Search

We evaluate the isovector nucleon electromagnetic form factors in quenched and unquenched QCD on the lattice using Wilson fermions. In the quenched theory we use a lattice of spatial size 3 fm at beta=6.0 enabling us to reach low momentum transfers and a lowest pion mass of about 400 MeV. In the unquenched theory we use two degenerate flavors of dynamical Wilson fermions on a lattice of spatial size 1.9 fm at beta=5.6 and lowest pion mass of about 380 MeV enabling comparison with the results obtained in the quenched theory. that unquenching effects are small for the pion masses considered in this work. We compare our lattice results to the isovector part of the experimentally measured form factors.

We present a determination of the ratio of kaon and pion leptonic decay constants in isosymmetric QCD (ISOQCD), fK/f��, 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, fISOQCD �� = 130.4(2) MeV, at the ISOQCD pion point, MISOQCD �� = 135.0(2) MeV, obtaining for the gradient-flow scales the values w0 = 0.17383(63) fm, .....t0 = 0.14436(61) 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 MISOQCD K = 494.2(4) MeV we get (fK/f��)ISOQCD = 1.1995(44), where the error includes both statistical and systematic uncertainties. Implications for the Cabibbo-Kobayashi-Maskawa matrix element jVusj and for the first-row Cabibbo-Kobayashi-Maskawa unitarity are discussed.

We discuss an extension of the QUDA library for the Wilson twisted mass operator. A performance analysis is presented for both degenerate and non-degenerate flavor doublets. The degenerate twisted mass fermion operator runs at up to 190, 487 and 856 Gflops, for double, single and half precisions respectively on recent NVIDIA Kepler GPUs, while our implementation for the non-degenerate flavor doublet allows to reach 163, 516 and 879 GFlops, respectively. The code is currently in production for the hadron structure study., Comment: 7 pages, 2 figures, presented at the 31st International Symposium on Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013, Mainz, Germany

Physical review / D 97(9), 094504 (2018). doi:10.1103/PhysRevD.97.094504, We evaluate the strange nucleon electromagnetic form factors using an ensemble of gauge configurations generated with two degenerate maximally twisted mass clover-improved fermions with mass tuned to approximately reproduce the physical pion mass. In addition, we present results for the disconnected light quark contributions to the nucleon electromagnetic form factors. Improved stochastic methods are employed leading to high-precision results. The momentum dependence of the disconnected contributions is fitted using the model-independent z-expansion. We extract the magnetic moment and the electric and magnetic radii of the proton and neutron by including both connected and disconnected contributions. We find that the disconnected light quark contributions to both electric and magnetic form factors are nonzero and at the few percent level as compared to the connected. The strange form factors are also at the percent level but more noisy yielding statistical errors that are typically within one standard deviation from a zero value., Published by APS, Woodbury, NY

Physical review / D D 101(3), 031501 (2020). doi:10.1103/PhysRevD.101.031501, The role of the strange quarks on the low-energy interactions of the proton can be probed through the strange electromagnetic form factors. Knowledge of these form factors provides essential input for parity-violating processes and contributes to the understanding of the sea quark dynamics. We determine the strange electromagnetic form factors of the nucleon within the lattice formulation of Quantum Chromodynamics using simulations that include light, strange and charm quarks in the sea all tuned to their physical mass values. We employ state-of-the-art techniques to accurately extract the form factors for values of the momentum transfer square up to 0.8~GeV$^2$. We find that both the electric and magnetic form factors are statistically non-zero. We obtain for the strange magnetic moment $\mu^s=-0.017(4)$, the strange magnetic radius $\langle r^2_M \rangle^s=-0.015(9)$~fm$^2$, and the strange charge radius $\langle r^2_E \rangle^s=-0.0048(6)$~fm$^2$., Published by Inst.302363, Melville, NY

We present results on the nucleon scalar, axial, and tensor charges as well as on the momentum fraction, and the helicity and transversity moments. The pion momentum fraction is also presented. The computation of these key observables is carried out using lattice QCD simulations at a physical value of the pion mass. The evaluation is based on gauge configurations generated with two degenerate sea quarks of twisted mass fermions with a clover term. We investigate excited states contributions with the nucleon quantum numbers by analyzing three sink-source time separations. We find that, for the scalar charge, excited states contribute significantly and to a less degree to the nucleon momentum fraction and helicity moment. Our result for the nucleon axial charge agrees with the experimental value. Furthermore, we predict a value of 1.027(62) in the MS¯¯¯¯¯ scheme at 2 GeV for the isovector nucleon tensor charge directly at the physical point. The pion momentum fraction is found to be ⟨x⟩π±u−d=0.214(15)(+12−9) in the MS¯¯¯¯¯ at 2 GeV.

The electromagnetic form factors of the proton and the neutron are computed within lattice QCD using simulations of maximally twisted mass fermions with masses fixed to their physical values. We analyse two new ensembles of $N_f=2$ and $N_f=2+1+1$ and use several values of the sink-source time separation in the range of 1.0 fm to 1.6 fm to ensure ground state identification. The light quark disconnected contributions are evaluated to very high accuracy enabling us to include them in the determination of the proton and neutron form factors, the electric and magnetic radii, and the magnetic moments. We find that disconnected contributions are non-negligible and contribute up to 15% for the case of the neutron electric charge radius.

We present a calculation of the up, down, strange, and charm quark masses performed within the lattice QCD framework. We use the twisted-mass fermion action and carry out simulations that include in the sea two light mass-degenerate quarks, as well as the strange and charm quarks. In the analysis, we use gauge ensembles simulated at three values of the lattice spacing and with light quarks that correspond to pion masses in the range from 350 MeV to the physical value, while the strange and charm quark masses are tuned approximately to their physical values. We use several quantities to set the scale in order to check for finite lattice spacing effects, and in the continuum limit, we get compatible results. The quark mass renormalization is carried out nonperturbatively using the (modified) Regularization Independent Momentum Subtraction (RI0-MOM) method converted into the MS scheme. For the determination of the quark masses, we use physical observables from both the meson and the baryon sectors, obtaining mud = 3.636(66) ("60���57) MeV and ms = 98.7(2.4) (+4.0���3.2) MeV in the MS(2 GeV) scheme and mc = 1036(17) (+15���8) MeV in the MS(3 GeV) scheme, where the first errors are statistical and the second ones are combinations of systematic errors. For the quark mass ratios, we get ms/mud = 27.17(32) (+56���38) and mc/ms =11.48(12) (+25���19).

We present a study of gauge invariant density-density correlators. Density-density correlators probe hadron wave functions and thus can be used to study hadron deformation. Their zero momentum projection requires the computation of all-to-all propagators, which are evaluated with the standard stochastic technique, the dilution method and the stochastic sequential technique. We compare the results to a previous analysis that did not employ the zero momentum projection., Comment: 6 pages, 10 figures. Poster presented at Lattice 2005

Physical review / D 104(7), 074515 (2021). doi:10.1103/PhysRevD.104.074515, We present a calculation of the up, down, strange, and charm quark masses performed within the lattice QCD framework. We use the twisted-mass fermion action and carry out simulations that include in the sea two light mass-degenerate quarks, as well as the strange and charm quarks. In the analysis, we use gauge ensembles simulated at three values of the lattice spacing and with light quarks that correspond to pion masses in the range from 350 MeV to the physical value, while the strange and charm quark masses are tuned approximately to their physical values. We use several quantities to set the scale in order to check for finite lattice spacing effects, and in the continuum limit, we get compatible results. The quark mass renormalization is carried out nonperturbatively using the (modified) Regularization Independent Momentum Subtraction (RI���-MOM) method converted into the $\overline{MS}$ scheme. For the determination of the quark masses, we use physical observables from both the meson and the baryon sectors, obtaining $m_{ud}$=3.636(66)($_{-57}^{+60}$) MeV and $m_s$=98.7(2.4)($_{-3.2}^{+4.0}$) MeV in the $\overline{MS}$(2 GeV) scheme and $m_c$=1036(17)($_{-8}^{+15}$) MeV in the $\overline{MS}$(3 GeV) scheme, where the first errors are statistical and the second ones are combinations of systematic errors. For the quark mass ratios, we get $m_s/m_{ud}$=27.17(32)($_{-38}^{+56}$) and $m_c/m_s$=11.48(12)($_{-19}^{+25}$)., Published by Inst., Melville, NY

Physical review / D 96(3), 034503(2017). doi:10.1103/PhysRevD.96.034503, We present results for the nucleon electromagnetic form factors using an ensemble of maximally twisted mass clover-improved fermions with pion mass of about 130 MeV. We use multiple sink-source separations and three analysis methods to probe ground-state dominance. We evaluate both the connected and disconnected contributions to the nucleon matrix elements. We find that the disconnected quark loop contributions to the isoscalar matrix elements are small, giving an upper bound of up to 2% of the connected and smaller than its statistical error. We present results for the isovector and isoscalar electric and magnetic Sachs form factors and the corresponding proton and neutron form factors. By fitting the momentum dependence of the form factors to a dipole form or to the z expansion, we extract the nucleon electric and magnetic radii, as well as the magnetic moment. We compare our results to experiment as well as to other recent lattice QCD calculations., Published by APS, Woodbury, NY

34th annual International Symposium on Lattice Field Theory, Southampton, UK, 24 Jul 2016 - 30 Jul 2016 ; 7 pp., (2016)., We present results on the spin and quark content of the nucleon using $N_f=2$ twisted mass clover-improved fermion simulations with a pion mass close to its physical value. We use recently developed methods to obtain accurate results for both connected and disconnected contributions. We provide results for the axial charge, quark and gluon momentum fraction as well as the light, strange and charm $\sigma$-terms.

Physical review / D 103(3), 034509 (2021). doi:10.1103/PhysRevD.103.034509, 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., Published by Inst.302363, Melville, NY

The role of the strange quarks on the low-energy interactions of the proton can be probed through the strange electromagnetic form factors. Knowledge of these form factors provides essential input for parity-violating processes and contributes to the understanding of the sea quark dynamics. We determine the strange electromagnetic form factors of the nucleon within the lattice formulation of Quantum Chromodynamics using simulations that include light, strange and charm quarks in the sea all tuned to their physical mass values. We employ state-of-the-art techniques to accurately extract the form factors for values of the momentum transfer square up to 0.8~GeV$^2$. We find that both the electric and magnetic form factors are statistically non-zero. We obtain for the strange magnetic moment $\mu^s=-0.017(4)$, the strange magnetic radius $\langle r^2_M \rangle^s=-0.015(9)$~fm$^2$, and the strange charge radius $\langle r^2_E \rangle^s=-0.0048(6)$~fm$^2$.

We review the status of an ongoing long-term lattice investigation of the spectrum and structure of tetraquark candidates. We focus on the light scalar meson $a_0(980)$. First steps regarding the study of a possibly existing $c c \bar{c} \bar{c}$ tetraquark are also outlined., 5 pages, 3 figures, talk given at FAIRNESS 2013, September 15-21 2013, Berlin, Germany

We present lattice results for the a_0(980) state using a variational approach with a variety of creation operators: quark-antiquark, mesonic molecule, diquark-antidiquark as well as two-meson type. Focus is put on recent technical advances, in particular the computation of singly disconnected diagrams., 7 pages, 3 figures, talk given at the 31st International Symposium on Lattice Field Theory (Lattice 2013), July 29-August 3 2013, Mainz, Germany