Narrow Results

We discuss a possible interpretation of the scalar mesons f₀(980) and a₀(980) as hadronic molecules - bound states of K and mesons. Using a phenomenological Lagrangian approach we calculate strong as well as the electromagnetic decay properties of both scalars. The covariant and gauge invariant model, which also allows for finite size effects of the hadronic molecule, delivers results in good agreement with experimental data.

We discuss how to use Morgan's pole counting rule to distinguish a molecular state from an 'elementary' particle. As two examples we focus on X(3872) and f₀(980) particles. A molecule may be generated from a meson loop bubble chain, and an 'elementary' particle is related to an explicit interaction field in the effective lagrangian and propagates with a Breit–Wigner propagator. For X(3872) it is found that the data favor the 'elementary' particle explanation. For f₀(980) the study becomes much more difficult, since highly nonperturbative dynamics is involved. A unitarization model analysis suggests that f₀(980)'s property is quite exotic. Unlike other light scalars, it does not behave like a state, and could be interpreted as a molecule.

We discuss the possible interpretation of X(3872) as a DD* hadronic molecule with J^PC = 2^-+. Using the phenomenological Lagrangian approach, we studied its radiative and strong decay properties. We find that our model with about 97.6% isospin zero component explains the existing data nicely. We predict the partial widths of the radiative and strong decays of X(3872) and show that the measurement of the ratio may signal the nature of X(3872).

The first two pentaquark candidates P_c(4380) and P_c(4450) were observed by LHCb Collaboration in 2015. Recently, they presented their improved measurements in the P_c energy range, that a new narrow signal denoted as P_c(4312) is reported and the previous observed structure P_c(4450) is resolved into two narrow states, P_c(4440) and P_c(4457) while there is no obvious signal for the broad state P_c(4380). We analyze the strong decays of these P_c states systematically in the hadronic molecule pictures. With the effective Lagrangian method, the partial widths of these P_c decaying into all allowed channels are estimated. Our results show that these P_c states are good candidates of the S-wave $\overline{D}{\mathrm{\Sigma }}_c,\overline{D}{\mathrm{\Sigma }}_c^{*}$ and $\overline{D}*{\mathrm{\Sigma }}_c$ molecules. Also other possible flavor and spin partners of P_c states are investigated in the present work.

We study the structure of the σ meson, the lowest-lying resonance of the ππ scattering in the scalar-isoscalar channel, through the softening phenomena associated with the partial restoration of chiral symmetry. We build dynamical chiral models to describe the ππ scattering amplitude, in which the σ meson is described either as the chiral partner of the pion or as the dynamically generated resonance through the ππ attraction. It is shown that the internal structure is reflected in the softening phenomena; the softening pattern of the dynamically generated σ meson is qualitatively different from the behavior of the chiral partner of the pion. On the other hand, in the symmetry restoration limit, the dynamically generated σ meson behaves similarly to the chiral partner.

We discuss how to use Morgan's pole counting rule to distinguish a molecular state from an ‘elementary’ particle. As two examples we focus on X(3872) and f₀(980) particles. A molecule may be generated from a meson loop bubble chain, and an ‘elementary’ particle is related to an explicit interaction field in the effective lagrangian and propagates with a Breit–Wigner propagator. For X(3872) it is found that the data favor the ‘elementary’ particle explanation. For f₀(3872) the study becomes much more difficult, since highly non-perturbative dynamics is involved. A unitarization model analysis suggests that f₀(980)'s property is quite exotic. Unlike other light scalars, it does not behave like a state, and could be interpreted as a molecule.