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Register a new userScalar mesons and tetraquarks by means of lattice QCD
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We study the light scalar mesons a_0(980) and kappa using N_f = 2+1+1 flavor lattice QCD. In order to probe the internal structure of these scalar mesons, and in particular to identify, whether a sizeable tetraquark component is present, we use a large set of operators, including diquark-antidiquark, mesonic molecule and two-meson operators. The inclusion of disconnected diagrams, which are technically rather challenging, but which would allow us to extend our work to e.g. the f_0(980) meson, is introduced and discussed.
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We investigate whether the lightest scalar mesons sigma and kappa have a large tetraquark component, as is strongly supported by many phenomenological studies. A search for possible light tetraquark states with J^PC=0^++ and I=0, 2, 1/2, 3/2 on the lattice is presented. We perform the two-flavor dynamical simulation with Chirally Improved quarks and the quenched simulation with overlap quarks, finding qualitative agreement between both results. The spectrum is determined using the generalized eigenvalue method with a number of tetraquark interpolators at the source and the sink, and we omit the disconnected contractions. The time-dependence of the eigenvalues at finite temporal extent of the lattice is explored also analytically. In all the channels, we unavoidably find lowest scattering states pi(k)pi(-k) or K(k)pi(-k) with back-to-back momentum k=0, 2*pi/L,... However, we find an additional light state in the I=0 and I=1/2 channels, which may be interpreted as the observed resonances sigma and kappa with a sizable tetraquark component. In the exotic repulsive channels I=2 and I=3/2, where no resonance is observed, we find no light state in addition to the scattering states.
Charmed tetraquarks $T_{cc}=(ccbar{u}bar{d})$ and $T_{cs}=(csbar{u}bar{d})$ are studied through the S-wave meson-meson interactions, $D$-$D$, $bar{K}$-$D$, $D$-$D^{*}$ and $bar{K}$-$D^{*}$, on the basis of the (2+1)-flavor lattice QCD simulations with the pion mass $m_{pi} simeq $410, 570 and 700 MeV. For the charm quark, the relativistic heavy quark action is employed to treat its dynamics on the lattice. Using the HAL QCD method, we extract the S-wave potentials in lattice QCD simulations, from which the meson-meson scattering phase shifts are calculated. The phase shifts in the isospin triplet ($I$=1) channels indicate repulsive interactions, while those in the $I=0$ channels suggest attraction, growing as $m_{pi}$ decreases. This is particularly prominent in the $T_{cc} (J^P=1^+,I=0)$ channel, though neither bound state nor resonance are found in the range $m_{pi} =410-700$ MeV. We make a qualitative comparison of our results with the phenomenological diquark picture.
Since gluons in QCD are interacting fundamental constituents just as quarks are, we expect that in addition to mesons made from a quark and an antiquark, there should also be glueballs and hybrids (bound states of quarks, antiquarks and gluons). In general, these states would mix strongly with the conventional q-bar-q mesons. However, they can also have exotic quantum numbers inaccessible to q-bar-q mesons. Confirmation of such states would give information on the role of dynamical color in low energy QCD. In the quenched approximation we present a lattice calculation of the masses of mesons with exotic quantum numbers. These hybrid mesons can mix with four quark (q-bar-q-bar-q-q) states. The quenched approximation partially suppresses this mixing. Nonetheless, our hybrid interpolating fields also couple to four quark states. Using a four quark source operator, we demonstrate this mixing for the 1-+ meson. Using the conventional Wilson quark action, we calculate both at reasonably light quark masses, intending to extrapolate to small quark mass, and near the charmed quark mass, where we calculate the masses of some c-bar-c-g hybrid mesons. The hybrid meson masses are large --- over 4 GeV for charmonium and more than twice the vector meson mass at our smallest quark mass, which is near the strange quark mass.
The spectrum of orbitally excited $D_s$ mesons is computed in the continuum limit of quenched lattice QCD. The results are consistent with the interpretation that the narrow resonance in the $D_s pi^0$ channel discovered by the BABAR Collaboration is a $J^P=0^+$ $cbar{s}$ meson. Furthermore, within statistical errors, the $1^+-1^-$ and the $0^+-0^-$ mass splittings are equal, in agreement with the chiral multiplet structure predicted by heavy hadron chiral effective theory. On our coarsest lattice we present results from the first study of orbitally excited $D_s$ mesons with two flavors of dynamical quarks, with mass slightly larger than the strange quark mass. These results are consistent with the quenched data.
We present ground state spectra of mesons containing a charm and a bottom quark. For the charm quark we use overlap valence quarks while a non-relativistic formulation is utilized for the bottom quark on a background of 2+1+1 flavors HISQ gauge configurations generated by the MILC collaboration. The hyperfine splitting between $1S$ states of $B_c$ mesons is found to be $56^{+4}_{-3}$ MeV. We also study the baryons containing only charm and bottom quarks and predict their ground state masses. Results are obtained at three lattice spacings.
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