Feynman diagrams are graphical representations used in particle physics to calculate scattering amplitudes, which describe the probabilities of various particle interactions. The calculation process involves the following steps:
Identify the initial and final states: Determine the particles involved in the scattering process and specify their initial and final states. For example, in electron-electron scattering, the initial state might consist of two incoming electrons, while the final state could involve two outgoing electrons.
Assign external lines: Draw the external lines on the Feynman diagram to represent the particles in the initial and final states. These lines correspond to the incoming and outgoing particles and are typically straight lines.
Identify interaction vertices: Identify the interaction vertices in the diagram where the particles interact with each other. These vertices correspond to the fundamental interactions between particles, such as electromagnetic or weak interactions.
Assign internal lines: Draw internal lines connecting the vertices to represent the propagation of virtual particles. These virtual particles can be photons, gluons, or other force carriers, depending on the specific interaction being studied. Internal lines are typically wavy or curly to distinguish them from external lines.
Assign momenta and energies: Assign momenta and energies to the particles flowing through the diagram, including both external and internal lines. These values are determined by the kinematics of the scattering process and are often denoted by arrows on the lines.
Write down the Feynman amplitude: Associate a mathematical expression with each Feynman diagram. This expression is known as the Feynman amplitude and represents the quantum mechanical probability amplitude for the scattering process. The Feynman amplitude includes factors related to the particle interactions, momenta, and other properties of the particles involved.
Evaluate the Feynman amplitude: Calculate the Feynman amplitude using the rules of quantum field theory. This involves combining the contributions from the external lines, internal lines, and interaction vertices, taking into account the appropriate mathematical expressions and rules associated with each element of the diagram.
Sum over all diagrams: For a given scattering process, there can be multiple Feynman diagrams contributing to the amplitude. Sum up the amplitudes of all relevant diagrams to obtain the total scattering amplitude.
Square the amplitude: Once you have the total scattering amplitude, square it to obtain the probability density for the scattering process. This step accounts for both interference effects between different Feynman diagrams and the statistical nature of quantum mechanics.
Calculate physical observables: Use the squared amplitude to calculate measurable quantities, such as cross-sections or decay rates, which provide information about the likelihood of specific particle interactions occurring in experiments.
It's worth noting that the actual calculations can be quite involved, especially for complex processes involving multiple particles and interactions. Simplified Feynman rules and computational techniques are often employed to streamline the calculations and make them more tractable.