ar X iv : h ep - p h / 01 11 39 9 v 1 2 9 N ov 2 00 1 Excitation of non - quark degrees of freedom in N * ∗

We study the contribution of glueball and σ-meson degrees of freedom to nucleon excited states in the framework of a chiral version of the chromodielectric model. We have found that these degrees of freedom considerably lower the energy of the Roper and may substantially weaken the electroexcitation amplitudes for the N(1440) and in particular for the N(1710). Among the excited states of the nucleon the Roper resonance, N(1440), plays a rather special role since, due to its relatively low excitation energy, a simple picture in which one quark populates the 2s level does not work here. The relatively low energy of the N* has been explained by • the residual interaction originating from chiral mesons exchange [1], • allowing the confining potential to vibrate (e.g. in the MIT bag model [2], or in models with dynamically generated confinement, [3]), • describing the N* as a pure sigma meson excitation rather than the excitation of the quark core [4], • explicit excitations of glue-field [5]. We present a simple model, the chromodielectric model (CDM), which is particularly suitable to describe the interplay of glueball and meson excitations together with quark radial excitations. The model includes a chromodielectric field χ which assures quark dynamical confinement, and the chiral fields, σ and π. The Lagrangian takes the form [6]: L = iψ̄γ∂μψ + g χ ψ̄(σ̂ + i~τ · ~̂πγ5)ψ + Lσ,π + Lχ , (1) where Lχ = 12∂μχ̂ ∂χ̂− 1 2 M χ̂ , Lσ,π = 1 2∂μσ̂∂μσ̂ + 12∂μ~̂π · ∂~̂π − U(~̂π 2 + σ̂) , (2) Talk delivered by B. Golli at the workshop on Few body problems in hadronic and atomic physics at Bled, Slovenia, July 7–14, 2001. E-mail: [email protected]