Tambe Ebai, Norbert2016-02-122016-02-122015-11https://hdl.handle.net/11299/177092University of Minnesota Ph.D. dissertation. 2015. Major: Physics. Advisor: Yuichi Kubota. 1 computer file (PDF); x, 151 pages.We perform a search for long-lived neutral particles in final state with delayed photons and large missing transverse energy produced in LHC proton-proton collisions at center-of-mass energy, $\sqrt{s} = 8$TeV. Capitalizing on the excellent timing resolution of the CMS Electromagnetic Calorimeter the search uses photon time measurements made by the Electromagnetic Calorimeter as the main search quantity. We found a single event consistent with our background expectations from the Standard Model and set a model-independent upper limit of 4.37 on number of signal events. We also interpret our results in the context of the SPS8 benchmark GMSB model and show that neutralinos with mean lifetime, $\tau_{\PSneutralinoOne} \leq 45$ns, and mass, $m_{\PSneutralinoOne} \leq 300$GeV/c^{2}, or effective Supersymmetry breaking energy scale, $\Lambda \leq 220TeV$, are ruled out of existence at 95\% $CL_{S}$ confidence level. The exclusion limit on the product of the production cross-section and branching ratio of the neutralino to photon and gravitino decay channel, $\sigma_{(\PSneutralinoOne \rightarrow \gamma + \tilde{G})}\times BR$, for different lifetimes and masses is derived. Our results confirm for the first time that the CMS Electromagnetic Calorimeter provides good sensitivity to search for long-lived neutral particles with lifetimes up to $40$ns and masses up to $300$GeV/c^{2}using only timing measurements.enBeam Halo Muon-Induced PhotonCosmic Muon-Induced PhotonSpike Seeded PhotonCERNLHCCMSECAL TimingDelayed PhotonsElectronsNeutralinoGravitinoGauge Mediated Supersymmetry BreakingLong-Lived Neutral ParticlesSPS8 Benchmark ModelCross-Section LimitsThesis or Dissertation