Abstract
The energy-momentum tensor (EMT) form factors pave new ways for exploring hadron structure. Especially the D-term related to the EMT form factor D(t) has received a lot of attention due to its attractive physical interpretation in terms of mechanical properties. We study the nucleon EMT form factors and the associated densities in the bag model which we formulate for an arbitrary number of colors Nc and show that the EMT form factors are consistently described in this model in the large-Nc limit. The simplicity of the model allows us to test in a lucid way many theoretical concepts related to EMT form factors and densities including recently introduced concepts like normal and tangential forces, or monopole and quadrupole contributions to the angular momentum distribution. We also study the D-terms of rho-meson, Roper resonance, other N* states and Delta-resonances. Among the most interesting outcomes is the lucid demonstration of the deeper connection of EMT conservation, stability, the virial theorem and the negative sign of the D-term.
Highlights
We present a study of energy momentum tensor (EMT) properties in one of the simplest hadronic models: the bag model [59,60,61]
We have explored the bag model to study the EMT form factors AaðtÞ, JaðtÞ, DaðtÞ, and caðtÞ and the EMT densities
The quark contributions (a 1⁄4 u, d) to the EMT form factors are defined in terms of the single-quark wave functions and the SU(4) spin-flavor factors needed to construct the nucleon wave functions
Summary
The version of the bag model used in this work is at variance with chiral symmetry which is a drawback This model has, important advantages: it is a consistent theoretical framework. We include an insightful study of hypothetical highly excited bag model states This is the only study of EMT properties of excited states available in the literature besides Q-balls [35], and we make the interesting observation that in both systems asymptotically the D-term grows as D 1⁄4 −const × M8=3 with the mass M of the excitation, even though the excited states have much different internal structures in the two frameworks. Some of our results were previously mentioned in Refs. [80,81]
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