A group-theoretic construction yields complete form factor bases for scalar, vector, and tensor operators on spin-1/2 to spin-2 particles, with new P and T structures for higher spins and identification of a redundant conserved structure for spin-2 in existing literature.
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The angular momentum controversy: What's it all about and does it matter?
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abstract
The general question, crucial to an understanding of the internal structure of the nucleon, of how to split the total angular momentum of a photon or gluon into spin and orbital contributions is one of the most important and interesting challenges faced by gauge theories like Quantum Electrodynamics and Quantum Chromodynamics. This is particularly challenging since all QED textbooks state that such an splitting cannot be done for a photon (and a fortiori for a gluon) in a gauge-invariant way, yet experimentalists around the world are engaged in measuring what they believe is the gluon spin! This question has been a subject of intense debate and controversy, ever since, in 2008, it was claimed that such a gauge-invariant split was, in fact, possible. We explain in what sense this claim is true and how it turns out that one of the main problems is that such a decomposition is not unique and therefore raises the question of what is the most natural or physical choice. The essential requirement of measurability does not solve the ambiguities and leads us to the conclusion that the choice of a particular decomposition is essentially a matter of taste and convenience. In this review, we provide a pedagogical introduction to the question of angular momentum decomposition in a gauge theory, present the main relevant decompositions and discuss in detail several aspects of the controversies regarding the question of gauge invariance, frame dependence, uniqueness and measurability. We stress the physical implications of the recent developments and collect into a separate section all the sum rules and relations which we think experimentally relevant. We hope that such a review will make the matter amenable to a broader community and will help to clarify the present situation.
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Derives relativistic spatial distributions of transverse orbital angular momentum, intrinsic spin, and total angular momentum in the transverse plane for spin-0 and spin-1/2 targets via quantum phase-space formalism and verifies the transverse spin sum rule.
Relativistic EMT distributions in polarized nucleons recover good and bad light-front components in the IMF after including polarization effects.
A light-front spectator model yields the first calculation of Im(F^g_{1,4}) and the resulting sin(2φ) asymmetry in ep → epπ⁰ at EIC kinematics.
The quantum phase-space formalism derives transverse energy-momentum tensor distributions in polarized nucleons and reproduces standard light-front distributions including bad components in the infinite-momentum frame.
In the bag model, GTMD calculations are consistent, orbital angular momentum is tied to F_{1,4}^q through the Ji sum rule, and a deeper link to pretzelosity TMD is established.
The EIC Yellow Report specifies the science goals, required detector capabilities, and technology concepts needed to realize a high-luminosity electron-ion collider program.
A hadronic approach based on dispersion relations and meson dominance achieves a successful description of lattice QCD data for gravitational form factors of pions and nucleons.
The review summarizes developments in spin hydrodynamics, polarization from spin-vorticity coupling, pseudo-gauge freedom, and heavy-flavor spin dynamics in relativistic systems.
citing papers explorer
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Covariant Construction of Generalized Form Factors
A group-theoretic construction yields complete form factor bases for scalar, vector, and tensor operators on spin-1/2 to spin-2 particles, with new P and T structures for higher spins and identification of a redundant conserved structure for spin-2 in existing literature.
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Mapping the transverse spin sum rule in position space
Derives relativistic spatial distributions of transverse orbital angular momentum, intrinsic spin, and total angular momentum in the transverse plane for spin-0 and spin-1/2 targets via quantum phase-space formalism and verifies the transverse spin sum rule.
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Relativistic energy-momentum tensor distributions in a polarized nucleon
Relativistic EMT distributions in polarized nucleons recover good and bad light-front components in the IMF after including polarization effects.
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Accessing gluon GTMD $F^g_{1,4}$ via the $\langle\sin(2\phi)\rangle$ azimuthal asymmetry of exclusive $\pi^0$ production in $ep$ collisions
A light-front spectator model yields the first calculation of Im(F^g_{1,4}) and the resulting sin(2φ) asymmetry in ep → epπ⁰ at EIC kinematics.
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Transverse energy-momentum tensor distributions in polarized nucleons
The quantum phase-space formalism derives transverse energy-momentum tensor distributions in polarized nucleons and reproduces standard light-front distributions including bad components in the infinite-momentum frame.
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GTMDs, orbital angular momentum, and pretzelosity
In the bag model, GTMD calculations are consistent, orbital angular momentum is tied to F_{1,4}^q through the Ji sum rule, and a deeper link to pretzelosity TMD is established.
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Science Requirements and Detector Concepts for the Electron-Ion Collider: EIC Yellow Report
The EIC Yellow Report specifies the science goals, required detector capabilities, and technology concepts needed to realize a high-luminosity electron-ion collider program.
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Particle seismology: mechanical and gravitational properties from parton-hadron duality
A hadronic approach based on dispersion relations and meson dominance achieves a successful description of lattice QCD data for gravitational form factors of pions and nucleons.
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Spin dynamics and polarization in relativistic systems: recent developments
The review summarizes developments in spin hydrodynamics, polarization from spin-vorticity coupling, pseudo-gauge freedom, and heavy-flavor spin dynamics in relativistic systems.