The processes that are responsible for the abundance of matter in the Universe remain elusive to cosmologists and particle physicists alike. Violation of charge-parity (CP) - the symmetry between matter and antimatter in response to physical interactions - is thought to be a necessary requirement for prevalence of matter. CP-violation in composite particle decays mediated by the charged weak force has been observed, however, in the Standard Model (SM) of high energy physics, such processes are too rare to account for cosmological measurements. It remains an outstanding challenge of the high energy physics community to identify and constrain sources of CP-violation, in the SM and beyond. Since the discovery of the Higgs boson by the ATLAS and CMS experiments at CERN, the search for new physics is ongoing at the Large Hadron Collider (LHC). The associated production of the heaviest known particles can now be studied, giving direct access to fundamental couplings which are indispensable to the continued scrutiny of SM predictions. One sector is the associated production of top quarks with a Higgs boson, where the top quark polarisation and spin correlations between top quark matter-antimatter pairs can be used to probe the CP structure of the top quark coupling to the scalar field (the strongest amongst all particles in the SM). This work compares SM predictions for single top and top pair associated production with a Higgs boson at the LHC against those of the SM Effective Field Theory (SMEFT). This alternative framework expands upon the efficacy of the SM at low energies via a basis of higher dimension operators that parameterise effective interactions between SM fields. A reduced basis of dimension-six SMEFT operators for the top-Higgs sector is defined, which introduce effective electroweak and strong couplings between bosons and with third-generation quarks. Focus is placed on operators which parameterise CP-odd or CP-violating interactions. Proton-proton collision events are generated, at âs = 13 TeV and leading order in the perturbative QCD expansion, to compute differential cross-section predictions, using kinematic and angular observables which are sensitive to top polarisation and $t\bar{t}$ spin correlations and/or are manifestly CP-odd. SMEFT predictions for a set of these observables are injected into a $\chi^2$-fit to SM simulation, to obtain indicative limits on operators in the reduced basis. This fit uses $th$ and $t\bar{t}h$ signal strength measurements from the LHC as guidelines. The ability to identify and establish marginalised bounds on sources of anomalous CP in top Higgs production is demonstrated in this way, using physically motivated kinematic observables. These studies are contextualised by efforts towards a global SMEFT interpretation of CP-violation at the LHC, and aim to provide recommendations for analysis on effective operators relevant to top Higgs associated production alongside observables that can be used to constrain them.
Date of Award | 6 Jan 2025 |
---|
Original language | English |
---|
Awarding Institution | - The University of Manchester
|
---|
Supervisor | Eleni Vryonidou (Supervisor) & Yvonne Peters (Supervisor) |
---|
Sensitivity to CP-violating effective couplings in the top-Higgs sector
Winter, J. (Author). 6 Jan 2025
Student thesis: Master of Philosophy