Browsing by Author "Gray, Heather M"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Open Access
    The reconstruction of high-pt photons with the electromagnetic calorimeter of the ALICE experiment at the LHC.
    (2005) Gray, Heather M; Cleymans, Jean
    Careful jet and jet quenching analyses require calibration with the parton energy prior to its passage though the nuclear matter. This can only be obtained via analysis of y-jet processes because photons, once produced, are essentially unaffected by a colour-charged medium, and pass through the medium retaining their original energy. The experimental challenges include identifying photons within the high multiplicity heavy-ion environment and reducing the background from the photonic decays of neutral pions. This thesis details algorithms developed for photon reconstruction in the heavy-ion environment for the electromagnetic calorimeter (EMCal) of the ALICE experiment. Shower shape analysis was used to optimise cuts to discriminate between the direct photon signal and the background from the photonic decays of neutral pions.
  • Thumbnail Image
    Item
    Open Access
    Single top channel in association with H→bb̄ production at ATLAS : a feasible study
    (2015) Antel, Claire; Hamilton, Andrew; Gray, Heather M
    A degeneracy in the minima of the profile likelihood function of the Higgs boson coupling to fermions and bosons, where boson couplings are assumed to be positive, is reported by both ATLAS and CMS. Although one minimum lies in the region of positive fermionic coupling scale factors, consistent with the Standard Model, the second minimum lies in the negative region-a region which would indicate new physics. This degeneracy can potentially be resolved by studying a fermionic-bosonic interference that takes place in the single top quark production in association with a Higgs particle, in which the cross-section is significantly enhanced for anomalous coupling values. A truth level feasibility study of the tH process with H→bb̄ is conducted at 8 TeV and 21 fb-¹ of integrated luminosity. the equivalent of the 2012 run at the LHC. A sensitivity of l.7σ is found for the enhanced case of a coupling scale factor of cF = -1. This is insufficient for a detection. A projection to 14 TeV centre-of-mass energies indicates that at truth-level a 3σ significance with 15 fb-¹ of integrated luminosity can be reached for the enhanced case. However, assuming as low as 5% systematic uncertainty lowers this to below 2σ across the whole luminosity range, indicating that the upcoming run at LHC will also be insufficient to resolve the degeneracy.
  • No Thumbnail Available
    Item
    Open Access
    The reconstruction of high-pt photons with the electromagnetic calorimeter of the ALICE experiment at the LHC
    (2005) Gray, Heather M; Cleymans, Jean; Odyniec, Grazyna
    In two years, the frontier of relativistic heavy ion physics will move to the Large Hadron Collider (LHC), which will explore matter with energy densities greater than 10 times that at the Relativistic Heavy Ion Collider (RHIC), providing an opportunity to test predictions of Quantum Chromodynamics (QCD). At the LHC, the cross-sections for high momentum processes are expected to be sufficiently large for jet physics to play an important role. Some first hints of this exciting physics have already been reported at RHIC energies, namely observation of jet quenching, as indicated by the suppression of the yield of high Pt hadrons and the suppressed yield of back-to-back correlated hadrons from jets, in central Au+Au collisions. However, careful jet and jet quenching analyses require calibration with the parton energy prior to its passage though the nuclear matter. This can only be obtained via analysis of 1 -jet processes because photons, once produced, are essentially unaffected by a colour-charged medium, and pass through the medium retaining their original energy. The experimental challenges include identifying photons within the high multiplicity heavy-ion environment and reducing the background from the photonic decays of neutral pions. This thesis details algorithms developed for photon reconstruction in the heavy-ion envi-ronment for the electromagnetic calorimeter (EMCal) of the ALI experiment. Shower shape analysis was used to optimise cuts to discriminate between the direct photon signal and the background from the photonic decays of neutral pions. Photon energy resolution of approxi-mately 5% at 10 GeV and 3% at 30 GeV was obtained in simulations. An enhancement in the ratio by a factor of 50 relative to theoretical predictions at 20 GeV was obtained using the techniques of shower shape analysis