Internal structure of exotic hadron candidate $f_0$(980) by using fragmentation functions

Abstract

High-energy hadron reactions could be appropriate to find exotic evidences in exotic hadron candidates instead of global observables such as masses, spins, parities, and decay widths, because quarks and gluons are explicit degrees of freedom. One of possible methods is to use fragmentation functions (FFs) by taking advantage of properties on favored and disfavored functions, which corresponds to valence-quark and sea-quark distributions in parton distribution functions. Looking at these FFs, we should be able to find the exotic nature as the valence-quark distributions reflect the nature of valence constituents as shown in the pion and the proton. Recent accurate measurements of the $f_0$(980) FFs by the Belle collaboration made it possible to find its internal configuration by looking at their second moments and functional forms. Global analysis results of the $f_0$(980) FFs indicate that its internal configuration looks like $s\bar s$, which is different from a tetraquark or $K\bar K$-molecule like configuration suggested from low-energy studies. This fact indicates that the internal configuration looks different depending on the energy. At low energies, it looks like a tetraquark ($K\bar K$ molecule) hadron, but it looks like a $q\bar q$ hadron at high energies. In the similar way, some exotic hadron candidates could become ordinary $q\bar q$ or $qqq$ hadrons at high energies, although they are interpreted as exotic at low energies. This kind of new idea should be tested by future experiments especially by looking at energy or momentum dependencies in high energy hadron reactions.