Study of charmless $B^0_{d,s}\to h^+h'^-\pi^0$ decays in LHCb

This thesis work is about the identification of charmless three-body decays of neutral B mesons with a $\pi^0$ in the final state ($B^0_{d,s}\to h^+h'^-\pi^0$) through the analysis of a 1.0 fb$^{-1}$ data sample collected in 2011 by the LHCb experiment at the LHC. Those modes are sensitive to CKM parameters and allow to test Standard Model predictions related to CP violation. Some of them, such as $B^0_s\to K^-\pi^+\pi^0$ and $B^0_s\to K^+K^-\pi^0$, have not been observed previously. This is the first analysis with energetic $\pi^0$ in LHCb. To evaluate the detector performance for $\pi^0$, $D^{*+}\to D^0(\to K^-\pi^+\pi^0)\pi^+$ decays have been selected. This provides a clean and high statistics sample of neutral pions used to study their energy calibration as well as the effectiveness of the $\pi^0$/$\gamma$ separation tool of the experiment. Radiative decays are indeed the most dangerous backgrounds for the study of $B^0_{d,s}\to h^+h'^-\pi^0$ modes. To select $B^0_{d,s}\to h^+h'^-\pi^0$ events, a dedicated trigger algorithm has been implemented. It complements the standard online selections and significantly improves the efficiency. Offline, obvious backgrounds, mainly from charm decays, are rejected applying simple criteria. The sample is then further purified using a multivariate classifier. At last, information from the ring imaging Cherenkov detectors allows to separate $\pi^+\pi^-\pi^0$, $K^{\pm}\pi^{\mp}\pi^0$ and $K^+K^-\pi^0$ final states. A simultaneous fit of the reconstructed B mass spectra in the three subsamples is used to extract the signal yields. It accounts for the different backgrounds remaining as well as for the signal crossfeeds from one subsample to the other. The $B^0\to \pi^+\pi^-\pi^0$, $B^0\to K^+\pi^-\pi^0$ and $B^0_s\to K^+K^-\pi^0$ contributions are clearly established. This is the first observation of the $B^0_s\to K^+K^-\pi^0$ decay mode.