Summary
Electroproduction of pions is calculated at the mass shell of theN*(1236) in a model where seven Feynman diagram are used, namely the direct isobar term, the crossed isobar term, the three Born terms, terms with ρ and ω exchange. Three parameters,i.e. coupling constants for the γNN*, ρNN and ωNN vertices, were obtained through a best fit of the angular distributions in photoproduction of π+ and π0. On the other hand, a universal electromagnetic form factor of the dipole type was used at all vertices connecting the virtual photon with hadrons. Recent results of single-arm and double-arm experiments on electro-production of pions are fitted with this model, and the agreement is satisfactory. We notice that the value found for the magnetic-dipole moment of the γNN* vertex is in very good agreement with theSU 6 prediction.
Riassunto
Si calcola l’elettroproduzione di pioni sullo strato delle masse delNN*(1236) in un modello in cui si usano sette diagrammi di Feynman, cioè il termine isobaro diretto, il termine isobaro incrociato, i tre termini di Born ed i termini con scambio di ρ e ω. Si sono ottenuti tre parametri, cioè le costanti di accoppiamento dei vertici γNN*, ρNN e ωNN, per mezzo della migliore approssimazione delle distribuzioni angolari nella fotoproduzione di π+ e π0. D’altra parte, si è usato un fattore di forma elettromagnetico universale in tutti i vertici che connettono il fotone virtuale con adroni. Con questo modello si approssimano i risultati degli esperimenti con un solo o due bracci sull’elettroproduzione di pioni, e l’accordo è soddisfacente. Si nota che il valore trovato per il momento di dipolo magnetico del vertice γNN* è in ottimo accordo con la predizione diSU 6.
Реэюме
Проводится вычисление злектророждения пионов на массовой поверхностиNN* (1236) в модели, в которой испольэуются семь диаграмм Фейнмана, а именно, прямой иэобарический член, перекрестный иэобарический член, три борновских члена, и члены с ρ- и -обменом. Три параметра, т.е. константы свяэи для γNN*, ρNN и ωNN верщин, были получены с помошью наилучщей подгонки угловых распределений в фоторождении π− и π0. С другой стороны испольэовался универсальный злектромагнитный форм-фактор дипольного типа во всех верщинах, свяэываюших виртуальный фотон с адронами. Реэультаты недавних зкспериментов по злектророждению пионов подгоняются с помошью зтой модели, получается удовлетворительное согласие. Отмечается, что полученная величина для магнитного дипольного момента γNN* верщины очень хорощо согласуется с предскаэаниямиSU 6.
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Same as (25), p. 273.
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This work was partly supported by the Commissiariat à l’Energie Atomique.
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Cochard, G., Karatchentzeff, M., Kessler, P. et al. Electroproduction at theN*(1236). Nuov Cim A 12, 909–926 (1972). https://doi.org/10.1007/BF02747856
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DOI: https://doi.org/10.1007/BF02747856