Syllabus

High-Energy Astroparticle Physics - Theory (2025/2026)

Course Synopsis

This course introduces the theoretical foundations of high-energy astroparticle physics, with a primary focus on cosmic rays and the physical mechanisms that regulate their transport, acceleration, and radiative output. Starting from the motion of charged particles in turbulent magnetic fields, students will develop the transport formalism from first principles and understand how diffusive propagation arises in astrophysical environments. The course then applies this framework to major problems in Galactic cosmic-ray physics, including confinement, large-scale transport, and particle acceleration at collisionless shocks.

The course also examines the radiative signatures of high-energy particles in both Galactic and extragalactic settings. Students will learn how to describe the main leptonic processes, including synchrotron and inverse Compton emission, together with hadronic interactions such as pion production and spallation, and to relate these mechanisms to gamma-ray and neutrino observations within the broader multi-messenger picture.

Course Syllabus

Lectures: 15 2-hour lectures

• Lecture 1. Introduction to High-Energy Astroparticle Physics
  • Scope of high-energy astroparticle physics
  • Major open questions in the field
  • Course structure, objectives, and practical information
• Lecture 2. Radiative Processes I: Synchrotron Emission
  • Kinetic equation for the evolution of relativistic electrons
  • Synchrotron power and radiative loss timescales
  • The cooling break: physical origin and observational significance
  • Characteristic and critical synchrotron frequencies
  • Synchrotron emission from electron populations
• Lecture 3. Radiative Processes II: Inverse Compton Scattering and Gamma-Ray Absorption
  • Fundamentals of Compton scattering
  • Inverse Compton scattering and energy transfer by relativistic electrons
  • Single-particle radiated power in inverse Compton scattering
  • The Klein-Nishina regime: cross-section and physical implications
  • Gamma-ray absorption via pair production: threshold energy and cross-section
• Lecture 4. Hadronic Radiative Processes
  • Interaction timescales for hadronic processes
  • Pion production in proton-proton collisions
  • Pion emissivity and derivation of the gamma-ray spectrum
  • Neutral-pion decay and the origin of the pion bump
• Lecture 5. Diffuse Emission and Multi-Messenger Connections
  • Diffuse Galactic gamma-ray emission: observational overview and theoretical interpretation
  • Multi-messenger relations among cosmic rays, gamma rays, and neutrinos
  • The Waxman-Bahcall bound and its physical meaning
• Lecture 6. Cosmic Ray Transport I: Primaries and Grammage
  • Introduction to the cosmic ray transport equation
  • The diffusion equation: solutions and limitations in cosmic ray propagation
  • Transport equation for primary nuclei: formulation and analytical solutions
  • Cosmic ray composition and the role of grammage as a key observational diagnostic
• Lecture 7. Cosmic Ray Transport II: Secondaries
  • Transport equation for secondary nuclei
  • Secondary-to-primary ratios as probes of cosmic ray transport
  • Unstable secondary nuclei and their observational relevance
• Lecture 8. Cosmic Ray Scattering in Turbulent Magnetic Fields
  • The supernova remnant paradigm for Galactic cosmic rays
  • Charged particle transport in turbulent magnetic fields: quasi-linear theory
  • Resonant scattering and wave-particle interactions
  • Pitch-angle diffusion coefficient
• Lecture 9. From Pitch-Angle Scattering to Spatial Diffusion
  • From pitch-angle diffusion to the spatial diffusion coefficient
  • Implications for the Galactic mean free path
  • The Kolmogorov spectrum of magnetic turbulence and its consequences for transport
• Lecture 10. Magnetohydrodynamic Foundations
  • Fundamentals of ideal magnetohydrodynamics
  • Flux freezing and magnetic field advection
  • Magnetohydrodynamic waves and their physical properties
• Lecture 11. Transport Theory and Cosmic Ray Anisotropy
  • Derivation of the transport equation from the Fokker-Planck formalism
  • Physical interpretation of the transport coefficients
  • Implications for large-scale cosmic ray anisotropy
• Lecture 12. General Principles of Particle Acceleration
  • Basic requirements for astrophysical particle acceleration
  • The Hillas criterion and candidate Galactic accelerators
  • Overview of stochastic acceleration mechanisms
  • Second-order Fermi acceleration
• Lecture 13. Shock Waves in Astrophysics
  • Formation and propagation of shock waves
  • Interstellar shock waves
  • Rankine-Hugoniot jump conditions
• Lecture 14. Supernova Remnants and Diffusive Shock Acceleration
  • Dynamical evolution of supernova remnants
  • Free-expansion phase
  • Sedov-Taylor phase
  • Diffusive shock acceleration as first-order Fermi acceleration
• Lecture 15. Diffusive Shock Acceleration: Transport Viewpoint and Observational Tests
  • Transport-equation formulation of diffusive shock acceleration
  • Spectral predictions and physical interpretation
  • X-ray filaments as observational tests of particle acceleration in supernova remnants