Hadronic energy-momentum tensor revisited

TL;DR

This paper revisits the hadronic energy-momentum tensor, proposing a fluid-like form that clarifies the definitions of energy density, pressure, and shear, and explores different factorization schemes revealing connections to known density definitions.

Contribution

It introduces a relativistic continuum form of the hadronic EMT with spin, providing new insights into its structure and relation to quantum expectation values and density schemes.

Findings

Hadronic EMT can be expressed as a relativistic continuum with spin.

A spin tensor contribution is identified without total angular momentum reference.

Different factorization schemes connect to Breit-frame and light-front densities.

Abstract

Everything gravitates and they do so through the energy-momentum tensor (EMT). However, there is no consensus on how to define the EMT of a hadron, e.g. the proton, the fundamental building blocks of the visible world. In this work, we show that the hadronic EMT can be cast into the form of relativistic continua with spin. The fluid-like form allows a clear identification of the proper hadronic energy density, pressure and shear. A spin tensor contribution is also identified without the reference to the total angular momentum operator, in alignment with the recent development in relativistic spin hydrodynamics. The hadronic EMT is related to the quantum expectation value of the EMT operator through the convolution with the hadronic wavepacket. To factorize out the hadronic EMT, different multipole expansion schemes are explored. We show that the popular Breit-frame densities and light-front densities emerge as the monopole momentum densities from different factorization schemes.