Eur. Phys. J. A (2008) 35: 317-319
https://doi.org/10.1140/epja/i2008-10569-6

Regular Article - Experimental Physics

Excitation of the Roper resonance in single- and double-pion production in nucleon-nucleon collisions^⋆

¹ Physikalisches Institut der Universität Tübingen, Auf der Morgenstelle 14, D-72076, Tübingen, Germany
² Joint Institute for Nuclear Research, Dubna, Russia
³ The Svedberg Laboratory, Uppsala, Sweden
⁴ Uppsala University, Uppsala, Sweden
⁵ Hamburg University, Hamburg, Germany
⁶ Budker Institute of Nuclear Physics, Novosibirsk, Russia
⁷ Institute of Theoretical and Experimental Physics, Moscow, Russia
⁸ Soltan Institute of Nuclear Studies, Warsaw, Poland
⁹ Forschungszentrum Jülich, Germany
¹⁰ High Energy Accelerator Research Organization, Tsukuba, Japan
¹¹ Soltan Institute of Nuclear Studies, Lodz, Poland

^* e-mail: clement@pit.physik.uni-tuebingen.de

Received: 18 January 2008
Accepted: 21 January 2008
Published online: 9 April 2008

Abstract

In most investigations the Roper resonance is sensed only very indirectly via complex partial-wave analyses. We find indications for its excitation in the invariant nπ⁺ mass spectrum of the pp → npπ⁺ reaction at M ≈ 1360 MeV with a width of ≈ 150 MeV . The values fit very favorably to the most recent phase shift results as well as to the observations at BES. In the near-threshold two-pion production pp → ppπ⁰π⁰ , where the Roper excitation and its subsequent decays via the routes N ^* → Δπ → Nππ and N ^* → Nσ are the only dominant processes, we find its direct decay into the Nσ channel to be the by far dominating decay process --in favor of a monopole excitation of the Roper resonance.