Amplitude analysis and the nature of the Zc(3900)

The microscopic nature of the XYZ states remains an unsettled topic. We show how a thorough amplitude analysis of the data can help constraining models of these states. Specifically, we consider the case of the Zc(3900) peak and discuss possible scenarios of a QCD state, virtual state, or a kinematical enhancement. We conclude that current data are not precise enough to distinguish between these hypotheses, however, the method we propose, when applied to the forthcoming high-statistics measurements should shed light on the nature of these exotic enhancements.Comment: 14 pages, 10 figures, 3 tables. Version accepted for publication on Phys.Lett.

On the η\eta and η′\eta' Photoproduction Beam Asymmetry at High Energies

We show that, in the Regge limit, beam asymmetries in $\eta$ and $\eta'$ photoproduction are sensitive to hidden strangeness components. Under reasonable assumptions about the couplings we estimate the contribution of the $\phi$ Regge pole, which is expected to be the dominant hidden strangeness contribution. The ratio of the asymmetries in $\eta'$ and $\eta$ production is estimated to be close to unity in the forward region $0 < -t/\text{GeV}^2 \leq 1$ at the photon energy $E_\text{lab} = 9$~GeV, relevant for the upcoming measurements at Jefferson Lab.Comment: 9 pages, 4 figure

On the Equivalence of Three-Particle Scattering Formalisms

In recent years, different on-shell $\mathbf{3}\to\mathbf{3}$ scattering formalisms have been proposed to be applied to both lattice QCD and infinite volume scattering processes. We prove that the formulation in the infinite volume presented by Hansen and Sharpe in Phys.~Rev.~D92, 114509 (2015) and subsequently Brice\~no, Hansen, and Sharpe in Phys.~Rev.~D95, 074510 (2017) can be recovered from the $B$-matrix representation, derived on the basis of $S$-matrix unitarity, presented by Mai {\em et al.} in Eur.~Phys.~J.~A53, 177 (2017) and Jackura {\em et al.} in Eur.~Phys.~J.~C79, 56 (2019). Therefore, both formalisms in the infinite volume are equivalent and the physical content is identical. Additionally, the Faddeev equations are recovered in the non-relativistic limit of both representations.Comment: 13 pages, 5 figure

Determination of the pole position of the lightest hybrid meson candidate

Mapping states with explicit gluonic degrees of freedom in the light sector is a challenge, and has led to controversies in the past. In particular, the experiments have reported two different hybrid candidates with spin-exotic signature, pi1(1400) and pi1(1600), which couple separately to eta pi and eta' pi. This picture is not compatible with recent Lattice QCD estimates for hybrid states, nor with most phenomenological models. We consider the recent partial wave analysis of the eta(') pi system by the COMPASS collaboration. We fit the extracted intensities and phases with a coupled-channel amplitude that enforces the unitarity and analyticity of the S-matrix. We provide a robust extraction of a single exotic pi1 resonant pole, with mass and width 1564 +- 24 +- 86 MeV and 492 +- 54 +- 102 MeV, which couples to both eta(') pi channels. We find no evidence for a second exotic state. We also provide the resonance parameters of the a2(1320) and a2'(1700).Comment: 6 pages + 3 pages of supplemental material. Version to appear on Phys.Rev.Let

Analyticity constraints for hadron amplitudes : going high to heal low energy issues

Analyticity constitutes a rigid constraint on hadron scattering amplitudes. This property is used to relate models in different energy regimes. Using meson photoproduction as a benchmark, we show how to test contemporary low-energy models directly against high-energy data. This method pinpoints deficiencies of the models and treads a path to further improvement. The implementation of this technique enables one to produce more stable and reliable partial waves for future use in hadron spectroscopy and new physics searches

Finite-Energy Sum Rules in Eta Photoproduction off the Nucleon

The reaction ${\gamma}N \to {\eta}N$ is studied in the high-energy regime (with photon lab energies $E_{\gamma}^{\textrm{lab}} > 4$ GeV) using information from the resonance region through the use of finite-energy sum rules (FESR). We illustrate how analyticity allows one to map the t-dependence of the unknown Regge residue functions. We provide predictions for the energy dependence of the beam asymmetry at high energies.Comment: Joint Physics Analysis Cente

Khuri-Treiman equations for 3π decays of particles with spin

Khuri-Treiman equations have proven to be a useful theoretical tool in the analysis of three-body decays, especially into the 3π final state. In this work we present in full detail the necessary generalization of the formalism to study the decays of particles with arbitrary spin, parity, and charge conjugation. To this extent, we find it most convenient to work with helicity amplitudes instead of the so-called invariant amplitudes, especially when dealing with the unitarity relations. The isobar expansions in the three possible (s-, t-, and u-) final channels are related with the appropriate crossing matrices. We pay special attention to the kinematical singularities and constraints of the helicity amplitudes, showing that these can be derived by means of the crossing matrix

Dalitz-plot decomposition for three-body decays

We present a general formalism to write the decay amplitude for multibody reactions with explicit separation of the rotational degrees of freedom, which are well controlled by the spin of the decay particle, and dynamic functions on the subchannel invariant masses, which require modeling. Using the three-particle kinematics we demonstrate the proposed factorization, named the Dalitz-plot decomposition. The Wigner rotations, which are subtle factors needed by the isobar modeling in the helicity framework, are simplified with the proposed decomposition. Consequently, we are able to provide them in an explicit form suitable for the general case of arbitrary spins. The only unknown model-dependent factors are the isobar line shapes that describe the subchannel dynamics. The advantages of the new decomposition are shown through three examples relevant for the recent discovery of the exotic charmonium candidate Zc(4430), the pentaquarks Pc, and the intriguing Λc+→pK-π+ decay

ω→3π\omega \to 3\pi and ωπ0\omega\pi^{0} transition form factor revisited

In light of recent experimental results, we revisit the dispersive analysis of the $\omega \to 3\pi$ decay amplitude and of the $\omega\pi^0$ transition form factor. Within the framework of the Khuri-Treiman equations, we show that the $\omega \to 3\pi$ Dalitz-plot parameters obtained with a once-subtracted amplitude are in agreement with the latest experimental determination by BESIII. Furthermore, we show that at low energies the $\omega\pi^0$ transition form factor obtained from our determination of the $\omega \to 3\pi$ amplitude is consistent with the data from MAMI and NA60 experiments