Speaker
Description
In recent years, there are experimental reports on exotic-hadron candidates, which have different quark configurations from ordinary $q\bar q$ and $qqq$ constituents. However, it is not easy to confirm their exotic nature from global observables such as masses, spins, parities, and decay widths. At high energies, internal quark and gluon configurations could become more apparent because hadrons should be described by fundamental degrees of freedom of quarks and gluons in quantum chromodynamics. Therefore, the internal structure of exotic hadrons coould be investigated by various high-energy reactions.
First, there is a constituent-counting rule suggested in perturbative QCD for hard exclusive reactions to count the number of elementary constituents. The rule could be used for finding the number of constituents in exotic hadrons. It is expected that the number is four and five for tetraquark and pentaquark hadrons, respectively. We proposed this idea in 2013 and have been investigating future possibilities, for example, $\Lambda$(1405) production at the high-momentum beamline of J-PARC [1]. We also analyzed available data on hard hyperon productions including $\Lambda$(1405) in hadron and charged-lepton reactions [1]. We found that the ground $\Lambda$ production is consistent with the three-quark nature. However, it is difficult to determine the number of constituents for $\Lambda$(1405) because the current data are not accurate enough and they are within a limited energy range. Nevertheless, we reported an interesting tendency that $\Lambda$(1405) looks like pentaquark at low energies but it seems be a three-quark baryon at high energies. The energy dependence could be important for probing the internal configuration, especially if it is a mixture of three-quark and five-quark states.
Second, another possibility is to use the fragmentation functions (FFs) [2]. In the work of 2025, accurate FFs of an exotic hadron candidate $f_0$(980) are determined for the first time by an global analysis of experimental data on $e^+ + e^- \to f_0 (980)+X$ with recent precise measurements of the Belle collaboration. From the global analysis, we found that their second moments have a relation $M_u = M_d \ll M_s \sim M_g $ for up-quark, down-quark, strange-quark, and gluon FFs. Furthermore, the function $D_s^{f_0}(z)$ is distributed in the larger-$z$ region in comparison with the functions $D_{u}^{f_0}(z)$, $D_{d}^{f_0}(z)$, and $D_g^{f_0}(z)$. These facts support that the $f_0 (980)$ has the $s\bar s$ configuration at high energies. This is a new finding that $f_0 (980)$ should be considered mainly as the $s\bar s$ state, which is different from our usual understanding as a tetraquark (or $K\bar K$) hadron from low-energy studies. My results could indicate the transition of the internal configuration picture that $f_0 (980)$ looks like a $q\bar q$ state at high energies although it is described by tetra-quark or $K\bar K$ molecule state at low energies. It sheds light on a new direction in exotic hadron physics.
[1] W.-C. Chang, S. Kumano, T. Sekihara, Phys.Rev. D93 (2016) 034006; H. Kawamura, S. Kumano, T. Sekihara, Phys. Rev. D88 (2013) 034010; H. Kawamura, S. Kumano, Phys. Rev. D89 (2014) 054007.
[2] M. Hirai, S. Kumano, M. Oka, and K. Sudoh, Phys. Rev. D77, 017504 (2008); S. Kumano, arXiv:2511.20217.