Threshold cusps and triangle singularities in hadronic reactions

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.

arXiv commentVersion accepted for publication in Prog. Part. Nucl. Phys.; 84 pages, 34 figures

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• Discover the GellMann-Okubo formula with machine learning
• Binding of the three-hadron $D{D}^{\ast }K$ system from the lattice effective field theory
• Triangle singularity in $B^0\to {\pi }^{-}{K}^{+}X(3872)$ via the $D_{s1}\bar{D}{D}^{\ast }$ loop and possible precise measurement of the $X(3872)$ mass
• $X$ structures in $B^{+}\to J/\psi \varphi K^{+}$ as one-loop and double-triangle threshold cusps
• Molecular pentaquark states with open charm and bottom flavors
• Universal non-resonant explanation to charmoniumlike structures $Z_c(3885)$ and $Z_c(4025)$
• Investigating the Isospin Property of $T_{cc}^{+}$ from its Dalitz Plot Distribution
• $X$ and $Z_{cs}$ in $B^{+}\to J/\psi \varphi K^{+}$ as $s$-wave threshold cusps and alternative spin-parity assignments to $X(4274)$ and $X(4500)$
• Production of vector mesons in pentaquark states resonance channel in $p$-$A$ ultraperipheral collisions
• Molecular states in $D_s^{(\ast )+}{\Xi }_c^{{(}^{\prime },\ast )}$ systems
• Two-pion exchange for coupled-channel scattering of two heavy mesons
• Weak-decay searches for $Qs\bar{u}\bar{d}$ tetraquarks
• $P_c(4457)$ interpreted as a $J^P=1/2^{+}$ state by ${\bar{D}}^0{\Lambda }_c^{+}(2595)$-${\pi }^0{P}_c(4312)$ interaction
• Classification of Coupled-Channel Near-Threshold Structures
• The role of triangle singularity in isospin breaking process $J/\psi \to \Lambda \bar{\Lambda }\pi$ and the possible evidence of ${\Sigma }^{\ast }({\frac{1}{2}}^{-})$ states
• Constraining the $DD{D}^{\ast }$ three-body bound state via the $Z_c(3900)$ pole
• Nature of excited $\Xi$ baryons with threshold effects
• ${\bar{B}}^0\to {\bar{K}}^{(\ast )0}X$, $B^{-}\to K^{(\ast )-}X$, ${\bar{B}}_s^0\to \eta ({\eta }^{\prime },\varphi )X$ from the $X(3872)$ molecular perspective
• Interpretation of near-threshold peaks using the method of independent S-matrix poles
• Studying triangle singularity through spin observables
• $P$-wave states $T_{bb}^{-}$ from diquarks
• Charmoniumlike Channels $1^{+}$ with Isospin $1$ from Lattice and Effective Field Theory
• $\Lambda (1405)$ mediated triangle singularity in the $K^{-}d\to p{\Sigma }^{-}$ reaction
• Production of $1^{-+}$ exotic charmonium-like states in electron-positron collisions
• Pole trajectories from $S$- and $P$-wave $D{\bar{D}}^{\ast }$ interactions
• Radiative decays of the neutral $Z_c(3900)$ and $Z_c(4020)$
• Light tetraquark states with the exotic quantum number $J^{PC}=3^{-+}$
• Pole analysis for the $D^{\ast }\bar{K}$-$D\overline{K^{\ast }}$ coupled-channel system
• An alternative way to decipher the nature of the doubly charmed tetraquark $T_{cc}(3875{)}^{+}$: its antiparticle $T_{\bar{c}\bar{c}}(3875{)}^{-}$ photoproduction off nuclei near threshold
• XYZ spectroscopy at electron-hadron facilities III: Semi-inclusive processes with vector exchanges
• Production of open-charm pentaquark molecules in decay $B^0\to {\bar{D}}^{0}p\bar{p}$
• Photoproduction of the $X(3872)$ beyond vector meson dominance: the open-charm coupled-channel mechanism
• Study of the $Y(4660)$ from a light-quark perspective
• The role of triangle singularity in the decay process $D^0\to {\pi }^{+}{\pi }^{-}{f}_0(980),f_0\to {\pi }^{+}{\pi }^{-}$
• Strong decays of $P_{\psi }^N(4440{)}^{+}$ and $P_{\psi }^N(4457{)}^{+}$ within the Bethe-Salpeter framework
• Photon-induced production of the exotic charged charmonium-like state $Z_c(3900)$ off nuclear targets and its internal structure
• First observation of the ${\Lambda }_b^0\to {\Lambda }_c^{+}{D}_s^{-}{K}^{+}{K}^{-}$ decay and search for pentaquarks in the ${\Lambda }_c^{+}{D}_s^{-}$ system
• Analysis of the hidden-charm pentaquark candidates in the $J/\psi \Xi$ mass spectrum via the QCD sum rules
• Unraveling $K(1690)$ as a pseudoscalar $ud\bar{d}\bar{s}$ tetraquark state
• Possible ${\Sigma }_c^{\ast }\bar{\Sigma }$ molecular states
• Hints of the $J^{PC}=0^{--}$ and $1^{--}$ $K^{\ast }{\bar{K}}_1(1270)$ Molecules in the $J/\psi \to \varphi \eta {\eta }^{\prime }$ Decay
• Production of the bottomonium-like states $Z_b$ states at $e$-$h$ and ultraperipheral $h$-$h$ collisions
• $Z_c(3900)$ in a hadronic molecule and a triangle singularity approach at finite temperature
• Doubly-charmed pentaquark states in a mass splitting model
• The $B^{-}\to J/\psi {\eta }^{\prime }{K}^{-}$ reaction and the $Y(4710)$ and $K_0^{\ast }(1430)$ contributions
• Analytic Properties of Triangle Feynman Diagrams in Quantum Field Theory
• Possible bound states in the triple-${\eta }_c$ and triple-$J/\psi$ systems
• Hunting for the heavy quark spin symmetry partner of $Z_{cs}$
• Learning Pole Structures of Hadronic States using Predictive Uncertainty Estimation
• Charting the new physics through one-loop effects in the muon magnetic dipole moment
• Hadronic molecules and multiquark states
• Production of hidden-heavy and double-heavy hadronic molecules at the $Z$ factory of CEPC
• New Types of Hydrogenlike matter Composed of Electron(s) and Meson(s)
• Coupled-channel meson-meson scattering in the diabatic framework
• Box Singularity Conditions in Box Diagrams of Decay Processes
• Dispersive Analysis of $D$- and $B$-Meson Form Factors with Chiral and Heavy-Quark Constraints
• Decoding the near-threshold $X_{0,1}(4140)$ and $X_1(4685)$ states via OZI-suppressed coupled-channel scattering
• Insights into the exotic charged states $Z_b(10610)$ and $Z_b(10650)$ from their photoproduction off nuclei
• Observation and investigation of the $T_{c\bar{c}1}(4430{)}^{+}$ structure in $B^{+}\to \psi (2S)K_{\text{S}}^0{\pi }^{+}$ decays
• What can we learn from the radiative decays of the $D_{s1}(2460)$ meson?
• $X(3860)$ production in association with $J/\psi$ via $e^{+}{e}^{-}$ annihilation at Belle