Z boson mass reconstruction
Caroline Steiblin Prof. Al Goshaw Dr. Andrea Bocci Duke University
1 Purpose
• Comparing Monte-Carlo (MC) simulations of Z boson mass reconstruction from muons, photons, and electrons to LHC data, to find agreement and qualitative proof of electron-photon fake rates
• Identify the Z boson as a true photon source for identification tests
2 ATLAS Detector A Toroidal LHC ApparatuS
• The ATLAS detector consists of four major components: • The inner detector to measure the momentum of charged particles • A calorimeter to measure particle energy (main part used) • A muon spectrometer to identify muons and measure their momenta • A magnet system to bend charged particles for measurement
• Reconstruction algorithms are used to identify different particle trajectories for identification and analysis
3 Standard Model
• The Standard Model allows the Z boson to decay into a lepton and anti-lepton (eg.+/- muon) and a photon, but not three leptons (eg. +/- muon and an electron).
• Data can show a violation of the Standard Model with three leptons, which may demonstrate the possibility of an electron faking a photon.
4 Z boson
• Charge-less, spin 1, 91 GeV particle • Decays to a an fermion/anti-fermion pair • Experimentally well understood and easy to reconstruct with low background
• Focused on Z μ+μ-γ and Z μ+μ-e- decay, as muons are efficiently reconstructed, and offer a sample of pure photons • Data used from full 2012 8 TeV data and simulated Monte Carlo program
5 Photon reconstruction
• Reconstruction of photons, which do not leave tracks in the calorimeter
• Electrons leave tracks though, and are placed with similar electromagnetic clusters, so interchanging one for the other is not uncommon
• While traversing a material, a photon can decay into an electron and positron, which leads to misidentification
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Fake rate of electrons and photons
• Misidentification of photons during Z boson reconstruction can lead to anomalies in data, which can lead to inaccurate results, and mass predictions
• Number of electrons present in both full data sample and Monte Carlo is much lower than that of photons produced in the muon channel
7 ISR and FSR
• Initial State Radiation (ISR)- not used in this project, but creates a Z boson with a radiated photon before decay
• Final State Radiations (FSR)- used for research to identify pure photons and measure photon energies after Z boson decay
8 Monte Carlo (MC) Simulation
• MC used to simulate events from pp collision and particles produced
• “Data” reconstructed similarly to that of LHC data
• Can find agreement with LHC to test the performance of the ATLAS detector
9 Cut Flow
Monte Carlo LHC Data
10 Analysis Cuts Specific
• Vertex where two particle tracks are present within 200 mm of each-other (MC: 99.99% Data: 99.95%)
• Muon where both a positive and negative muon exist in an event, with a transverse momentum over 25 GeV, eta under 2.4 radians, and energy ratio is under 0.2 (MC: 22.06% Data: 11.7%)
• Photon where a photon exists with a transverse energy over 10 GeV, eta in the range of 0<|eta|<1.37, 1.52<|eta|<2.37 radians, isolation under 4 GeV (MC: 13.85% Data: 3.56%) Symbol: P1
• Photon Invariant Mass where the reconstructed events yield results in the range of the Z boson 80 GeV < Mass < 96 GeV (MC: 9.36% Data: 1.38%) Symbol: P2
• Electron when an electron exists with a transverse energy over 10 GeV, eta in the range of 0<|eta|<1.37, 1.52<|eta|<2.37 radians, isolation under 4 GeV (MC: 5.50% Data: 0.86%) Symbol: E1
• Electron Invariant Mass where the reconstructed events yield results in the range of the Z boson 80 GeV < Mass < 96 GeV (MC: 4.27% Data: 0.69%) Symbol: E2
11 Cut Flow
Monte Carlo LHC Data
12 Invariant Mass from Z(mumug) Before Photon Selection (After P1)
Monte Carlo LHC Data
13 Invariant Mass from Z(mumug) After Photon Selection (After P2)
Monte Carlo LHC Data
14 Invariant Mass from Z(mumue) Before Electron Selection (After E1)
Monte Carlo LHC Data
15 Invariant Mass from Z(mumue) After Electron Selection (After E2)
Monte Carlo LHC Data
16 deltaR (muon-photon) After Photon Selection (After P2)
Monte Carlo LHC Data
17 deltaR (muon-electron) After Electron Selection (After E2)
Monte Carlo LHC Data
18 Photon Eta After Photon Selection (After P2)
Monte Carlo LHC Data
19 Electron Eta After Electron Selection (After E2)
Monte Carlo LHC Data
20 Photon transverse energy After Photon Selection (After P2)
Monte Carlo LHC Data
21 Electron transverse energy After Electron Selection (After E2)
Monte Carlo LHC Data
22 Photon & Electron transverse energy After Photon & Electron Cuts (After P2 & E2)
Monte Carlo LHC Data
23 Photon isolation After Photon Selection (After P2)
Monte Carlo LHC Data
24 Electron isolation After Electron Selection (After E2)
Monte Carlo LHC Data
25 Summary
• No way to accurately measure the fake-rate quantitatively
• Monte Carlo and LHC Data results demonstrate similar trends
• Standard Model predictions reaffirmed
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