ﬁve-quark hadron, where s and t are Mandelstam variables. Such a measurement will be
possible, for example, by using the high-momentum beamline at J-PARC. In addition,
another exclusive process γ + p → K
+
+ Λ(1405) could be investigated at LEPS and JLab
for ﬁnding the nature of Λ (1405). We indicated that the constituent-counting rule could
be used as a valuable observable in determining internal structure of exotic hadrons by
high-energy exclusive processes, where quark-gluon degrees of freedom explicitly appear.
Furthermore, it is interesting to investigate the transition from hadron degrees of freedom
to quark-gluon ones for exclusive exotic-hadron production processes.
Second, we analyzed the JLab-CLAS data on the photoproduction of hyperon resonances,
as well as the available data for the ground state Λ and Σ
0
of the CLAS and SLAC-
E84 collaborations, by considering constituent-counting rule suggested by perturbative
QCD [17]. From the analyses of the γ p → K
+
Λ and K
+
Σ
0
reactions, we found that the
number of the elementary constituents is consistent with n
γ
= 1, n
p
= 3, n
K
+
= 2, and
n
Λ
= n
Σ
0
= 3. Then, the analysis was made for the photoproductions of the hyperon
resonances Λ(1405), Σ(1385)
0
, and Λ(1520), where Λ(1405) could be considered to be
a
¯
KN molecule and hence its constituent number could be ﬁve. However, we found
that the current data are not enough to conclude the numbers of their constituent. It is
necessary to investigate the higher-energy region at
√
s > 2.8 GeV experimentally beyond
the energy of the available CLAS data for counting the number of constituents clearly. We
also mentioned that our results indicate energy dependence in the constituent number,
especially for Λ(1405). Namely, Λ(1405) looked like a penta-quark state at lower energies,
but it became a three-quark one at high energies. If an excited hyperon is a mixture of
three-quark and ﬁve-quark states, the energy dependence of the scaling behavior could be
valuable for ﬁnding its composition and mixture. We expect to have much progress in
future on the internal structure of exotic hadron candidates by using high-energy hadron
reactions, as a new ﬁeld on exotic hadrons.
14. Compositeness of exotic hadron candidates
In recent years, there are a number of reports on exotic hadron candidates. They are theo-
retically described by ordinary hadrons (q¯q, qqq), exotic conﬁgurations, hadron molecules,
or their mixtures. These models contain parameters which are adjusted to explain ex-
perimental observables, so that their descriptions are not necessarily appropriate ways
to understand their internal structure. As a possible method to understand the hadron
structure is to investigate compositeness of bound-state systems.
In our studies, structure of the a
0
(980) and f
0
(980) resonances was investigated with the
a
0
(980)-f
0
(980) mixing intensity from the viewpoint of compositeness [18], which corre-
sponds to the amount of two-body states composing resonances as well as bound states.
If it is one, the hadron is a bound state of a hadron molecule, and it is an ordinary hadron
described by the basic quark model if the compositeness is zero. For example, since it
is not possible to explain the strong decay width of f
0
→ ππ by quark models with the
q¯q conﬁguration [75] and also from experimental measurements on the radiative decay
ϕ → f
0
γ [63] and the two-photon decay width (f
0
→ γγ), f
0
(980) could be considered as
a K
¯
K molecule.
For this purpose, we ﬁrst formulated the a
0
(980)-f
0
(980) mixing intensity as the ratio of
two partial decay widths of a parent particle, in the same manner as the recent analysis in
BES (Beijing Spectrometer) experiments. Calculating the a
0
(980)-f
0
(980) mixing intensity
with the existing Flatte parameters from experiments, we found that many combinations
10