What do 4-lepton final states tell us about the Standard Model and the Higgs boson?

Each analysis consists of (i) selecting and studying particular final states made of particles measured by the detector, (ii) identifying the underlying proton-proton collision process(es), and (iii) interpreting the results in terms of a SM measurement or within some new theory. Specific to this project:

(i) Four charged lepton final state. (ii) pp→ ZZ+X → 4l+X or pp→ H+X→ ZZ+X → 4l+X. X indicates particles needed to satisfy the basic conservation rules. (iii) Test of the electroweak theory and production and study of the Higgs boson.

An introduction and short demo/tutorial will be given at the beginning to all students involved in the ATLAS-related projects.

Detailed description

This project consists of 3 steps:

(i) Select and study four charged lepton final states produced in proton-proton collisions at the LHC and collected by the ATLAS detector.

Go to the Z-path web pages and download the event display program HYPATIA and one arbitrary data sample consisting of 50 events (dirXX/groupX.zip). Unzip your data sample and open it with HYPATIA (File -> Read Event Locally and open the first event “event001.xml”). Navigate through the data sample using the “Next Event” button and look for events with four charged leptons (e⁺e⁻e⁺e⁻, µ⁺µ⁻µ⁺µ⁻, e⁺e⁻µ⁺µ⁻) in the final state (these are quite rare). Use HYPATIA to calculate the invariant mass of individual electron and/or muon pairs as well as of the full 4-lepton system by inserting the leptons into the “Invariant Mass Window” using the “Electron” and/or “Muon” buttons.
Go to the web-based Histograms Analyzer to learn how “cuts” are used in a particle physics analysis to select events of interest. Place cuts on any variable directly by clicking on the x-axis of the corresponding histogram, and see how the composition of the data sample in terms of the different physics processes changes.
Go to the ATLAS Open Data portal and download the samples and analysis code to a computer which has PyROOT installed and at least 7 GB of free disk space. Follow the instructions to analyze the data and plot results, first using the predefined “ZZAnalysis”, and later modifying this analysis to complete the below exercises.

(ii) Analyse the data in terms of the following processes (the Z boson can be on- or off-shell) by building the invariant masses, of 2 pairs of oppositely charged leptons, or of the four leptons. Information about the angular distribution of the leptons in the centre of mass of the decaying Z or H bosons may also be used.

pp→ ZZ+X → 4l+X.
pp→ H+X→ ZZ+X → 4l+X.

(iii) Test of the electroweak theory and study of the Higgs boson. Describe the features of the invariant mass distributions within a wide range of masses. Compare the right- (l⁺l⁻) and wrong- (l⁺l⁺, l⁻l⁻, l⁺l’⁻, ... ) lepton combinations. Does the SM describe well the data? Compare data and available MC. Draw your conclusions. Make use of statistical tools to interpret the results and quantify the agreement or disagreement. Try to fit the data using combinations of functions describing the particle resonances and the continuum (non-resonant combinations). Extract the properties of particle resonances: mass and width.

The SM predicts a coupling between the Z boson and quarks or leptons, allowing the scattering process quark-antiquark → ZZ through a quark exchange. The Z-self coupling and γ-Z coupling, leading to the annihilation process quark-antiquark→ γ,Z→ ZZ, are forbidden.
The SM predicts the existence of a scalar (spin-0) boson. The Higgs boson is observed at a mass of 125 GeV by the ATLAS and CMS collaborations. The Higgs couples to massive particles.

Published May 11, 2017 10:28 PM - Last modified Feb. 20, 2024 6:14 PM