Quark quasi-Sivers function and quasi-Boer-Mulders function in a spectator diquark model

Abstract

We compute the leading-twist T-odd quasi-distributions of the proton in a spectator model with scalar and axial-vector diquarks: the quasi Sivers function $\tilde{f}_{1T}^\perp(x, k_T^2; P_z)$ and the quasi Boer-Mulders function $\tilde{h}_1^\perp(x, k_T^2; P_z)$. We obtain the quark-quark correlators in the four-dimensional Euclidian space by replacing $\gamma^+$ and $\sigma^{i+}$ in the light-cone frame with $\gamma_z$ and $\sigma_{iz}$. We show by analytical calculation that the results of $\tilde{f}_{1T}^\perp$ and $\tilde{h}_1^\perp$ derived from the correlators can reduce to the expressions of the corresponding standard T-odd distributions $f_{1T}^\perp(x, k_T^2)$ and $h_1^\perp(x, k_T^2)$ in the limit $P_z\rightarrow\infty$. The numerical results for these quasi-distributions and their first transverse moments for the $u$ and $d$ quarks in different $x$ and $P_z$ regions are also presented. We find that $\tilde{f}_{1T}^{\perp(1)}(x, P_z)$ and $\tilde{h}_1^{\perp(1)}(x, P_z)$ in the spectator model are fair approximations to the standard ones (within 20-30\%) in the region $0.1<x<0.5$ when $P_z \geq 2.5-3$ GeV. This supports the idea of using T-odd quasi-distributions to obtain standard distributions in the region $P_z>2.5$ GeV as fair approximation.