Is the world supersymmetric and/or where is Dark Matter?

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) Dilepton and missing transverse energy (MET) final state. (ii) pp→~l+~l-+X→ l+l-+MET+X, pp→Z+MET+X→ l+l-+MET+X, pp→W+W-+X→ l+l-+MET+X, pp→Z(→l+l-)  Z(→νν)+X→ l+l-+MET+X. (iii) Search for SUSY particles (sleptons) and/or Dark Matter.  

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 di-lepton final states - featuring important missing energy/momentum taken by weakly interacting particles such as Dark Matter - 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 lepton pairs (e⁺e⁻, µ⁺µ⁻). Use HYPATIA to calculate the invariant mass of the pairs by inserting the particles into the “Invariant Mass Window” using the “Electron” and “Muon” buttons. Check the value of the MET in each event. If the MET is large enough, the direction will be shown as a red dashed line.

  • 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 “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 mass of 2 pairs of leptons  (ee, mumu, emu). In this particular search the information about the missing transverse energy (MET) will be decisive in separating signal from background

  • pp→~l+~l-+X→ l+l-+MET+X (possible signal for supersymmetry)

  • pp→Z+MET+X→ l+l-+MET+X (possible signal of DM)

  • pp→W+W-+X→ l+l-+MET+X, pp→Z(→l+l-)  Z(→νν)+X→ l+l-+MET+X (SM background processes)

(iii) Describe the features of the dilepton invariant mass and MET distributions. Does the SM describe well the data? Compare data and the available MC samples. Compare the right- (l⁺l⁻) and wrong- (l⁺l⁺, l⁻l⁻, l⁺l’⁻, ... ) lepton combinations. Draw your conclusions.

Interpret the data in terms of searches for supersymmetric particles (sleptons) and/or Dark Matter. 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.

Published May 11, 2017 10:29 PM - Last modified Feb. 20, 2024 6:15 PM