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Register a new userStudy of the Zc+ channel in lattice QCD
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Several charged charmonium-like hadrons called $Z_c$ have been recently discovered by different experiments. In contrast to conventional hadrons these contain at least two valence quarks and antiquarks ($bar{c}cbar{d}u$). We perform a lattice QCD simulation of the $I^G(J^{PC})=1^+(1^{+-})$ channel including all relevant two-meson operators under 4.3 GeV: $J/psi pi$, $psi_{2S}pi$, $psi_{1D}pi$, $D bar{D}^*$, $D^* bar{D}^*$, $eta_c rho$ as well as additional diquark anti-diquark operators. In our $N_f = 2$ simulation with pion mass at 266 MeV we are able to identify all two-meson levels within the energy region of interest. However we find no additional level identifiable as a candidate for $Z_c$.
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Recently experimentalists have discovered several charged charmonium-like hadrons $Z_c^+$ with unconventional quark content $bar ccbar d u$. We perform a search for $Z_c^+$ with mass below $4.2~$GeV in the channel $I^G(J^{PC})=1^+(1^{+-})$ using lattice QCD. The major challenge is presented by the two-meson states $J/psi, pi$, $psi_{2S}pi$, $psi_{1D}pi$, $Dbar D^*$, $D^*bar D^*$, $eta_crho$ that are inevitably present in this channel. The spectrum of eigenstates is extracted using a number of meson-meson and diquark-antidiquark interpolating fields. For our pion mass of 266~MeV we find all the expected two-meson states but no additional candidate for $Z_c^+$ below $4.2~$GeV. Possible reasons for not seeing an additional eigenstate related to $Z_c^+$ are discussed. We also illustrate how a simulation incorporating interpolators with a structure resembling low-lying two-mesons states seems to render a $Z_c^+$ candidate, which is however not robust after further two-meson states around $4.2~$GeV are implemented.
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 interaction among $pi J/psi$, $rho eta_{c}$ and $bar{D}D^{*}$ channels is derived from (2+1)-flavor QCD simulations at $m_{pi}=410$-$700$ MeV. The interaction is dominated by the off-diagonal $pi J/psi$-$bar{D}D^{*}$ and $rho eta_{c}$-$bar{D}D^{*}$ couplings, which indicates that the $Z_{c}(3900)$ is not a usual resonance but a threshold cusp. Semi-phenomenological analyses with the coupled-channel interaction are presented to confirm this conclusion.
We present a lattice QCD study of $Npi$ scattering in the positive-parity nucleon channel, where the puzzling Roper resonance $N^*(1440)$ resides in experiment. The study is based on the PACS-CS ensemble of gauge configurations with $N_f=2+1$ Wilson-clover dynamical fermions, $m_pi simeq 156~$MeV and $Lsimeq 2.9~$fm. In addition to a number of $qqq$ interpolating fields, we implement operators for $Npi$ in $p$-wave and $Nsigma$ in $s$-wave. In the center-of-momentum frame we find three eigenstates below 1.65 GeV. They are dominated by $N(0)$, $N(0)pi(0)pi(0)$ (mixed with $N(0)sigma(0)$) and $N(p)pi(-p)$ with $psimeq 2pi/L$, where momenta are given in parentheses. This is the first simulation where the expected multi-hadron states are found in this channel. The experimental $Npi$ phase-shift would -- in the approximation of purely elastic $Npi$ scattering -- imply an additional eigenstate near the Roper mass $m_Rsimeq 1.43~$GeV for our lattice size. We do not observe any such additional eigenstate, which indicates that $Npi$ elastic scattering alone does not render a low-lying Roper. Coupling with other channels, most notably with $Npipi$, seems to be important for generating the Roper resonance, reinforcing the notion that this state could be a dynamically generated resonance. Our results are in line with most of previous lattice studies based just on $qqq$ interpolators, that did not find a Roper eigenstate below $1.65~$GeV. The study of the coupled-channel scattering including a three-particle decay $Npipi$ remains a challenge.
We study the coupled pion-nucleon system (negative parity, isospin 1/2) based on a lattice QCD simulation for nf=2 mass degenerate light quarks. Both, standard 3-quarks baryon operators as well as meson-baryon (4+1)-quark operators are included. This is an exploratory study for just one lattice size and lattice spacing and at a pion mass of 266 MeV. Using the distillation method and variational analysis we determine energy levels of the lowest eigenstates. Comparison with the results of simple 3-quark correlation studies exhibits drastic differences and a new level appears. A clearer picture of the negative parity nucleon spectrum emerges. For the parameters of the simulation we may assume elastic s-wave scattering and can derive values of the phase shift.
We study infrared conformality of the twelve-flavor QCD on the lattice. Utilizing the highly improved staggered quarks (HISQ) type action which is useful to study the continuum physics, we analyze the lattice data of the mass and the decay constant of a pseudoscalar meson and the mass of a vector meson as well at several values of lattice spacing and fermion mass. Our result is consistent with the conformal hypothesis for the mass anomalous dimension $gamma_m sim 0.4-0.5$.
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