# Point-Particle Effective Field Theory III: Relativistic Fermions and the Dirac Equation

## TL;DR

This paper develops a point-particle effective field theory for relativistic fermions interacting with finite-sized sources, enabling precise modeling of finite-source effects and other short-range interactions on bound-state energies.

## Contribution

It introduces a first-quantized PPEFT framework for relativistic fermions, providing a systematic way to incorporate finite-source effects and other short-range interactions into bound-state energy calculations.

## Key findings

- Provides a boundary condition formulation for the Dirac field near sources.
- Enables model-independent parameterization of finite-source effects.
- Captures various short-range interactions affecting bound states.

## Abstract

We formulate point-particle effective field theory (PPEFT) for relativistic spin-half fermions interacting with a massive, charged finite-sized source using a first-quantized effective field theory for the heavy compact object and a second-quantized language for the lighter fermion with which it interacts. This description shows how to determine the near-source boundary condition for the Dirac field in terms of the relevant physical properties of the source, and reduces to the standard choices in the limit of a point source. Using a first-quantized effective description is appropriate when the compact object is sufficiently heavy, and is simpler than (though equivalent to) the effective theory that treats the compact source in a second-quantized way. As an application we use the PPEFT to parameterize the leading energy shift for the bound energy levels due to finite-sized source effects in a model-independent way, allowing these effects to be fit in precision measurements. Besides capturing finite-source-size effects, the PPEFT treatment also efficiently captures how other short-distance source interactions can shift bound-state energy levels, such as due to vacuum polarization (through the Uehling potential) or strong interactions for Coulomb bound states of hadrons, or any hypothetical new short-range forces sourced by nuclei.