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The absorption model and production of 2 mesons

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Il Nuovo Cimento A (1965-1970)

Summary

The absorption model for high-energy peripheral production of resonances is applied to the production of spin-parity 2+ mesons in pion-nucleon interaction with the twofold purpose of testing the model against the experimental data and of discussing some approximations made within the model. By a detailed numerical investigation of the partial-wave expansion we show how the impact parameter approximation, often used in the application of the model, may significantly affect both the predicted cross-section and the decay distribution. moreover, the cross-section, but not so much the decay distribution, is sensitive to the detailed way in which each partial wave in the model is modified by the absorption. We trace these features back to the fact that the low partial waves, although strongly suppressed as compared to the unmodified Born approximation, still play a major role. The comparison with experiment for production of fo (pion exchange) shows that the slope of the cross-section is correctly predicted but that the absolute magnitude is too big by a factor of around two. For A2 (1310) production (rho exchange) the slope of the cross-section can be made to agree with experiment only after the introduction of a fairly strong form factor; the absolute ma gntude and the energy dependence of the cross-section are wrong. These results strengthen the conclusion that the absorption model is unable to reproduce the experimental data for production of high-spin resonances, especially if vector meson exchange has to be assumed. The spin-2 formalism, including decay widths and decay distributions, is presented in the Appendices.

Riassunto

Si applica alla produzione di mesoni con spin-parità 2+ il modello di assorbimento per la produzione periferica di risonanze di alta energia nella interazione pione-nucleone col duplice proposito di verificare il modello rispetto ai dati sperimentali e di discutere alcune approssimazioni fatte nel modello. Con una dettagliata indagine numerica dello sviluppo dell'onda parziale si mostra come l'approssimazione del parametro d'urto, spesso usata nell'applicazione del modello, possa influire in modo significativo sia sulla sezione d'urto predetta sia sulla distribuzione di decadimento. Inoltre la sezione d'urto, ma non altrettanto la distribuzione di decadimento, è sensibile al modo dettagliato in cui ciascuna onda parziale del modello è modificata dall'assorbimento. Si riallacciano queste caratteristiche al fatto che le onde parziali basse, anche se fortemente soppresse in confronto all'approssimazione di Born non modificata, ricoprono ancora un ruolo importante. Il confronto con gli esperimenti nel caso della produzione di fo (scambio di pioni) mostra che la pendenza della sezione d'urto è predetta correttamente ma che la grandezza assoluta è troppo grande per un fattore di circa 2. Per la produzione di A2 (1310) (scambio di σ) si può fare in modo che la pendenza della sezione d'urto concordi con gli esperimenti solo dopo l'introduzione di un fattore di forma adeguato; la grandezza assoluta e la dipendenza della sezione d'urto dall'energia sono errate. Questi risultati riconfermano la conclusione che il modello di assorbimento è inadatto a riprodurre i dati sperimentali della produzione di risonanze di spin elevato, specialmente se si deve presupporre lo scambio di mesoni vettoriali. Nelle Appendici si presenta il formalismo dello spin 2, incluse le ampiezze e le distribuzioni di decadimento.

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Author notes

  1. J. Högaasen

    Present address: Laboratoire de Physique Théorique, Départment de Physique Nucléaire, Orsay

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  1. CERN, Geneva

    H. Högaasen, J. Högaasen, R. Keyser & B. E. Y. Svensson

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  1. H. Högaasen
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  3. R. Keyser
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  4. B. E. Y. Svensson
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Högaasen, H., Högaasen, J., Keyser, R. et al. The absorption model and production of 2 mesons. Nuovo Cimento A (1965-1970) 42, 323–350 (1966). https://doi.org/10.1007/BF02717923

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