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Title: Searching for supersymmetry with same-sign dimuons
Author: Grant, Nicholas Ian
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2009
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A study was made of Monte Carlo simulations of the channel pp -> ḡḡ -> bbμ+μ+(or b¯b¯μ¯μ¯) + jets + missing ET which offers a realistic possibility of searching for supersymmetry. This has a distinctive signature of two same-sign muons in half of the events. After reconstruction, requirements were imposed on the PT, isolation and impact parameter of the muons. Events were also required to have either four jets with ET greater than a threshold that was varied or missing ET greater than a variable threshold. With these requirements, the backgrounds were reduced to a level equal to or less than that of the signal. If four jets with ET > 35 GeV were required, the signal-to-background ratio was 1.2 ± 0.3 (statistical)+0.6/-0.4(systematic) +0.8/-0.5(theoretical) when low luminosity pile-up was added to the signal. The minimum integrated luminosity required to obtain a 50" excess of signal events was estimated to be [5.6 +1.4 (statistical) +2.3 (systematic) +1.9 (theoretical)] fb-1. A larger signal-to-background ratio of 2.3 ± 0.5 (statistical)+0.6/-0.3(systematic) +1.5/-1.1(theoretical) could be obtained by increasing the jet ET threshold to 50 GeV. Testbeam data were analysed on lead tungstate crystals and vacuum phototriodes for the CMS electromagnetic calorimeter endcaps. A potential instability of the vacuum phototriodes was found, which is believed to be due to movement of caesium within the electrode coatings; this was reduced by illumination with a 100 Hz laser. The energy resolution for electron reconstruction in a 3x3 matrix of endcap crystals was (1.21 ± 0.05)% at 120 GeV, which was worse than expected. This was partly due to additional noise from barrel-type VFE cards. Position resolution using the log weighting method applied to the same 3x3 matrix was (0.96 ± 0.02) mm. Formation of colour centres was investigated by repeatedly irradiating the crystals with 120 GeV electrons, alternating with injection of a 440 nm monitoring laser. For six crystals, the exponents a of power law fits to electron and laser amplitudes had a mean of 1.15 and a standard deviation of 0.06. The intercalibration error due to absorption of scintillation light by colour centres was restricted to 0.2% provided that the correct value of a for a crystal or its mean value for a set of crystals was used
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available