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Precision Measurements of B0s Mesons in the CMS Experiment At HIP Internal Report Series HIP-2017-03 B0 Precision measurements of s mesons in the CMS experiment at the LHC Terhi Järvinen Department of Physics Faculty of Science UNIVERSITY OF HELSINKI ACADEMIC DISSERTATION To be presented with the permission of the Faculty of Science of the University of Helsinki, for public criticism in the auditorium E204 at 10 o’clock, on December 18th, 2017. Helsinki 2017 Supervisors Professor Paula Eerola, University of Helsinki Dr. Giacomo Fedi, University of Pisa and INFN Pisa Pre-examiners Reader Lars Eklund, University of Glasgow Dr. Weiming Yao, Lawrence Berkeley National Laboratory Opponent Professor Ulrik Egede, Imperial College London ISSN 1455-0563 ISBN 978-951-51-1271-2 (paperback) Helsinki University Print (Unigrafia Oy) ISBN 978-951-51-1272-9 (pdf) http://ethesis.helsinki.fi Electronic Publications at the University of Helsinki HELSINGIN YLIOPISTO — HELSINGFORS UNIVERSITET — UNIVERSITY OF HELSINKI Tiedekunta — Fakultet — Faculty Laitos — Institution — Department Faculty of Science Department of Physics Tekijä — Författare — Author Terhi Järvinen Työn nimi — Arbetets titel — Title 0 Precision measurements of Bs mesons in the CMS experiment at the LHC Oppiaine — Läroämne — Subject Physics Työn laji — Arbetets art — Level Aika — Datum — Month and year Sivumäärä — Sidoantal — Number of pages PhD Thesis November 2017 181 pages Tiivistelmä — Referat — Abstract 0 Two precision measurements are performed to Bs data. The primary measurement of the thesis 0 is the effective lifetime of the Bs meson decaying in the J/ψφ(1020) state. The effective lifetime analysis uses 2012 data recorded by the Compact Muon Solenoid (CMS) experiment. The data are collected with a center-of-mass energy of 8 TeV and correspond to an integrated luminosity of −1 19.7 fb . The effective lifetime is found to be cτeff = 443.9 ± 2.0 (stat.) ± 1.2 (syst.) μm. In addition to the lifetime measurement, the design, validation and performance of the two flavour tagging algorithms are described. The algorithms are developed to improve the precision of the weak mixing phase measurement. Lastly, the results from the weak phase analysis are briefly covered. The flavour tagging studies and the weak phase measurement are also performed with the 2012 data recorded by the CMS experiment. Avainsanat — Nyckelord — Keywords B physics, b-hadron lifetime measurements, CP violation Säilytyspaikka — Förvaringsställe — Where deposited Muita tietoja — övriga uppgifter — Additional information Contents Preface I Acknowledgements II 1 Introduction 1 2 Physics of neutral B mesons 5 2.1 Standard model of particle physics ................... 5 2.2 CKM matrix ............................... 6 2.2.1 Unitarity triangles ........................ 8 2.3 Neutral B meson systems ......................... 10 2.3.1 Time evolution of the neutral B meson states ......... 12 2.3.2 Matrix elements of the Hamiltonian ............... 13 2.3.3 Decay widths and masses ..................... 15 2.3.4 Mass and CP eigenstates ..................... 17 2.3.5 Decay rates and mixing probabilities .............. 18 2.3.6 CP violation in the meson decays ................ 20 0 → 2.4 Decay rate in the Bs J/ψφ channel .................. 24 0 2.4.1 Effective lifetime of Bs meson .................. 28 2.4.2 Flavour tagging .......................... 30 2.4.3 Mixing asymmetry ........................ 36 iii 3 Proton-proton collisions 37 3.1 Physics concepts ............................. 37 3.1.1 Luminosity ............................ 37 3.1.2 Rapidity and pseudorapidity ................... 38 3.2 The Large Hadron Collider ........................ 39 3.2.1 Acceleration chain ........................ 39 3.2.2 Magnets .............................. 41 3.2.3 The LHC experiments ...................... 41 4 The Compact Muon Solenoid 42 4.1 Tracking system . ............................ 43 4.1.1 Pixel tracker ............................ 44 4.1.2 Strip tracker ............................ 44 4.2 Calorimeters ................................ 46 4.3 Magnet .................................. 47 4.4 Muon detectors .............................. 47 4.4.1 Drift tubes . ............................ 48 4.4.2 Resistive plate chambers ..................... 48 4.4.3 Cathode strip chambers ..................... 49 4.5 Triggering system ............................. 49 4.5.1 L1 trigger ............................. 50 4.5.2 HLT trigger ............................ 52 5 Event reconstruction 54 5.1Tracks................................... 54 5.1.1 Track categorisation ....................... 56 5.2 Vertex reconstruction ........................... 59 5.2.1 Pixel vertices ........................... 59 5.2.2 Primary and secondary vertices ................. 59 5.3 Beam spot ................................. 61 5.4 Muons ................................... 62 5.4.1 Global muon reconstruction ................... 63 5.4.2 Tracker muon reconstruction ................... 63 5.4.3 Muon categorisation ....................... 64 5.5 Electrons .................................. 65 5.6 Particle flow algorithm .......................... 67 5.7 Composite particles ............................ 68 6 Flavour tagging in the CMS experiment 70 6.1 Data and simulated samples ....................... 70 6.2 Tagging method .............................. 71 6.3 Event reconstruction and selection .................... 71 6.3.1 Trigger selection ......................... 72 6.3.2 J/ψ mesons ............................ 72 6.3.3 φ(1020) mesons .......................... 73 6.3.4 K∗(892)0 mesons ......................... 73 0 6.3.5 Bs mesons ............................. 74 + 0 6.3.6 B and Bd mesons ........................ 74 6.4 B+ mass fit ................................ 74 6.5 Simulation bias .............................. 75 6.6 Cut-based tagging algorithm ....................... 78 6.6.1 Input variables .......................... 79 6.6.2 Optimisation ........................... 80 6.6.3 Tagging performance of the cut-based algorithm ........ 81 6.7 Neural network-based tagging algorithm ................ 87 6.7.1 Multilayer perceptron network .................. 87 6.7.2 Lepton preselection ........................ 88 6.7.3 Input variables .......................... 90 6.7.4 Network structure ......................... 93 6.7.5 Tagging performance of the MLP algorithm . ........ 93 0 6.8 Bd mixing asymmetry ..........................100 0 6.8.1 Likelihood function for Bd mixing asymmetry . ........101 0 6.8.2 Cross-checks with simulated Bd sample .............103 0 → 0∗ 6.8.3 Mistag fraction in Bd J/ψK decay .............103 0 → 0∗ 6.8.4 Testing the fitted mistag fraction in Bd J/ψK decay . 105 0 6.8.5 Other cross-checks with the simulated Bd sample .......106 0 → 0∗ 6.8.6 Tagging results with Bd J/ψK data ............106 6.8.7 Physics results from the asymmetry fit .............108 6.9 Summary and outlook of the tagging studies ..............111 7 Weak phase and decay width difference measurement 114 7.1 Maximum likelihood fit ..........................114 7.2 Results and systematic uncertainties ..................116 7.3 Summary and outlook of the φs analysis ................117 0 → 8 Measurement of the effective lifetime in the Bs J/ψφ decay 120 8.1 Feasibility studies .............................120 8.1.1 Upper limit of the fit range ...................121 8.1.2 Lower limit of the fit range ....................122 8.2 Data and simulated samples .......................124 8.3 Event reconstruction and selection ....................128 8.3.1 Trigger selection .........................128 8.3.2 Muons and J/ψ mesons .....................128 8.3.3 φ(1020) mesons ..........................129 0 8.3.4 Bs mesons .............................129 8.4 Fit observables . .............................129 8.5 Background studies ............................131 8.6 Vertex reconstruction and selection ...................131 8.7 Vertex and ct resolutions .........................133 8.8 ct efficiency ................................135 8.8.1 Apparent efficiency ........................135 8.8.2 Reconstruction efficiency .....................140 8.8.3 Total ct efficiency .........................142 8.9 Likelihood function ............................144 8.9.1 Fit procedure ...........................146 8.9.2 Closure tests of the likelihood function .............147 8.10 Fit results .................................148 8.11 Systematic uncertainties .........................151 8.11.1 Likelihood function bias .....................151 8.11.2 Modelling assumptions ......................151 8.11.3 Primary vertex selection .....................152 8.11.4 ct resolution ............................153 8.11.5 MC sample size ..........................154 8.11.6 Efficiency modelling .......................154 8.11.7 Tracker alignment .........................155 8.11.8 ct range ..............................157 8.11.9 S-wave contribution ........................157 8.12 Results ...................................161 8.13 Summary and outlook of the effective lifetime analysis ........162 9 Conclusions 164 A Data-simulation comparisons 166 Bibliography 171 Preface 0 In this thesis, neutral Bs meson system is studied using the data collected by the CMS experiment. In the first part of my PhD degree, I designed and optimised the first flavour tagging algorithm used in the early weak phase measurement of 0 → the Bs J/ψφ decay mode. I also contributed to validation of the second, more advanced tagging algorithm that was created at INFN and the University of Padua and used in the CMS
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