Search results for "ISOVECTOR"
showing 10 items of 82 documents
Exotic bcq¯q¯ four-quark states
2019
We carry out a systematic study of exotic $Q{Q}^{\ensuremath{'}}\overline{q}\overline{q}$ four-quark states containing distinguishable heavy flavors, $b$ and $c$. Different generic constituent models are explored in an attempt to extract general conclusions. The results are robust, predicting the same sets of quantum numbers as the best candidates to lodge bound states independently of the model used, the isoscalar ${J}^{P}={0}^{+}$ and ${J}^{P}={1}^{+}$ states. The first state would be strong and electromagnetic-interaction stable, while the second would decay electromagnetically to $\overline{B}D\ensuremath{\gamma}$. Isovector states are found to be unbound, preventing the existence of ch…
Break-up and Coherent Photoproduction ofηMesons on the Deuteron
1997
We present new break-up and coherent data for $\ensuremath{\eta}$ meson photoproduction on the deuteron, using a deuterium target and tagged bremsstrahlung photons up to 1 GeV. The differential cross sections for the coherent process were measured from threshold to 800 MeV. They are much smaller than those previously reported. The break-up channel provides a direct measurement of the neutron to proton differential cross section ratios. At the ${S}_{11}(1535)$ resonance peak, ${\ensuremath{\sigma}}_{n}/{\ensuremath{\sigma}}_{p}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.68\ifmmode\pm\else\textpm\fi{}0.06$ leading to an isoscalar to isovector amplitude ratio of ${A}_{s}{/A}_{\ensurem…
Chiral symmetry amplitudes in the S-wave isoscalar and isovector channels and the σ, f[sub 0](980), a[sub 0](980) scalar mesons
1998
We use a nonperturbative approach which combines coupled channel Lippmann Schwinger equations with meson-meson potentials provided by the lowest order chiral Lagrangian. By means of one parameter, a cut off in the momentum of the loop integrals, which results of the order of 1 GeV, we obtain singularities in the S-wave amplitudes corresponding to the σ, f0 and a0 resonances. The ππ→ππ, ππ→KK phase shifts and inelasticities in the T=0 scalar channel are well reproduced as well as the π0η and KK mass distributions in the T=1 channel. Furthermore, the total and partial decay widths of the f0 and a0 resonances are properly reproduced. The results seem to indicate that chiral symmetry constraint…
Computing the nucleon Dirac radius directly at $Q^2=0$
2016
We describe a lattice approach for directly computing momentum derivatives of nucleon matrix elements using the Rome method, which we apply to obtain the isovector magnetic moment and Dirac radius. We present preliminary results calculated at the physical pion mass using a 2HEX-smeared Wilson-clover action. For removing the effects of excited-state contamination, the calculations were done at three source-sink separations and the summation method was used.
AN OPERATOR PRODUCT EXPANSION ANALYSIS OF e+e-ANNIHILATION DATA
2013
Perturbative Quantum Chromodynamics combined with the operator product expansion is expected to provide a framework for the description of phenomena in hadron interactions including contributions of nonperturbative origin. Applied to the correlator of two electromagnetic currents, this framework can be confronted with e+e-annihilation into hadrons. Data from the total hadronic e+e-cross-section have become much more precise in recent years and the power corrections in the operator product expansion, i.e. the vacuum condensates are expected to be determined with higher precision than previously. We present an analysis of the condensates of dimensions d = 2, 4 and 6 and find reasonably stable…
Charge transport and vector meson dissociation across the thermal phase transition in lattice QCD with two light quark flavors
2015
We compute and analyze correlation functions in the isovector vector channel at vanishing spatial momentum across the deconfinement phase transition in lattice QCD. The simulations are carried out at temperatures $T/T_c=0.156, 0.8, 1.0, 1.25$ and $1.67$ with $T_c\simeq203$MeV for two flavors of Wilson-Clover fermions with a zero-temperature pion mass of $\simeq270$MeV. Exploiting exact sum rules and applying a phenomenologically motivated ansatz allows us to determine the spectral function $\rho(\omega,T)$ via a fit to the lattice correlation function data. From these results we estimate the electrical conductivity across the deconfinement phase transition via a Kubo formula and find eviden…
Nucleon average quark momentum fraction with Nf = 2+1 Wilson fermions
2018
We report on an analysis of the average quark momentum fraction of the nucleon and related quantities using Nf = 2 + 1 Wilson fermions. Computations are performed on four CLS ensembles covering three values of the lattice spacing at pion masses down to Mπ ≈ 200 MeV. Several source-sink separations (~ 1:0 fm to ~ 1:4 fm) are used to assess the excited-state contamination. To gain further insight, the generalized pencil-of-functions approach has been implemented to reduce the excited-state contamination in the relevant two-and three-point functions. Preliminary results are shown for the isovector nucleon charges from vector, axial vector and tensor derivative (twist-2) operators.
The $I=1$ pion-pion scattering amplitude and timelike pion form factor from $N_{\rm f} = 2+1$ lattice QCD
2019
The elastic $I=1$ $p$-wave $\pi\pi$ scattering amplitude is calculated together with the isovector timelike pion form factor using lattice QCD with $N_{\rm f}=2+1$ dynamical quark flavors. Wilson clover ensembles generated by the Coordinated Lattice Simulations (CLS) initiative are employed at four lattice spacings down to $a = 0.05\,\mathrm{fm}$, several pion masses down to $m_{\pi} = 200\,\mathrm{MeV}$, and spatial volumes of extent $L = 3.1-5.5\,\mathrm{fm}$. The set of measurements on these ensembles, which is publicly available, enables an investigation of systematic errors due to the finite lattice spacing and spatial volume. The $\pi\pi$ scattering amplitude is fit on each ensemble b…
Multi-hadron spectroscopy in a large physical volume
2017
We demonstrate the efficacy of the stochastic LapH method to treat all-to-all quark propagation on a $N_f = 2+1$ CLS ensemble with large linear spatial extent $L = 5.5$ fm, allowing us to obtain the benchmark elastic isovector p-wave pion-pion scattering amplitude to good precision already on a relatively small number of gauge configurations. These results hold promise for multi-hadron spectroscopy at close-to-physical pion mass with exponential finite-volume effects under control.
Nuclear matrix elements for0νββdecays with light or heavy Majorana-neutrino exchange
2015
We compute the nuclear matrix elements (NMEs) corresponding to the neutrinoless double beta ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$) decays of nuclei which attract current experimental interest. We concentrate on ground-state-to-ground-state decay transitions mediated by light (l-NMEs) or heavy (h-NMEs) Majorana neutrinos. The computations are done in realistic single-particle model spaces using the proton-neutron quasiparticle random-phase approximation (pnQRPA) with two-nucleon interactions based on the Bonn one-boson-exchange $G$ matrix. Both the l-NMEs and the h-NMEs include the appropriate short-range correlations, nucleon form factors, and higher-order nucleonic weak …