The unexpected success of the Standard Model in explaining collider phenomenology, together with the undeniable cosmological evidence for its incompleteness, leads us wondering where new physics may hide. If the SM is valid up to the scale of gravity, 10^19 GeV, new discoveries lie in the far future. If, however, Beyond-the-SM effects reside close to the electroweak scale, they could be within reach of current experiments. Given that after more than a decade of successful operation of the Large Hadron Collider all the low hanging fruits have already been collected, how can we ensure that we are exploiting the full potential of the LHC? In this work we explore novel ideas and strategies in this direction, focusing on the phenomenology of the top quark, the heaviest known fundamental particle. In the first part of this thesis, "The Effective Field Theory Pathaway", we present developments in theoretical predictions and Monte Carlo tools in the Standard Model Effective Field Theory, a framework where small deviations from the SM can be collected, parameterized, and analyzed together consistently. One of the current aims of the SMEFT community is calculation and automation of Next-to-Leading-Order accurate theoretical predictions. Corrections of QCD origin are already available and automated for arbitrary processes. However, up to now, electroweak corrections, that become important at high energy, were not available in general. In Chapter 2 we present a calculation and automation of NLO EW corrections for a sector of the SMEFT in the high-energy limit, the region where these effects are most important. Next, in Chapter 3 we focus on another ingredient required for accurate theory predictions, the Renormalization Group evolution of the SMEFT. In Chapter 3 we describe how, for the first time, we implemented the differential equations governing such evolution in a Monte Carlo tool, allowing for a simple and convenient inclusion of SMEFT RG effects in theoretical predictions for experimental analyses. In the second part of this work, "The quantum frontier", we explore a novel trend that emerged, where fundamental aspects of quantum mechanics are employed to look for new phenomena. Between 2023 and 2024, the ATLAS and CMS experiments at the LHC observed entanglement between top quarks, sparking a quickly growing program of TeV-scale "quantum" measurements. This new class of analyses can provide a new and unconventional way to search for new physics: in Chapter 4 we provide a calculation of SMEFT effects in spin correlations and entanglement measurements between top quarks, for the first time at NLO accuracy in QCD and in 1/Lambda, where Lambda is the SMEFT cutoff scale. Next, in Chapter 5 we investigate the possibility of detecting new particles, such as scalar or pseudoscalar bosons, by indirectly observing their footprint in the quantum state of top quark pairs. We find that quantum observables provide complementary information to regular measurements, and show several examples where they extend the reach of existing searches. Finally, in Chapter 6 we study the phenomenology of similar quantum measurements of top quarks at the proposed FCC-ee machine, providing for the first time theoretical predictions and a phenomenological study.
- SMEFT
- Top Physics
- Quantum Information
- Collider Physics
New methods and results for searches of new physics in the top sector
Severi, C. (Author). 1 Aug 2025
Student thesis: Phd