Search for direct CP violation in charm neutral meson decays at LHCb

A search for time-integrated violation of the $C\!P$ symmetry, $\mathcal{A}_{C\!P}(K^-K^+)$, in the Cabibbo-suppressed $D^{0}\rightarrow K^-K^+$ decays is performed at the LHCb detector using proton-proton collisions recorded from 2015 to 2018 at the centre of mass energy of 13 TeV. The data used corresponds to an integrated luminosity of 5.7 fb$^{-1}$. The flavour of the charm mesons is defined from the charge of the pion in \mbox{$D^{*+} \rightarrow D^{0}\pi^+$} and \mbox{$D^{*-} \rightarrow \overline{D}^{0} \pi^-$} decays. Nuisance asymmetries are constrained from \mbox{$D^{*+}\rightarrow D^{0}(\rightarrow K^-\pi^+)\pi^+$}, $D^{+}\rightarrow K_{S}^{0}\pi^+$, $D^{+}\rightarrow K^-\pi^+\pi^+$, $D_{s}^+\rightarrow K_{S}^{0} K^+$ and $D_{s}^+\rightarrow \phi\pi^+$ decays. The $\mathcal{A}_{C\!P}(K^-K^+)$ asymmetry is measured to be \begin{align*} \mathcal{A}_{C\!P}(K^-K^+) = \left[6.8 \pm 5.4 \mathrm{\,(stat)} \pm 1.6 \mathrm{\,(syst)}\right]\cdot 10^{-4}, \end{align*} in agreement with the previous LHCb results and the current world average. This represents the world's most precise measurement of this quantity to date. Combining $\mathcal{A}_{C\!P}(K^-K^+)$ with the time-integrated $C\!P$ asymmetry difference, $\Delta\mathcal{A}_{C\!P}=\mathcal{A}_{C\!P}(K^-K^+)-\mathcal{A}_{C\!P}(\pi^-\pi^+)$, and the time-dependent $C\!P$ asymmetry, $\Delta Y$, measured with $D^{0}\rightarrow K^-K^+$ and $D^{0}\rightarrow \pi^-\pi^+$ decays, the direct $C\!P$ asymmetries in $D^{0}\rightarrow K^-K^+$ and $D^{0}\rightarrow \pi^-\pi^+$ decays, $a^d_{KK}$ and $a^d_{\pi\pi}$, result to be \begin{align*} a^d_{KK} &= (7.7 \pm 5.7)\cdot 10^{-4},\\ a^d_{\pi\pi} &= (23.2 \pm 6.1)\cdot 10^{-4}, \end{align*} where the errors include systematic and statistical uncertainties and the correlation between the two values is $\rho(a^d_{KK} ,a^d_{\pi\pi}) = 0.88$. The values differ from zero for 1.4 and 3.8 standard deviations, respectively. In particular, $a^d_{\pi\pi}$ shows an evidence for direct $C\!P$ violation in $D^{0}\rightarrow \pi^-\pi^+$ decays. In this thesis, a preliminary study regarding the performance evaluation for a future silicon tracker around the interaction region (Vertex Locator) for Upgrade-II is also reported. In particular, the possibility of using a 3D silicon sensor developed within the TimeSpot project is explored. Considering a preliminary parametrisation of the sensor with a time resolution per pixel of 50~ps, the track reconstruction efficiency of charged particles is estimated to be 97\% while the fake track rate is 1.6\%. Those preliminary results are not far from the current performance of the LHCb-Upgrade VELO detector and show the importance of time measurements for a vertex detector working in the High-Luminosity era.