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Giannis Koutsou

Publications and source records attributed to Giannis Koutsou.

At least 73 records · Page 4Linked to original sources
arXiv2015

Strangeness of the nucleon from Lattice Quantum Chromodynamics

We present a non-perturbative calculation of the strangeness of the nucleon $y_N$ within the framework of lattice QCD. This observable is known to be an important cornerstone to interpret results from direct dark matter detection experiments. We perform a lattice computation for $y_N$ with an analysis of systematic effects originating from discretization, finite size, chiral extrapolation and excited state effects leading to the value of $y_N= 0.173(50)$. The rather large uncertainty of this value of $y_N$ is dominated by systematic uncertainties which we were able to quantify in this work.

hep-lat↗
arXiv2015

Extraction of the isovector magnetic form factor of the nucleon at zero momentum

The extraction of the magnetic form factor of the nucleon at zero momentum transfer is usually performed by adopting a parametrization for its momentum dependence and fitting the results obtained at finite momenta. We present position space methods that rely on taking the derivative of relevant correlators to extract directly the magnetic form factor at zero momentum without the need to assume a functional form for its momentum dependence. These methods are explored on one ensemble using $N_f=2+1+1$ Wilson twisted mass fermions.

hep-lat↗
arXiv2015

Nucleon electromagnetic form factors from twisted mass lattice QCD

Results on the electromagnetic form factors of the nucleon using twisted mass fermion configurations are presented. These include a gauge field ensemble simulated with two degenerate light quarks yielding a pion mass of around 130 MeV, as well as two ensembles that include strange and charm quarks in the sea yielding pion masses of 210 MeV and 373 MeV. Details of the methods used and systematic errors are discussed, such as noise reduction techniques and the effect of excited states contamination.

hep-lat↗
arXiv2014

Results on the disconnected contributions for hadron structure

We present results on the disconnected contributions to three point functions entering in studies of hadron structure. We use $N_F = 2+1+1$ twisted mass fermions and give a detailed description on the results of the nucleon σ-terms, isoscalar axial charge and first moments of bare parton distributions for a range of pions masses. In addition we give the σ-terms and the computations are performed using QUDA code implemented on GPUs.

hep-lat↗
arXiv2014

Nucleon observables and axial charges of other baryons using twisted mass fermions

We present results on the nucleon scalar, axial and tensor charges, as well as, on the first moments of the unpolarized, polarized and transversity parton distributions using $N_f=2$ and $N_f=2+1+1$ twisted mass fermions. These include an ensemble that yields the physical value of the ratio of the nucleon to the pion mass. Results on the axial charges of hyperons and charmed baryons are also presented for a range of pion masses including the physical one.

hep-lat↗
arXiv2014

A QUDA-branch to compute disconnected diagrams in GPUs

Although QUDA allows for an efficient computation of many QCD quantities, it is surprinsingly lacking tools to evaluate disconnected diagrams, for which GPUs are specially well suited. We aim to fill this gap by creating our own branch of QUDA, which includes new kernels and functions required to calculate fermion loops using several methods and fermionic regularizations.

hep-lat↗
arXiv2014

Computation of disconnected contributions to nucleon observables

We compare several methods for computing disconnected fermion loops contributing to nucleon three-point functions. The comparison is carried out using one ensemble of $N_f=2+1+1$ twisted mass fermions with pion mass of 373 MeV. The complete set of operators up to one-derivative are examined by developing optimized code for mutli-GPUs. Simple guidelines are given as to the preferable method for each class of operators.

hep-lat↗
arXiv2013

Nucleon transversity generalized form factors with twisted mass fermions

We present results on the nucleon tensor form factors and first moment of the transversity distribution using maximally twisted mass fermions. We analyze two Nf=2+1+1 ensembles having pion masses of 213 MeV and 373 MeV with lattice spacing a=0.064 fm and a=0.082 fm, respectively. First results using an Nf=2 ensemble of twisted mass fermions with a clover term at a physical pion mass and lattice spacing a=0.094 fm are also presented. The renormalization function for the local tensor form factors is evaluated non-perturbatively with a perturbative subtraction of O(a^2)-terms, while for the first moment of the transversity we use a perturbative estimate. Results are given in the MS-bar scheme at a scale of mu=2 GeV, and are compared with recent results obtained using different discretization schemes.

hep-lat↗
arXiv2013

Implementation of the twisted mass fermion operator in the QUDA library

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.

hep-lat↗
arXiv2013

Study of the a_0(980) on the lattice

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.

hep-lat↗
arXiv2013

Meson and Baryon dispersion relations with Brillouin fermions

We study the dispersion relations of mesons and baryons built from Brillouin quarks on one N_f=2 gauge ensemble provided by QCDSF. For quark masses up to the physical strange quark mass, there is hardly any improvement over the Wilson discretization, if either action is link-smeared and tree-level clover improved. For quark masses in the range of the physical charm quark mass, the Brillouin action still shows a perfect relativistic behavior, while the Wilson action induces severe cut-off effects. As an application we determine the masses of the Ω_c^0, Ω_{cc}^+ and Ω_{ccc}^{++} baryons on that ensemble.

hep-lat↗
arXiv2012

Brillouin improvement for Wilson fermions

We present a parameter-free Wilson-type lattice Dirac operator with an 81-point stencil for the covariant derivative and the Laplacian which attempts to minimize the breaking of rotational symmetry near the boundary of the Brillouin zone. The usefulness of this "Brillouin operator" in practical applications is explored by studying the scaling of pseudoscalar decay constants in quenched QCD, with rather good results in the physical charm region. We also investigate the suitability of this operator as a kernel to the overlap procedure. Here, the resulting overlap operator is found to be cheaper to construct and significantly better localized than the variety with the standard Wilson kernel.

hep-lat↗
arXiv2012

The ratio m_c/m_s with Wilson fermions

We determine the quark mass ratio m_c/m_s on the lattice, using Wilson-type fermions. Configurations with N_f=2 dynamical clover-improved fermions by the QCDSF collaboration are used, which were made available through the ILDG. In the valence sector we use a sophisticated, mass-independently O(a)-improved Wilson-type action with small cut-off effects even in the charm mass region. After an extrapolation to the physical pion mass, to zero lattice spacing and to infinite box volume, we find m_c/m_s=11.27(30)(26).

hep-lat↗
arXiv2011

$m_c/m_s$ with Brillouin fermions

We present a calculation of the ratio of the charm quark mass to the strange quark mass. Using the Brillouin improved Wilson action, we are able to calculate this ratio in a single framework, using a relativistic fermionic action throughout. The calculation is carried out on selected ensembles of two flavor clover improved lattices produced by the QCDSF collaboration, allowing an extrapolation to the continuum, to infinite volume and to the physical pion mass.

hep-lat↗
arXiv2010

Axial and pseudoscalar form-factors of the Delta^+(1232)

We present first results on the axial and pseudoscalar $Δ$ form factors. The analysis is carried out in the quenched approximation where statistical errors are small and the lattice set-up can be investigated relatively quickly. We also present an analysis with a hybrid action using staggered sea quarks and domain-wall valence fermions.

hep-lat↗
arXiv2009

Quark transverse charge densities in the $Δ(1232)$ from lattice QCD

We extend the formalism relating electromagnetic form factors to transverse quark charge densities in the light-front frame to the case of a spin-3/2 baryon and calculate these transverse densities for the $Δ(1232)$ isobar using lattice QCD. The transverse charge densities for a transversely polarized spin-3/2 particle are characterized by monopole, dipole, quadrupole, and octupole patterns representing the structure beyond that of a pure point-like spin-3/2 particle. We present lattice QCD results for the $Δ$-isobar electromagnetic form factors for pion masses down to approximatively 350 MeV for three cases: quenched QCD, two-degenerate flavors of dynamical Wilson quarks, and three flavors of quarks using a mixed action that combines domain wall valence quarks and dynamical staggered sea quarks. We extract transverse quark charge densities from these lattice results and find that the $Δ$ is prolately deformed, as indicated by the fact that the quadrupole moment $G_{E2}(0$) is larger than the value -3 characterizing a point particle and the fact that the transverse charge density in a $Δ^+$ of maximal transverse spin projection is elongated along the axis of the spin.

hep-ph↗
arXiv2008

Nucleon form factors with dynamical twisted mass fermions

The electromagnetic and axial form factors of the nucleon are evaluated in twisted mass QCD with two degenerate flavors of light, dynamical quarks. The axial charge g_A, magnetic moment and the Dirac and Pauli radii are determined for pion masses in the range 300 MeV to 500 MeV.

hep-lat↗