Ricerca del decadimento X(3872) J= ! nell’esperimento LHCb al PDF file 1 Exotic charmonia: ......

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  • Facoltà di Scienze Matematiche, Fisiche e Naturali

    Ricerca del decadimento X(3872)→ J/ψω

    nell’esperimento LHCb al CERN Tesi di Laurea Magistrale in Fisica

    Candidato: Relatore: Lorenzo Capriotti Roberta Santacesaria

    Correlatore: Antonio Augusto Alves Jr.

    Anno Accademico 2013/2014

  • Contents

    1 Exotic charmonia: theoretical and experimental status 8 1.1 Exotic quarkonia in the charm sector . . . . . . . . . . . . . . 9 1.2 X(3872) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    1.2.1 Experimental status . . . . . . . . . . . . . . . . . . . 10 1.2.2 Theoretical models . . . . . . . . . . . . . . . . . . . . 13 1.2.3 The J/ψω decay channel . . . . . . . . . . . . . . . . . 15

    2 The LHCb experiment at CERN 17 2.1 Large Hadron Collider . . . . . . . . . . . . . . . . . . . . . . 17 2.2 LHCb detector . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    2.2.1 Vertex Locator (VELO) . . . . . . . . . . . . . . . . . 20 2.2.2 Ring-Imaging Cherenkov Detectors (RICH) . . . . . . 20 2.2.3 Tracking System . . . . . . . . . . . . . . . . . . . . . 21 2.2.4 Magnet . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.2.5 ECAL and HCAL . . . . . . . . . . . . . . . . . . . . . 24 2.2.6 Muon system . . . . . . . . . . . . . . . . . . . . . . . 26 2.2.7 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

    2.3 Analysis tools . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.3.1 Stripping and reconstruction . . . . . . . . . . . . . . . 27 2.3.2 Monte Carlo simulations . . . . . . . . . . . . . . . . . 28

    3 Data Analysis 29 3.1 Data samples . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.2 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . 30

    3.2.1 Stripping cuts . . . . . . . . . . . . . . . . . . . . . . . 30 3.2.2 Decay reconstruction . . . . . . . . . . . . . . . . . . . 31 3.2.3 Preselection cuts . . . . . . . . . . . . . . . . . . . . . 32

    3.3 Monte Carlo sample . . . . . . . . . . . . . . . . . . . . . . . 35 3.4 Multivariate analysis . . . . . . . . . . . . . . . . . . . . . . . 39

    3.4.1 Input variables and training phase . . . . . . . . . . . . 42 3.5 B+ mass fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

    3.5.1 Application phase of MVA . . . . . . . . . . . . . . . . 48 3.6 Background subtraction: sPlot technique . . . . . . . . . . . . 50

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  • 3.6.1 Application of sPlot technique . . . . . . . . . . . . . . 50 3.7 J/ψω mass spectrum . . . . . . . . . . . . . . . . . . . . . . . 52 3.8 Resonances contribution . . . . . . . . . . . . . . . . . . . . . 53 3.9 Hypotheses for the excess of events at 5350 MeV/c2 . . . . . . 57 3.10 Efficiency corrections . . . . . . . . . . . . . . . . . . . . . . . 61 3.11 J/ψω mass resolution . . . . . . . . . . . . . . . . . . . . . . . 70

    4 Results 72 4.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

    Appendices 76

    A The sPlot technique 77 A.1 Preliminary step: total correlation . . . . . . . . . . . . . . . . 78 A.2 The sPlot formalism . . . . . . . . . . . . . . . . . . . . . . . 78

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  • List of Figures

    1.1 Charmonium spectrum . . . . . . . . . . . . . . . . . . . . . . 10 1.2 Peak observed by Belle (2003) . . . . . . . . . . . . . . . . . . 11 1.3 Result of the X(3872) particle angular analysis from CDF . . . 11 1.4 Determination of X(3872) quantum numbers: BaBar analysis

    on the left, LHCb analysis on the right . . . . . . . . . . . . . 12 1.5 Distribution of the test statistic t for the simulated experiments

    with 2−+ and 1++ at LHCb . . . . . . . . . . . . . . . . . . . 12 1.6 Simulation of the prompt production cross section as a function

    of the relative centre of mass momentum of the system D0−D̄∗0 (the allowed phase space region is highlighted) . . . . . . . . . 13

    1.7 Spectrum of [cq][c̄q̄] states according to the tetraquark model . 14 1.8 Corrected M(J/ψω) distribution for B+ (top) and B0 (bottom)

    decays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

    2.1 The CERN accelerator complex . . . . . . . . . . . . . . . . . 18 2.2 Polar angles of the b- and b̄-hadrons calculated with PYTHIA 19 2.3 LHCb detector, lateral view (non-bending plane) . . . . . . . . 19 2.4 VELO system configuration and VELO sensors . . . . . . . . 20 2.5 RICH detectors layout . . . . . . . . . . . . . . . . . . . . . . 21 2.6 Inner Tracker layout . . . . . . . . . . . . . . . . . . . . . . . 22 2.7 Trigger Tracker layout . . . . . . . . . . . . . . . . . . . . . . 23 2.8 Tracking system layout . . . . . . . . . . . . . . . . . . . . . . 23 2.9 LHCb magnet, perspective view . . . . . . . . . . . . . . . . . 24 2.10 Calorimeters segmentation: ECAL on the left, HCAL on the

    right. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.11 Resolved π0 reconstruction efficiency for the channel B0 →

    π+π−π0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.12 Muon system configuration . . . . . . . . . . . . . . . . . . . . 26

    3.1 B mass spectrum after the application of preselection cuts . . 35 3.2 Dalitz plot for K+ω squared mass vs J/ψω squared mass,

    generator level (MC) . . . . . . . . . . . . . . . . . . . . . . . 36 3.3 B+ mass spectrum (MC), before and after fixing the π0 mass . 38 3.4 π0 mass spectrum (MC) . . . . . . . . . . . . . . . . . . . . . 39

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  • 3.5 B+ mass vs π0 mass (MC) . . . . . . . . . . . . . . . . . . . . 39 3.6 ω mass (MC) . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.7 Dalitz plot for K+ω squared mass vs J/ψω squared mass, after

    the reconstruction and preselection phases (MC) . . . . . . . . 40 3.8 Input variables, with signal and background distributions . . . 43 3.9 Correlation matrices for signal and background samples . . . . 44 3.10 MVA methods performance (ROC curve) . . . . . . . . . . . . 44 3.11 PLE method response . . . . . . . . . . . . . . . . . . . . . . 45 3.12 B+ mass spectrum, after the cut on rL . . . . . . . . . . . . . 45 3.13 B+ mass spectrum, before and after the cut on rL . . . . . . . 46 3.14 B+ mass fit, without the bump contribution, for rL > 0.65 . . 47 3.15 B+ mass fit, with the bump contribution, for rL > 0.65 . . . . 47 3.16 Statistical significance (blue histogram), signal efficiency

    (magenta histogram) and background rejection (black histogram) as a function of rcutL (left axis for the significance and right axis for the rest) . . . . . . . . . . . . . . . . . . . . 49

    3.17 Fit on B+ mass for both rcutL , with fit results . . . . . . . . . . 50 3.18 ω invariant mass distribution after the rL cut . . . . . . . . . 51 3.19 ω invariant mass signal distribution (background subtracted) . 51 3.20 π0 mass signal distribution (background subtracted) . . . . . . 52 3.21 J/ψω mass, MVA_LL > 0.65 . . . . . . . . . . . . . . . . . . 53 3.22 J/ψω mass, MVA_LL > 0.15 + M(ω) cut . . . . . . . . . . . 53 3.23 J/ψππ and Kπ invariant mass, MVA_LL > 0.65 . . . . . . . 54 3.24 J/ψππ and Kπ invariant mass (background subtracted),

    MVA_LL > 0.65 . . . . . . . . . . . . . . . . . . . . . . . . . 54 3.25 J/ψω mass without Ψ(2S) and K∗(892) contributions,

    MVA_LL > 0.65 . . . . . . . . . . . . . . . . . . . . . . . . . 55 3.26 J/ψω mass without Ψ(2S) and K∗(892) contributions,

    MVA_LL > 0.15 + M(ω) cut . . . . . . . . . . . . . . . . . . 55 3.27 M(J/ψω) vs M(Kω), MVA_LL > 0.65 . . . . . . . . . . . . . 56 3.28 M(J/ψω) vs M(Kω), MVA_LL > 0.15 + M(ω) cut . . . . . . 56 3.29 M(J/ψω), MVA_LL > 0.65, after cut on M(Kω) . . . . . . . 57 3.30 M(J/ψω) , MVA_LL > 0.15 + M(ω) cut, after cut on M(Kω) 57 3.31 B+ mass vs (J/ψKππ) mass . . . . . . . . . . . . . . . . . . . 58 3.32 B+ mass vs (K J/ψ) mass . . . . . . . . . . . . . . . . . . . . 59 3.33 B+ mass vs (J/ψππ) mass . . . . . . . . . . . . . . . . . . . . 59 3.34 B+ mass vs (K π) mass . . . . . . . . . . . . . . . . . . . . . . 60 3.35 B+ mass vs (K ω) mass . . . . . . . . . . . . . . . . . . . . . . 60 3.36 Effect of the cut flow on the J/ψω mass distribution (black

    histogram before the application of the cuts, red histogram after) 63 3.37 Effect of the cut flow on the J/ψω mass distribution (black

    histogram before the application of the cuts, red histogram after) 64

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  • 3.38 Efficiency of every step as a function of M(J/ψω). Asymmetric errors are computed according to the Agresti-Coull formula [48]. 65

    3.39 Total efficiency distributions for both MVA cut values . . . . . 66 3.40 Fit on the linear component of the total efficiency for both MVA

    cut values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 3.41 Fit on the M(Kω) cut efficiency for both MVA cut values . . . 67 3.42 Total efficiency distributions, data from MC and functional form 68 3.43 M(J/ψω), MVA_LL > 0.65, efficiency corrected . . . . . . . . 69 3.44 M(J/ψω) , MVA_LL > 0.15 + M(ω) cut, efficiency corrected 69 3.45 δM for Mreco(J/ψω) ∈ [4100, 4150] MeV/c2 . . . . . . . . . . . 70 3.46 J/ψω mass resolution . . . . . . . . . . . . . . . . . . . . . . . 71 3.47 J/ψω mass, threshold region . . . . . . . . . . . . . . . . . . . 71

    4.1 Corrected J/ψω mass with the new MC normalization, MVA_LL > 0.65 . . . . . . . . . . . . . . . . . . . . . . . . . 73

    4.2 Corrected J/ψω mass with the new MC normalization, MVA_LL > 0.15 + M(ω) cut . . . . . . . . . . . . . . . . . . 73

    6

  • Introduction

    In this thesis, a search for the decay X(387