Feasibility Study of Pion and Kaon Structure via the Sullivan Process at EicC

TL;DR

This work assesses the feasibility of measuring the pion and kaon structure functions $F_2^{\pi}$ and $F_2^{K}$ via the Sullivan process in deep-inelastic scattering at EicC. Using dedicated Monte Carlo simulations and a detailed detector model, the study projects high-precision extractions by tagging leading baryons (neutrons for pions, $\Lambda$ for kaons) and fitting the $t$-dependent yields to separate the meson structure from the meson flux. The results indicate statistical uncertainties below 5% for $F_2^{\pi}$ (for $Q^2>5$ GeV$^2$) and below 8% for $F_2^{K}$, with pion-systematics dominated by forward-baryon reconstruction and kaon-systematics mitigated by charged-decay tracking. The analysis demonstrates EicC's unique potential to constrain light-mos meson PDFs and, when combined with Drell–Yan data, to reduce flux-model uncertainties, thereby advancing our understanding of nonperturbative QCD in the meson sector.

Abstract

The Electron--Ion Collider in China (EicC) provides an excellent opportunity to explore the internal structure of pions and kaons via the Sullivan process in deep-inelastic scattering (DIS). In this study, we present detailed projections for the pion and kaon structure functions, $F_2^π$ and $F_2^K$, at EicC, with a focus on both statistical and systematic uncertainties. Leveraging EicC's high luminosity and broad kinematic coverage, the accessible kinematic region is extended beyond previous measurements. The projected statistical uncertainties for $F_2^π$ and $F_2^K$ are below 5\% and 8\%, respectively, across most kinematic bins. Systematic uncertainties arising from detector effects have been carefully evaluated. These results significantly enhance the precision of meson structure function measurements and provide important constraints on theoretical models of meson parton distributions. Moreover, this study bridges the gap between fixed-target and collider-era measurements, highlighting the pivotal role of EicC in advancing our understanding of hadronic structure.

Figures (9)

• Figure 1: Sullivan process in DIS. $(a)$ Pion case: the proton emits a virtual $\pi^+$ and transitions into a neutron. $(b)$ Kaon case: the proton emits a virtual $K^+$ and transitions into a $\Lambda$.
• Figure 2: Event rates for the LN-DIS process at the EicC, obtained from Monte Carlo simulation and shown as two-dimensional histograms of invariant kinematic variables. Left panel: distribution in $(x_B, Q^2)$. Right panel: distribution in $(x_\pi, |t|)$. The MC events were generated within the kinematic region $0.01\ \mathrm{GeV}^2<|t|<1\ \mathrm{GeV}^2$, $0.75<x_L<1$, $x_B<1$, $1\ \mathrm{GeV}^2<Q^2<50\ \mathrm{GeV}^2$, and $W^2>4\ \mathrm{GeV}^2$.
• Figure 3: For the Sullivan process $ep \to e n X$, the left panel shows the correlation between the scattered electron momentum and pseudorapidity $\eta$, while the right panel depicts the relationship between the detected neutron momentum and scattering (polar) angle. The distributions are approximately symmetric with respect to the incident proton beam, which is designed to have a crossing angle of 50 mrad (about 2.86°). All cases correspond to the energy setting of a 3.5 GeV electron colliding with a 20 GeV proton. The distributions for the $ep \to e \Lambda X$ process exhibit a similar pattern.
• Figure 4: Conceptual design of the EicC detector, with the central and ion far-forward detectors included.
• Figure 5: Distribution of the $\Lambda$ decay vertex along the $z$ direction for final-state $\Lambda$ at different collision energies.