Electroexcitation of the Roper resonance from CLAS data

The helicity amplitudes of the electroexcitation of the Roper resonance on proton are extracted at 1.7 < Q < 4.2 GeV 2 from recent high precision JLab-CLAS cross sections data and longitudinally polarized beam asymmetry for π electroproduction on protons. The analysis is made using two approaches, dispersion relations and unitary isobar model, which give consistent results. It is found that the transverse helicity amplitude for the γ∗p → P11(1440) transition, which is large and negative at Q 2 = 0, becomes large and positive at Q ≃ 2 GeV , and then drops slowly with Q. Longitudinal helicity amplitude, that was previously found from CLAS data as large and positive at Q = 0.4, 0.65 GeV , drops with Q. These results rule out the presentation of P11(1440) as a q G hybrid state, and provide strong evidence in favor of this resonance as a first radial excitation of the 3q ground state. PACS. 1 1.55.Fv, 13.60.Le, 13.40.Gp, 14.20.Gk In this talk we report our results on the electroexcitation of the Roper resonance (P11(1440)) extracted from a large body of CLAS data on differential cross sections and polarized beam asymmetries for the process ep → enπ in the range of invariant hadronic mass W = 1.15−1.69GeV and photon virtuality Q = 1.7 − 4.2 GeV 2 with full azimuthal and polar angle coverage [1]. Combined with the information obtained from the previous CLAS data at Q = 0.4, 0.65 GeV 2 [2,3] and that at Q = 0 [4], these results give us knowledge of the Roper electroexcitation in wide Q range and allow us to draw quite definite conclusions on the nature of P11(1440). It is known that the structure of the Roper resonance has attracted special attention since its discovery, because the simplest and most natural assumption that this is a first radial excitation of the 3q ground state led to the difficulties in the description of the resonance. To deal with shortcomings of the quark model, alternative, as well extended descriptions of P11(1440) were developed: as a hybrid (qG) state [5,6], that of a quark core dressed by a meson cloud [7,8], a dynamically generated πN resonance [9], and presentations that include 3q− qq̄ components, in particular, strong σN component (see Ref. [10] and references therein). The Q dependence of the electromagnetic transition form factors is highly sensitive to different descriptions of the Roper state. However, until recently, the data base needed to measure these form factors at relatively high Q was almost exclusively based on π production, and was very limited in kinematical coverage. Also, the πp final state is dominated by the nearby isospin 3/2∆(1232) resonance, whereas the isospin 1/2 Roper state couples more strongly to the πn channel. The data [1] allow us to extract the γ∗p → P11(1440) helicity amplitudes at Q = 1.7 − 4.2 GeV , and therefore to obtain complete picture of the electroexcitation of the Roper resonance in a wide Q region. The approaches we use are fixed-t dispersion relations (DR) and unitary isobar model (UIM), which both were successfully employed in Refs. [2,3,11] to analyze photoproduction and low Q electroproduction of pions. These approaches were presented and discussed in Refs. [11]. The imaginary parts of the amplitudes in both approaches are determined mainly by schannel resonance contributions which we parameterize in the Breit-Wigner form with energy-dependent widths [11,12]. The exception was made for the P33(1232) resonance which was treated in a special way. According to the phase-shift analyses of the πN scattering, the πN amplitude corresponding to the P33(1232) resonance is elastic up to W = 1.43 GeV (see, for example, the latest GWU(VPI) analyses [13,14]). In combination with DR and Watson’s theorem, this provides strict constraints on the multipole amplitudes M 3/2