Origin of the nucleon gravitational form factor $B_N(t)$: Exposition in light-front holographic QCD

TL;DR

This paper explains the smallness of the nucleon's gravitational form factor $B_N(t)$ at finite momentum transfer using light-front holographic QCD, attributing it to a fundamental cancellation in the nucleon's wave functions.

Contribution

It provides a theoretical explanation for the suppression of $B_N(t)$, linking it to the nucleon's S-wave dominance and antisymmetric longitudinal dynamics.

Findings

$B_N(t)$ is governed by an antisymmetric factor in the longitudinal dynamics.

The form factor vanishes in the symmetric limit and is suppressed for realistic nucleon structures.

The smallness of $B_N(t)$ is a signature of the nucleon's dominant S-wave character.

Abstract

Recent lattice QCD simulations and phenomenological models indicate that the nucleon's gravitational form factor $B_N(t)$ remains remarkably small at finite momentum transfer $t$. While $B_N(0) = 0$ is a known consequence of the equivalence principle, the physical origin of its suppression at finite $t$ has not been fully elucidated. In this work, we demonstrate that the smallness of $B_N(t)$ arises from a fundamental cancellation within the nucleon's wave functions. Using light-front holographic QCD, we show that $B_N(t)$ is governed by an antisymmetric factor in the longitudinal dynamics that leads to the exact vanishing of the form factor in the symmetric limit and significant suppression for realistic nucleon structures. Our results suggest that the smallness of $B_N(t)$ is a signature of the nucleon's dominant S-wave character, providing a formal justification for its frequent omission in practical applications like near-threshold $J/\psi$ production.