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Register a new userTheory of hadronic molecules applied to the XYZ states
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Authors
C. Hanhart
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In recent years data have been accumulated at various experiments about states in the heavy quarkonium mass range that seem to be inconsistent with the most simple variants of the quark model. In this contribution it is demonstrated that most of those data are consistent with a dominant molecular nature of those states. It is also discussed which kind of observables are sensitive to the molecular component and which are not.
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I give a brief comment on the LHCbs discovery of $P_c$ and $P_{cs}$ pentaquark states. These pentaquark states not only reveal a new kind of hadronic molecules, but also shed new light on the possible structure of pentaquark components in baryons, hence may play very important role for understanding un-quenching dynamics and the whole baryon spectroscopy.
We propose novel triangle relations, not the well-known triangle singularity, for better understanding of the exotic XYZ states. Nine XYZ resonances, X(3872), Y(4230), Zc(3900), X(4012), Y(4360/4390), Zc(4020), X(4274), Y(4660), and Zcs(3985) have been classified into triples to construct three triangles based on the assumption that they are all tetra-quark states. We also suggest some channels deserving search for with high priority based on this hypothesis, as well as predictions of a few production/decay rates of these channels. We hope further experimental studies of the XYZ states will benefit from our results.
Numerous exotic candidates containing a heavy quark and anti-quark (the so-called $XYZ$ states) have been reported since the observation of the $X(3872)$ in 2003. For these systems a study of the implications of the heavy quark spin symmetry and its breaking is expected to provide useful guidance towards a better understanding of their nature. For instance, since the formation of the complete spin multiplets is described with the same parameter sets, in some cases the currently available experimental data on the $XYZ$ states allows us to predict properties of spin partner states. To illustrate this point we extract the parameters of the two $Z_b$ states by analyzing the most recent experimental data within an effective-field theory approach which treats both short-ranged contact interactions and the long-ranged one-pion/one-eta Goldstone boson exchanges (OPE/OEE) dynamically. The line shapes and pole positions of their spin partners are then predicted in a parameter-free way and await to be tested by future experimental data.
In this proceeding, we present our recent work on decay behaviors of the $P_c$ hadronic molecules, which can help to disentangle the nature of the two $P_c$ pentaquark-like structures. The results turn out that the relative ratio of the decays of $P^+_c(4380)$ to $bar{D}^* Lambda_c$ and $J/psi p$ is very different for $P_c$ being a $bar D^*Sigma_c$ or $bar DSigma_c^*$ bound state with $J^P=frac{3}{2}^-$. And from the total decay width, we find that $P_c(4380)$ being a $bar DSigma_c^*$ molecule state with $J^P=frac{3}{2}^-$ and $P_c(4450)$ being a $bar D^*Sigma_c$ molecule state with $J^P=frac{5}{2}^+$ is more favorable to the experimental data.
The $P_c(4380)$ and $P_c(4450)$ states observed recently by LHCb experiment were proposed to be either $bar{D} Sigma_c^*$ or $bar{D}^* Sigma_c$ S-wave bound states of spin parity $J^P={frac32}^-$. We analyze the decay behaviors of such two types of hadronic molecules within the effective Lagrangian framework. With branching ratios of ten possible decay channels calculated, it is found that the two types of hadronic molecules have distinguishable decay patterns. While the $bar{D} Sigma_c^*$ molecule decays dominantly to $bar{D}^* Lambda_c$ channel with a branching ratio by 2 orders of magnitude larger than to $bar{D}Lambda_c$, the $bar{D}^* Sigma_c$ molecule decays to these two channels with a difference of less than a factor of 2. Our results show that the total decay width of $P_c(4380)$ as the spin-parity-${frac32}^-$ $bar{D} Sigma_c^*$ molecule is about a factor of 2 larger than the corresponding value for the $bar{D}^* Sigma_c$ molecule. It suggests that the assignment of $bar{D} Sigma_c^*$ molecule for $P_c(4380)$ is more favorable than the $bar{D}^* Sigma_c$ molecule. In addition, $P_c(4450)$ seems to be a $bar{D}^* Sigma_c$ molecule with $J^P={frac52}^+$ in our scheme. Based on these partial decay widths of $P_c(4380)$, we estimate the cross sections for the reactions $gamma p to J/psi p $ and $ pi pto J/psi p $ through the s-channel $P_c(4380)$ state. The forthcoming $gamma p$ experiment at JLAB and $pi p$ experiment at JPARC should be able to pin down the nature of these $P_c$ states.
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