Exploring Nucleon Structure and the Proton Mass Problem through Holographic QCD

TL;DR

This paper develops a unified holographic QCD framework to extract quark and gluon distributions in protons, linking their contributions to the proton mass and matching experimental and lattice data.

Contribution

It introduces a novel parameterization method based on Light-Front Holographic QCD to simultaneously derive quark PDFs, GPDs, and GFFs, and models gluon GPDs within a consistent framework.

Findings

Quark GPDs derived from LFHQCD match experimental data.

Gluon GPDs modeled successfully within the same framework.

Trace anomaly contributes approximately 23% to proton mass.

Abstract

Understanding the internal structure of the proton-including the distributions of quarks and gluons and their contributions to proton properties such as mass-remains a central challenge in quantum chromodynamics (QCD). While quark generalized parton distributions (GPDs) have been studied extensively, a unified approach that simultaneously extracts quark parton distribution functions (PDFs), gravitational form factors (GFFs), and gluon GPDs from experimental constraints is still lacking. Moreover, the role of gluons in proton mass generation, particularly through the trace anomaly mechanism, requires deeper theoretical and phenomenological exploration. In this study, we first extract quark GPDs in protons using parameterization method based on the electromagnetic form factors provided by Light-Front Holographic QCD (LFHQCD) from which we derive both quark PDFs and their GFFs. We then extend this approach to model gluon GPDs. Our calculations show consistency with experimental data and lattice QCD results and successfully reproduce soft Pomeron behavior. Furthermore, we investigate near-threshold $J/\psi$ production using gauge/string duality to quantify the contribution of the trace anomaly to the proton mass. Our results demonstrate that the parameterization method provides a consistent framework for describing both quark and gluon structure, bridging GPDs, PDFs, and GFFs. The analysis of $J/\psi$ production confirms that the trace anomaly contributes significantly ($\sim 23\%$) to the proton mass, with the calculated cross-section dependence on momentum transfer $t$ in agreement with experimental observations. This work advances the understanding of proton structure by integrating quark and gluon degrees of freedom and elucidating the origin of proton mass within QCD.