STUDY OF THE BREATHING MODE OF THE NUCLEON IN α-p AND p-p SCATTERING

Abstract

In α-p and p-p scattering at energies in the GeV region a strong "scalar" N* excitation has been observed in the mass region of the lowest P11 resonance, with characteristic properties quite different from the Roper resonance seen in π-N and γ-N. From the t-dependence in p-p scattering a P11 assignment is clearly supported. Further, from exclusive p+p → p p π+ π- data a dominant 2π-N decay of this resonance is deduced. In a DWBA description the differential cross sections were found to be sensitive to the nucleon transition density. To describe all p-p data at beam momenta between 9 and 30 GeV/c a surface peaked transition density had to be utilized. Equivalent information on the charge transition density can be obtained from the longitudinal electron scattering amplitude S1/2, deduced from polarized e-p scattering experiments. Combining the analysis of (p,p') and (e,e'), for the "scalar" P11 we obtain the following results: a) a valence quark excitation is clearly seen, b) strong sea-quark effects are observed, which are significantly larger than the valence quark contribution, c) the sea-quark transition density shows "breathing", indicating that these effects are already present in the g.s. nucleon.