Search for CP violation with the singly-Cabibbo-suppressed decays D+ → φπ+ and Ds+ → KS0π+ at LHCb

In this thesis work is described a preliminary measurement of the search for direct CP-violation in the singly-Cabibbo-suppressed decays Ds+ → KS0π± and D+ → φπ+ reconstructed using proton-proton collision recorded by the LHCb detector at a centre-of-mass energy of 13 TeV during the 2018 data taking campaign. The analysed data correspond to an integrated luminosity of about 2 fb-1 The measurements of the CP asymmetries, ACP, are determined from the so-called raw asymmetries obtained from fits to the invariant mass distribution of the Cabibbo-suppressed Ds+ → KS0π+ and D+ → φπ+ decays. The adopted fitting model is obtained by studying a large data sample of simulated events and represents a novelty for this measurement. Thanks to this study, the systematic uncertainties related to the fitting model improves with respect to the previous analysis performed by LHCb on a different data sample. The values of ACP are determined using the Cabibbo favourite D+ → KS0π+ and Ds+ → φπ+ decays. Given that ACP can be considered negligible, these decays allow to constrain the D+ production and the π+ detection asymmetries. The final results are ACP (Ds+ → φπ+) = −(0.57 ± 0.04)% ACP (Ds+ → KS0π+) = −(0.07 ± 0.25)% where the central values are blind, i.e. biased by a random shift, as this analysis still needs to be reviewed by the LHCb collaboration before being published. The errors reported are only statistical. The systematic uncertainties are discussed, but not evaluated as this study is beyond the aim of the thesis work.