Public Thesis Defense of Julien Touchèque - IRMP

Precision tests of the Standard Model: the Drell-Yan processes by Julien Touchèque -

Vendredi 11 septembre 2026 à 15h00 - Auditoire CYCL01 - Bâtiment de Hemptinne - Chemin du Cyclotron, 2 - 1348 Louvain-la-Neuve -

The discovery of the Higgs boson cemented the form of the Standard Model of particle physics (SM) as we know it and observations have so far confirmed SM predictions about the couplings of the Higgs with other particles. If the particle physics community used to face too few models other than the Higgs to explain the origin of the particle masses before its observation, too many beyond-the-Standard-Model (BSM) models have been proposed to overcome the remaining shortcomings of the SM to be tested experimentally. As a result, generic models present a useful intermediary step to point towards the potentialextension of the SM probing multiple Ultraviolet (UV) models simultaneously. In this thesis, two examples of generic models are investigated.
Off-shell effects of potential heavy new particles are provided in the Standard Model Effective Field Theory (SMEFT) through higher dimensional operators. In particular, charge-parity (CP) symmetry violating contributions from Wilson coefficients are ranked by using several U(1) global flavoursymmetries on CP-odd operators of the Warsaw basis. Asymmetries of simple observables,such as triple products, are proven to be more sensitive to the SMEFT operators compared to the cross-section and more efficient than most of the usual observables used in experiments. We use the dileptonic decays of the dibosonWZ and W\gamma processes to demonstrate the efficiency of these observables compared to the theoretical and measurable optimal observables.
On-shell effects of two heavy scalars are independently searched for in four-top production. These new scalars are singlet and octet of SU(3) that coupleto the top quark. The signal consists of the associated production of a scalar with a top quark pair and of pair production in the case of the octet where the fully hadronic, the leptonic and the same-sign-lepton dileptonic decay channels are considered. The decays of the massive scalars generate boosted top-quark pairs producing event topologies that are suppressed in the SM. The processes are generated at next-to-leading orderin quantum chromodynamics with a careful renormalisationprocedure and the correct selection of loop diagrams. Using improvements in top-jet tagging techniques and in the main background simulation as well as a new technique to select the events, anew particle search is performed to constrain the respective mass and coupling to top quarks of new resonances. The analysis yields improved limits by up to an order of magnitude compared to previous analyses.

Jury members :
Prof. DEGRANDE Céline (UCLouvain) (Supervisor)
Prof. MALTONI Fabio (UCLouvain) (Chairperson)
Prof. LEMAITRE Vincent (UCLouvain) (Secretary)
Prof. VRYONIDOU Eleni (University of Cyprus)
Prof. FUKS Benjamin (LPTHE, Paris, Fr.)

Pay attention : the public defense of Julien Touchèque will also take place in the form of a videoconference