Search

It is shown that the data on the pion charge form factor admit the possibility for a substantial sea-quark components in the pion wave function. If the charge form factor is calculated with instant form kinematics in a constituent quark model that is extended to include explicit $(q\bar q)^2$ components in the pion wave function, that component will give the dominant contribution to the calculated $\pi^+$ charge form factor at large values of momentum transfer. The present experimental values $Q^2$ can be described well with $(q\bar q)^2$ component admixtures of up to 50%. The sensitivity of the calculated $\pi^+$ charge form factor to whether one of the quarks or one of the antiquarks is taken to be in the P-state is small., Comment: 14 pages

The contributions from the qqqq (q) over bar components to the magnetic moments of the octet as well as the Delta(++) and Omega(-) decuplet baryons are calculated for the configurations that are expected to have the lowest energy if the hyperfine interaction depends on both spin and flavor. The contributions from the u (u) over bar ,d (d) over bar, and the s (s) over bar components are given separately. It is shown that addition of qqqq (q) over bar admixtures to the ground state baryons can improve the overall description of the magnetic moments of the baryon octet and decuplet in the quark model without SU(3) flavor symmetry breaking, beyond that of the different constituent masses of the strange and light-flavor quarks. The explicit flavor (and spin) wave functions for all the possible configurations of the qqqq (q) over bar components with light and strange q (q) over bar pairs are given for the baryon octet and decuplet. Admixtures of similar to 10% of the qqqq (q) over bar configuration where the flavor-spin symmetry is [4](FS)[22](F)[22](S), which is likely to have the lowest energy, in particular reduces the deviation from the empirical values of the magnetic moments Sigma(-) and the Xi(0) compared with the static qqq quark model.

The role of 5-quark components in the pion and electromagnetic decays and transition form factors of the N(1440) is explored. The $qqqq\bar q$ components, where the 4-quark subsystem has the flavor-spin symmetries $[4]_{FS}[22]_F[22]_S$ and $[4]_{FS}[31]_F[31]_S$, which are expected to have the lowest energy of all $qqqq\bar q$ configurations, are considered in detail with a nonrelativistic quark model. The matrix elements between the 5-quark components of the N(1440) and the nucleon, $qqqq\bar q\to qqqq\bar q$, play a minor role in these decays, while the transition matrix elements $qqqq\bar q\to qqq$ and $qqq\to qqqq\bar q$ that involve quark antiquark annihilation are very significant. Both for the electromagnetic and strong decay the change from the valence quark model value is dominated by the confinement triggered $q\bar q$ annihilation transitions. In the case of pion decay the calculated decay width is enhanced substantially both by the direct $q\bar q \to \pi$ and also by the confinement triggered $q\bar q\to \pi$ transitions. Agreement with the empirical value for the pion decay width may be reached with a $\sim$ 30% $qqqq\bar q$ component in the N(1440).

Five-quark $\mathit{qqqq}\overline{q}$ components in the $\ensuremath{\Delta}(1232)$ are shown to contribute significantly to $\ensuremath{\Delta}(1232)\ensuremath{\rightarrow}N\ensuremath{\pi}$ decay through quark-antiquark annihilation transitions. These involve the overlap between the qqq and the $\mathit{qqqq}\overline{q}$ components and may be triggered by the confining interaction between the quarks. With a ~10% admixture of five-quark components in the $\ensuremath{\Delta}(1232)$ the decay width can be larger by factors ~2 -- 3 over that calculated in the nonrelativistic quark model with three valence quarks, depending on the details of the confining interaction. The effect of transitions between the $\mathit{qqqq}\overline{q}$ components themselves on the calculated decay width is however small. The large contribution of the quark-antiquark annihilation transitions thus may compensate the underprediction of the width of the $\ensuremath{\Delta}(1232)$ by the valence quark model, once the $\ensuremath{\Delta}(1232)$ contains $\mathit{qqqq}\overline{q}$ components with ~10% probability.

In the framework of the resonating-group method, the interactions of decuplet baryon-baryon systems with strangeness numbers s= -1 and s= -5 are investigated by employing the chiral SU(3) quark model. The effective baryon-baryon interactions deduced from quark-quark interactions and scattering cross sections of the Sigma*Delta and Xi*Omega systems are calculated.