/ Experiment-Nucl nucl-ex/0612015

Evidence for a 'narrow' roper resonance: The Breathing mode of the nucleon

Clement, H. ; Skorodko, T. ; Bashkanov, M. ; Bogoslawsky, D. ; Calen, H. ; Cappellaro, F. ; Demiroers, L. ; Doroshkevich, E. ; Ekstrom, C. ; Fransson, K. ; Gustafsson, L. ; Hoistad, B. ; Ivanov, G. ; Jacewicz, M. ; Jiganov, E. ; Johansson, T. ; Kaskulov, M. ; Khakimova, O. ; Keleta, S. ; Koch, I. ; Kren, F. ; Kullander, S. ; Kupsc, A. ; Kuznetsov, A. ; Marciniewski, P.

Page No: 23-32
Pages: 11

Abstract: All the time since its discovery the N$^*$(1440) baryon state, commonly known as Roper resonance, has been a state with many question marks - despite of its 4-star ranking in the particle data book. One reason is that it does not produce any explicit resonance-like structures in the observables of $\pi$N or $\gamma$N reactions. Only in partial wave analyses of $\pi$N scattering data a clear resonance strcuture gets obvious in the $P_{11}$ partial wave. Very recent measurements of the J/$\Psi$ decay by the BES collaboration and of the $pp \to np\pi^+$ reaction at 1.3 GeV by the CELSIUS-WASA collaboration show for the first time a clear resonance structure in the invariant $n\pi^+$ mass spectrum for the Roper resonance at M $\approx$ 1360 MeV with a width of about 150 MeV. These values agree very favorably with the pole position results of recent $\pi$N phase shift analyses. In consequence of this very low-lying pole postion, which is roughly 100 MeV below the nominal value, the decay properties have to be reinvestigated. From our two-pion production data we see that the decay mainly proceeds via N$^* \to$N$\sigma$, i.e. a monopole transition as expected for the breathing mode of the nucleon.

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