https://doi.org/10.1140/epja/s10050-021-00634-1
Regular Article - Experimental Physics
Observation of a structure in the 
invariant mass distribution near 1700 
in the 
reaction
The CBELSA/TAPS Collaboration
1
II. Physikalisches Institut, Universität Gießen, Giessen, Germany
2
Helmholtz-Institut für Strahlen-und Kernphysik, Universität Bonn, Bonn, Germany
3
Department of Physics, Florida State University, Tallahassee, FL, USA
4
Departement Physik, Universität Basel, Basel, Switzerland
5
Physikalisches Institut, Universität Bonn, Bonn, Germany
6
National Research Centre “Kurchatov institute”, Petersburg Nuclear Physics Institute Gatchina, Gatchina, Russia
7
Physikalisches Institut, Universität Bochum, Bochum, Germany
8
Institute of Experimental Physics, University of Hamburg, Hamburg, Germany
9
Institut für Kernphysik, Forschungszentrum Jülich, Jülich, Germany
10
Institut für Kernphysik, Universität Mainz, Mainz, Germany
11
Paul Scherrer Institut, Villigen PSI, Villigen, Switzerland
a
Volker.Metag@exp2.physik.uni-giessen.de
Received:
9
August
2021
Accepted:
18
November
2021
Published online:
9
December
2021
The reaction 
has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the 
invariant mass distribution at a mass of 
. The existence of this structure cannot be confirmed in the present work. Instead, for 
= 1400–1500 MeV and the cut 
a statistically significant structure in the invariant mass distribution near 1700 is observed with a width of 
while the mass resolution is 
= 5 . Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from 
1700 to 1725 ; the width increases to about 50 and decreases thereafter. The cross section associated with this structure reaches a maximum of 100 nb around 
1490 MeV (W 1920 MeV), which coincides with the 
threshold. Three scenarios are discussed which might be the origin of this structure in the invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the 
reaction.
© The Author(s), under exclusive licence to Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2021
