Dr Yanrui Liu (Tokyo Institute of Technology)
Exotic heavy quark hadrons near one or several two-body thresholds triggered lots of discussions about hadronic molecule problem in the heavy quark sector. Here the larger reduced mass reduces the kinetic energy and is advantageous for the bound state. It is interesting to investigate possible molecules containing the lowest charmed baryon, Lambda_c (Lc). We present the study of two systems, Lc-N and Lc-Lc, in a one-boson-exchange potential model. The heavy quark symmetry leads to the nearly degenerate Sigma_c and Sigma_c^* and therefore the intermediate excited baryons may have important contributions to such systems. We explore these coupled channel effects. For our study, we first construct the effective Lagrangian reflecting the heavy quark symmetry, chiral symmetry, and hidden local symmetry and then derive the non-relativistic potentials. The numerical results are obtained by solving the Schrodinger equation. The coupling constants in the model are determined by decay of heavy baryons, quark model symmetry, chiral multiplet assumption, vector meson dominance assumption, and QCD sum rule results. To consider the extended structure of the hadrons, we introduce additional cutoff parameters at each vertex. Since we do not have enough information to determine the values, they are treated as free parameters. In studying the Lc-N system, we use both one-boson-exchange potential (OBEP) model and one-pion-exchange potential (OPEP) model. We find that the channel coupling is essential to the molecular bound states. Loosely bound molecules are obtained with the cutoff around 1.2~1.3 GeV in the OPEP model. To get molecule solutions, the cutoffs in the OBEP model can be smaller. In addition, both the binding energy and corresponding root-mean-square radius in the OMEP model may be roughly reproduced in the OPEP model. We use only the OPEP model in studying the Lc-Lc system. The channel coupling, especially the tensor force induced S-D wave mixing, plays a crucial role in binding two Lc's. According to our calculation, molecule-like solutions exist with the cutoff around 1.0~1.2 GeV. Tightly bound solutions with higher cutoffs may be beyond our model since shorter distance interactions contribute. To conclude, the molecular bound states in Lc-N and Lc-Lc are plausible, although the results are sensitive to the chosen cutoff parameters.
Mr Wakafumi Meguro (Tokyo Institute of Technology)