Radiation damage studies in the LHCb VELO detector and measurement of the flavour-specific asymmetry in semileptonic B-decays

This thesis presents several studies of data collected at the LHCb detector during its first two years of operation. A detector upgrade study is first presented, using simulated events at an increased luminosity. A second study involves radiation damage to the silicon sensors of the LHCb Vertex Locator. During $2010$ and $2011$ the silicon sensors were exposed to a range of fluences, with sensors in the most active regions exposed to fluences of up to approximately $\rm{45 \times 10^{12}\,1\,MeV}$ neutron equivalent ($\rm{1\,MeV\,n_{eq}}$). The first observation of ${\rm {\it{n^+}}\mbox{-}on\mbox{-}{\it{n}}}$ sensor type inversion at the Large Hadron Collider is reported, occurring at a fluence of around $(10 - 15) \times 10 ^{12}$ of $1\,\rm{MeV\,n_{eq}}$. The effective depletion voltages of the only ${\rm {\it{n^+}}\mbox{-}on\mbox{-}{\it{p}}}$ sensors in use at the Large Hadron Collider have also been studied, with decreases of around $25\,$V observed after initial irradiation. Following this, the effective depletion voltage in ${\rm {\it{n^+}}\mbox{-}on\mbox{-}{\it{p}}}$ type sensors is observed to increase at a comparable rate to type inverted ${\rm {\it{n^+}}\mbox{-}on\mbox{-}{\it{n}}}$ type sensors. A reduction in the charge collection efficiency due to an unexpected effect involving the sensor readout lines is also observed. A third study relates to $CP$ violation in neutral $B$-meson mixing, by the measurement of the flavour-specific asymmetry. In the Standard Model, $CP$ violation from this source is expected to be of order $10^{-4}$. Any measured enhancement of this would be a strong indication of new physics. The D0 collaboration has measured the flavour specific asymmetry from $B^{0}$ and $B_{s}^{0}$ mixing, and found it to be inconsistent with the Standard Model at a confidence level of $3.9$ standard deviations, thus motivating an independent measurement from the LHCb experiment. Using the full $2011$ LHCb dataset, corresponding to $\rm{1.0\,fb^{-1}}$ of recorded luminosity, the $B_{s}^{0}$-meson component of the flavour-specific asymmetry is measured to be $a_{{\rm fs}}^{s} = (-0.12 \pm 0.48 \pm 0.34)\%$, where the first uncertainty is statistical and the second is systematic. This is the single most accurate measurement of $a_{{\rm fs}}^{s}$, and is consistent with both the D0 measurement and the Standard Model prediction.