Melosh rotation: source of the proton's missing spin

TL;DR

This paper explains the proton's missing spin by showing that Melosh rotation affects the observed spin structure, reconciling the naive quark model with experimental results.

Contribution

It introduces Melosh rotation as a key factor in understanding the proton's spin structure within the naive quark model.

Findings

Melosh rotation explains the small observed proton spin.

Deep inelastic scattering probes light-cone quarks.

Proton spin is the sum of Melosh rotated quark spins.

Abstract

It is shown that the observed small value of the integrated spin structure function for protons could be naturally understood within the naive quark model by considering the effect from Melosh rotation. The key to this problem lies in the fact that the deep inelastic process probes the light-cone quarks rather than the instant-form quarks, and that the spin of the proton is the sum of the Melosh rotated light-cone spin of the individual quarks rather than simply the sum of the light-cone spin of the quarks directly.