CP violation in D0→K+K- and D0→π+π- decays and lepton-flavour universality test with the decay B0→D*-τ+ντ

Abstract

The LHCb experiment has been designed to exploit the potential of heavy-flavour production in highly energetic pp collisions at the Large Hadron Collider, in order to look for indirect signs of physics beyond the Standard Model of particle physics. This thesis presents two distinct measurements performed using LHCb data. The first is that of the ratio of branching fractions R(D*)=B(B→D*-τ+ντ)/B(B→D*-l+νl), with the τ lepton decaying to final states containing three charged pions, performed using a data sample of pp collisions at the centre-of-mass energies of 7 and 8 Tev, corresponding to 3fb-1 of integrated luminosity. The result is R(D*)=0.291±0.019±0.026±0.013, where the first uncertainty is statistical, the second systematic and the third due to the knowledge of the B(B→D*-π+π-π+) branching fraction, as this decay is used in the analysis as a normalisation for the intermediate measurement of B(B→D*-τ+ντ). This corresponds to one of the most precise single measurements of R(D*) and to the first performed with the three-prong decay of the τ lepton to date. The measured value of R(D*) is compatible with previous determinations and with the Standard Model expectation. The second measurement presented in this thesis is that of the difference between the CP asymmetries in D0→K+K- and D0→π+π- decays, performed using a data sample of pp collisions at the centre-of-mass energy of 13 Tev, corresponding to 6fb-1 of integrated luminosity. The value of ΔACP=ACP(K+K-)-ACP(π+π-), measured by reconstructing D0 mesons which originate from D*+→D0π+ decays or from B→D0μνX decays, is ΔACP=(-17.1±3.0±1.0)x10-4, where the first uncertainty is statistical and the second systematic. By combining this result with previous LHCb measurements based on Run-1 data, the value of ΔACP results to be ΔACP=(-15.4±2.9)x10-4, which differs from zero by 5.3 standard deviations. This is the first observation of CP violation in the decay of a charm hadron.