Evidence for a 'Narrow' Roper Resonance - the Breathing Mode of the Nucleon

Abstract

All the time since its discovery the N${}^{\ast }$(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 $\ddot{I}€$N or ${\hat{I}}^3$N reactions. Only in partial wave analyses of $\ddot{I}€$N scattering data a clear resonance strcuture gets obvious in the $P_{11}$ partial wave. Very recent measurements of the J/$\hat{I}¨$ decay by the BES collaboration and of the $pp\to np\ddot{I}{€}^{+}$ reaction at 1.3 GeV by the CELSIUS-WASA collaboration show for the first time a clear resonance structure in the invariant $n\ddot{I}{€}^{+}$ 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 $\ddot{I}€$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${}^{\ast }\to$N$\ddot{I}ƒ$, i.e. a monopole transition as expected for the breathing mode of the nucleon.

CRIMEA06