Tutorial: Custom Four Momentum Vector

This tutorial explains how to define a custom AbstractFourMomentum type that can be used to represent, for example, particle momenta in an AbstractParticleStateful.

Defining a Custom Four Momentum Type

The FourMomentum interface in QEDbase.jl is designed to be extendable. By implementing a simple API, you can make any custom type behave like a Lorentz vector and unlock access to various kinematic functions in the package.

Step 1: Define Your Custom Type

To define a valid AbstractFourMomentum type, create a type with the fields .E, .px, .py and .pz.

using QEDbase

struct CustomFourMomentum <: AbstractFourMomentum{Float64}
    E::Float64  # energy component
    px::Float64 # x component
    py::Float64 # y component
    pz::Float64 # z component
end

Step 2: Add the necessary accessor functions

QEDbase.getT(p::CustomFourMomentum) = p.E
QEDbase.getX(p::CustomFourMomentum) = p.px
QEDbase.getY(p::CustomFourMomentum) = p.py
QEDbase.getZ(p::CustomFourMomentum) = p.pz

Step 3: Register the type as a LorentzVectorLike

register_LorentzVectorLike(CustomFourMomentum)

A mutable version can also be implemented by also defining the interface functions setT!, setX!, setY!, and setZ!.

Example Usage:

mom = CustomFourMomentum(1.0, 0.0, 0.0, 1.0)
println("Defined momentum $mom is on shell: $(isonshell(mom))")

Defined momentum [1.0, 0.0, 0.0, 1.0] is on shell: true

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