Tetraquark and Pentaquark Systems in Lattice QCD

Abstract

Motivated by the recent experimental discoveries of multi-quark candidates, e.g., the $\Theta^+(1540)$, we study multi-quark systems in lattice QCD. First, we perform accurate mass measurements of low-lying 5Q states with $J=1/2$ and I=0 in both positive- and negative-parity channels in anisotropic lattice QCD. The lowest positive-parity 5Q state is found to have a large mass of about 2.24GeV after the chiral extrapolation. To single out the compact 5Q state from $NK$ scattering states, we develop a new method with the hybrid-boundary condition (HBC), and find no evidence of the compact 5Q state below 1.75GeV in the negative-parity channel. Second, we perform the first study of the multi-quark potential in lattice QCD to clarify the inter-quark interaction in multi-quark systems. The 5Q potential $V_{\rm 5Q}$ for the QQ-${\rm \bar{Q}}$-QQ system is found to be well described by the ``OGE Coulomb plus multi-Y Ansatz": the sum of the one-gluon-exchange (OGE) Coulomb term and the multi-Y-type linear term based on the flux-tube picture. The 4Q potential $V_{\rm 4Q}$ for the QQ-${\rm \bar{Q}\bar{Q}}$ system is also described by the OGE Coulomb plus multi-Y Ansatz, when QQ and $\rm \bar Q \bar Q$ are well separated. The 4Q system is described as a "two-meson" state with disconnected flux tubes, when the nearest quark and antiquark pair is spatially close. We observe a lattice-QCD evidence for the ``flip-flop'', i.e., the flux-tube recombination between the connected 4Q state and the ``two-meson'' state. On the confinement mechanism, the lattice QCD results indicate the flux-tube-type linear confinement in multi-quark hadrons.Comment: 22 pages, 3 tables, 16 figures. Talk given at International Workshop on Quark Nuclear Physics 2005 (QNP05), Phoenix Park, Korea, 22-24, Feb., 200

The heavy tetra-quark states after the discovery of tetra-charm states.

“…As a consequence the production cross section should be at least by two orders of magnitude larger than that in the compact case. The compactness choice has been already assumed in many papers among which [1,4,10,11]. The results of the present paper are in strong agreement with this choice.…”

“…We shall denote by the capital letter Q the a e-mail: becchi@ge.infn.it (corresponding author) heavy Quarks. The physics of these compounds is expected to give important QCD tests because Quarks can be considered fixed sources and the binding forces could be easily identified in lattice QCD also accounting for the many body forces [1]. For recent review articles see [2,3].…”

“…There are theoretical papers, partly based on lattice QCD, discussing this problem [4][5][6][7][8][9]. The role of many body forces is however rarely discussed [1].…”

The heavy tetra-quark states after the discovery of tetra-charm states

“…The physics of these compounds is expected to give important QCD tests if the internal kinetic energy is smaller than the Quark mass and hence the system can be considered non-relativistic. Thus Quarks can be considered fixed sources and the binding forces could be easily identified in lattice QCD also considering the many body forces [1].…”

“…There are theoretical papers, partly based on lattice QCD, discussing this problem [2] [3]. The role of many body forces is however rarely discussed [1].…”