arXiv subjects

G. Koutsou

Publications and source records attributed to G. Koutsou.

At least 55 records · Page 3Linked to original sources

arXiv2012

Calculation of fermion loops for $η^\prime$ and nucleon scalar and electromagnetic form factors

The exact evaluation of the disconnected diagram contributions to the flavor-singlet pseudoscalar meson mass, the nucleon sigma term and the nucleon electromagnetic form factors, is carried out utilizing GPGPU technology with the NVIDIA CUDA platform. The disconnected loops are also computed using stochastic methods with several noise reduction techniques. Various dilution schemes as well as the truncated solver method are studied. We make a comparison of these stochastic techniques to the exact results and show that the number of noise vectors depends on the operator insertion in the fermionic loop.

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arXiv2011

The $Δ(1232)$ axial charge and form factors from lattice QCD

We present the first calculation on the $Δ$ axial-vector and pseudoscalar form factors using lattice QCD. Two Goldberger-Treiman relations are derived and examined. A combined chiral fit is performed to the nucleon axial charge, N to $Δ$ axial transition coupling constant and $Δ$ axial charge.

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arXiv2010

Nucleon to Delta transition form factors with $N_F=2+1$ domain wall fermions

We calculate the electromagnetic, axial and pseudo-scalar form factors of the Nucleon to $Δ(1232)$ transition using two dynamical light degenerate quarks and a dynamical strange quark simulated with the domain wall fermion action. Results are obtained at lattice spacings $a = 0.114$ fm and $a=0.084$ fm, with corresponding pion masses of $330$ MeV and $297$ MeV, respectively. High statistics measurements are achieved by utilizing the coherent sink technique. The dominant electromagnetic dipole form factor, the axial form factors and the pseudo-scalar coupling are extracted to a good accuracy. This allows the investigation of the non-diagonal Goldberger-Treiman relation. Particular emphasis is given on the extraction of the sub-dominant electromagnetic quadrupole form factors and their ratio to the dominant dipole form factor, $R_{EM}$ and $R_{SM}$, measured in experiment.

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arXiv2010

The electromagnetic form factors of the Omega in lattice QCD

We present results on the Omega baryon electromagnetic form factors using $N_f=2+1$ domain-wall fermion configurations for three pion masses in the range of about 350 to 300 MeV. We compare results obtained using domain wall fermions with those of a mixed-action (hybrid) approach, which combine domain wall valence quarks on staggered sea quarks, for a pion mass of about 350 MeV. We pay particular attention in the evaluation of the subdominant electric quadrupole form factor to sufficient accuracy to exclude a zero value, by constructing a sequential source that isolates it from the dominant form factors. The $Ω^-$ magnetic moment, $μ_{Ω^{-}}$, the electric charge and magnetic radius, $\langle r^{2}_{E0/M1} \rangle$, are extracted for these pion masses. The electric quadrupole moment is determined for the first time using dynamical quarks.

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arXiv2009

Axial Nucleon to Delta transition form factors on 2+1 flavor hybrid lattices

We correct the values of the dominant nucleon to Delta axial transition form factors CA_5 and CA_6 published in C. Alexandrou et.al., Phys. Rev. D 76,094511 (2007). The analysis error affects only the values obtained when using the hybrid action in the low Q^2 regime bringing them into agreement with those obtained with Wilson fermions.

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arXiv2009

N to Delta transition form factors with Nf=2+1 domain wall fermions

The electromagnetic, axial and pseudoscalar nucleon to Delta form factors are calculated using dynamical domain wall fermions at a lattice spacing of a = 0.114 fm on a lattice of spatial size 2.74 fm and pion mass of 331 MeV. Pion pole dominance and the Goldberger-Treiman relations are examined.

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arXiv2009

Nucleon form factors with Nf=2 dynamical twisted mass fermions

We present results on the electromagnetic and axial nucleon form factors using two degenerate flavors of twisted mass fermions on lattices of spatial size 2.1 fm and 2.7 fm and a lattice spacing of about 0.09 fm. We consider pion masses in the range of 260-470 MeV. We chirally extrapolate results on the nucleon axial ch arge, the isovector Dirac and Pauli root mean squared radii and magnetic moment to the physical point and co mpare to experiment.

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arXiv2009

Omega baryon electromagnetic form factors from lattice QCD

We present results on the Omega baryon electromagnetic form factors using N_f= 2 + 1 dynamical domain-wall fermion configurations corresponding to a pion mass of about 330 MeV. We construct appropriate sequential sources for the determination of the two dominant form factors GE0 and GM1 as well as a sequential source that isolates the subdominant electric quadrupole form factor GE2. We calculate the Omega baryon magnetic moment, mu_omega, and electric charge radius, ^2, and compare to experiment, for the case of mu_omega, and to other lattice calculations.

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arXiv2009

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 $Δ$ 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.

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arXiv2009

Partially quenched study of strange baryon with Nf = 2 twisted mass fermions

We present results on the mass of the baryon octet and decuplet using two flavors of light dynamical twisted mass fermions. The strange quark mass is fixed to its physical value from the kaon sector in a partially quenched set up. Calculations are performed for light quark masses corresponding to a pion mass in the range 270-500 MeV and lattice sizes of 2.1 fm and 2.7 fm. We check for cut-off effects and isospin breaking by evaluating the baryon masses at two different lattice spacings. We carry out a chiral extrapolation for the octet baryons and discuss results for the Omega.

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arXiv2008

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.

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arXiv2008

Light baryon masses with dynamical twisted mass fermions

We present results on the mass of the nucleon and the Delta using two dynamical degenerate twisted mass quarks and the tree-level Symanzik improved gauge action. The evaluation is performed at four quark masses corresponding to a pion mass in the range of about 300-600 MeV on lattices of 2.1-2.7 fm. We check for cut-off effects by evaluating these baryon masses on lattices of spatial size 2.1 fm at beta=3.9 and beta=4.05 and on a lattice of 2.4 fm at beta=3.8. The values we find are compatible within our statistical errors. Lattice results are extrapolated to the physical limit using continuum chiral perturbation theory. Performing a combined fit to our lattice data at beta=3.9 and beta=4.05 we find a nucleon mass of 964\pm 28 (stat.) \pm 8 (syst.) MeV. The nucleon mass at the physical point provides an independent determination of the lattice spacing. Using heavy baryon chiral perturbation theory at O(p^3) we find a_{β=3.9}=0.0890\pm 0.0039(stat.) \pm 0.0014(syst.) fm, and a_{β=4.05}= 0.0691\pm 0.0034(stat.) \pm 0.0010(syst.) fm, in good agreement with the values determined from the pion decay constant. Isospin violating lattice artifacts in the Delta-system are found to be compatible with zero for the values of the lattice spacings used in this work. Performing a combined fit to our lattice data at beta=3.9 and beta=4.05 we find for the masses of the Delta^{++,-} and Delta^{+,0} 1316 \pm 60 (stat.) MeV and 1330 \pm 74 (stat.) MeV respectively. We confirm that in the continuum limit they are also degenerate.

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arXiv2007

The nucleon to Delta electromagnetic transition form factors in lattice QCD

The electromagnetic nucleon to Delta transition form factors are evaluated using two degenerate flavors of dynamical Wilson fermions and using dynamical sea staggered fermions with domain wall valence quarks. The two subdominant quadrupole form factors are evaluated for the first time in full QCD to sufficient accuracy to exclude a zero value, which is taken as a signal for deformation in the nucleon-Delta system. For the Coulomb quadrupole form factor the unquenched results show deviations from the quenched results at low q^2 bringing dynamical lattice results closer to experiment, thereby confirming the importance of pion cloud contributions on this quantity.

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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.

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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 $1.61(2)$.

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arXiv2006

Hadron form factors using density-density correlators

Gauge invariant density-density correlators yield detailed information on hadron structure. Hadron deformation and form factors can be extracted for momentum transfers up to about 6 GeV$^2$. We use stochastic techniques and dilution to compute the all to all propagator required for the exact evaluation of density-density correlators. We present first results for the pion form factor.

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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.

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arXiv2005

The static tetraquark and pentaquark potentials

We evaluate the static $qq\bar{q}\bar{q}$ and $qqqq\bar{q}$ potentials in the quenched theory at $β=5.8$ and $β=6.0$ on a lattice of size $16^3\times 32$. We compare the static potentials to the sum of two meson potentials for the tetraquark system and to the sum of the baryonic and mesonic potentials for the pentaquark state, as well as, with the confining potential obtained in the strong coupling expansion.

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