Measurement of the CKM angle gamma in the B⁰->DK*⁰ decays using the Dalitz method in the LHCb experiment at CERN and photon reconstruction optimisation for the LHCb detector upgrade

Quark mixing is described in the standard model of particle physics with the Cabibbo-Kobayashi-Maskawa mecanism. The angle gamma of the unitarity triangle is one of the parameters of this mecanism that is still determined with a large uncertainty. It can be measured without significant contribution of new physics, making it a standard model key measurement. The current precision of the best direct measurement of gamma is approximately 10°, whereas the global fits of the CKM parameters determine this angle up to a few degrees. Therefore precise measurement of this quantity is needed to further constrain the Unitarity Triangle of the CKM matrix, and check the consistency of the theory. This thesis reports a measurement of gamma with a Dalitz analysis of the B0->DK*0 channel where the D meson decays into K0Spipi, based on the 3 fb⁻¹ of proton-proton collision data collected by LHCb during the LHC Run I, at the centre-of-mass energy of 7 and 8 TeV. This channel is sensitive to gamma through the interference between the b->u and b->c transitions. The CP violation observables are measured to be x- = -0.09 ^{+0.13}_{-0.13} ± 0.09 ± 0.01 , x+ = -0.10 ^{+0.27}_{-0.26} ± 0.06 ± 0.01 , y- = 0.23 ^{+0.15}_{-0.16} ± 0.04 ± 0.01 , y+ = -0.74 ^{+0.23}_{-0.26} ± 0.07 ± 0.01 , where the first uncertainty is statistical, the second is the experimental systematic uncertainty and the third is the systematic uncertainty due to the Dalitz model. A frequentist interpretation of these observables leads to rB0 = 0.39±0.13 , deltaB0 = ( 186^{+24}_{-23} )°, gamma = ( 77^{+23}_{-24} )° , where rB0 is the magnitude of the ratio between the suppressed and favoured decays and deltaB0 the strong phase difference between these two decays. In addition, the work performed on the optimisation of the photon reconstruction for the upgraded LHCb detector is reported. During LHC Run III, the LHCb instantaneous luminosity will be increased by a factor five, implying a larger shower overlap in the electromagnetic calorimeter. The study shows that reducing the cluster size used in the photon reconstruction limits the effect of the overlap between the showers, without inducing a significant energy leakage. With some dedicated corrections, the new cluster reconstruction improves the Bs->Phi gamma mass resolution by 7 to 12%, depending on the calorimeter region.