Lattice results for the longitudinal spin structure and color forces on quarks in a nucleon

Abstract

Using lattice QCD, we calculate the twist-2 contribution $a_2$ to the third Mellin moment of the spin structure functions $g_1$ and $g_2$ in the nucleon. In addition we evaluate the twist-3 contribution $d_2$. Our computations make use of $N_f=2+1$ gauge field ensembles generated by the Coordinated Lattice Simulations (CLS) effort. Neglecting quark-line disconnected contributions we obtain as our best estimates $a_2^{(p)}=0.069(17)$, $d_2^{(p)}=0.0105(68)$ and $a_2^{(n)}=0.0068(88)$, $d_2^{(n)}=-0.0009(70)$ for the proton and the neutron, respectively, where we use the normalizations given in Eqs. (58) and (59). While the $a_2$ results have been converted to the $\overline{\mathrm{MS}}$ scheme using three-loop perturbation theory, the numbers for $d_2$ are given in the regularization independent momentum subtraction (RI${}^{\prime }$-MOM) scheme, i.e., the conversion has been performed only in tree-level perturbation theory. The $d_2$ results can be interpreted as corresponding to a transverse color Lorentz force on a quark in a transversely polarized proton of size $F^{(u)}=116(61)$ MeV/fm and $F^{(d)}=-38(66)$ MeV/fm for $u$ and $d$ quarks, respectively. The error estimates quoted include statistical and systematic uncertainties added in quadrature.

arXiv comment17 pages, 8 figures; v2: 1 figure, 1 reference and a few clarifications added, analysis corrected, results slightly changed

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• Transverse force distributions in the proton from lattice QCD
• Transverse structure of the proton beyond leading twist: A light-front Hamiltonian approach
• A Journey of Seeking Pressure and Forces in the Nucleon
• Higher Twists
• NLO corrections and factorization for transverse single-spin asymmetries
• Transverse Nucleon Single-Spin Asymmetry for Single-Inclusive Hadron and Jet Production at NLO Accuracy
• Valence Quark PDFs of the Proton from Two-Current Correlations in Lattice QCD