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 $Wγ$. 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 $Z, W, γ$+jets. Almost no interference suppression happens for these operators, but they can virtually affect any process at NLO.

Figures (26)

• Figure 1: Notation for the colour indices $i,j,k,\ell$ in a photon ( left) and a gluon ( right) exchanges between a quark and an antiquark lines. $T^a$ with $a=1,\ldots,8$ are the $SU(3)$ generators
• Figure 2: Examples of corrections to the fundamental Feynman diagrams that need to be computed to renormalise QED at one-loop: fermion ( a) and photon ( b) propagators, and fermion-photon interaction vertex ( c). The second row shows examples of a real-emission diagram ( d) and a virtual one ( e)
• Figure 3: Feynman diagrams for the muon decay into an electron in the SM ( left) and in Fermi theory ( right). $p$ is the muon momentum
• Figure 4: The plot on the left shows examples of a SM and a $\mathcal{O}(1/\Lambda^2)$ amplitudes as functions of the CoM-scattering angle $\theta$ in a $2\rightarrow 2$ process. The plot on the right sketches the corresponding differential interference cross section, signed and in absolute value, as a function of the same angle
• Figure 5: LO differential distributions of the leading-jet $p_T$ ( top), the transverse sphericity ( centre) and the leading-jet absolute rapidity ( bottom) in three-jet production, for the SM divided by 100 ( black), the linear ( orange) and quadratic ( green) orders. They all show the $p_T^j>200$ GeV case. The positive-and negative-weighted contributions to the linear term are shown separately through the shaded histograms. Numerical uncertainties are included. $C_G$ is set to 1 and $\Lambda$ to 5 TeV. In the top plot, the last bin contains the overflow; in the central one, the dotted line is the inverse of the negative part