The Nucleon's Virtual Meson Cloud and Deep Inelastic Lepton Scattering

TL;DR

It is found that large meson loop momenta, {parallel},ital k}{parallel}{approx_equal}0.8 GeV, dominate in the calculation of the nucleon{close_quote}s sea quark distributions in the meson cloud model, which indicates the limitations of the applicability of the physical picture of a mesonCloud around theucleon in high-energy processes.

Abstract

We address the question whether the nucleon's antiquark sea can be attributed entirely to its virtual meson cloud and, in essence, whether there exists a smooth transition between hadronic and quark-gluon degrees of freedom. We take into account contributions from $π$ and $K$ mesons and compare with the nucleon's antiquark distributions which serve as a non-perturbative input to the QCD evolution equations. We elucidate the different behavior in the flavor singlet and non-singlet channels and study the dependence of our results on the scale $Q^2$. The meson-nucleon cut-offs that we determine give not only an indication on the size of the region within which quarks are confined in a nucleon, but we find that the scale of these form factors is closely related to the four-momentum transfer, $Q^2$, where gluons are resolved by a high energy probe, and that large meson loop momenta, $|{\bf k}| \approx 0.8$ GeV, contribute significantly to the sea quark distributions. While the agreement of our calculations with data-based parametrizations is satisfactory and scale independent for the flavor breaking share of the nucleon's antiquark sea, the flavor singlet component is quite poorly described. This hints the importance of gluon degrees of freedom.

Figures (6)

• Figure 1: The meson cloud contribution to the nucleon's structure functions in deep inelastic lepton scattering, where $B \in \{N, \Delta, \Lambda, \Sigma, \Sigma^*\}$ refers to an octet or a decuplet baryon accessible from the nucleon through emission of a meson, $M \in \{\pi , K\}$.
• Figure 2: The flavor breaking components of the nucleon's antiquark sea at $Q^2=1$ GeV$^2$. The solid [dot-long-dashed, dot-short-dashed, short-dashed, long-dashed] curves correspond to the MRS(A') [CTEQ3M, GRV94(HO), BM(A), MRS(D'$_-$)] PDFs, and the dotted lines refer to our meson cloud calculations using an exponential form factor and varying $\Lambda_e$ between 700 and 1000 MeV.
• Figure 3: The flavor breaking components of the nucleon's antiquark sea. The solid and dashed curves show the PDFs at $Q^2=1$ and 4 GeV$^2$, and the dot-dashed and dotted lines refer to the corresponding meson cloud calculations using the NA24 pion structure function and employing Gaussian cut-offs of $\Lambda_e=870$ and 880 MeV for the MRS(A') and CTEQ3M PDFs, respectively. The range of $x$-values considered in our fits is indicated through the vertical lines.
• Figure 4: The various components of the nucleon's antiquark sea. The solid and dashed curves show the MRS(A) and CTEQ3M PDFs, and the dotted lines refer to our meson cloud calculations using the NA24 pion structure function and the cut-offs $\Lambda^e_{\pi NN}=1030$ MeV, $\Lambda^e_{\pi N\Delta}=800$ MeV and $\Lambda^e_{KNY}=1200$ MeV that were adjusted at a scale of $Q^2=1$ GeV$^2$.
• Figure 5: The different relative contributions to the convolution integral of Eq. (\ref{['eq:su3a']}) for the $N \to N\pi$ process from the various regions in the space spanned by the quantities $y$, $t$ and $|{\bf k}|$, and for a typical $x$-value of $x=0.3$.