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Pavao, R., & Oset, E. (2018). Coupled channels dynamics in the generation of the Omega (2012) resonance. Eur. Phys. J. C, 78(10), 857–8pp.
Abstract: We look into the newly observed Omega (2012) state from the molecular perspective in which the resonance is generated from the (K) over bar Xi*, eta Omega and (K) over bar Xi channels. We find that this picture provides a natural explanation of the properties of the Omega (2012) state. We stress that the molecular nature of the resonance is revealed with a large coupling of the Omega (2012) to the (K) over bar Xi* channel, that can be observed in the Omega (2012) -> (K) over bar pi Xi decay which is incorporated automatically in our chiral unitary approach via the use of the spectral function of Xi* in the evaluation of the (K) over bar Xi* loop function.
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Molina, R., Geng, L. S., & Oset, E. (2019). Comments on the dispersion relation method to vector-vector interaction. Prog. Theor. Exp. Phys., (10), 103B05–16pp.
Abstract: We study in detail the method proposed recently to study the vector-vector interaction using the N/D method and dispersion relations, which concludes that, while, for J = 0, one finds bound states, in the case of J = 2, where the interaction is also attractive and much stronger, no bound state is found. In that work, approximations are done for N and D and a subtracted dispersion relation for D is used, with subtractions made up to a polynomial of second degree in s – s(th), matching the expression to 1 – VG at threshold. We study this in detail for the rho rho interaction and to see the convergence of the method we make an extra subtraction matching 1 – VG at threshold up to (s – s(th))(3). We show that the method cannot be used to extrapolate the results down to 1270 MeV where the f(2)(1270) resonance appears, due to the artificial singularity stemming from the “on-shell” factorization of the rho exchange potential. In addition, we explore the same method but folding this interaction with the mass distribution of the rho, and we show that the singularity disappears and the method allows one to extrapolate to low energies, where both the (s – s(th))(2) and (s – s(th))(3) expansions lead to a zero of Re D(s), at about the same energy where a realistic approach produces a bound state. Even then, the method generates a large Im D(s) that we discuss is unphysical.
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Oset, E., & Roca, L. (2022). Exotic molecular meson states of B(*) K(*) nature. Eur. Phys. J. C, 82(10), 882–9pp.
Abstract: We evaluate theoretically the interaction of the open bottom and strange systems (B) over bar (K) over bar, (B) over bar * (K) over bar, (B) over bar (K) over bar * and (B) over bar* (K) over bar* to look for possible bound states which could correspond to exotic non-quark-antiquark mesons since they would contain at least one b and one s quarks. The s-wave scattering matrix is evaluated implementing unitarity by means of the Bethe-Salpeter equation, with the potential kernels obtained from contact and vector meson exchange mechanisms. The vertices needed are supplied from Lagrangians derived from suitable extensions of the hidden gauge symmetry approach to the bottom sector. We find poles below the respective thresholds for isospin 0 interaction and evaluate the widths of the different obtained states by including the main sources of imaginary part, which are the B *-> B gamma decay in the (B) over bar * (K) over bar channels, the K *-> K pi in the channels involving a K *, plus the box diagrams with (B) over bar (K) over bar and (B) over bar * (K) over bar intermediate states for the (B) over bar * (K) over bar * channels.
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Li, H. P., Zhang, G. J., Liang, W. H., & Oset, E. (2023). Theoretical interpretation of the Ξ(1620) and Ξ(1690) resonances seen in Ξc+ → Ξ-π+π+ decay. Eur. Phys. J. C, 83(10), 954–7pp.
Abstract: We study the Belle reaction Xi(+)(c) -> Xi(-)pi(+)pi(+) looking at the mass distribution of pi(+)Xi, where clear signals for the Xi(1620) and Xi(1690) resonances are seen. These two resonances are generated dynamically from the interaction in coupled channels of pi Xi, (K) over bar Lambda, (K) over bar Xi and eta Xi within the chiral unitary approach. Yet, the weak decay process at the quark level, together with the hadronization to produce pairs of mesons, does not produce the pi pi Xi final state. In order to produce this state one must make transitions from the (K) over bar Lambda, (K) over bar Xi and eta Xi components to pi Xi, and this interaction is what produces the resonances. So, the reaction offers a good test for the molecular picture of these resonances. Adding the contribution of the Xi*(1530) and some background we are able to get a good reproduction of the mass distribution showing the signatures of the two resonances as found in the experiment.
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Dai, L. R., Abreu, L. M., Feijoo, A., & Oset, E. (2023). The isospin and compositeness of the Tcc(3875) state. Eur. Phys. J. C, 83(10), 983–11pp.
Abstract: We perform a fit to the LHCb data on the T-cc(3875) state in order to determine its nature. We use a general framework that allows to have the (DD & lowast;+)-D-0, (D+D & lowast;0) components forming a molecular state, as well as a possible nonmolecular state or contributions from missing coupled channels. From the fits to the data we conclude that the state observed is clearly of molecular nature from the (DD & lowast;+)-D-0, (D+D & lowast;0) components and the possible contribution of a nonmolecular state or missing channels is smaller than 3%, compatible with zero. We also determine that the state has isospin I=0 with a minor isospin breaking from the different masses of the channels involved, and the probabilities of the (DD & lowast;+)-D-0, (D+D & lowast;0) channels are of the order of 69% and 29% with uncertainties of 1%. The differences between these probabilities should not be interpreted as a measure of the isospin violation. Due to the short range of the strong interaction where the isospin is manifested, the isospin nature is provided by the couplings of the state found to the (DD & lowast;+)-D-0, (D+D & lowast;0) components, and our results for these couplings indicate that we have an I=0 state with a very small isospin breaking. We also find that the potential obtained provides a repulsive interaction in I=1, preventing the formation of an I=1 state, in agreement with what is observed in the experiment.
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Ramos, A., & Oset, E. (2013). The role of vector-baryon channels and resonances in the gamma p -> K-0 Sigma(+) and gamma n -> K-0 Sigma(0) reactions near the K*Lambda threshold. Phys. Lett. B, 727(1-3), 287–292.
Abstract: We have studied the gamma p -> K-0 Sigma(+) reaction in the energy region around the K*Lambda and K*Sigma thresholds, where the CBELSA/TAPS cross section shows a sudden drop and the differential cross section experiences a transition from a forward-peaked distribution to a flat one. Our coupled-channel model incorporates the dynamics of the vector meson-baryon interaction which is obtained from the hidden gauge formalism. We find that the cross section in this energy region results from a delicate interference between amplitudes having K*Lambda and K*Sigma intermediate states. The sharp downfall is dictated by the presence of a nearby N* resonance produced by our model, a feature that we have employed to predict its properties. We also show results for the complementary gamma n -> K-0 Sigma(0) reaction, the measurement of which would test the mechanism proposed in this work.
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Nagahiro, H., Hirenzaki, S., Oset, E., & Ramos, A. (2012). eta '-Nucleus optical potential and possible eta ' bound states. Phys. Lett. B, 709(1-2), 87–92.
Abstract: Starting from a recent model of the eta'N interaction, we evaluate the eta'-nucleus optical potential, including the contribution of lowest order in density, t rho/2m(eta'), together with the second-order terms accounting for eta' absorption by two nucleons. We also calculate the formation cross section of the eta' bound states from (pi(+), p) reactions on nuclei. The eta'-nucleus potential suffers from uncertainties tied to the poorly known eta'N interaction, which can be partially constrained by the experimental modulus of the eta'N scattering length and/or the recently measured transparency ratios in eta' nuclear photoproduction. Assuming an attractive interaction and taking the claimed experimental value vertical bar a(eta'N)vertical bar = 0.1 fm, we obtain an eta' optical potential in nuclear matter at saturation density of V eta' = -(8.7 + 1.8i) MeV, not attractive enough to produce eta' bound states in light nuclei. Larger values of the scattering length give rise to deeper optical potentials, with moderate enough imaginary parts. For a value vertical bar a(eta'N)vertical bar = 0.3 fm, which can still be considered to lie within the uncertainties of the experimental constraints, the spectra of light and medium nuclei show clear structures associated to eta'-nuclear bound states and to threshold enhancements in the unbound region.
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Gamermann, D., Nieves, J., Oset, E., & Ruiz Arriola, E. (2010). Couplings in coupled channels versus wave functions: Application to the X(3872) resonance. Phys. Rev. D, 81(1), 014029–14pp.
Abstract: We perform an analytical study of the scattering matrix and bound states in problems with many physical coupled channels. We establish the relationship of the couplings of the states to the different channels, obtained from the residues of the scattering matrix at the poles, with the wave functions for the different channels. The couplings basically reflect the value of the wave functions around the origin in coordinate space. In the concrete case of the X(3872) resonance, understood as a bound state of D-0(D) over bar*(0) and D+D*(-) (and c.c. From now on, when we refer to D-0(D) over bar*(0), D+D*(-), or D (D) over bar* we are actually referring to the combination of these states with their complex conjugate in order to form a state with positive C-parity), with the D-0(D) over bar*(0) loosely bound, we find that the couplings to the two channels are essentially equal leading to a state of good isospin I = 0 character. This is in spite of having a probability for finding the D-0(D) over bar*(0) state much larger than for D+D*(-) since the loosely bound channel extends further in space. The analytical results, obtained with exact solutions of the Schrodinger equation for the wave functions, can be useful in general to interpret results found numerically in the study of problems with unitary coupled channels methods.
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Molina, R., Branz, T., & Oset, E. (2010). New interpretation for the D*(s2)(2573) and the prediction of novel exotic charmed mesons. Phys. Rev. D, 82(1), 014010–17pp.
Abstract: In this manuscript we study the vector-vector interaction within the hidden-gauge formalism in a coupled channel unitary approach. In the sector C = 1, S = 1, J = 2 we get a pole in the T matrix around 2572 MeV that we identify with the D*(s2)(2573), coupling strongly to the D*K*(D-s(*)phi(omega)) channels. In addition we obtain resonances in other exotic sectors which have not been studied before such as C = 1, S = -1, C = 2, S = 0 and C = 2, S = 1. These "flavor-exotic'' states are interpreted as D*(K) over bar*, D*D*, and (DsD)-D-** molecular states but have not been observed yet. In total we obtain nine states with different spin, isospin, charm, and strangeness of non-C = 0, S = 0 and C = 1, S = 0 character, which have been reported before.
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Yamagata-Sekihara, J., Nieves, J., & Oset, E. (2011). Couplings in coupled channels versus wave functions in the case of resonances: Application to the two A(1405) states. Phys. Rev. D, 83(1), 014003–15pp.
Abstract: In this paper we develop a formalism to evaluate wave functions in momentum and coordinate space for the resonant states dynamically generated in a unitary coupled channel approach. The on-shell approach for the scattering matrix, commonly used, is also obtained in quantum mechanics with a separable potential, which allows one to write wave functions in a trivial way. We develop useful relationships among the couplings of the dynamically generated resonances to the different channels and the wave functions at the origin. The formalism provides an intuitive picture of the resonances in the coupled channel approach, as bound states of one bound channel, which decays into open ones. It also provides an insight and practical rules for evaluating couplings of the resonances to external sources and how to deal with final state interaction in production processes. As an application of the formalism we evaluate the wave functions of the two A(1405) states in the pi Sigma, (K) over barN, and other coupled channels. It also offers a practical way to study three-body systems when two of them cluster into a resonance.
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