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  • τ → ππ0ν event reconstruction and measurement of branching ratio

    Zuzana Gruberova, Charles University, Czech Republic

    September 4, 2019

    Supervisors: Ilya Komarov, Armine Rostomyan, Francesco Tenchini, Petar Rados

    Abstract

    This note reviews the summer project on τ → ππ0ν branching ratio measurement for detector performance study. We analyzed data from Belle II experiment using Monte Carlo simulations. We optimized the selections to get a pure sample and calculated the branching ratio, BR(τ → ππ0ν) = (24.5 ± 0.5)%. Only statistical uncertainties were taken into account. Despite this fact, agreement with PDG value is good. We used results of related projects to calculate ratios of branching ratios.

    1

  • Contents

    List of Figures 3

    List of Tables 4

    1. Introduction 5

    2. Theory 5 2.1. Belle II detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2. τ lepton decay modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3. Event characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4. Branching ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    3. Data sets 9 3.1. Experimental data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2. Candidate event selection . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3. Monte Carlo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    4. Signal selection 10 4.1. Cut performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.2. Cut selections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.3. Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    5. Data and Monte Carlo comparison 14 5.1. Plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

    6. Branching ratio 18 6.1. ππ0 sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.2. Combined results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    7. Conclusion 21

    References 22

    A. Attachments 23 A.1. MC decay modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 A.2. Cut efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 A.3. Cut flow of MC plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 A.4. Exp7 data and MC comparison plots . . . . . . . . . . . . . . . . . . . . 26

    2

  • List of Figures

    2.1. Belle II detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

    2.2. Feynman diagram of τ → ππ0ν decay. . . . . . . . . . . . . . . . . . . . . 7 2.3. Diagram of the signal event. . . . . . . . . . . . . . . . . . . . . . . . . . 8

    4.1. Distribution of number of photons on 1-prong side after preselection. . . 11

    4.2. Thrust and visible energy distribution after preselection. . . . . . . . . . 12

    4.3. Thrust and visible energy distribution after cuts. . . . . . . . . . . . . . 12

    4.4. ECL trigger efficiency dependence on visible energy. . . . . . . . . . . . . 13

    4.5. ECL trigger efficiency dependence on thrust. . . . . . . . . . . . . . . . . 14

    5.1. Exp8 data and MC comparison - thrust after preselection. . . . . . . . . 15

    5.2. Exp8 data and MC comparison - visible energy after preselection. . . . . 15

    5.3. Exp8 data and MC comparison - thrust after cuts. . . . . . . . . . . . . . 15

    5.4. Exp8 data and MC comparison - visible energy after cuts. . . . . . . . . 15

    5.5. Exp7 data and MC comparison - thrust after cuts. . . . . . . . . . . . . . 16

    5.6. Exp7 data and MC comparison - visible energy after cuts. . . . . . . . . 16

    5.7. Exp8 data and MC comparison - τ 1-prong invariant mass after cuts. . . 16

    5.8. Exp8 data and MC comparison - π0 1-prong invariant mass after cuts. . . 16

    5.9. Exp8 data and MC comparison - 1-prong π0 energy after cuts. . . . . . . 17

    5.10. Exp8 data and MC comparison - 1-prong π energy after cuts. . . . . . . 17

    5.11. 1-prong MCMode distribution after cuts. . . . . . . . . . . . . . . . . . . 17

    6.1. Branching ratios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

    6.2. Ratios of branching ratios. . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    A.1. Thrust and visible energy cut flow - step 1. . . . . . . . . . . . . . . . . . 25

    A.2. Thrust and visible energy cut flow - step 2. . . . . . . . . . . . . . . . . . 25

    A.3. Exp7 data and MC comparison - τ 1-prong invariant mass after cuts. . . 26

    A.4. Exp7 data and MC comparison - 1-prong π0 invariant mass after cuts. . . 26

    A.5. Exp7 data and MC comparison - 1-prong π0 energy after cuts. . . . . . . 26

    A.6. Exp7 data and MC comparison - 1-prong π energy after cuts. . . . . . . 26

    3

  • List of Tables

    2.1. τ lepton decay modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    3.1. Simulated processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    4.1. Cut values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

    6.1. Branching ratios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

    6.2. Ratios of branching ratios. . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    A.1. MC decay modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

    A.2. Cut efficiencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

    4

  • 1. Introduction

    Particle experiment Belle II, developed at High Energy Accelerator Research Organ- isation (KEK) in Tsukuba, Japan, is operated on SuperKEKB accelerator, a 3 km circumference asymmetric e+e− collider with

    √ s = 10.58GeV. It is an extension of the

    previous experiment Belle, which run on KEKB accelerator until 2010. Belle II is a B-factory, it aims at producing large amount of B mesons, but also D mesons and τ pairs for precision measurements.

    SuperKEKB is expected to reach the instantaneous luminosity of 8× 1035 cm−2s−1 and total integrated luminosity of 50 ab−1 in 2020’s, the Belle II detector is designed to record data with performance similar or better than in Belle in a much more severe beam background environment. [1]

    One of the studies performed on Belle II data focuses on properties of τ leptons, the heaviest known leptons. The precision measurements of τ decays can provide an insight into such problems as CP violation, lepton flavour violation or lepton universality. Due to their mass, τ leptons can also decay hadronically, which allows us to study strong interactions as well.

    This note presents a study of τ → ππ0ν event reconstruction using Monte Carlo simu- lations and measurement of the branching ratio of this decay. This project is one of the four projects on similar topics:

    • τ → eνν • τ → πν / µνν • τ → ππ0ν • τ → 3πν

    The final parts of this note incorporate output from the other three projects.

    2. Theory

    Most of the elementary particles are not stable and decay via the weak interaction. One type of a particle can decay in different ways and produce different new particles. The probability of a particle decaying in certain way is characterized by a branching ratio.

    Data analysed during this project were collected at the Belle II detector, this section gives a brief overview of the Belle II subdetectors and describes the decay modes of τ lepton with focus on the τ → ππ0ν decay, how to recognize this event and how to determine the branching ratio.

    5

  • 2.1. Belle II detector

    SuperKEKB collides electrons with positrons, where positrons circulate in the Low Ener- gy Ring (LER) of the accelerator at the energy of 4 GeV and electrons travel the opposite direction in the High Energy Ring (HER) at 7 GeV. When two bunches collide, the centre of mass moves in the direction of the electron beam. Therefore, most of the interaction products are detected in the forward direction and thus the layout of the detector is asymmetric. 3D model of the Belle II detector is shown in Figure 2.1.

    The tracking system comprises the Pixel Detector (PXD), Strip Vertex Detector (SVD), and the Central Drift Chamber (CDC). The Time Of Propagation counters (TOP) and Aerogel Ring-Imaging Cherenkov detector counters (ARICH) are responsible for particle identification. Crystals of the Electromagnetic Calorimeter (ECL) detect neutral particles and measure the energy of electromagnetically interacting particles. The KL and muon detector (KLM) surrounds the whole system and detects muons and kaons. [2]

    Figure 2.1: Belle II detector. The figure shows major Belle II subdetectors. The forward side of Belle II is to the right in the picture. [3]

    2.2. τ lepton decay modes

    τ leptons are the heaviest of known leptons with mass (1776.86±0.12) MeV and lifetime (2.903± 0.005)× 10−13 s. Therefore, they can decay both leptonically and hadronically. Table 2.1 lists the most dominant decay modes with their branching ratios. In total, τ leptons decay mostly hadronically, and leptonically only in around 35 %.

    The decay studied in this note is the most common one. Figure 2.2 shows the Feynman diagram of the τ → ππ0ν with the intermediate resonance ρ. Other resonances yielding ππ0ν are negligibly rare. [4]

    6

  • Figure 2.2: Feynman diagram of τ → ππ0ν decay. The picture shows the ρ resonance, which i