Lattice QCD Calculation of the Subtraction Function in Forward Compton Amplitude.

Area of Science:

• * Nuclear Physics
• * Particle Physics
• * Quantum Chromodynamics

Background:

• * The subtraction function is essential for calculating the forward Compton amplitude.
• * This amplitude is critical for predicting the Lamb shift in muonic atoms and the proton-neutron mass difference.
• * Lattice QCD provides a framework for ab initio calculations of these fundamental quantities.

Purpose of the Study:

• * To perform a lattice QCD calculation of the subtraction function.
• * To utilize a novel subtraction point to minimize statistical and systematic uncertainties.
• * To determine the contributions of intermediate states and calculate nucleon isovector subtraction functions.

Main Methods:

• * Employed two domain wall fermion gauge ensembles near the physical pion mass.
• * Utilized a recently proposed subtraction point, avoiding ground-state subtractions.
• * Incorporated Nπ intermediate state contributions.
• * Computed subtraction functions for the proton and neutron.

Main Results:

• * Demonstrated the advantage of the new subtraction point in reducing uncertainties.
• * Identified significant contributions from Nπ intermediate states.
• * Calculated the proton, neutron, and nucleon isovector subtraction functions across a range of momentum transfers (Q²).
• * Compared lattice results for the proton subtraction function with chiral perturbation theory and perturbative operator-product expansion.

Conclusions:

• * The subtraction function was successfully computed using lattice QCD.
• * The inclusion of Nπ intermediate states is crucial for accurate calculations.
• * These results contribute to understanding two-photon exchange effects in the Lamb shift and nucleon electromagnetic self-energy.