Search for Long-lived Chargino via Gluino Pair Production in pp Collisions at √s = 13 TeV with the ATLAS Detector

Supersymmetry (SUSY) is one of the most promising theories beyond the Standard Model. SUSY models which predict that the Lightest SUSY Particle (LSP) is the partner of the neutral W boson (wino) are especially attractive because they are compatible with the cosmological observations. Such models also predict that the second lightest SUSY particle is the charged wino whose mass is nearly degenerated to the neutral wino mass. The charged wino has a long lifetime of about 0.2 ns due to the degeneracy and therefore the charged wino trajectory may be reconstructed by the inner detector of the ATLAS detector. The short wino track is a characteristic signature of the charged wino, and no such event signature is expected from the SM processes. This thesis presents a search for the long-lived charged wino in pp collisions at √s = 13 TeV at the LHC with the ATLAS detector. The integrated luminosity of 36.1 fb−1 data is used for this analysis. Gluino pair production followed by gluino decay into a charged or neutral wino and two jets is focused. The search uses pixel tracklets which is reconstructed from only four hits in the pixel detector. It is the first attempt in the ATLAS Collaboration to use the tracklets of the pixel detector as a physics object. The observed distribution of the transverse momentum of candidate events is consistent with the background-only hypothesis. For the wino lifetime of 0.2 ns, the gluino masses less than 1650 GeV are excluded at the chargino mass less than 460 GeV, which is the lower chargino mass limit from searches for electroweak direct production of winos. The chargino mass less than 1050 GeV is excluded for the region with the mass difference between wino and gluino larger than 100 GeV. For the wino lifetime of 1.0 ns, the gluino mass less than 1750 GeV is excluded at the chargino mass of 580 GeV. The chargino mass less than 1200 GeV is excluded for the region with the mass difference between wino and gluino larger than 100 GeV.