Internal structure of exotic hadrons by high-energy exclusive reactions

TL;DR

This paper proposes using high-energy exclusive reactions, constituent counting rules, and hadron tomography via GPDs and GDAs to probe the internal structure of exotic hadron candidates, offering a new approach to identify their nature.

Contribution

It introduces a novel method combining constituent counting, GPDs, and GDAs in exclusive reactions to investigate the internal structure of exotic hadrons.

Findings

Constituent counting rule can determine the number of constituents in exotic hadrons.

Hadron tomography with GPDs and GDAs reveals exotic nature through form factors and invariant mass dependence.

Proposes experimental setups at J-PARC and KEKB for these investigations.

Abstract

We propose to use high-energy exclusive reactions for probing internal structure of exotic hadron candidates. First, the constituent counting rule of perturbative QCD can be used for finding internal configurations of an exotic hadron candidate. It is because the number of constituents ($n$), which participate in the exclusive reaction, is found by the scaling behavior of the cross section $d\sigma/dt \propto 1/s^{n-2}$ at large momentum transfer, where $s$ is the center-of-mass energy squared. As an example, we show that the internal structure of $\Lambda \, (1405)$ should be found, for example, by the reaction $\pi^- + p \to K^0 + \Lambda (1405)$. Second, the internal structure of exotic hadron candidates should be investigated by hadron tomography with generalized parton distributions (GPDs) and generalized distribution amplitudes (GDAs) in exclusive reactions. Exotic nature should be reflected in the GPDs which contain two factors, longitudinal parton distributions as indicated by the constituent counting rule and transverse form factors as suggested by the hadron size. The GDAs should be investigated by the two-photon process $\gamma^* \gamma \to h\bar h$, for example $h=f_0$ or $a_0$, in electron-positron annihilation. Since the GDAs contain information on a time-like form factor, exotic nature should be found by studying the $h\bar h$ invariant mass dependence of the cross section. The internal structure of exotic hadron candidates should be clarified by the exclusive reactions at facilities such as J-PARC and KEKB.