Public Thesis defense - IRMP

New physics probes through loop computation in effective theories by Andres Felipe VASQUEZ TOCORA

Pour l’obtention du grade académique de Doctorat en sciences

Given the current situation, where no new particles or direct signs for Beyond the Standard Model (BSM) physics have been found, we consider the indirect signs that such states would leave on measurement at LHC and foreseen collider energies. The Standard Model Effective Field Theory (SMEFT) is a powerful framework useful in the search of such deviations. Higher-order SM predictions allow to correctly establish a deviation, while higher-order SMEFT predictions allow to enhance sensitivity by correctly describing correlations, by accounting for new dependencies and allow to precisely interpret deviations (towards the correct family of UV models). It is for this reasons that one-loop contributions should be taken into consideration when making predictions in the SMEFT. Given the fact that the top quark is the heaviest elementary particle known to date, BSM particles are often expected to have sizable couplings to the top. Moreover, studies in the top sector are motivated by the fact that the top is the least known particle from the strong sector and the LHC is a top factory. The top sector is thus of interest in the search of new physics signals. This thesis aims at presenting several theoretical and phenomenological aspects of the SMEFT. We discuss the challenges and subtleties appearing in the computation of the top-pair production in the SMEFT at one-loop in QCD, and the corresponding sensitivity of this process to new physics. As a result, we provide bounds on four-heavy-quark operators from the top-pair production data, which are complementary to other bounds found in the literature. The effects of BSM physics coupling to the top can also be studied in processes without tops in the final state. Because of the large Yukawa coupling of the top, the one-loop induced double Higgs production in lepton colliders offers the possibility of studying such couplings. We perform a sensitivity study for several scenarios of future lepton colliders such as CLIC, ILC and FCC-ee.

While the SMEFT focuses on BSM effects via deviations at high energy we also investigate near-threshold effects through the Sommerfeld enhancement. As a matter of fact, light mediators from hidden sectors could couple to final state particles, leading to the origin of resonances due to non-perturbative effects. Experimentally, the near-threshold-energy regions of several production processes are not yet well measured, such as the top-pair and double Higgs production. On the theoretical side, an identification in terms of UV divergences in the relativistic QFT of the divergences of the calculation of P-wave Sommerfeld enhancement in processes with unstable particles in the final state was missing in the literature. To obtain a finite result, we implement a renormalization procedure, showing that these divergences can be removed by adding proper counterterms. This procedure offers the advantage of not having the arbitrariness of introducing a cutoff and thus is safe of spoiling the physical significance of the enhancement.

Jury members :

• Prof. Céline Degrande (UCLouvain), supervisor
• Prof. Rogerio Rosenfeld (UNESP, Brazil), supervisor
• Prof. Fabio Maltoni (UCLouvain), chairperson
• Prof. Giacomo Bruno (UCLouvain), secretary
• Prof. Oscar Eboli (Universidade de Sao Paulo, Brazil)
• Dr. Gauthier Durieux (CERN, Switzerland)

Pay attention :

The public defense of Andres Felipe Vasquez Tocora scheduled for Thirsday 22 December at 03:00 p.m. will also take place in the form of a video conference

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