Prog. Part. Nucl. Phys. 112, 103757 (2020)
Threshold cusps and triangle singularities in hadronic reactions
Feng-Kun Guoa,c, Xiao-Hai Liub, and Shuntaro Sakaia
a CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190, China
b Center for Joint Quantum Studies and Department of Physics, School of Science, Tianjin University, Tianjin 300350, China
c School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
The spectrum of hadrons is the manifestation of color confinement of quantum chromodynamics. Hadronic resonances correspond to poles of the S-matrix. Since 2003, lots of new hadron resonant structures were discovered in the mass regions from light mesons to hadrons containing a pair of a heavy quark and an antiquark. Many of them are candidates of exotic hadrons, and they are usually observed as peaks in invariant mass distributions. However, the S-matrix also has kinematical singularities due to the on-shellness of intermediate particles for a process, such as two-body thresholds and triangle singularities (TSs), and they can produce peaks as well. On the one hand, such singularities may be misidentified as resonances; on the other hand, they can be used as tools for precision measurements. In this paper, we review the threshold cusps and various triangle singularities in hadronic reactions, paying attention to their manifestations in phenomena related to exotic hadron candidates.