Measurement of the CP asymmetries difference between $D^+ \rightarrow K^-K^+\pi^+$ and $D^+ \rightarrow \pi^-\pi^+\pi^+$ decays in the LHCb experiment

The violation of the combined charge conjugation and parity symmetry, or simply CP violation, is one of the necessary conditions to explain the matter-antimatter asymmetry observed in the Universe. In the Standard Model, CP violation arises due to an irreducible complex phase in the Cabibbo-Kabayashi-Maskawa (CKM) matrix. However, this mechanism is not enough to explain the observed baryogenesis. New sources of CP violation are necessary. The Standard Model predicts very small CP violation in the charm sector, $\mathcal{O}(10^{-3})$ or less. This makes decays of D mesons an excellent tool in the search for these new sources. With the large sample collected at LHCb during Run I, corresponding to 1.0 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}= 7$ TeV and 2.0 fb$^{-1}$ at $\sqrt{s}= 8$ TeV, we can reach the upper limits of Standard Model predictions with several decay modes. In the Standard Model, direct CP violation in the charm sector can only occur in Cabibbo-supressed (CS) decays. In this work, we measure the difference between the CP asymmetries $A_{CP}$ of the CS decays $D^+\rightarrow K^-K^+\pi^+$ and $D^+\rightarrow\pi^-\pi^+\pi^+$ ($\Delta A_{\rm CP}$), under the assumption that instrumental and production asymmetries cancel between the two modes. The analysis uses as control channels, for the study of instrumental and production asymmetries, the Cabibbo-favoured decays $D_s^+\rightarrow K^-K^+\pi^+$ and $D_s^+\rightarrow \pi^-\pi^+\pi^+$, where no CP violation is expected, and obtains the $D^+$ asymmetries in a blind way, by randomly inserting offsets to the actual fitted values. Once the study, which is currently under the Charm Working group review, is approved by the collaboration, the offsets will be removed and the $\Delta A_{\rm CP}$ values will be known.