PYTHIA 6.4 Physics and Manual Torbjörn Sjöstrand, Stephen Mrenna, Peter Skands PYTHIA is a program for generating high-energy-physics events, i.e., sets of outgoing particles produced in interactions between two incoming particles. The goal is to provide as accurate as possible a representation of event properties in a wide range of reactions, both within and beyond the Standard Model, with emphasis on those involving strong interactions. The program combines analytical results with various QCD-based models to simulate physics input for areas such as hard subprocesses, initial- and final-state parton showers, underlying events, beam remnants, fragmentation, and decays. It also provides extensive information on all program elements, including subroutines, functions, switches, parameters, and particle data, allowing users to tailor the generation task to their interests. The code and further information are available at http://www.thep.lu.se/~torbjorn/Pythia.html. Keywords: Phenomenological Models, Hadronic Colliders, Standard Model, Beyond Standard Model. The report provides an overview of the physics and manual of PYTHIA 6.4, including its history, physics features, and program structure. It covers topics such as hard processes, parton distributions, initial- and final-state radiation, beam remnants, hadronization, and more. The report also includes detailed information on the program's components, including subroutines, switches, parameters, and particle data. It discusses the program's use in various physics scenarios, including electron-positron annihilation, resonance production, and non-standard physics. The report also covers the program's use in different types of collisions, such as e⁺e⁻, lepton-hadron, and hadron-hadron collisions. It includes a detailed description of the program's physics processes, including QCD processes, electroweak gauge bosons, Higgs production, and non-standard physics. The report also discusses the program's use in event generation, including the generation of weighted events, varying energies, and external processes. It includes a detailed description of the program's fragmentation and decay processes, as well as its use in event study and analysis. The report concludes with a summary and outlook for future developments.PYTHIA 6.4 Physics and Manual Torbjörn Sjöstrand, Stephen Mrenna, Peter Skands PYTHIA is a program for generating high-energy-physics events, i.e., sets of outgoing particles produced in interactions between two incoming particles. The goal is to provide as accurate as possible a representation of event properties in a wide range of reactions, both within and beyond the Standard Model, with emphasis on those involving strong interactions. The program combines analytical results with various QCD-based models to simulate physics input for areas such as hard subprocesses, initial- and final-state parton showers, underlying events, beam remnants, fragmentation, and decays. It also provides extensive information on all program elements, including subroutines, functions, switches, parameters, and particle data, allowing users to tailor the generation task to their interests. The code and further information are available at http://www.thep.lu.se/~torbjorn/Pythia.html. Keywords: Phenomenological Models, Hadronic Colliders, Standard Model, Beyond Standard Model. The report provides an overview of the physics and manual of PYTHIA 6.4, including its history, physics features, and program structure. It covers topics such as hard processes, parton distributions, initial- and final-state radiation, beam remnants, hadronization, and more. The report also includes detailed information on the program's components, including subroutines, switches, parameters, and particle data. It discusses the program's use in various physics scenarios, including electron-positron annihilation, resonance production, and non-standard physics. The report also covers the program's use in different types of collisions, such as e⁺e⁻, lepton-hadron, and hadron-hadron collisions. It includes a detailed description of the program's physics processes, including QCD processes, electroweak gauge bosons, Higgs production, and non-standard physics. The report also discusses the program's use in event generation, including the generation of weighted events, varying energies, and external processes. It includes a detailed description of the program's fragmentation and decay processes, as well as its use in event study and analysis. The report concludes with a summary and outlook for future developments.