https://doi.org/10.1140/epjc/s10052-020-08620-5
Regular Article – Theoretical Physics
Deciphering the mechanism of near-threshold 
photoproduction
1
Helmholtz-Institut für Strahlen- und Kernphysik and Bethe Center for Theoretical Physics, Universität Bonn, 53115, Bonn, Germany
2
Institute for Theoretical and Experimental Physics NRC “Kurchatov Institute”, 117218, Moscow, Russia
3
P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Leninskiy Prospect 53, 119991, Moscow, Russia
4
CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Zhong Guan Cun East Street 55, 100190, Beijing, China
5
School of Physical Sciences, University of Chinese Academy of Sciences, 100049, Beijing, China
6
Institute for Advanced Simulation, Institut für Kernphysik and Jülich Center for Hadron Physics, Forschungszentrum Jülich, 52425, Jülich, Germany
7
Tbilisi State University, 0186, Tbilisi, Georgia
8
Moscow Institute of Physics and Technology, Institutsky lane 9, 141700, Dolgoprudny, Moscow Region, Russia
9
National Research Nuclear University MEPhI, Kashirskoe highway 31, 115409, Moscow, Russia
10
Department of Physics, Institute for Nuclear Studies, The George Washington University, 20052, Washington, DC, USA
Received:
11
October
2020
Accepted:
30
October
2020
Published online:
16
November
2020
The photoproduction of the off the proton is believed to deepen our understanding of various physics issues. On the one hand, it is proposed to provide access to the origin of the proton mass, based on the QCD multipole expansion. On the other hand, it can be employed in a study of pentaquark states. The process is usually assumed to proceed through vector-meson dominance, that is the photon couples to a which rescatters with the proton to give the 
final state. In this paper, we provide a compelling hint for and propose measurements necessary to confirm a novel production mechanism via the 
intermediate states. In particular, there must be cusp structures at the thresholds in the energy dependence of the photoproduction cross section. The same mechanism also implies the -nucleon scattering lengths of order 1 mfm. Given this, one expects only a minor contribution of charm quarks to the nucleon mass.
© The Author(s) 2020
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