https://doi.org/10.1140/epjc/s10052-019-6552-7
Regular Article - Theoretical Physics
The 
structures in a coupled-channels model
1
Grupo de Física Nuclear and Instituto Universitario de Física Fundamental y Matemáticas (IUFFyM), Universidad de Salamanca, 37008, Salamanca, Spain
2
Grup de Física Teòrica, Dept. Física and IFAE-BIST, Universitat Autònoma de Barcelona, 08193, Bellaterra, Barcelona, Spain
* e-mail: pgortega@usal.es
Received:
3
August
2018
Accepted:
31
December
2018
Published online:
28
January
2019
The 
and 
are two charmonium-like structures discovered in the 
and 
invariant mass spectra. Their nature is puzzling due to their charge, which forces its minimal quark content to be 
(
). Thus, it is necessary to explore four-quark systems in order to understand their inner structure. Additionally, their strong coupling to channels such as and the closeness of their mass to 
-thresholds stimulates both a molecular interpretation or a coupled-channels threshold effect. In this work we perform a coupled-channels calculation of the 
sector including 
, and 
channels in the framework of a constituent quark model which satisfactorily describes a wide range of properties of (non-)conventional hadrons containing heavy quarks. The meson–meson interactions are dominated by the non-diagonal 
and 
couplings which indicates that the and are unusual structures. The study of the analytic structure of the S-matrix allows us to conclude that the point-wise behavior of the line shapes in the and 
invariant mass distributions is due to the presence of two virtual states that produce the peaks.
© The Author(s), 2019
