Detailed description
This project consists of 3 steps:
(i) Select and study di-lepton and di-photon final states produced in proton-proton collisions at the LHC and collected by the ATLAS detector.
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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 lepton pairs (e⁺e⁻, µ⁺µ⁻) or photon pairs (γγ). Use HYPATIA to calculate the invariant mass of the pairs by inserting the particles into the “Invariant Mass Window” using the “Electron”, “Muon”, and “Photon” buttons.
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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.
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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 “ZAnalysis”, and later modifying this analysis to complete the below exercises.
(ii) Analyse the data in terms of the following processes by building the invariant masses, of 2 pairs of oppositely charged leptons, or of 2 photons. Information about the angular distribution of the leptons in the centre of mass of the decaying Z or H bosons may also be used.
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pp→ γ,Z, Z’,G→ l+l- +X
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pp→H,G→γγ + X.
(iii) Study the known particle resonances and determine their properties, search for new ones in dilepton and diphoton invariant mass distributions. Make use of statistical tools to interpret the results and quantify the agreement or disagreement. When applicable 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, if any: mass and width.
Make use of spin to distinguish various outcomes. (This task may be difficult, in which case just give it a try). In addition to mass and electric charge, particles have a fundamental property called spin or “intrinsic angular momentum”. The Higgs boson has spin 0, the Z' and Z bosons spin 1, and the Graviton, the hypothetical mediator of gravity, spin 2. Knowing the spin is a way to further characterise a newly observed resonance. This can be done by studying, in addition to the invariant mass, the angular distribution of the decay products in the reference frame of the decaying particle. A spin-0 particle would lead to an isotropic distribution. The higher the spin, the more complicated would be the pattern. The conservation of angular momentum and spin come in addition, and imposes constraints on allowed particle decays.
Parts of this project, making use of the di-photon final states, depend on the availability of some data and MC samples. The di-photons can however be replaced by 2 lepton pairs stemming from di-Z-bosons (ZZ or ZZ*).