Structure of the breathing mode of the nucleon from high-energyp−pscattering

Abstract

Spectra of $p\text{\ensuremath{-}}p$ and $\ensuremath{\pi}\text{\ensuremath{-}}p$ scattering at beam momenta between 6 and 30 GeV/c have been reanalyzed. These show strong excitation of ${N}^{*}$ resonances, the strongest one corresponding to the ``scalar'' ${P}_{11}$ excitation (breathing mode) at ${m}_{0}=1400\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.3em}{0ex}}\text{MeV}$ with $\ensuremath{\Gamma}=200\ifmmode\pm\else\textpm\fi{}20\phantom{\rule{0.3em}{0ex}}\text{MeV}$. The result of a strong scalar excitation is supported by a large longitudinal amplitude ${S}_{1/2}$ extracted from $e\text{\ensuremath{-}}p$ scattering. From exclusive data on $p+p\ensuremath{\rightarrow}\mathit{pp}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$, a large $2\ensuremath{\pi}\text{\ensuremath{-}}N$ decay branch for the ${P}_{11}$ resonance of ${B}_{2\ensuremath{\pi}}=75\ifmmode\pm\else\textpm\fi{}20$% has been extracted.The differential cross sections were described in a double-folding approach, assuming multigluon exchange as the dominant part of the effective interaction between the constituents of projectile and target. First, the parameters of the interaction were fitted to elastic scattering; then, with this interaction, the inelastic cross sections were described in the distorted wave Born approximation (DWBA). A good description of the data requires a surface peaked transition density, quite different from that of a pure radial mode. In contrast, the electron scattering amplitude ${S}_{1/2}$ is quite well described by a breathing mode transition density with radial node. This large difference between charge and matter transition densities suggests that in $p\text{\ensuremath{-}}p$ scattering the coupling to the multigluon field is much more important than the coupling to the valence quarks. A multigluon (or sea-quark) transition density is derived, which also shows breathing, indicating a rather complex multiquark structure of N and ${N}^{*}$ including additional multigluon (or ${q}^{2n}{\overline{q}}^{2n}$) creation out of the ground state vacuum.