Searches for $B\rightarrow D\mu^+\mu^-$ decays and test-beam studies of TORCH detector prototypes

Two complementary pieces of research, linked by the LHCb experiment, are presented. The first uses 8.7fb$^{-1}$ of LHCb data to search for a set of rare decays in the $B \rightarrow D\mu^+\mu^-$ family: $B^{0} \rightarrow \bar{D^{0}}\mu^+\mu^-$, $B^{0} \rightarrow \bar{D^{0}}J/\psi$, $B^{0}_{s} \rightarrow \bar{D^{0}}\mu^+\mu^-$, $B^{0}_{s} \rightarrow \bar{D^{0}}J/\psi$, $B^{+} \rightarrow D^{+}_{s}\mu^+\mu^-$, $B^{+} \rightarrow D^{+}_{s}J/\psi$, and $B^{+}_{c} \rightarrow D^{+}_{s}\mu^+\mu^-$. Although no signals are observed, the following world-best upper limits are set on the branching ratios at the 95\% confidence level: $\phantom{and}\,\,\mathcal{B}\left(B^{0} \rightarrow \bar{D^{0}}\mu^+\mu^-\right)\,\,<\,\,6.1\times 10^{-8}$, $\phantom{and}\,\,\mathcal{B}\left(B^{0} \rightarrow \bar{D^{0}}J/\psi\right)\,\,<\,\,1.4\times 10^{-6}$, $\phantom{and}\,\,\mathcal{B}\left(B^{0}_{s} \rightarrow \bar{D^{0}}\mu^+\mu^-\right)\,\,<\,\,1.8\times 10^{-7}$, $\phantom{and}\,\,\mathcal{B}\left(B^{0}_{s} \rightarrow \bar{D^{0}}J/\psi\right)\,\,<\,\,1.4\times 10^{-6}$, $\phantom{and}\,\,\mathcal{B}\left(B^{+} \rightarrow D^{+}_{s}\mu^+\mu^-\right)\,\,<\,\,3.2\times 10^{-8}$, $\phantom{and}\,\,\mathcal{B}\left(B^{+} \rightarrow D^{+}_{s}J/\psi\right)\,\,<\,\,3.5\times 10^{-7}$, and $\frac{f_{c}}{f_{u}}.\mathcal{B}\left(B^{+}_{c} \rightarrow D^{+}_{s}\mu^+\mu^-\right)\,\,<\,\,9.6\times 10^{-8}$, where $f_{c}$ and $f_{u}$ are the probabilities that a $b$ quark will hadronize into a $B^{+}_{c}$ or $B^{+}$ mesons respectively. Additionally, the branching ratio of the previously-observed $B^{+}_{c} \rightarrow D^{+}_{s}J/\psi$ decay is measured to be \begin{equation} \frac{f_{c}}{f_{u}}. \mathcal{B}\left( B^{+}_{c} \rightarrow D^{+}_{s}J/\psi \right) = \left(1.63 \pm 0.15 \pm 0.13 \right) \times 10^{-5}\,, \end{equation} 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.