[Paper Review] ``Electromagnetic'' Answer to the Nucleon Spin Puzzle

Douglas Singleton|arXiv (Cornell University)|Oct 1, 1997

Particle physics theoretical and experimental studies3 references3 citations

TL;DR

The paper proposes that a portion of the nucleon's spin not accounted for by valence quarks arises from the electromagnetic field angular momentum generated by the interaction between quark electric charges and magnetic dipoles. Order-of-magnitude estimates suggest this field angular momentum could contribute significantly to the nucleon's spin, with a similar mechanism potentially existing in QCD via color charges and color magnetic dipoles.

ABSTRACT

A possible resolution to the question of where the nucleon gets some portion of its internal angular momentum, not carried by the valence quarks, is given in terms of the electromagnetic field angular momentum of the nucleon. This field angular momentum is similiar in origin to that of the Thomson dipole (an electric charge and a magnetic monopole), but arises from the interaction of the quark electric charges and magnetic dipoles. Rough, order of magnitude calculations show that such a field angular momentum could provide some of the nucleon's spin. Under certain assumptions QCD should also exhibit a similiar field angular momentum coming from the interaction of color charges with color magnetic dipoles.

Motivation & Objective

• To address the long-standing nucleon spin puzzle by identifying a source of angular momentum beyond valence quarks.
• To explore whether the electromagnetic field angular momentum from quark electric charges and magnetic dipoles can account for a portion of the nucleon's spin.
• To investigate the possibility of a similar field angular momentum mechanism in QCD through color charges and color magnetic dipoles.
• To provide a qualitative but physically motivated explanation for the missing spin using electromagnetic analogies.

Proposed method

• Model the nucleon as a system of quarks with electric charges and magnetic dipoles to compute the resulting electromagnetic field angular momentum.
• Use the Thomson dipole analogy—where an electric charge and magnetic monopole generate angular momentum—to estimate the field angular momentum in the nucleon.
• Perform rough order-of-magnitude calculations to assess the magnitude of the electromagnetic field angular momentum contribution.
• Extend the reasoning to QCD by considering the interaction between color charges and color magnetic dipoles as a potential source of analogous field angular momentum.
• Apply symmetry and field-theoretic reasoning to argue for the plausibility of such angular momentum in quantum chromodynamics.

Experimental results

Research questions

• RQ1 Can the electromagnetic field angular momentum from quark charges and magnetic dipoles account for a measurable portion of the nucleon's spin?
• RQ2 How does the angular momentum from the electromagnetic field compare to the spin carried by valence quarks?
• RQ3 Is there a QCD analog of the electromagnetic field angular momentum arising from the interaction of color charges and color magnetic dipoles?
• RQ4 What is the order of magnitude of the field angular momentum in the nucleon under plausible assumptions?

Key findings

• The electromagnetic field angular momentum arising from quark electric charges and magnetic dipoles is estimated to be of the right order of magnitude to contribute significantly to the nucleon's spin.
• The mechanism is analogous to the Thomson dipole, where a charge and magnetic monopole generate angular momentum, suggesting a plausible physical origin for the missing spin.
• The model suggests that such field angular momentum could account for part of the nucleon's spin not carried by valence quarks.
• A similar field angular momentum mechanism may exist in QCD, arising from the interaction of color charges and color magnetic dipoles.