Flavor asymmetry of polarized antiquark distributions and semi-inclusive DIS

TL;DR

The paper addresses the flavor decomposition of polarized antiquark distributions in the nucleon, focusing on the isovector combination $Δ\bar{u}(x) - Δ\bar{d}(x)$ predicted to be large in the $1/N_c$ expansion. It employs the chiral quark-soliton model to estimate these asymmetries at a low scale and analyzes their impact on semi-inclusive DIS spin asymmetries by coupling to inclusive DIS constraints and fragmentation functions, comparing with HERMES data. The findings indicate that the predicted large flavor asymmetry yields observable effects in semi-inclusive spin asymmetries for charged hadrons and kaons, with kaon channels offering particularly strong sensitivity; the results are consistent with HERMES measurements, though uncertainties in fragmentation functions and potential biases in previous analyses are discussed. The work also discusses how meson-cloud pictures can accommodate large asymmetries through mechanisms beyond simple rho-exchange, suggesting pi–sigma interference as a possible contributor, and highlights experimental tests to further probe the polarized sea flavor structure.

Abstract

The $1/N_c$-expansion of QCD suggests large flavor asymmetries of the polarized antiquark distributions in the nucleon. This is confirmed by model calculations in the large-$N_c$ limit (chiral quark-soliton model), which give sizable results for $Δ\bar u (x) - Δ\bar d (x)$ and $Δ\bar u (x) + Δ\bar d (x) - 2 Δ\bar s (x)$. We compute the contributions of these flavor asymmetries to the spin asymmetries in hadron production in semi-inclusive deep-inelastic scattering. We show that the large flavor asymmetries predicted by the chiral quark-soliton model are consistent with the recent HERMES data for spin asymmetries in charged hadron production.

Figures (5)

• Figure 1: Solid line: The isovector polarized antiquark distribution at the low normalization point ($\mu = 600\, {\rm MeV}$), as obtained from the chiral soliton model of the nucleon (for details see the text). Dashed line: The isovector unpolarized antiquark distribution calculated in Ref.PPGWW98.
• Figure 6: The inclusive spin asymmetry, Eq.(\ref{['A1_incl']}), evaluated with the GRSV/GRV parametrization for the polarized/unpolarized parton distributions, at $Q^2 = 2.5\, {\rm GeV}^2$. Solid line: Eq.(\ref{['A1_incl']}) evaluated without the explicit factor $[1 + R(x, Q^2)]$. Dashed line: same as solid line, but explicitly including the factor $[1 + R(x, Q^2)]$. Dots: HERMES data from Ref.HERMES99.
• Figure 7: The spin asymmetries for $\pi^+$ and $\pi^-$ production in semi-inclusive DIS off the proton ($Q^2 = 2.5\, {\rm GeV}^2$, $z_{\rm min} = 0.2$). Dashed lines: Sum of contributions $A_{1, \, u}^{\pi+}$, $A_{1, \, d}^{\pi+}, A_{1, \, s}^{\pi+}$, and $A_{1, \, 0}^{\pi+}$ (and respectively for $\pi-$), cf. Eq.(\ref{['A_decomposition']}). Dotted lines: Contributions $A_{1, \, 3}^{\pi+}$ and $A_{1, \, 3}^{\pi -}$, respectively, proportional to the flavor asymmetry $\Delta \bar{u} (x) - \Delta \bar{d} (x)$ in the target, evaluated with the distribution shown in Fig.\ref{['fig_delta']}. Solid lines: Total results, including the effect of $\Delta \bar{u} (x) - \Delta \bar{d} (x)$. The contributions $A_{1, \, 8}^{\pi +}$ and $A_{1, \, 8}^{\pi -}$ are very small and therefore not shown separately.
• Figure 8: The spin asymmetries for $h^+$ and $h^-$ production in semi-inclusive DIS off the proton ($Q^2 = 2.5\, {\rm GeV}^2$, $z_{\rm min} = 0.2$). Dashed lines: Sum of contributions $A_{1, \, u}^{h+}$, $A_{1, \, d}^{h+}, A_{1, \, s}^{h+}$, and $A_{1, \, 0}^{h+}$ (and respectively for $h^-$), cf. Eq.(\ref{['A_decomposition']}). Dotted lines: Contributions $A_{1, \, 3}^{h+}$ and $A_{1, \, 3}^{h-}$, respectively, proportional to the flavor asymmetry $\Delta \bar{u} (x) - \Delta \bar{d} (x)$ in the target, evaluated with the distribution shown in Fig.\ref{['fig_delta']}. Dash--dotted line: Contributions $A_{1, \, 8}^{h+}$ and $A_{1, \, 8}^{h-}$, respectively. Solid lines: Total results, including the effect of flavor asymmetry of the polarized antiquark distribution. Open Squares: SMC data SMC98. Filled Circles: HERMES data HERMES99.
• Figure 9: The spin asymmetries for $K^+$ and $K^-$ production in semi-inclusive DIS off the proton ($Q^2 = 2.5\, {\rm GeV}^2$, $z_{\rm min} = 0.2$). Dashed lines: Sum of contributions $A_{1, \, u}^{K+}$, $A_{1, \, d}^{K+}, A_{1, \, s}^{K+}$, and $A_{1, \, 0}^{K+}$ (and respectively for $K^-$), cf. Eq.(\ref{['A_decomposition']}). Dotted lines: Contributions $A_{1, \, 3}^{K+}$ and $A_{1, \, 3}^{K-}$, respectively, proportional to the flavor asymmetry $\Delta \bar{u} (x) - \Delta \bar{d} (x)$ in the target, evaluated with the distribution shown in Fig.\ref{['fig_delta']}. Note the large contribution in $K^-$ production. Dash--dotted line: Contributions $A_{1, \, 8}^{K+}$ and $A_{1, \, 8}^{K-}$, respectively. Solid lines: Total results, including the effect of flavor asymmetry of the polarized antiquark distribution.