Searches for Supersymmetry and Dark Matter
Symmetries play a crucial role in physics. Supersymmetry (SUSY) relates integer spin particles (bosons) and half-integer spin particles (fermions). It allows unification of the electroweak and strong interactions, proposes dark matter candidates, and predicts five Higgs bosons (3 neutral and 2 charged ones). Processes of interest involve superpartners of the leptons (superpartners have a "~" above the particle), of the gauge and Higgs boson(s), as well as a dark matter particle, which is predicted to be the lightest supersymmetric particle (LSP).
- Students can search for sleptons, hypothetical supersymmetric partners of leptons, in the process pp→~l+~l-+X→ l+l-+χχ+X, where the DM candidate χ is the lightest neutralino (a mixture of photino, zino and higgsino), leading to a pair of leptons (e⁺e⁻or µ⁺µ⁻) and missing transverse energy (MET). The main SM background comes from, pp→W+W-+X→ l+l-+ννbar (MET)+X, pp→Z(→l+l-) Z(→ννbar)+X→ l+l-+MET+X and top quark-pair production. Among others, the following variables help extracting the signal: di-lepton invariant mass Mll, missing transverse energy (MET), transverse mass. The results will be interpreted within SUSY simplified models having slepton and DM masses as free parameters.
- The same final state (l+l-+MET+X) above may be due to the production of a pair of lightest charginos χ±1 (mixture of wino (W-superpartner) and charged higgsino (charged Higgs super-partner) through the process pp→ χ+1χ-1+X, followed by (i) ~l+~l-+ννbar+χχ+X or (ii) W+W-+χχ+MET+X. Further variables and analyses, including multivariate techniques based on neural networks and machine learning (ML) help separate various new physics scenarios on one hand and known SM physics on the other hand.
- Another promising search is for electro-weakinos (superpartners of electroweak gauge bosons, 4 neutralinos and 4 charginos) through the process pp→ χ±1χ02+X, followed by W±Z0+χχ+MET+X ~ ending with the final states 2-jets+l+l-+χχ+X or l± ν+l+l-+χχ+X.