A lattice study of the pentaquark states

We present a study of the pentaquark system in quenched lattice QCD using diquark-diquark and kaon-nucleon local and smeared interpolating fields. We examine the volume dependence of the spectral weights of local correlators on lattices of size $16^3\times 32$, $24^3\times32$ and $32^3\times 64$ at $\beta=6.0$. We find that a reliable evaluation of the volume dependence of the spectral weights requires accurate determination of the correlators at large time separations. Our main result from the spectral weight analysis in the pentaquark system is that within our variational basis and statistics we can not exclude a pentaquark resonance. However our data also do not allow a clear identification of a pentaquark state since only the spectral weights of the lowest state can be determined to sufficient accuracy to test for volume dependence. In the negative parity channel the mass extracted for this state is very close to the KN threshold whereas in the positive parity channel is about 60% above.Comment: Manuscript expanded, discussion of two-pion system included, a comment regarding Ref.13 was corrected, version to appear in Phys. Rev. D, 19 figure

Delta-baryon electromagnetic form factors in lattice QCD

We develop techniques to calculate the four Delta electromagnetic form factors using lattice QCD, with particular emphasis on the sub-dominant electric quadrupole form factor that probes deformation of the Delta. Results are presented for pion masses down to approximately 350 MeV for three cases: quenched QCD, two flavors of dynamical Wilson quarks, and three flavors of quarks described by a mixed action combining domain wall valence quarks and dynamical staggered sea quarks. The magnetic moment of the Delta is chirally extrapolated to the physical point and the Delta charge density distributions are discussed.Comment: 4 pages, 5 figure

The Matter and the Pseudoscalar Densities in Lattice QCD

The matter and the pseudoscalar densities inside a hadron are calculated via gauge-invariant equal-time correlation functions. A comparison is made between the charge charge and the matter density distributions for the pion, the rho, the nucleon and the $\Delta^+$ within the quenched theory, and with two flavours of dynamical quarks.Comment: Typos corrected; 13 pages, 16 figure

Field emission properties of nano-composite carbon nitride films

A modified cathodic arc technique has been used to deposit carbon nitride thin films directly on n+ Si substrates. Transmission Electron Microscopy showed that clusters of fullerene-like nanoparticles are embedded in the deposited material. Field emission in vacuum from as-grown films starts at an electric field strength of 3.8 V/micron. When the films were etched in an HF:NH4F solution for ten minutes, the threshold field decreased to 2.6 V/micron. The role of the carbon nanoparticles in the field emission process and the influence of the chemical etching treatment are discussed.Comment: 22 pages, 8 figures, submitted to J. Vac. Sc. Techn.

Delta electromagnetic form factors and quark transverse charge densities from lattice QCD

We discuss the techniques to extract the electromagnetic Delta form factors in Lattice QCD. We evaluate these form factors using dynamical fermions with smallest pion mass of about 350 MeV. We pay particular attention to the extraction of the electric quadrupole form factor that signals a deformation of the Delta. The magnetic moment of the $\Delta$ is extrapolated using a chiral effective field theory. Using the form factors we evaluate the transverse density distributions in the infinite momentum frame showing deformation in the Delta.Comment: 9 pages, 6 figures, Invited Talk at the 6th International Workshop on Chiral Dynamics, CD09, July 6-10, 2009, Bern, Switzerlan

Non-Perturbative Mass Renormalization in Quenched QED from the Worldline Variational Approach

Following Feynman's successful treatment of the polaron problem we apply the same variational principle to quenched QED in the worldline formulation. New features arise from the description of fermions by Grassmann trajectories, the supersymmetry between bosonic and fermionic variables and the much more singular structure of a renormalizable gauge theory like QED in 3+1 dimensions. We take as trial action a general retarded quadratic action both for the bosonic and fermionic degrees of freedom and derive the variational equations for the corresponding retardation functions. We find a simple analytic, non-perturbative, solution for the anomalous mass dimension gamma_m(alpha) in the MS scheme. For small couplings we compare our result with recent four-loop perturbative calculations while at large couplings we find that gamma_m(alpha) becomes proportional to (alpha)^(1/2). The anomalous mass dimension shows no obvious sign of the chiral symmetry breaking observed in calculations based on the use of Dyson-Schwinger equations, however we find that a perturbative expansion of gamma_m(alpha) diverges for alpha > 0.7934. Finally, we investigate the behaviour of gamma_m(alpha) at large orders in perturbation theory.Comment: 18 pages, 1 Figure, RevTeX; the manuscript has been substantially revised and enlarged in order to make it selfcontained; accepted for publication in Phys. Rev.

Nucleon form factors and moments of parton distributions in twisted mass lattice QCD

We present results on the electroweak form factors and on the lower moments of parton distributions of the nucleon, within lattice QCD using two dynamical flavors of degenerate twisted mass fermions. Results are obtained on lattices with three different values of the lattice spacings, namely a=0.089 fm, a=0.070 fm and a=0.056 fm, allowing the investigation of cut-off effects. The volume dependence is examined by comparing results on two lattices of spatial length L=2.1 fm and L=2.8 fm. The simulations span pion masses in the range of 260-470 MeV. Our results are renormalized non-perturbatively and the values are given in the MS-scheme at a scale mu=2 GeV.Comment: Talk presented in the XXIst International Europhysics Conference on High Energy Physics, 21-27 July 2011, Grenoble, Rhones Alpes Franc

The nucleon spin and momentum decomposition using lattice QCD simulations

We determine within lattice QCD, the nucleon spin carried by valence and sea quarks, and gluons. The calculation is performed using an ensemble of gauge configurations with two degenerate light quarks with mass fixed to approximately reproduce the physical pion mass. We find that the total angular momentum carried by the quarks in the nucleon is $J_{u+d+s}{=}0.408(61)_{\rm stat.}(48)_{\rm syst.}$ and the gluon contribution is $J_g {=}0.133(11)_{\rm stat.}(14)_{\rm syst.}$ giving a total of $J_N{=}0.54(6)_{\rm stat.}(5)_{\rm syst.}$ consistent with the spin sum. For the quark intrinsic spin contribution we obtain $\frac{1}{2}\Delta \Sigma_{u+d+s}{=}0.201(17)_{\rm stat.}(5)_{\rm syst.}$. All quantities are given in the $\overline{\textrm{MS}}$ scheme at 2~GeV. The quark and gluon momentum fractions are also computed and add up to $\langle x\rangle_{u+d+s}+\langle x\rangle_g{=}0.804(121)_{\rm stat.}(95)_{\rm syst.}+0.267(12)_{\rm stat.}(10)_{\rm syst.}{=}1.07(12)_{\rm stat.}(10)_{\rm syst.}$ satisfying the momentum sum.Comment: Version published in PR

Electromagnetic form factors of the Delta with D-waves

The electromagnetic form factors of the Delta baryon are evaluated within the framework of a covariant spectator quark model, where S and D-states are included in the $\Delta$ wave function. We predict all the four Delta multipole form factors: the electric charge GE0, the magnetic dipole GM1, the electric quadrupole GE2 and the magnetic octupole GM3. We compare our predictions with other theoretical calculations. Our results are compatible with the available experimental data and recent lattice QCD data.Comment: Accepted for publication in PRD. Version with typographic and orthographic corrections. 30 pages, 11 figure