Renormalization Group and Pade Applications to Quantum Field Theory

Pade approximants (PA) have been widely applied in practically all areas of physics. This work focuses on developing PA as tools for both perturbative and non-perturbative Quantum Field Theory (QFT). In perturbative QFT, a systematic estimation of higher (unknown) loop terms via asymptotic Pade approximation procedure (APAP) is established and enhanced by application to renormalization-group-(RG-)invariant quantities. This methodology is applied to hadronic Higgs decay rates (both within the Standard Model and its MSSM extension), Higgs-sector cross-sections, inclusive semileptonic decays, QCD (Quantum Chromodynamics) correlation functions and the QCD static potential. APAP is also applied directly to RG functions in massive phi-4 theory. In non-perturbative QFT Pade summation methods are employed to probe the large coupling regions of QCD. In analysing all the possible Pade-approximants to truncated Beta-function for QCD, the singularity structure corresponding to the all-orders Beta-function is analyzed. Noting the consistent ordering of poles and roots for such approximants, these are found free of defective (pole) behaviour and physical conclusions are then drawn.