The electromagnetic form factors of the proton and the neutron are computed within lattice QCD using simulations with quark masses fixed to their physical values. Both connected and disconnected ...contributions are computed. We analyze two new ensembles of Nf = 2 and Nf = 2 + 1 + 1 twisted mass clover-improved fermions and determine the proton and neutron form factors, the electric and magnetic radii, and the magnetic moments. We use several values of the sink-source time separation in the range of 1.0 to 1.6 fm to ensure ground state identification. Disconnected contributions are calculated to an unprecedented accuracy at the physical point. Although they constitute a small correction, they are non-negligible and contribute up to 15% for the case of the neutron electric charge radius.
We determine the nucleon axial, scalar and tensor charges within lattice quantum chromodynamics including all contributions from valence and sea quarks. We analyze three gauge ensembles simulated ...within the twisted mass formulation at approximately physical value of the pion mass. Two of these ensembles are simulated with two dynamical light quarks and lattice spacing a = 0.094 fm and the third with a = 0.08 fm includes in addition the strange and charm quarks in the sea. After comparing the results among these three ensembles, we quote as final values our most accurate analysis using the latter ensemble. For the nucleon isovector axial charge we find 1.286(23) in agreement with the experimental value. We provide the flavor decomposition of the intrinsic spin 1/2 ΔΣq carried by quarks in the nucleon obtaining for the up, down, strange and charm quarks 1/2 ΔΣu = 0.431 (8), 1/2 ΔΣd = − 0.212 (8) , 1/2 ΔΣs = − 0.023 (4) and 1/2 ΔΣc = − 0.005 (2) , respectively. The corresponding values of the tensor and scalar charges for each quark flavor are also evaluated providing valuable input for experimental searches for beyond the standard model physics. In addition, we extract the nucleon σ -terms and find for the light quark content σπN = 41.6 (3.8) MeV and for the strange σs = 45.6 (6.2) MeV . The y-parameter that is used in phenomenological studies we find y = 0.078 (7) .
We compute the nucleon axial and induced pseudoscalar form factors using three ensembles of gauge configurations, generated with dynamical light quarks with mass tuned to approximately their physical ...value. One of the ensembles also includes the strange and charm quarks with their mass close to physical. The latter ensemble has large statistics and finer lattice spacing and it is used to obtain final results, while the other two are used for assessing volume effects. The pseudoscalar form factor is also computed using these ensembles. We examine the momentum dependence of these form factors as well as relations based on pion pole dominance and the partially conserved axial-vector current hypothesis.
We present results on the isoscalar form factors including the disconnected contributions, as well as on the strange and charm quark form factors. Using previous results on the isovector form ...factors, we determine the flavor decomposition of the nucleon axial form factors. These are computed using an ensemble of Nf = 2 + 1 + 1 twisted mass fermions simulated with physical values of quark masses. We investigate the SU(3) flavor symmetry and show that there is up to 10% breaking for the axial and up to 50% for the induced pseudoscalar form factors. By fitting the Q2-dependence, we determined the corresponding root mean square radii. The pseudoscalar coupling of the η meson and the nucleon is found to be gηNN = 3.7 (1.0) (0.7), and the Goldberger-Treiman discrepancy for the octet combination about 50%.
We present results for the moments of nucleon isovector vector and axial generalized parton distribution functions computed within lattice QCD. Three ensembles of maximally twisted mass ...clover-improved fermions simulated with a physical value of the pion mass are analyzed. Two of these ensembles are generated using two degenerate light quarks. A third ensemble is used having, in addition to the light quarks, strange and charm quarks in the sea. A careful analysis of the convergence to the ground state is carried out that is shown to be essential for extracting the correct nucleon matrix elements. This allows a controlled determination of the unpolarized, helicity, and tensor second Mellin moments. The vector and axial-vector generalized form factors are also computed as a function of the momentum transfer square up to about 1 GeV2. The three ensembles allow us to check for unquenching effects and to assess lattice finite volume effects.
We present a determination of the ratio of kaon and pion leptonic decay constants in isosymmetric QCD (ISOQCD), fK/fπ, making use of the gauge ensembles produced by the Extended Twisted Mass ...Collaboration with Nf = 2 + 1 + 1 flavors of Wilson-clover twisted-mass quarks, including configurations close to the physical point for all dynamical flavors. The simulations are carried out at three values of the lattice spacing ranging from ∼ 0.068 to ∼ 0.092 fm with linear lattice size up to L ~ 5.5 fm. The scale is set by the particle data group (PDG) value of the pion decay constant, ... , at the ISOQCD pion point, ... , obtaining for the gradient-flow scales the values w 0 = 0.17383 (63) fm, ... and t0/w0 = 0.11969 (62) fm. The data are analyzed within the framework of SU(2) chiral perturbation theory without resorting to the use of renormalized quark masses. At the ISOQCD kaon point ... we get (fK/fπ) ISO QCD = 1.1995 (44), where the error includes both statistical and systematic uncertainties. Implications for the Cabibbo-Kobayashi-Maskawa matrix element |Vus| and for the first-row Cabibbo-Kobayashi-Maskawa unitarity are discussed.(ProQuest: … denotes formulae omitted.)
We present an investigation of the electromagnetic pion form factor, Fπ(Q2), at small values of the four-momentum transfer Q2 (≲0.25 GeV2), based on the gauge configurations generated by the ...European Twisted Mass Collaboration with Nf=2 twisted-mass quarks at maximal twist including a clover term. Momentum is injected using nonperiodic boundary conditions and the calculations are carried out at a fixed lattice spacing (a≃0.09 fm) and with pion masses equal to its physical value, 240 MeV and 340 MeV. Our data are successfully analyzed using chiral perturbation theory at next-to-leading order in the light-quark mass. For each pion mass two different lattice volumes are used to take care of finite size effects. Our final result for the squared charge radius is ⟨r2⟩π=0.443(29) fm2, where the error includes several sources of systematic errors except the uncertainty related to discretization effects. The corresponding value of the SU(2) chiral low-energy constant ℓ¯6 is equal to ℓ¯6=16.2(1.0).
We present results for the isospin-0 ππ s-wave scattering length calculated with Osterwalder-Seiler valence quarks on Wilson twisted mass gauge configurations. We use three Nf=2 ensembles with ...unitary (valence) pion mass at its physical value (250 MeV), at 240 MeV (320 MeV) and at 330 MeV (400 MeV), respectively. By using the stochastic Laplacian Heaviside quark smearing method, all quark propagation diagrams contributing to the isospin-0 ππ correlation function are computed with sufficient precision. The chiral extrapolation is performed to obtain the scattering length at the physical pion mass. Our result Mπa0I=0=0.198(9)(6) agrees reasonably well with various experimental measurements and theoretical predictions. Since we only use one lattice spacing, certain systematics uncertainties, especially those arising from unitary breaking, are not controlled in our result.
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