Exploring fundamental physics at high-energy colliders : minimal supersymmetric standard model analyses and parton transverse momentum fits with dynamical resolution scales
Authors:   , Sadeghi Barzani Safura 
Published: July 2024

Investigations of fundamental interactions at current and future hadron colliders, from explorations into the origin of electroweak symmetry breaking to dark matter searches to studies of deep aspects of the strong interactions, require complex computational tools, which allow one to interpret the results of experimental measurements by performing detailed comparisons of the large amount of data produced in the experiments with theoretical predictions. This thesis presents studies of such computational techniques in two areas of collider physics: searches for supersymmetry through the minimal supersymmetric standard model (MSSM); determination of hadron structure in terms of transverse momentum dependent (TMD) parton distributions. Currently most successful and best-motivated extension of the standard model is supersymmetry (SUSY) with the Minimal Supersymmetric Standard Model(MSSM) being its best-known model. The first part of this thesis focuses on the MSSM, exploring its prospects and implications for future collider experiments. In this part of the study, an analysis of 30 MSSM points demonstrated that the proposed XENONnT direct search for neutralino dark matter (DM) will cover a substantial portion of the MSSM parameter space. However, the experiment is hindered by neutrino irreducible backgrounds that obscure some MSSM regions. To address this, complementary searches for SUSY at future colliders are crucial. By scanning the 30 free parameters of the MSSM to find regions consistent with limits from the LHC, SUSY particle searches (SModelS), the electron’s dipole moment, and cold dark matter relic density measurements, benchmark points were identified that feature long-lived charginos and scenarios where the lightest CP-even Higgs decays into BSM particles. The second part of the thesis investigates Quantum Chromodynamics (QCD) effects in the partonic transverse-momentum distributions of hadrons, using the recently proposed formalism of parton branching (PB) TMD evolution, which is coded in the Monte Carlo event generator Cascade and evolution program uPDFevolv. In this formalism, QCD coherence effects in the radiation of soft gluons are taken into account partly through the dependence of the QCD strong coupling on the radiated transverse momenta, and partly by introducing a soft-gluon resolution scale to separate resolvable and non-resolvable branchings, and allowing for the resolution scale to be dynamical, i.e., to depend on the mass scale at which the branching takes place. The objective of the thesis is to perform the first analysis of the precision HERA data for deep inelastic scattering (DIS) using the PB TMD framework with dynamical resolution scale at next-to-leading order (NLO) in QCD perturbation theory. To this end, we use the QCD fitting package xFitter. We find a good fit of precision DIS data, and perform an extraction of collinear and TMD parton distributions from this fit. We compare this with earlier extractions which do not include dynamical resolution scales, employing the web-based library platform TMDlib. We carry out validation checks of the extracted distributions, study the variation of the NLO level of accuracy compared to the leading order, and give a method to estimate uncertainties on the extracted distributions, due to both model and experimental errors. The parton distributions thus determined can be used to make predictions for production processes at the Large Hadron Collider (LHC), and are applicable in parton-shower Monte Carlo event generators employing showering algorithms with dynamical resolution scales. We present an application to Drell-Yan lepton-pair production at the LHC, and a sensitivity study to intrinsic transverse momentum from LHC Drell-Yan spectra.