Ph.D Thesis


Ph.D StudentEnrique Kajomovitz
SubjectSearch for Heavy Long-Lived Particles in ATLAS
DepartmentDepartment of Physics
Supervisor Full Professors Tarem Shlomit
Full Thesis textFull thesis text - English Version


Abstract


This thesis presents a search for long-lived particles through a measurement of their mass performed on a data sample of 4.70 fb−1 from proton-proton collisions at a centre-of-mass energy
 of  √s = 7TeV collected by the ATLAS detector at the LHC in 2011. The mass of the long-lived particles is estimated from a measurement of their speed, β, and their momentum measurements using the relation m = p/βγ.

Two distinct types of long-lived particles are sought after; sleptons, the supersymmetric partners of the Standard Model leptons, and R-hadrons, colorless bound states that include a colored supersymmetric long-lived particle in addition to colored Standard Model particles. Sleptons are expected to interact with the detector as if they were heavy muons, charged throughout the detector and penetrating. In contrast, R-hadrons may change charge in the detector due to the strong interactions between the light quarks in the R-hadrons with the detector material. Thus, R-hadrons may be charged in the Inner Detector and neutral in the Muon Spectrometer or vice versa, and therefore the signature of a colored long-lived colored particle might be different than the signature of a non-colored long-lived particle.

No excess is observed above the estimated background and upper limits, at 95% confidence level, are set on the mass of the long-lived particles in different scenarios, based on their possible interactions in the inner detector, the calorimeters and the muon spectrometer.

Long-lived staus in gauge-mediated supersymmetry-breaking models are excluded up to a mass of 300 GeV for tanβ = 5-20. Directly produced long-lived sleptons are excluded up to a mass of 278 GeV. Gluino (stop, sbottom) R-hadrons are excluded up to a mass of 1003 GeV (698 GeV, 634 GeV) when using a generic interaction model.