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Albaladejo, M., Guo, F. K., Hidalgo-Duque, C., Nieves, J., & Pavon Valderrama, M. (2015). Decay widths of the spin-2 partners of the X(3872). Eur. Phys. J. C, 75(11), 547–26pp.
Abstract: We consider the X(3872) resonance as a J(PC) = 1(++) D (D) over bar* hadronic molecule. According to heavy quark spin symmetry, there will exist a partner with quantum numbers 2(++), X-2, which would be a D*(D) over bar* loosely bound state. The X-2 is expected to decay dominantly into D (D) over bar, D (D) over bar* and (D) over barD* in d-wave. In this work, we calculate the decay widths of the X-2 resonance into the above channels, as well as those of its bottom partner, X-b2, the mass of which comes from assuming heavy flavor symmetry for the contact terms. We find partial widths of the X-2 and X-b2 of the order of a few MeV. Finally, we also study the radiative X-2 -> D (D) over bar*gamma. and X-b2 -> (B) over bar B*gamma decays. These decay modes are more sensitive to the long-distance structure of the resonances and to the D (D) over bar* or B (B) over bar* final state interaction.
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Albaladejo, M., Guo, F. K., Hidalgo-Duque, C., & Nieves, J. (2016). Z(c)(3900): What has been really seen? Phys. Lett. B, 755, 337–342.
Abstract: The Z(c)(+/-)(3900)/Z(c)(+/-)(3885) resonant structure has been experimentally observed in the Y(4260) -> J/Psi pi pi and Y(4260) -> (D) over bar* D pi decays. This structure is intriguing since it is a prominent candidate of an exotic hadron. Yet, its nature is unclear so far. In this work, we simultaneously describe the (D) over bar* D and J/Psi pi invariant mass distributions in which the Z(c) peak is seen using amplitudes with exact unitarity. Two different scenarios are statistically acceptable, where the origin of the Z(c) state is different. They correspond to using energy dependent or independent (D) over bar *D S-wave interaction. In the first one, the Z(c) peak is due to a resonance with a mass around the D (D) over bar* threshold. In the second one, the Z(c) peak is produced by a virtual state which must have a hadronic molecular nature. In both cases the two observations, Z(c)(+/-)(3900) and Z(c)(+/-)(3885), are shown to have the same common origin, and a (D) over bar *D bound state solution is not allowed. Precise measurements of the line shapes around the D (D) over bar* threshold are called for in order to understand the nature of this state.
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Bayar, M., Aceti, F., Guo, F. K., & Oset, E. (2016). Discussion on triangle singularities in the Lambda(b) -> J/psi K(-)p reaction. Phys. Rev. D, 94(7), 074039–10pp.
Abstract: We have analyzed the singularities of a triangle loop integral in detail and derived a formula for an easy evaluation of the triangle singularity on the physical boundary. It is applied to the Lambda(b) -> J/psi K(-)p process via Lambda*-charmonium-proton intermediate states. Although the evaluation of absolute rates is not possible, we identify the chi(c1) and the psi(2S)as the relatively most relevant states among all possible charmonia up to the psi(2S). The Lambda(1890)chi(c1)p loop is very special, as its normal threshold and triangle singularities merge at about 4.45 GeV, generating a narrow and prominent peak in the amplitude in the case that the chi(c1)p is in an S wave. We also see that loops with the same charmonium and other Lambda* hyperons produce less dramatic peaks from the threshold singularity alone. For the case of chi(c1)p -> J/psi p and quantum numbers 3/2(-) or 5/2(+), one needs P and D waves, respectively, in the chi(c1)p, which drastically reduce the strength of the contribution and smooth the threshold peak. In this case, we conclude that the singularities cannot account for the observed narrow peak. In the case of 1/2(+), 3/2(-) quantum numbers, where chi(c1)p -> J/psi p can proceed in an S wave, the Lambda(1890)chi(c1)p triangle diagram could play an important role, though neither can assert their strength without further input from experiments and lattice QCD calculations.
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Guo, F. K., Meissner, U. G., Nieves, J., & Yang, Z. (2016). Remarks on the P-c structures and triangle singularities. Eur. Phys. J. A, 52(10), 318–6pp.
Abstract: It was proposed that the narrow P-c(4450) structure observed by the LHCb Collaboration in the reaction Lambda(b) -> J/psi pK might be due to a triangle singularity around the chi(c1)-proton threshold at 4.45 GeV. We discuss the occurrence of a similar triangle singularity in the J/psi p invariant mass distribution for the decay Lambda(b) -> J/psi p pi, which could explain the bump around 4.45 GeV in the data. More precise measurements of this process would provide valuable information towards an understanding of the P-c structures.
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Wang, E., Xie, J. J., Liang, W. H., Guo, F. K., & Oset, E. (2017). Role of a triangle singularity in the gamma p -> K+Lambda (1405) reaction. Phys. Rev. C, 95(1), 015205–9pp.
Abstract: We show the effects of a triangle singularity mechanism for the gamma p -> K+Lambda(1405) reaction. The mechanism has a N-* resonance around 2030 MeV, which decays into K*Sigma. The K-* decays to K+ pi, and the pi Sigma merge to form the Lambda (1405). This mechanism produces a peak around root s = 2110 MeV, and has its largest contribution around cos theta= 0. The addition of this mechanism to other conventional ones leads to a good reproduction of d sigma/dcos theta and the integrated cross section around this energy, providing a solution to a problem encountered in previous theoretical models.
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