Resurrecting the Standard Model Effective Field Theory interferences at colliders

Abstract

Even if some experimental evidence suggests the existence of physics beyond the SM, no clues of new resonances can be found in the data. In the case their masses are much larger than the energies of current experiments, the SMEFT formalism can be used to introduce new operators that parametrise small deviations from the SM predictions at the LHC, induced by interactions between the known and new states. This thesis focuses on some operators and processes for which the leading correction to the SM, namely its interference with dimension-6 operators, is suppressed, either because it is small all over the phase space as a result of a helicity mismatch in the SM and SMEFT amplitudes, or because a cancellation between large cross-section contributions with opposite sign occurs. Several useful quantities are introduced to distinguish among these two cases, and a phenomenological strategy to revive the interferences is developed. They are applied to the cases of the $O_G$ and $O_W$ operators, respectively in three-jet production and EW processes like VBF $Zjj$, $WZ$ and $W\gamma$. The comparison among them highlights how different procedures can be followed to restore the interference. The quantities introduced in this thesis can be used to find simple kinematic observables that are sensitive to the suppression and can yield competitive bounds on the coefficients of the operators, even outside the SMEFT validity region. In the last chapter, a study of ten four-light quark operators is presented at LO matched to parton shower. Individual and marginalised limits on them are obtained through multijet production and processes where the jets are generated together with EW bosons, like $Z, W, \gamma$+jets. Almost no interference suppression happens for these operators, but they can virtually affect any process at NLO.