followed by disappoint-

    ment and even depression, and then by

    renewed enthusiasm.

    Murray Gell-Man

  • Abstract

    The investigation of the measurement of the relative phase between strong and electro-

    magnetic resonant and non-resonant amplitudes could shed new light on many different

    open questions for Charmonia. Recently a relative phase of |∆φ| = 90◦ for J/ψ has been observed by BESIII Collaboration, and the relation suggests the hypothesis that the J/ψ

    and ψ(2S) could be a combination of two resonances, one decaying solely through EM

    processes and another one decaying strongly; in this work the process ψ(2s) → pp̄ is investigated.

    After an initial focus on the related theoretical issues, the experimental scenario, the

    BESIII spectrometer and BOSS, the official software framework adopted for reconstruc-

    tion and event selection, will be described in detail. After the reconstruction and the

    event selection are performed, the number of the pp̄ pairs are evaluated by means of p

    and p̄ simultaneous fit of momenta distributions. The pp̄ pairs produced in the investi-

    gated channel should be emitted back-to-back in the center of mass frame, once taken

    into account possible misalignments, detector resolution and contribution of Initial State

    Radiation, which was naively implemented by mean of a custom algorithm. The com-

    plete procedure to extract the cross section and the branching fraction for the inclusive

    process will be reported, from the event selection to the final result. Finally the relative

    phase between the strong and electromagnetic (resonant and non-resonant) amplitudes,

    has been extracted considering the pp̄ final state. This analysis has been performed on

    the full set of events collected in 2018, to perform both the ψ(2S) lineshape scan and a

    detailed investigation of the off-resonance interference pattern.

    5

  • Contents

    1 Introduction 1

    1.1 The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

    1.2 Quantum ChromoDynamics . . . . . . . . . . . . . . . . . . . . . . . . . . 2

    1.3 Running Coupling Constant and Confinement . . . . . . . . . . . . . . . . 2

    1.4 Charmonium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    1.5 Cross Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

    1.6 Initial State Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    1.7 Phase Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    1.8 A Different Theoretical Approach . . . . . . . . . . . . . . . . . . . . . . . 8

    2 The BESIII Experiment 9

    2.1 The BEPCII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

    2.2 The BESIII Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    2.2.1 The Superconductor Solenoid Magnet . . . . . . . . . . . . . . . . . 11

    2.2.2 Multilayer Drift Chamber . . . . . . . . . . . . . . . . . . . . . . . 11

    2.2.3 The Time-Of-Flight System . . . . . . . . . . . . . . . . . . . . . . 12

    2.2.4 Electromagnetic Calorimeter . . . . . . . . . . . . . . . . . . . . . . 14

    2.2.5 Muon Identifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

    2.2.6 Triggers and Data Acquisition . . . . . . . . . . . . . . . . . . . . . 15

    2.3 The BESIII Offline Software System . . . . . . . . . . . . . . . . . . . . . 16

    2.3.1 Simulation, Reconstruction and Analysis . . . . . . . . . . . . . . . 16

    2.3.2 Event Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    2.3.3 Particle Transport and Digitalization . . . . . . . . . . . . . . . . . 17

    2.3.4 Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

    2.4 BESIII Physics Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    2.4.1 Charmonium Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 18

    2.4.2 Open Charm Decays . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    2.4.3 Light Hadron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . 18

    2.4.4 τ , R, and QCD Studies . . . . . . . . . . . . . . . . . . . . . . . . . 19

    2.4.5 New Physics beyond the Standard Model . . . . . . . . . . . . . . . 19

    3 Data Analysis 21

    3.1 Experimental Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

    3.2 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

    3.3 Simulations Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

    7

  • 3.4 ISR Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

    3.5 Background Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

    3.6 Simultaneous Fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

    3.7 Systematic Uncertanties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

    3.8 Inclusive Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

    4 Results 43

    4.1 Cross Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

    4.2 Relative Phase Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

    5 Conclusions 47

    List of Figures 49

    List of Tables 51

    Bibliography 53

  • Chapter 1

    Introduction

    Elementary particle physics is trying to address the question, “What is matter made of?”

    on the most fundamental level. There are just three fundamental forces in nature: the

    strong force, carried by gluons, the electroweak force, carried by photons and W and

    Z bosons, and the gravitational force, dominant at macroscopic scale. Each of these

    forces is described by a (sometimes unfortunately different) physical theory. According

    to the Standard Model, the strong interaction acts on particles carrying color charge e.g.

    quarks and gluons. Together the elementary quarks and gluons form hadrons, a class of

    composite particles that interact via the strong force. In this work, the strong interaction is

    investigated by observing decays into hadrons of quarkonium states, produced in electron-

    positron collision provided by the Beijing Electron-Positron Collider II (BEPCII), and

    observed by mean of the Beijing Electromagnetic Spectrometer III (BESIII).

    1.1 The Standard Model

    The Standard Model of fundamental interactions is a theoretical model that led to the uni-

    fication of Quantum Electro-Dynamics (QED) and Quantum Chromodynamics (QCD).

    QED describes both electromagnetic and weak forces, while QCD the strong force (for

    more details see Section 1.2) [1–3]. The Standard Model is mathematically formulated

    in terms of renormalizable gauge theories. A gauge theory is a quantum field theory

    (QFT) where gauge fields emerge as a consequence of the internal symmetries [4]. In the

    Standard Model, these gauge fields are the force mediators.

    The basic Lagrangian may be expressed as:

    L = Lvct + LHiggs + Llep + Lquark,

    where Lvct corresponds to gauge bosons, LHiggs to Higgs scalar, Llep to leptonic fermions and Lquark to hadronic fermions, respectively.

    The total gauge group of the Standard Model is:

    SU(3)C × SU(2)L × U(1)Y (1.1)

    The symmetry group of the electroweak interaction is SU(2)L × U(1)Y , while the gauge symmetry of the strong interaction is SU(3)C .

    1

  • The theory is commonly viewed as containing the fundamental set of particles: the

    leptons, quarks, gauge bosons and the Higgs particle.

    The fermions, spin 1/2 particles which follow the Fermi-Dirac statistics, are divided

    into quarks and leptons. Leptons l can be subdivided in three families, each one composed

    of one of the three electrically charged leptons — the electron (e), the muon (µ) and the

    tauon (τ) — and of one of the three electrically neutral leptons — the electron-neutrino

    (νe), the muon-neutrino (νµ) and the tau-neutrino (ντ ). All the charged leptons have

    electric charge −e while their antiparticles have charge +e. All leptons interact via the weak interaction while only the charged ones interact also electromagnetically. None of

    the leptons interact via the strong interaction.

    Quarks q come in six flavours: down (d), up (u), strange (s), charm (c), bottom (b)

    and top (t). In contrast to the charged leptons, the quarks carry fractional electric charge:

    −1 3 e for d, s and b, and +2

    3 e for u, c and t. In addition to the electric charge, quarks also

    carry color charge, a further degree of freedom. While electric charge is either positive or

    negative, color charge comes in thre