Proton Gravitational Structure and Mass Decomposition on the Light Front

TL;DR

This paper calculates the proton's gravitational form factors using a relativistic light-front QCD approach, revealing detailed internal mass and pressure distributions and providing insights into the proton's mass decomposition consistent with lattice QCD.

Contribution

It introduces a nonperturbative light-front Hamiltonian method to compute quark and gluon GFFs, advancing understanding of the proton's internal structure and mass contributions.

Findings

Predicted GFFs agree with lattice QCD and experiments.

Quantified contributions to proton mass: quark energy 31.5%, gluon energy 34.7%, condensate 11.3%, trace anomaly 22.5%.

Determined proton's mass and mechanical radii.

Abstract

Gravitational form factors (GFFs) of hadrons encode essential information about the internal distributions of mass, spin, pressure, and shear among their quark and gluon constituents. We compute the quark and gluon GFFs of the proton using a fully relativistic, nonperturbative framework based on a light-front quantized Hamiltonian with quantum chromodynamics (QCD) input. This allows us to quantify the impact of a dynamical gluon on the proton's mechanical properties, such as pressure and shear distributions. Our predictions agree well with recent lattice QCD results and experimental extractions. We also determine the proton's mass and mechanical radii and address the long-standing puzzle of its mass decomposition. At the scale $\mu^2 = 4~\mathrm{GeV}^2$, we find that quark energy, gluon field energy, the quark condensate, and the QCD trace anomaly contribute $31.5\%$, $34.7\%$, $11.3\%$, and $22.5\%$, respectively, which are consistent with lattice QCD findings.