Searches for B → Dμ+μ− decays and test-beam studies of TORCH detector prototypes

Thesis

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 , B_s⁰ → D⁰μ⁺μ^-, B_s⁰ → D0J/ψ, B+ → Ds⁺μ⁺μ−, B⁺ → D_s⁺J/ψ, and B_c⁺ → 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(B_s⁰ → D0μ⁺μ^-) < 1.8 × 10⁻⁷,

B(B_s⁰ → 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 B_c⁺ → D_s⁺J/ψ decay is measured to be

(fc/fu).B(B_c⁺ → D_s⁺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.

Authors: Hancock, TH

• DOI: 10.5287/ora-o8oommxyx
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Physics; Flavour Physics; Particle Detectors; LHCb; Particle Physics
• Subjects: Particle Physics