70 •mil in FR9807007 The Roper resonance M. P. Rekalo National Science Center KFTI, 310108 Kharkov, Ukraine and Laboratoire National Saturnt, CE Saclay, 91191 Gif-sur-Yvette Cedex, France

1 Electromagnetic Interaction

The Roper resonance, JV*(1440), with Jp = l/2+, / = 1/2 was first observed in the phase shift analysis of TVN elastic scattering [1], then it was studied in reactions of photo- and electro- production of pions on nucleons, 7 + N —> N + TX and e~ + N —t e~ + N + TT. In the general case the electroexcitation of the Roper resonance (RR) on the nucleon, i. e. a transition 7* + N —> N* ( 7* is a virtual photon) is characterized by two electromagnetic formfactors: magnetic (absorption of virtual photons with transverse polarization) and electric (absorption of virtual photons with longitudinal polarization). Due to the isospin non-conservation in the electromagnetic hadron interaction, each of these formfactors has isovector and isoscalar components. Therefore four independent ( real in the space-like region of momentum transfer) formfactors describe the elecro-excitation of the RR. These formfactors contain the complete information about the structure of N* (the N* wave function) and about the mechanism of the interaction of the virtual photon with quarks and gluons in a compound system as N*. It is a microscopic information which is more rich then the global characteristics such as Jp, I, mass and width, branching ratios for different decays (JV* -> Nn, N* —> NTTTT, N* -> An, N* -» Na.....), which can be obtained from the phase shift analysis of 7r + JVT~elastic scattering, 2n- production in 7riV-collisions, TTN —>• n -f 7r + iV,photoproduction of pions on nucleons etc.. The electromagnetic formfactors of the vertex 7 + N -f JV*(1440) can be extracted from different polarization observables for the electroproduction of pions on protons and neutrons, e~ + N —v e~ + N -\- 7c. Adequate methods of analysis of such data allow to extract the k2 — dependence of formfactors of the electroexcitation of different nucleonic resonances. Informa- tions about masses, widths, JF and / for N* must be taken from other reactions such as, for example, elastic nN scattering. Of course any multipole analysis of e~ + N —>• e~ + N + -n data is not free from model- dependent assumptions, however it has to be considered as the most direct source of informa- tions about the electromagnetic structure of the N*. This is due to the well known properties of the electromagnetic interaction. In case of e + N —> e + N* (Fig. 1), we know precisely:

• the reaction mechanism -4 the one-photon exchange; • the mediator of the interaction vertex between leptons and hadrons —> the photon; • one vertex —*• the fee-interaction given by QED.

Therefore CEBAF will give new and interesting informations about the electromagnetic form- factors of the excitation of different nucleonic resonances, including the RR. These informa- tions will be essential for our understanding of the internal structure of the nucleon and of the nucleonic excitations in the non-perturbative regime. 71

2 Strong interaction

A different picture holds for the excitation of the RR in the strong interaction. Let us con- sider first the ap inelastic scattering with RR excitation, more exactly the inclusive reaction p(a, a')X. Different problems appear in the description of this process, in particular concern- ing the reaction mechanism. In case of a + p —> a + p + TT0 the 7r-meson can be produced in both vertexes ( Fig. 2). Instead of a single constant e for the -jee-vertex e + N -4- e + N* many different vertexes can contribute, which are charactrized not only by formfactors but also by different amplitudes ( with eventually a complicated spin structure). All these different con- tributions to the total amplitude interfere strongly with large polarization effects. Therefore the interpretation of the process p(a, a')X in terms of excitation of RR is not so simple. A large 'resonance-like' contribution in the X-mass spectrum from the pion production in the a-vertex (Fig. 2a), (classical Deck effect, [2] which is sometimes interpreted as A excitation of the projectile), constitute a very large background in the resonances region. In such situation polarization effects may be very important in order to find the correct mechanism. In this respect processes like d(d. d')X or d(p, d')X, may be more interesting then (a, a') scattering, still conserving the advantages of being an isoscalar probe. The first experimental data about the easiest measurable polarization observable in d+p —> d + X, the tensor analyzing power T20, obtained in Saturne and Dubna in 1995 [3], show clearly that the main mechanism for iV*-excitation can not be related to a exchange. The experimental data show large negative values of T2o, independent of energy and of the target. Models based on cr-exchange cannot reproduce these features, for any choice of the interaction constants and formfactors (at least models which do not contain derivatives in the dda-vertex). On the other hand, these properties are naturally reproduced in the framework of u ex- change (Fig. 3), as they are typical for a spin one particle exchange. Moreover, the spin structure of the u>dd vertex is identical to the *y*dd vertex, and all three possible formfactors of the u>dd vertex coincide with the corresponding electromagnetic formfactors of the deuteron (charge, quadrupole and magnetic ) because the deuteron isospin is equal to zero. The avail- able data on T20 from different reactions as e~ + d —> e~ + d [4] and d(d, d')X [3] confirm this prediction. As a consequence T20 is not very sensitive to the properties of the Roper excitation. A polarized target in p(d,d) X seems more adapted to the study of the properties of the proton vertex in the RR excitation. Future polarization experiments in p(d, d')X or d(p,d)X will allow a deeper understanding of the reaction mechanism giving informations about the isoscalar formfactors of u;NN transitions. In this case we will have a new method for the study of the isotopic structure of the electroexcitation of N*, as efficient as the elec- troproduction experiments. However the complete reconstruction of the isotopic structure of the electro-excitation amplitudes needs a neutron target. However the best experiment to study the isoscalar excitation of any nucleonic resonances with / = 1/2 is the diffractive dissociation of high energy protons scattered by a nuclear target, for example p + p -> p + N* . At small t and large s the Pomeron exchange (Fig. 4) is the main reaction mechanism, as the recent experiments in HERA seem to confirm. 72

Figure Caption

Fig. 1 One-photon mechanism for the electroexcitation of N*. Fig. 2 Possible mechanisms for the a+p-^a + N + v: a) Deck mechanism; b) Roper excitation. Fig. 3 u>—exchange for the process d-\-p—> d -{• TT + N. Fig. 4 Pomeron exchange for the diifractive dissociation of the proton.

References

[1] L.D. Roper Phys. Rev. Lett. 12 (1964) 340.

[2] R.T. Deck, Phys. Rev. Lett. 13 (1964) 169.

[3] L.S. Azhgirey et al. Phys. Lett. B364 (1995) 21.

[4] M. Garcpn et al. Phys. Rev. C49 (1994) 2516. 73

e- e-.

N N' Fig. 1 a a a a A ia,cov -4, N N N N n (a) Fig. 2 (b)

A CO

N Fig. 3 7i

N*

-4J- Fig.4