2025/12/07 by Chen, Yi
Physics and Astronomy · #FOS: Physical sciences #High Energy Physics - Phenomenology (hep-ph) #High Energy Physics - Theory (hep-th) #Nuclear Theory (nucl-th) #Nuclear physics research studies #Particle physics theoretical and experimental studies #Quantum Chromodynamics and Particle Interactions
paper · doi:10.48550/arxiv.2512.06801
openalex publication_date 2025/12/07 · openalex created_date 2025/12/10 · openalex updated_date 2026/07/28
Relativistic 3D weak-neutral axial-vector four-current and spin distributions inside a nucleon (or a general spin-(1)/(2) hadron) including three weak-neutral axial-vector form factors are investigated for the first time. We clarify that the relativistic 3D axial charge distribution in the Breit frame is completely described by the induced pseudotensor form factor GTZ(Q2) rather than by the axial form factor GAZ(Q2). We demonstrate that RA ≡ √ (-6)/(GAZ(0) )(d GAZ(Q2) )/(d Q2) |Q2=0 can not be interpreted as the physically meaningful 3D root-mean-square axial radius of a spin-(1)/(2) hadron. The genuine axial radius for any spin-(1)/(2) hadron in fact does not exist. We also show that the relativistic 3D weak-neutral spin radius rspin = √(⟨ rspin2 ⟩), with ⟨ rspin2 ⟩ ≡ RA2 + ( 1 )/( 4M2 )[ 1 + ( 2 GPZ(0) )/( GAZ(0) ) ] based on the relativistic and intrinsic 3D weak-neutral spin distribution in the Breit frame, is a physically meaningful radius that can be unambiguously defined for the nucleon, which provides an additional key motivation for the further determination of the induced pseudoscalar form factor GPZ(Q2). Numerically, we find that RA ≈ 0.6510~fm, rspin ≈ 2.1054~fm and rspin ≡ rspin/3 ≈ 0.7018~fm. For future experimental measurements of GAZ(Q2) and GTZ(Q2), we also derive the full tree-level unpolarized differential cross sections for neutrino-proton and antineutrino-proton elastic scattering in the lab frame, in hoping to provide a complementary and new perspective to unveil the nucleon spin structure by using (anti)neutrino-based facilities.