Session 1WF: Exotic Hadrons and Strangeness Nuclear Physics across the Date Line I

Search for Hybrid Mesons via Photoproduction at Jefferson Lab

Studies of meson spectra via strong decays provide insight regarding QCD at the confinement scale. These studies have led to phenomenological models for QCD such as the constituent quark model. However, QCD allows for a much richer spectrum of meson states which include extra states such as hybrids, exotics, multi-quarks, and glueballs. Within the past two decades a number of experiments have put forth tantalizing evidence for the existence of exotic hybrid mesons in the mass range below 2 GeV. Theoretical calculations indicate that photoproduction should provide an ideal hunting ground for gluonic hybrid mesons. I report on a search for photoproduced mesons using the CLAS at Jefferson Lab. CLAS has acquired the largest statistics to date for peripheral meson photoproduction at intermediate photon energies. Results on on several final states will be presented. I will conclude with a overview of future efforts to search for exotic mesons utilizing the 12 GeV CEBAF accelerator in conjunction with upgraded and new experimental facilities.   

Heavy quark in exotic hadron and nuclear systems

In recent years, it has turned out that heavy hadrons with charm and bottom flavors have rich structures, which are different from simple quark-antiquark or three-quark systems. The new states of heavy hadrons are called exotic hadrons X, Y and Z. The subjects are now covering not only exotic hadrons but also exotic ``nuclei'' in which heavy hadrons are bound. The purpose of the presentation is to discuss the general properties of exotic states of hadrons and nuclei with heavy quarks [1]. We begin our discussion by the heavy quark spin (HQS) symmetry in the heavy quark limit, and show that all heavy hadrons are classified by the HQS symmetry, i.e. either HQS singlet or doublet. Next, in order to discuss the long-range physics of exotic hadrons, we introduce the heavy hadron effective theory according to the HQS symmetry in heavy quark sector as well as by chiral symmetry in light quark sector. As examples, we investigate the theoretically possible states of hadronic molecules with an anti-D meson (B meson) and nucleons with baryon number one [2], two [3] and infinity (i.e. nuclear matter) [4]. Calculating the energies, we show that many of them exhibit the HQS doublets. Beyond the leading order in heavy quark limit, we further discuss the 1/M corrections with heavy hadron mass M, and show that finding the HQS-breaking (non-breaking) terms at 1/M is important to investigate the magnetic (electric) gluons in the heavy hadrons in nuclear medium [1,5].\\[4pt] [1] S. Yasui, K. Sudoh, Y. Yamaguchi, S. Ohkoda, A. Hosaka, T. Hyodo, Phys. Lett. B727, 185 (2013); Y. Yamaguchi, S. Ohkoda, T. Hyodo, A. Hosaka, S. Yasui, arXiv:1402.5222 [hep-ph].\\[0pt] [2] S. Yasui, K. Sudoh, Phys. Rev. D80, 034008 (2009); Y. Yamaguchi, S. Ohkoda, S. Yasui, A. Hosaka, Phys. Rev. D84, 014032 (2011), ibid. 85, 054003 (2013).\\[0pt] [3] Y. Yamaguchi, S. Yasui, A. Hosaka, Nucl. Phys. A927, 110 (2014).\\[0pt] [4] S. Yasui, K. Sudoh, Phys. Rev. C87, 015202 (2013).\\[0pt] [5] S. Yasui, K. Sudoh, Phys. Rev. C89, 015201 (2014).   

The XYZ Mesons at BESIII

The XYZ mesons in the charmonium sector do not fit within the normal patterns of charm and anti-charm quark bound states; their precise interpretation remains unsettled. In December of 2012, the BESIII Collaboration undertook a new program to study this class of mesons by directly producing the Y(4260) and Y(4360) states in electron-positron collisions. Since that time, BESIII has collected the world's largest samples of Y(4260) and Y(4360) decays, which has led to a number of new discoveries. I will review the current status of XYZ studies at BESIII.   

Charmed baryon spectroscopy at Belle

We report recent results on the charmed baryon spectroscopy at the Belle experiment. We searched for doubly charmed baryon $\Xi_{cc}^+$ in the $\Lambda_c^+K^-\pi^+$ and $\Xi_c^0\pi^+$ final states. No significant signal was found and we set upper limit on the production cross section. We searched for two excited $\Xi_c$ states, $\Xi_c(3055)^+$ and $\Xi_c(3123)^+$ whose evidence are previously reported by BaBar experiment. We found evidence of the $\Xi_c(3055)^+$ but not for the $\Xi_c(3123)^+$. The production rate of hadrons in the $e^+e^-$ collision as a function of the mass is known to lie on the exponential curve. If we find a hadron whose production rate is deviated from this line, it is a signature of the exotic state. We report measurement of the production rate of various charm and strange baryons.   

Heavy quarkonium potential from lattice QCD

We have recently proposed a novel method for the determination of the interaquark potential together with quark ``kinetic mass'' from the equal-time $Q\bar{Q}$ Bethe-Salpeter (BS) amplitude in lattice QCD. Our approach allows us to calculate spin-dependent $Q\bar{Q}$ potentials, spin-spin, spin-orbit and tensor potentials, as well. In this talk, we will give a short review of the BS amplitude method on the lattice and present results for both spin-independent and -dependent parts of charmonium potential, which is calculated in 2+1 flavor dynamical lattice QCD using the PACS-CS gauge configurations with a lattice cutoff of $a^{-1}\approx 2.2$ GeV. Our simulations are performed with a relativistic heavy-quark action for the charm quark at the lightest pion mass, $M_\pi = 156(7)$ MeV, in a spatial volume of $(3\; {\rm fm})^3$. We observe that the spin-independent charmonium potential obtained from lattice QCD with almost physical quark masses is quite similar to the Cornell potential used in nonrelativistic potential models. We also present preliminary results for a full set of spin-dependent potentials (spin-spin, spin-orbit and tensor potentials), which is calculated in full lattice QCD for the first time. From the viewpoint of phenomenology, greater knowledge of the r-dependence of the spin-dependent potentials paves way for making more accurate theoretical predictions about the higher-mass quarkonium states, where nonrelativistic potential models fail to reproduce properties of newly discovered charmonium-like $XYZ$ states.