electroproduction amplitudes in a model with dynamical confinement

The Roper resonance is described in a chiral version of the chromodielectric model as a cluster of three quarks in radial-orbital configuration (1s)2(2s)1, surrounded by π and σ-meson clouds and by a chromodielectric field which assures quark dynamical confinement. Radial profiles for all fields are determined self-consistently for each baryon. Transverse A1/2 and scalar S1/2 helicity amplitudes for the nucleon-Roper transition are calculated. The contribution of glueball and σ-meson vibrations is estimated; although small for N(1440), the σ contribution can be large for N(1710). (PACS 12.39.Fe, 13.40.Gp, 14.20.Gk) The new facilities for intermediate energy nuclear physics, operating with continuous electron beams, make more accessible accurate measurements of electromagnetic properties of both the nucleon and excited states, thus providing more and better information on the structure of baryons, and stimulating theoretical research on the structure of the nucleons and its resonances. The Roper resonance, N(1440), is of particular interest since, due to its relatively low excitation energy, a simple picture in which one quark populates the 2s level does not work here. The difficulties to predict electromagnetic properties in the constituent quark model further supports the idea that additional degrees of freedom, such as explicit excitations of glue-field [1], glueball field [2], or chiral fields [3,4] are important for formation of the Roper resonance. 1 E-mail:[email protected] 2 E-mail:[email protected] 3 E-mail:[email protected] 4 E-mail:[email protected] Preprint submitted to Elsevier Preprint 11 February 2008 In this letter we use a simple model, the chromodielectric model (CDM), which is particularly suitable to describe the interplay of glueball and meson excitations together with the usual quark radial excitation. In contrast to the nonrelativistic or relativistic versions of the constituent quark model, in the CDM the electromagnetic current operator is derived directly from the Lagrangian, hence no additional assumptions have to be introduced in the calculation of electroexcitation amplitudes. The electromagnetic current contains an explicit contribution from the pion field which has been shown to play an important role in the description of the N–∆ electroproduction [5]. The Roper has been considered in a non-chiral version of the CDM using the RPA techniques to describe coupled vibrations of valence quarks and the background chromodielectric field [2]. The energy of the lowest excitation turned out to be 40 % lower than the pure 1s–2s excitations. A similar result was obtained by Guichon [6], using the MIT bag model and considering the Roper as a collective vibration of valence quarks and the bag. In our approach we describe the nucleon and the Roper as chiral solitons resulting from the non-linear interactions between quarks and a scalar-isoscalar chiral singlet field χ which, through the peculiar way it couples to the quarks, provides a mechanism for confinement. In addition, the quarks interact with scalar-isoscalar (σ) and pseudoscalar-isovector (~π) mesons similarly as in the linear σ-model, though in the CDM the chiral fields are weaker and similar to the solution in the CBM for bag radius above 1 fm. The Lagrangian of the model can be written as [7] L = Lq + Lσ,π + Lq−meson + Lχ , (1) where Lq = iψ̄γ∂μψ , Lσ,π = 1 2∂μσ̂∂μσ̂ + 1 2∂μ~̂π · ∂~̂π − U(~̂π 2 + σ̂) , (2) U(~̂π2 + σ̂) being the usual Mexican hat potential, and the quark meson interaction is given by Lq−meson = g χ ψ̄(σ̂ + i~τ · ~̂πγ5)ψ . (3) The last term in (1) contains the kinetic and the potential piece for the χ-field: Lχ = 1 2∂μχ̂ ∂χ̂− 1 2 M χ̂ . (4) Other versions of the CDM consider a quartic potential in (4). By taking just 2 the mass term the confinement is imposed in the smoothest way, which seems to be the most appropriate choice for the quark matter sector of the CDM [8]. The parameters of the model have been fixed by requiring that the calculated static properties of the nucleon agree best with the experimental values: we take g = 0.03 GeV and M = 1.4 GeV [5,8,9]. The pion decay constant and the chiral meson masses are fixed to fπ = 0.093 GeV and mπ = 0.14 GeV, while for the mass of the σ-meson we consider values between mσ = 0.7 GeV and mσ = 1.2 GeV. We have checked that our results depend very weakly on the variations of these parameters. The starting point to describe a baryon is the hedgehog coherent state, which we write in the form: