Threshold cusps and triangle singularities in hadronic reactions

TL;DR

This paper reviews how kinematical singularities like threshold cusps and triangle singularities can produce peaks in hadronic reaction spectra, affecting the identification of exotic hadrons and serving as tools for precision measurements.

Contribution

It provides a comprehensive review of threshold cusps and triangle singularities in hadronic reactions, highlighting their roles in interpreting experimental data on exotic hadron candidates.

Findings

Kinematical singularities can mimic resonance signals.

Triangle singularities influence the interpretation of hadronic spectra.

Understanding these singularities aids in accurate exotic hadron identification.

Abstract

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, 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.