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Threshold cusps and triangle singularities in hadronic reactions
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Threshold cusps and triangle singularities in hadronic reactions
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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.
Forward citations
Cited by 24 Pith papers
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The isovector D/D*/B/B* electromagnetic form factors are reconstructed dispersively with ππ rescattering, yielding ρ(770) coupling constants from pole residues.
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The $a_1(1420)$ in a Unitary Coupled-Channel Three-Body Approach
Nine-channel unitary three-body fits to COMPASS lineshapes reproduce the a1(1420) enhancement by triangle singularity without requiring a genuine a1(1420) pole.
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Three-body unitary determination of the $f_1(1285)$ and $f_1(1420)$ pole positions
Fitting a spectator-isobar three-body unitary amplitude to BESIII K0S K0S pi0 data yields poles at (1277±2±1)-i(12±1±0) MeV for f1(1285) and (1435±2±7)-i(40±2±1) MeV for f1(1420), with the latter traced to a K Kbar* q...
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Symmetry Analysis of Compact Tetraquark States and Implications for the Level Ordering of the Fully Charmed Candidates $X(6600)$, $X(6900)$, and $X(7100)$
Counting symmetry-allowed states up to orbital angular momentum L=3 predicts low-lying compact tetraquarks prefer J^P=2^+, matching the observed 2^{++} fully charmed X states.
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Symmetry Analysis of Compact Tetraquark States and Implications for the Level Ordering of the Fully Charmed Candidates $X(6600)$, $X(6900)$, and $X(7100)$
Symmetry analysis of compact tetraquarks shows low-energy states favor J^P=2+ and places X(6600), X(6900), X(7100) among the lower levels of the fully charmed spectrum.
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The $a_1(1420)$ in a Unitary Coupled-Channel Three-Body Approach
Unitary coupled-channel three-body model fitted to COMPASS data reproduces the a1(1420) enhancement via triangle singularity, indicating no genuine resonance pole is required.
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Fully-heavy multiquarks in neural-network quantum states
Neural-network quantum states are used to compute spectra of fully-heavy multiquarks in a non-relativistic quark model, claiming to overcome dimensionality issues with superior accuracy over prior approximations.
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Searching for the pseudoscalar partner of $G(3900)$ via radiative $Y(4230)$ decays
Calculates B(Y(4230) → γ G0(3900)) = 3.8×10^{-5}–3.3×10^{-4} assuming P-wave molecular interpretations for both states and a triangle mechanism.
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Vector charmonium(-like) states in the energy range of 4.1-4.6 GeV
A coupled-channel framework is developed and fitted to BESIII data on vector charmonium-like states in the 4.1-4.6 GeV range, concluding that coupled-channel effects with dynamically generated poles explain the line shapes.
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Isospin-breaking effects on the threshold cusp structures in $\Lambda N$-$\Sigma N$ scattering
Isospin breaking splits threshold cusps in ΛN-ΣN scattering into constrained structures whose relative sharpness or type can change, as shown via K-matrix classification and N²LO chiral EFT calculations.
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Determination of the $Z_c(3900)$ and the $Z_{cs}(3985)$ states from joint analysis of experimental and lattice data
Joint analysis of experimental and lattice data confirms Z_c(3900) and Z_cs(3985) as SU(3) flavor partners with pole masses (3879.6 ± 4.8) MeV and (3976.9 ± 5.1) MeV, half-widths (32.2 ± 4.7) MeV and (28.8 ± 5.9) MeV,...
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Coupled-channel study of the three-body $DDK$ and $D^{*}D^{*}K$
The DDK system supports a deeply bound compact state across wide parameters and possibly a shallow three-body halo state near the D-DK threshold, with negligible D*D*K coupling and no resonances.
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Enhanced evidence of $X(7200)$ and improved measurements of $X(6900)$ parameters from a combined LHCb-ATLAS-CMS analysis
Simultaneous fits to LHCb-ATLAS-CMS data raise X(7200) evidence to 3.7–6.6σ (best 6.6σ) while confirming X(6900) at >12σ with improved mass and width precision.
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Enhanced evidence of $X(7200)$ and improved measurements of $X(6900)$ parameters from a combined LHCb-ATLAS-CMS analysis
A combined fit to LHCb, ATLAS, and CMS di-J/ψ data finds >12σ evidence for X(6900) and 3.7–6.6σ evidence for X(7200), with interference effects playing a critical role.
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Neutrinoless double-beta decay of the $\Delta^-$ resonance
Chiral EFT derivation of the Δ⁻ → p e⁻ e⁻ amplitude including long-range neutrino loops, short-range counterterms, pion-mass dependence for collinear electrons, and a long-range prediction in the degenerate Δ-nucleon ...
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Study of the molecular Properties of the $P_c$ and $P_{cs}$ States
Coupled-channel calculations show Pc and Pcs states as molecular bound states with RMS radii 0.5-2 fm when heavy-quark spin symmetry is respected across all channels.
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Decoding the near-threshold $X_{0,\,1}(4140)$ and $X_{1}(4685)$ states via OZI-suppressed coupled-channel scattering
X0(4140) is a dynamically generated J/psi phi pole with single-channel scattering length 1.11 fm; X1(4685) is interpreted as a psi(2S) phi hadronic molecule under heavy quark spin symmetry.
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What can we learn from the radiative decays of the $D_{s1}(2460)$ meson?
Measuring the ratio of radiative decay branching fractions from Ds1(2460) can probe the nature of Ds0*(2317) and Ds1(2460) mesons.
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Observation and investigation of the $T_{c\bar{c}1}(4430)^{+}$ structure in $B^{+} \to \psi(2S) K_{\text{S}}^{0} \pi^{+}$ decays
First 4D amplitude analysis of B+ → ψ(2S) K_S0 π+ confirms the exotic T_{c c-bar 1}(4430)+ structure with properties consistent with prior observations in the isospin-related channel.
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Effects of Final State Interactions on Landau Singularities
Triangle singularities mimicking resonances are analyzed in the presence of final-state rescattering using Landau equations and a scattering formalism enforcing two- and three-body unitarity.
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Decay constants of the two-pole $D_0^*(2300)$
Molecular two-pole D0*(2300) decay constants are 65 and 81 MeV—much smaller than compact c¯q estimates—and imply Cabibbo-favored b-hadron branching fractions of order 10^{-5}.
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Does $\psi(4660)$ exist?
Fits using coupled channels and bare states show the psi(4660) as a conventional charmonium state above the Lambda_c pair threshold in both Belle and BESIII datasets.
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Doubly charmed pentaquark states with strangeness $S=0, -1$
Parity-projected QCD sum rules predict that several doubly charmed pentaquark molecular states lie below their meson-baryon mass thresholds, including triply charged states with no ordinary baryon counterpart.
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