Measurement of indirect CP asymmetries in $D^0 \to K^− K^+$ and $D^0\to \pi^− \pi^+$ decays with the LHCb experiment

This thesis presents a measurement of indirect CP asymmetries in the charm system. The indirect CP violation for the charm-meson system is expected to be small in the Standard Model and has not been observed experimentally so far. Significant deviation from zero CP-violation would indicate New Physics and also a possible impact of non-perturbative processes. Effects of beyond Standard Model physics phenomena but as well perturbatively non-calculable processes can manifest themselves in a potential non-zero CP violation. The knowledge of the size of CP violation makes a significant contribution for the understanding of the Standard Model and has a direct impact on the effective parameters in the charm system. The $\mbox{LHCb}$ experiment has collected the world largest data samples of charm mesons. The proton-proton collisions data used in this thesis have been recorded in the first run period in 2011-2012. They correspond to an integrated luminosity of $3.0 {\mbox{fb}}^{-1}$. The measurement uses the singly Cabibbo-suppressed ${{D^{0}}}\rightarrow{K^{-}K^{+}}$ and ${{D^{0}}}\rightarrow{\pi^{-}\pi^{+}}$ decays. The charm mesons exploited in this analysis are produced in semi-muonic B-meson decays. Thereby, the charge of the created muon determines the flavour of the charm meson at production time. To perform this measurement, a robust method is developed that reaches a sensitivity at sub-permille level. The indirect CP asymmetry manifests itself in the asymmetries of effective lifetimes, $A_{\Gamma}$, of the ${{D^{0}}}$ and ${{{\kern 0.2em\overline{\kern -0.2em D}{}}^0}}$ decays and is measured for both decays $$ {{A_{\Gamma}(K^{-}K^{+})}} = (-0.134 \pm 0.077 \; {}^{+0.026}_{-0.034})\% \ , $$ $$ {{A_{\Gamma}(\pi^{-}\pi^{+})}} = (-0.092 \pm 0.145 \; {}^{+0.025}_{-0.033})\% \ ,$$ where the first uncertainty is statistical and the second systematic. The results are compatible with previous measurements and with the zero hypothesis of no CP violation. This analysis provides a significant contribution to the effective description of the charm meson system and to the world average. The results have been submitted for publication to the Journal of High Energy Physics (JHEP)[1].