Search for non-prompt photons in a diphoton and large missing transverse momentum final state at the ATLAS Detector

Lehan, Allan Search for non-prompt photons in a diphoton and large missing transverse momentum final state at the ATLAS Detector. PhD thesis, University of Liverpool.

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Abstract

A search for non-prompt photons in diphoton events with large missing transverse energy at the ATLAS detector is presented in this thesis. This search was performed using data collected from proton-proton collisions provided by the Large Hadron Collider at CERN at two energies: 4.8 fb^-1 collected at a center of mass energy of 7 TeV in 2011 and 20.3 fb^-1 collected at a center of mass energy of 8 TeV in 2012. The theoretical motivation for this search lies in the Gauge Mediated Symmetry Breaking models in which the lightest neutralino can be long-lived and decay to a photon and a gravitino, which is stable and the source of missing transverse momentum in these models. The photons produced in these decays have two unique properties due to the finite lifetime of the neutralino: they will not point back to the primary collision vertex and they will arrive at the electromagnetic calorimeter delayed compared to promptly produced photons. These two signatures were used to search for non-prompt photons in a simplified model in which the only two free parameters are the lifetime of the neutralino and the effective supersymmetry breaking scale, corresponding to the mass of the neutralino. In the two datasets, no significant excess over the Standard Model predictions was observed. These results were used to set exclusion limits on the lifetime and mass of the lightest neutralinos at the 95% confidence level: neutralinos with lifetimes varying from 250 ps to 50 ns in the mass range of 95 GeV to 240 GeV were excluded by the 7 TeV analysis, neutralinos with lifetimes varying from 250 ps to 100 ns in the mass range of 120 GeV to 440 GeV were excluded by the 8 TeV analysis.

Item Type:	Thesis (PhD)
Additional Information:	Date: 2015-09 (completed)
Depositing User:	Symplectic Admin
Date Deposited:	22 Dec 2015 11:14
Last Modified:	17 Dec 2022 01:15
DOI:	10.17638/02035539
URI:	https://livrepository.liverpool.ac.uk/id/eprint/2035539