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Z boson mass reconstruction

Caroline SteiblinProf. Al GoshawDr. Andrea BocciDuke University

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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

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ATLAS DetectorA 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

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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.

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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

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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

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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

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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

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Cut Flow

Monte Carlo LHC Data

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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

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Cut Flow

Monte Carlo LHC Data

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Invariant Mass from Z(mumug) Before Photon Selection (After P1)

Monte Carlo LHC Data

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Invariant Mass from Z(mumug) After Photon Selection (After P2)

Monte Carlo LHC Data

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Invariant Mass from Z(mumue) Before Electron Selection (After E1)

Monte Carlo LHC Data

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Invariant Mass from Z(mumue) After Electron Selection (After E2)

Monte Carlo LHC Data

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deltaR (muon-photon)After Photon Selection (After P2)

Monte Carlo LHC Data

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deltaR (muon-electron)After Electron Selection (After E2)

Monte Carlo LHC Data

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Photon EtaAfter Photon Selection (After P2)

Monte Carlo LHC Data

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Electron EtaAfter Electron Selection (After E2)

Monte Carlo LHC Data

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Photon transverse energy After Photon Selection (After P2)

Monte Carlo LHC Data

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Electron transverse energy

After Electron Selection (After E2)

Monte Carlo LHC Data

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Photon & Electron transverse energyAfter Photon & Electron Cuts (After P2 & E2)

Monte Carlo LHC Data

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Photon isolationAfter Photon Selection (After P2)

Monte Carlo LHC Data

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Electron isolationAfter Electron Selection (After E2)

Monte Carlo LHC Data

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Summary

• No way to accurately measure the fake-rate quantitatively

• Monte Carlo and LHC Data results demonstrate similar trends

• Standard Model predictions reaffirmed