Forces inside hadrons: pressure, surface tension, mechanical radius, and all that (1805.06596v3)

Abstract: The physics related to the form factors of the energy momentum tensor spans a wide spectrum of problems, and includes gravitational physics, hard exclusive reactions, hadronic decays of heavy quarkonia, and the physics of exotic hadrons described as hadroquarkonia. It also provides access to the "last global unknown property:" the D-term. We review the physics associated with the form factors of the energy-momentum tensor and the D-term, their interpretations in terms of mechanical properties, their applications, and the current experimental status.

Forces inside Hadrons: Pressure, Surface Tension, and Mechanical Properties

The paper "Forces inside hadrons: pressure, surface tension, mechanical radius, and all that" by Maxim V. Polyakov and Peter Schweitzer explores the deep interconnections between the form factors of the energy-momentum tensor (EMT) and the intrinsic mechanical properties of hadrons. Central to this exploration is the $D$-term, which encapsulates the intricate interplay of internal forces within hadrons, offering insights into the distribution of pressure and shear forces.

The energy-momentum tensor form factors provide comprehensive insights spanning gravitational physics, hard exclusive reactions, the decay processes of heavy quarkonia, and the exotica of hadroquarkonia. In this context, the EMT form factors are not mere mathematical constructs; they are fundamental in defining the hadron's mechanical properties, including pressure distributions and surface tensions.

Fundamental Aspects and Theoretical Implications

The mass and spin of particles are viewed traditionally as responses to external gravitational fields. However, another piece of the puzzle is the $D$-term, which emerges from spatial deformations and is closely tied to the mechanical structure through the stress tensor. This makes the $D$-term and the EMT form factors indispensable in understanding hadronic structure intricately.

The authors explore the behaviors of these mechanical properties in different frameworks. In non-interacting quantum field theories, a boson inherently possesses a negative $D$-term, while this term is absent in a free fermion. However, in interacting theories, the $D$-terms infuse significant sensitivity to the internal dynamics and correlations. For Goldstone bosons, chiral symmetry breaking governs the $D$-term to be $D = -1$ in the chiral limit.

Numerical Results and Bold Claims

From the large-$N_c$ QCD perspective, the $D$-term scales with color number, $\sim N_c^2$, distinguishing it fundamentally from other observables. This behavior is evident in hadrons where the $D$-term reflects substantial contributions to the mechanical radius and internal force distributions.

Moreover, nucleon $D$-terms calculated from disparate methods highlight the consistency in the negative sign, confirming theoretical expectations about stability and force distributions. In the chiral quark soliton model, the predictions aligned closely with lattice QCD and phenomenology derived from hard-exclusive data such as deeply virtual Compton scattering (DVCS).

Experimental Prospects and Future Developments

Despite the theoretical leaps, experimental determination of the $D$-term remains challenging. Recent analyses of DVCS data have started offering experimental glimpses into these form factors. These data support the intertwined structure of pressure and shear distributions, aligning with theoretical predictions, thus bolstering the significance of the $D$-term in explaining the strong forces inside hadrons.

The work also encourages further probing into complexities like the relation between the nucleon mass decomposition and confinement forces. It speculates on mechanical properties akin to macroscopic entities, hinting at broader applications that echo into modern quantitative experiments.

Conclusion

The exploration into the $D$-term and EMT form factors elucidates the profound connections dictating the hadron's internal landscape. Through a nexus of theory and experimental efforts, this work refines our understanding of hadronic structures, setting the stage for innovative inquiries into the quantum field. The intricate landscape of pressures and forces within hadrons is becoming increasingly accessible, promising rich insights into both fundamental physics and potential applications in quantum technologies.