Session M01: Challenges in Understanding Exotic Hadrons

Challenges in Understanding Exotic Hadrons

Exotic spectroscopy is a hot topic since there are several discoveries or observations per year. 

In this talk, the last main experimental discoveries will be presented, and then the main theoretical interpretations will be discussed.

Finally, some results with molecular models and other ones with compact models of which I am also one of the authors will be presented and discussed.   

The Search for Exotic Hadrons at GlueX

The GlueX experiment at Jefferson Lab was specifically designed for precision studies of the light-meson spectrum. A photon beam with energies of up to 12 GeV is directed onto a liquid hydrogen target contained within a hermetic detector with near-complete neutral and charged particle coverage. Linear polarization of the photon beam with a maximum around 9 GeV provides additional information about the production process. In 2018, the experiment completed its first phase, recording data with a total integrated luminosity above 400 pb^-1. We will highlight a selection of results from this world-leading data set with emphasis on the search for light hybrid mesons. In the mean time, the detector underwent significant upgrades and is currently recording data with an even higher luminosity. The future plans of the GlueX experiment to search for exotic hadrons with unprecedented precision will be summarized.   

Understanding exotic hadrons from the first-principle calculation in lattice QCD

The first-principle calculation using lattice QCD plays an important role in hadron physics. In particular, hadron interactions, for which the calculation methods in lattice QCD have been developed together with increasing computer resources in recent years, are vital inputs for understanding exotic hadrons through scattering processes. The HAL QCD method, one of the methods to extract hadron interactions, has the advantage that it provides us with an understanding of the structure of exotic hadrons via the corresponding hadron interaction potentials. 

In this presentation, after a brief review of the HAL QCD method, we overview the recent studies of exotic hadrons in the HAL QCD method, such as P_c pentaquark, Ω_cccΩ_ccc dibaryon, and Τ_cc tetraquark. Furthermore, we present one of our recent projects, the analyses of hadron scatterings containing light quark-antiquark pair creation and annihilation processes, which are essential to investigate hadron resonances and the exotic hadrons that appear in such scattering processes. We first show the results of the studies on ρ and Δ resonances and then report the preliminary results of our current study in this project: Λ(1405) from the meson-baryon interactions in flavor SU(3) limit.   

Study of the Lambda(1405) resonance via kaon-induced reactions

 The Λ(1405) hyperon resonance has been attracting us for a long time if it is an "exotic" meson-baryon molecular state. Recent experimental results on extracted anti-kaon-nucleon (KbarN) scattering amplitude in isospin I = 0 and the pole mass of Λ(1405) at J-PARC will be presented.

 We measured πΣ invariant mass spectra below and above the KbarN mass threshold in the K^-d -> NπΣ in order to study the KbarN interaction and the Λ(1405) resonance [1]. For this purpose, a negatively-charged kaon (K^-) beam of 1 GeV/c was irradiated on a deuterium target at the K1.8BR beam line in the J-PARC Hadron Experimental Facility. In the experiment, a nucleon (N: neutron or proton) knocked out from a deuteron (d) by an incident K^- was detected at a very forward angle, and four different final states of π⁺Σ^-, π^-Σ⁺, π⁰Σ⁰, and π^-Σ⁰ were identefied by measuring the charged particles emitted around the target.

 This reaction can be described by the following two-step process: (i) K^-"N₁"->KbarN and (ii) Kbar"N₂"->πΣ, where "N₁" and "N₂" are nucleons bound in the deuteron.

Since the nucleon emitted at the forward angle carries away most of the collision energy in (i), the center-of-mass energy in (ii) can be lower, even below the KbarN mass threshold. Since the recoiled kaon is slow down below 250 MeV/c, one expects that the S-wave Kbar"N₂"->πΣ scattering is dominant around the KbarN mass region. In order to separate the I=0 and 1 amplitudes in (ii), we demonstrate that an isospin relation of the cross sections between the four final πΣ states is satisfied as π⁰Σ⁰ = [π⁺Σ^- + π^-Σ⁺ - π^-Σ⁰]/2.

 The scattering amplitudes of step (i) has been well analyzed by Gopal et al. [2]. Employing their amplitudes in step (i), we could deduce the S-wave scattering amplitudes of step (ii): KbarN->KbarN as well as KbarN->πΣ in I = 0 in the framework of the KbarN-πΣ coupled channel by reproducing the πΣ spectrum of the I=0 channel. We find a resonance pole at 1417.7^+6.0_-7.4(fitting error)^+1.1_-1.0(systematic error) -i[26.9^+6.0_-7.9(fitting error)^+1.7_-2.0(systematic error)] MeV in the deduced KbarN->KbarN amplitude [3].