Multidimensional Measurements of Beam Single Spin Asymmetries in Semi-inclusive Deep-inelastic Charged Kaon Electroproduction off Protons in the Valence Region

TL;DR

This work tackles the problem of accessing twist-3 quark–gluon correlations in nucleon structure through beam single-spin asymmetries in kaon SIDIS. It leverages high-statistics, multidimensional binning of $K^+$ SIDIS off protons using the CLAS12 detector with two beam energies, extracting the $ m sin\phi$ moment $A_{LU}^{\sin{\phi}}$ and the structure-function ratio $F_{LU}^{\sin{\phi}}/F_{UU}$ across a broad kinematic range $Q^2\in[1,6]~\mathrm{GeV}^2$, $x_B\in[0.06,0.6]$, $z\in[0.1,0.7]$, and $P_T\in[0.1,0.8]~\mathrm{GeV}$. The results are compared to twist-3 TMD/FF models, with Model 2 reasonably capturing the $P_T$ dependence up to $P_T\approx0.7$ GeV, while Model 1 is disfavored due to a sign mismatch, providing new constraints for strange-quark TMDs and fragmentation functions in global fits. The measurements also probe the limits of factorization at higher $P_T$ and demonstrate the critical role of multidimensional kaon SIDIS data in flavor-separated TMD studies in the valence region.

Abstract

Measurements of beam single spin asymmetries in semi-inclusive deep inelastic electron scattering (SIDIS) with positively charged kaons off protons have been performed with 10.6 and 10.2 GeV incident electron beams using the CLAS12 spectrometer at Jefferson Lab. We report an analysis of the electroproduction of positively charged kaons over a large kinematic range of fractional energy, Bjorken $x$, transverse momentum, and photon virtualities $Q^2$ ranging from 1 GeV$^2$ up to 6 GeV$^2$. This is the first published multi-dimensionally binned CLAS12 measurement of a kaon SIDIS single spin asymmetry in the valence quark regime. The data provide constraints on the structure function ratio $F_{LU}^{\sinφ}/F_{UU}$, where $F_{LU}^{\sinφ}$ is a quantity with a leading twist of twist-3 that can reveal novel aspects of the quark-gluon correlations within the nucleon. The impact of the data on understanding the underlying reaction mechanisms and their kinematic variation is explored using theoretical models for the different contributing twist-3 parton distribution functions (PDFs) and fragmentation functions (FFs).

Figures (4)

• Figure 1: Schematic illustration of the reaction kinematics of the single kaon semi-inclusive deep inelastic scattering process.
• Figure 2: Distribution of $Q^2$ versus $x_B$ with the bin boundaries.
• Figure 3: $z$ dependence of $F_{LU}^{\sin{\phi}}/F_{UU}$ for $K^+$ for increasing $P_T$ bins (left to right) and for increasing $Q^2-x_B$-bins (bin 1: $\langle Q^2\rangle=1.68$ GeV$^2$, $\langle x_B\rangle=0.15$, bin 2: $\langle Q^2\rangle=2.52$ GeV$^2$, $\langle x_B\rangle=0.26$, bin 3: $\langle Q^2\rangle=4.36$ GeV$^2$, $\langle x_B\rangle=0.37$ (The 3 rows correspond to the 3 $Q^2-x_B$-bins shown in Fig. \ref{['fig:Q2_x_binning']}.)). The systematic uncertainty is given by the black histogram. The predictions of the different theoretical models are shown by the bold and dashed lines (blue: model 1, red: model 2). More details about the models can be found in the text and in Refs. Mao2013Mao2014
• Figure 4: $P_T$ dependence of $F_{LU}^{\sin{\phi}}/F_{UU}$ for $K^+$ for increasing $z$ bins (left to right) and for increasing $Q^2-x_B$-bins (see caption of Fig. \ref{['fig:Kp_z_4D']}). The systematic uncertainty is given by the black histogram. The predictions of the different theoretical models are shown by the bold and dashed lines (see caption of Fig. \ref{['fig:Kp_z_4D']}).