Measurement of CP asymmetries in $\lambda^0_b \to pk^-$ and $\lambda^0_b \to p \pi^-$ decays at LHCb

The LHCb experiment has been designed to perform precision measurements in the flavour physics sector at the Large Hadron Collider (LHC) located at CERN. After the recent observation of CP violation in the decay of the Bs0 meson to a charged pion-kaon pair at LHCb, it is interesting to see whether the same quark-level transition in Λ0b baryon decays gives rise to large CP-violating effects. Such decay processes involve both tree and penguin Feynman diagrams and could be sensitive probes for physics beyond the Standard Model. The measurement of the CP-violating observable defined as ∆ACP = ACP(Λ0b → pK−)−ACP(Λ0b →pπ−),where ACP(Λ0b →pK−) and ACP(Λ0b →pπ−) are the direct CP asymmetries in Λ0b → pK− and Λ0b → pπ− decays, is presented for the first time using LHCb data. The procedure followed to optimize the event selection, to calibrate particle identification, to parametrise the various components of the invariant mass spectra, and to compute corrections due to the production asymmetry of the initial state and the detection asymmetries of the final states, is discussed in detail. Using the full 2011 and 2012 data sets of pp collisions collected with the LHCb detector, corresponding to an integrated luminosity of about 3 fb−1, the value ∆ACP = (0.8 ± 2.1 ± 0.2)% is obtained. The first uncertainty is statistical and the second corresponds to one of the dominant systematic effects. As the result is compatible with zero, no evidence of CP violation is found. This is the most precise measurement of CP violation in the decays of baryons containing the b quark to date. Once the analysis will be completed with an exhaustive study of systematic uncertainties, the results will be published by the LHCb Collaboration.

Measurement of the Z and W production cross section in pp collisions at LHC using a bayesian approach

L’obiettivo di tutto il mio lavoro è stato quello di misurare le sezioni d’urto di produzione dei bosoni deboli W ± e Z nei loro decadimenti leptonici (e, μ) coi dati raccolti dal rivelatore ATLAS a LHC con un’energia del centro di massa di √s = 13 TeV relativi all’estate 2015. Gli eventi selezionati sono gli stessi di quelli del recente articolo della Collaborazione ATLAS sullo stesso argomento, in modo anche da poter operare un confronto tra i risultati ottenuti. Confronto peraltro necessario, poichè i risultati sono stati ottenuti con due metodologie differenti: tradizionale (classica) per l’articolo, bayesiana in questa tesi. L’approccio bayesiano permette di combinare i vari canali e di trattare gli effetti sistematici in modo del tutto naturale. I risultati ottenuti sono in ottimo accordo con le predizioni dello Standard Model e con quelli pubblicati da ATLAS.