Doubly Charmed Tetraquark T_{cc}

The doubly charmed tetraquark T_{cc}^{+} recently discovered by the LHCb Collaboration is studied on the basis of (2+1)-flavor lattice QCD simulations of the D^{*}D system with nearly physical pion mass m_{π}=146 MeV. The interaction of D^{*}D in the isoscalar and S-wave channel, derived from the hadronic spacetime correlation by the HAL QCD method, is attractive for all distances and leads to a near-threshold virtual state with a pole position E_{pole}=-59(_{-99}^{+53})(_{-67}^{+2}) keV and a large scattering length 1/a_{0}=0.05(5)(_{-2}^{+2}) fm^{-1}. The virtual state is shown to evolve into a loosely bound state as m_{π} decreases to its physical value by using a potential modified to m_{π}=135 MeV based on the pion-exchange interaction. Such a potential is found to give a semiquantitative description of the LHCb data on the D^{0}D^{0}π^{+} mass spectrum. Future study is necessary to perform physical-point simulations with the isospin-breaking and open three-body-channel effects taken into account.

Dibaryon with Highest Charm Number near Unitarity from Lattice QCD.

A pair of triply charmed baryons, Ω_{ccc}Ω_{ccc}, is studied as an ideal dibaryon system by (2+1)-flavor lattice QCD with nearly physical light-quark masses and the relativistic heavy-quark action with the physical charm quark mass. The spatial baryon-baryon correlation is related to their scattering parameters on the basis of the HAL QCD method. The Ω_{ccc}Ω_{ccc} in the ^{1}S_{0} channel, taking into account the Coulomb repulsion with the charge form factor of Ω_{ccc}, leads to the scattering length a_{0}^{C}≃-19 fm and the effective range r_{eff}^{C}≃0.45 fm. The ratio r_{eff}^{C}/a_{0}^{C}≃-0.024, whose magnitude is considerably smaller than that of the dineutron (-0.149), indicates that Ω_{ccc}Ω_{ccc} is located in the unitary regime.

Most Strange Dibaryon from Lattice QCD.

The ΩΩ system in the ^{1}S_{0} channel (the most strange dibaryon) is studied on the basis of the (2+1)-flavor lattice QCD simulations with a large volume (8.1 fm)^{3} and nearly physical pion mass m_{π}≃146 MeV at a lattice spacing of a≃0.0846 fm. We show that lattice QCD data analysis by the HAL QCD method leads to the scattering length a_{0}=4.6(6)(_{-0.5}^{+1.2}) fm, the effective range r_{eff}=1.27(3)(_{-0.03}^{+0.06}) fm, and the binding energy B_{ΩΩ}=1.6(6)(_{-0.6}^{+0.7}) MeV. These results indicate that the ΩΩ system has an overall attraction and is located near the unitary regime. Such a system can be best searched experimentally by the pair-momentum correlation in relativistic heavy-ion collisions.

Fate of the Tetraquark Candidate Z_{c}(3900) from Lattice QCD.

The possible exotic meson Z_{c}(3900), found in e^{+}e^{-} reactions, is studied by the method of coupled-channel scattering in lattice QCD. The interactions among πJ/ψ, ρη_{c}, and D[over ¯]D^{*} channels are derived from (2+1)-flavor QCD simulations at m_{π}=410-700 MeV. The interactions are dominated by the off-diagonal πJ/ψ-D[over ¯]D^{*} and ρη_{c}-D[over ¯]D^{*} couplings, which indicates that the Z_{c}(3900) is not a usual resonance but a threshold cusp. Semiphenomenological analyses with the coupled-channel interaction are also presented to confirm this conclusion.

Equation of state for nucleonic matter and its quark mass dependence from the nuclear force in lattice QCD.

Quark mass dependence of the equation of state (EOS) for nucleonic matter is investigated, on the basis of the Brueckner-Hartree-Fock method with the nucleon-nucleon interaction extracted from lattice QCD simulations. We observe saturation of nuclear matter at the lightest available quark mass corresponding to the pseudoscalar meson mass ≃469 MeV. Mass-radius relation of the neutron stars is also studied with the EOS for neutron-star matter from the same nuclear force in lattice QCD. We observe that the EOS becomes stiffer and thus the maximum mass of neutron star increases as the quark mass decreases toward the physical point.

Bound H dibaryon in flavor SU(3) limit of lattice QCD.

The flavor-singlet H dibaryon, which has strangeness -2 and baryon number 2, is studied by the approach recently developed for the baryon-baryon interactions in lattice QCD. The flavor-singlet central potential is derived from the spatial and imaginary-time dependence of the Nambu-Bethe-Salpeter wave function measured in N(f)=3 full QCD simulations with the lattice size of L≃2,3,4 fm. The potential is found to be insensitive to the volume, and it leads to a bound H dibaryon with the binding energy of 30-40 MeV for the pseudoscalar meson mass of 673-1015 MeV.

Extraction of hadron interactions above inelastic threshold in lattice QCD.

We propose a new method to extract hadron interactions above inelastic threshold from the Nambu-Bethe-Salpeter amplitude in lattice QCD. We consider the scattering such as A + B → C + D, where A, B, C, D are names of different 1-particle states. An extension to cases where particle productions occur during scatterings is also discussed.

Hybrid quantum annealing via molecular dynamics.

A novel quantum-classical hybrid scheme is proposed to efficiently solve large-scale combinatorial optimization problems. The key concept is to introduce a Hamiltonian dynamics of the classical flux variables associated with the quantum spins of the transverse-field Ising model. Molecular dynamics of the classical fluxes can be used as a powerful preconditioner to sort out the frozen and ambivalent spins for quantum annealers. The performance and accuracy of our smooth hybridization in comparison to the standard classical algorithms (the tabu search and the simulated annealing) are demonstrated by employing the MAX-CUT and Ising spin-glass problems.