Abstract

This talk covers both the improvements of luminosity measurements of pp collisions at ATLAS , underpinning all measurements made by the detector and a search for one of the most theoretically viable extensions to the SM: supersymmetry.

ATLAS uses mainly event-counting algorithms to measure luminosity, which break down at higher luminosities. If the ATLAS SCT (part of the ATLAS inner detector) can be employed as a luminometer using hit-counting algorithms, this issue may be mitigated. I will show that the SCT can feasibly operate as a luminometer when recording two-or-more strip clusters with the standard binary readout mode. Thus, the SCT can measure the luminosity with an accuracy within 10% of two of ATLAS ’s existing luminometers: LUCID and TileCal.

An analysis of an experimentally challenging region of phase space, where stop decays have a compressed mass spectrum, using the full Run-2 ATLAS dataset is presented. I show how the aMT2 kinematic variable, designed to give a lower limit on pair-produced particle masses, is found to be effective at differentiating SUSY decays from the SM background when used as an upper bound. Secondary vertex information and significance variables are also employed to design a search in this region of parameter space. Stop quarks are excluded up to 500 GeV and mass splittings between the stop and the neutralino are found to be excluded up to 130 GeV in the specific scenario considered, complimenting the exclusion limits found by other ATLAS searches for stop decays with one-lepton final states.