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Title: Searches for B → Dμ+μ− decays and test-beam studies of TORCH detector prototypes
Author: Hancock, Thomas H.
ISNI:       0000 0005 0291 505X
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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Two complementary pieces of research, linked by the LHCb experiment, are presented. The first uses 8.7 fb−1 of LHCb data to search for a set of rare decays in the B → Dμ+μ- family: B0 → D0μ+μ, B0 → D0J/ψ , Bs0 → D0μ+μ-, Bs0 → D0J/ψ, B+ → Ds+μ+μ−, B+ → Ds+J/ψ, and Bc+ → Ds+μ+μ-. Although no signals are observed, the following world-best upper limits are set on the branching ratios at the 95% confidence level: B(B0 → D0μ+μ-) < 6.1 × 10−8, B(B0 → D0J/ψ) < 1.4 × 10−6, B(Bs0 → D0μ+μ-) < 1.8 × 10−7, B(Bs0 → D0J/ψ) < 1.4 × 10−6, B(B+ → Ds+μ+μ-) < 3.2 × 10−8, B(B+ → Ds+J/ψ) < 3.5 × 10−7, and (fc/fu).B(Bc+ → Ds+μ+μ-) < 9.6 × 10−8, where fc and fu are the probabilities that a b quark will hadronize into a Bc+ or B+ mesons respectively. Additionally, the branching ratio of the previously-observed Bc+ → Ds+J/ψ decay is measured to be (fc/fu).B(Bc+ → Ds+J/ψ ) = (1.63 ± 0.15 ± 0.13) × 10−5, where the first error is statistical and the second systematic. The second piece of research details test-beam studies of novel time-of-flight detector for a future upgrade of LHCb, prototypes of the so-called TORCH detector. Three test-beam campaigns have been undertaken, two with a small- scale demonstrator and one with a half-sized LHCb module. Single-photon time resolutions better than 100 ps have been measured; a plan of future work required to reach the target 70 ps is discussed. The photon-counting efficiency of the half- sized demonstrator is also presented. A deficit of photons compared to the LHCb benchmark is seen, however with MCP-PMT photon detectors of improved quantum efficiency, photon yields close to requirement should be anticipated.
Supervisor: Harnew, Neville Sponsor: Department of Physics ; University of Oxford ; Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Particle Physics