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A. Tsapalis

Publications and source records attributed to A. Tsapalis.

At least 37 records · Page 2Linked to original sources
arXiv2007

The Lattice Free Energy of QCD with Clover Fermions, up to Three-Loops

We calculate the perturbative value of the free energy in Lattice QCD, up to three loops. Our calculation is performed using Wilson gluons and the Sheikholeslami - Wolhert (clover) improved action for fermions. The free energy is directly related to the average plaquette. To carry out the calculation, we compute all relevant Feynman diagrams up to 3 loops, using a set of automated procedures in Mathematica; numerical evaluation of the resulting loop integrals is performed on finite lattice, with subsequent extrapolation to infinite size. The results are presented as a function of the fermion mass m, for any SU(N_c) gauge group, and for an arbitrary number of fermion flavors. In order to enable independent comparisons, we also provide the results on a per diagram basis, for a specific mass value.

hep-lat↗
arXiv2007

Nucleon and Nucleon to Delta Axial form factors from Lattice QCD

We present results on the nucleon axial vector form factors $G_A(q^2)$ and $G_p(q^2)$ in the quenched theory and using two degenerate flavors of dynamical Wilson fermions for momentum transfer squared from about 0.1 to about 2 GeV^2 and for pion masses in the range of 380 to 600 MeV. We also present results on the corresponding N to Delta axial vector transition form factors $C_5^A(q^2)$ and $C_6^A(q^2)$ using, in addition to Wilson fermions, domain wall valence quarks and dynamical staggered sea quarks provided by the MILC collaboration.

hep-lat↗
arXiv2007

Axial Nucleon and Nucleon to Delta form fractors and the Goldberger-Treiman Relations from Lattice QCD

We evaluate the nucleon axial form factor, $G_A(q^2)$, and induced pseudoscalar form factor, $G_p(q^2)$, as well as the pion-nucleon form factor, $G_{πN N}(q^2)$, in lattice QCD. We also evaluate the corresponding nucleon to $Δ$ transition form factors, $C_5^A(q^2)$ and $C_6^A(q^2)$, and the pion-nucleon-$Δ$ form factor $G_{πNΔ}(q^2)$. The nucleon form factors are evaluated in the quenched theory and with two degenerate flavors of dynamical Wilson fermions. The nucleon to $Δ$ form factors, besides Wilson fermions, are evaluated using domain wall valence fermions with staggered sea quark configurations for pion masses as low as about 350 MeV. Using these form factors, together with an evaluation of the renormalized quark mass, we investigate the validity of the diagonal and non-diagonal Goldberger-Treiman relations. The ratios $G_{πNΔ}(q^2)/G_{πNN}(q^2)$ and $2C_5^A(q^2)/G_A(q^2)$ are constant as a function of the momentum transfer squared and show almost no dependence on the quark mass. We confirm equality of these two ratios consistent with the Goldberger-Treiman relations extracting a mean value of $1.61(2)$.

hep-lat↗
arXiv2007

The axial N to Delta transition form factors from Lattice QCD

We evaluate the N to Delta axial transition form factors in lattice QCD in the quenched theory, with two degenerate flavors of dynamical Wilson fermions and using domain wall valence fermions with staggered sea quark configurations. We predict the ratio $C_5^A(q^2)/C_3^V(q^2)$ relevant to the parity violating asymmetry and check the validity of the off-diagonal Goldberger-Treiman relation.

hep-lat↗
arXiv2006

The nucleon electromagnetic form factors from Lattice QCD

We evaluate the isovector nucleon electromagnetic form factors in quenched and full 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 full theory we use 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. We compare our lattice results to the isovector part of the experimentally measured form factors.

hep-lat↗
arXiv2006

Free Energy and Plaquette expectation value for gluons on the lattice, in three dimensions

We calculate the perturbative value of the Free Energy in Lattice QCD in three dimensions, up to three loops. Our calculation is performed using the Wilson formulation for gluons in SU(N) gauge theories. The Free Energy is directly related to the average plaquette. To carry out the calculation, we compute the coefficients involved in the perturbative expansion of the Free Energy up to three loops, using an automated set of procedures developed by us in Mathematica. The dependence on N is shown explicitly in our results. For purposes of comparison, we also present the individual contributions from every diagram. These have been obtained by means of two independent calculations, in order to cross check our results.

hep-lat↗
arXiv2005

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 $β=6.0$. 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.

hep-lat↗
arXiv2004

Momentum dependence of the N to Delta transition form factors

We present a new method to determine the momentum dependence of the N to Delta transition form factors and demonstrate its effectiveness in the quenched theory at $β=6.0$ on a $32^3 \times 64$ lattice. We address a number of technical issues such as the optimal combination of matrix elements and the simultaneous overconstrained analysis of all lattice vector momenta contributing to a given momentum transfer squared, $Q^2$.

hep-lat↗
arXiv2004

The pentaquark potential, mass and density-density correlator

We evaluate the static $qqqq\bar{q}$ potential in the quenched theory at $β=5.8$ and $β=6.0$ on a lattice of size $16^3\times 32$. The mass and density-density correlator for the $Θ^+$ is investigated in the quenched theory at $β=6.0$ on lattices of size $16^3\times 32$, $24^3\times 32$ and $32^3 \times 64$.

hep-lat↗
arXiv2004

Higher Loop Results for the Plaquette, Using the Clover and Overlap Actions

We calculate the perturbative value of the free energy in QCD on the lattice. This quantity is directly related to the average plaquette. Our calculation is done to 3 loops using the clover action for fermions; the results are presented for arbitrary values of the clover coefficient, and for a wide range of fermion masses. In addition, we calculate the 2 loop result for the same quantity, using the overlap action.

hep-lat↗
arXiv2004

The N to Delta electromagnetic transition form factors from Lattice QCD

The magnetic dipole, the electric quadrupole and the Coulomb quadrupole amplitudes for the transition γN\to Δare calculated in quenched lattice QCD at β=6.0 with Wilson fermions. Using a new method combining an optimal combination of interpolating fields for the $Δ$ and an overconstrained analysis, we obtain statistically accurate results for the dipole form factor and for the ratios of the electric and Coulomb quadrupole amplitudes to the magnetic dipole amplitude, R_{EM} and R_{SM}, up to momentum transfer squared 1.5 GeV^2. We show for the first time using lattice QCD that both R_{EM} and R_{SM} are non-zero and negative, in qualitative agreement with experiment and indicating the presence of deformation in the N- Delta system.

hep-lat↗
arXiv2004

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 $Δ^+$ within the quenched theory, and with two flavours of dynamical quarks.

hep-lat↗
arXiv2004

N to Delta electromagnetic transition form factors from Lattice QCD

The magnetic dipole, the electric quadrupole and the Coulomb quadrupole amplitudes for the transition $γN\to Δ$ are evaluated both in quenched lattice QCD at $β=6.0$ and using two dynamical Wilson fermions simulated at $β=5.6$. The dipole transition form factor is accurately determined at several values of momentum transfer. On the lattices studied in this work, the electric quadrupole amplitude is found to be non-zero yielding a negative value for the ratio, $ R_{EM}$, of electric quadrupole to magnetic dipole amplitudes at three values of momentum transfer.

hep-lat↗
arXiv2003

The Matter Density Distribution for Mesons and Baryons

The matter density distribution inside a hadron is evaluated using gauge-invariant correlation functions within the quenched and the unquenched theory. Comparison with the charge density distribution suggests that hadron deformation is a consequence of the relativistic motion of the quarks.

hep-lat↗
arXiv2003

Gauge-invariant two- and three- density correlators

Gauge-invariant spatial correlations between two and three quarks inside a hadron are measured within quenched and unquenched QCD. These correlators provide information on the shape and multipole moments of the pion, the rho, the nucleon and the $Δ$.

hep-lat↗
arXiv2003

Hadron wave functions and the issue of nucleon deformation

Using gauge invariant hadronic two- and three- density correlators we extract information on the spatial distributions of quarks in hadrons, and on hadron shape and multipole moments within quenched lattice QCD. Combined with the calculation of N to Delta transition amplitudes the issue of nucleon deformation can be addressed.

nucl-th↗