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Martha Constantinou

Publications and source records attributed to Martha Constantinou.

At least 109 records · Page 6Linked to original sources
arXiv2017

Nucleon electromagnetic form factors using lattice simulations at the physical point

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 contribution 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.

hep-lat↗
arXiv2017

Topological susceptibility from twisted mass fermions using spectral projectors and the gradient flow

We compare lattice QCD determinations of topological susceptibility using a gluonic definition from the gradient flow and a fermionic definition from the spectral projector method. We use ensembles with dynamical light, strange and charm flavors of maximally twisted mass fermions. For both definitions of the susceptibility we employ ensembles at three values of the lattice spacing and several quark masses at each spacing. The data are fitted to chiral perturbation theory predictions with a discretization term to determine the continuum chiral condensate in the massless limit and estimate the overall discretization errors. We find that both approaches lead to compatible results in the continuum limit, but the gluonic ones are much more affected by cut-off effects. This finally yields a much smaller total error in the spectral projector results. We show that there exists, in principle, a value of the spectral cutoff which would completely eliminate discretization effects in the topological susceptibility.

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arXiv2017

Gluon momentum fraction of the nucleon from lattice QCD

We perform a direct calculation of the gluon momentum fraction of the nucleon using maximally twisted mass fermion ensembles with $N_f=2+1+1$ flavors at a pion mass of about $370\,\mathrm{MeV}$ and a lattice spacing of $a\approx 0.082\,\mathrm{fm}$ and with $N_f=2$ flavors at the physical pion mass and a lattice spacing of $a\approx 0.093\,\mathrm{fm}$. In the definition of the gluon operator we employ stout smearing to obtain a statistically significant result for the bare matrix elements. In addition, we perform a lattice perturbative calculation including 2 levels of stout smearing to carry out the mixing and the renormalization of the quark and gluon operators. We find, after conversion to the $\overline{\mathrm{MS}}$ scheme at a scale of $2\,\mathrm{GeV}$: $\langle x\rangle^R_g {=} 0.284(23)(23)$ for pion mass of about $370\,\mathrm{MeV}$ and $\langle x\rangle^R_g {=} 0.283(23)(15)$ for the physical pion mass.

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arXiv2017

Updated Lattice Results for Parton Distributions

We provide an analysis of the x-dependence of the bare unpolarized, helicity and transversity iso-vector parton distribution functions (PDFs) from lattice calculations employing (maximally) twisted mass fermions. The x-dependence of the calculated PDFs resembles the one of the phenomenological parameterizations, a feature that makes this approach very promising. Furthermore, we apply momentum smearing for the relevant matrix elements to compute the lattice PDFs and find a large improvement factor when compared to conventional Gaussian smearing. This allows us to extend the lattice computation of the distributions to higher values of the nucleon momentum, which is essential for the prospects of a reliable extraction of the PDFs in the future.

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arXiv2017

Nucleon electromagnetic and axial form factors with N$_f$=2 twisted mass fermions at the physical point

We present results for the nucleon electromagnetic and axial form factors using an N$_f$=2 twisted mass fermion ensemble with pion mass of about 131 MeV. We use multiple sink-source separations to identify excited state contamination. Dipole masses for the momentum dependence of the form factors are extracted and compared to experiment, as is the nucleon magnetic moment and charge and magnetic radii.

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arXiv2017

Lattice Generalized Parton Distributions and Form Factors of the Nucleon

In these proceedings we discuss recent progress in nucleon structure using lattice QCD simulations at or near the physical value of the pion mass. Main focus will be given in observables such as the nucleon axial charge and the first moments of parton distributions, for both the valence and sea quark contributions, and discuss their implications on the spin content of the nucleon. We will will also report developments on the evaluation of the gluon momentum fraction, which contributes significantly to the nucleon spin.

hep-lat↗
arXiv2017

Parton Distributions from Lattice QCD with Momentum Smearing

In this work we continue our effort to explore a recent proposal, which allows light-cone distributions to be extracted from purely spatial correlations, being thus accessible to lattice methods. In order to test the feasibility of this method, we present our latest results from a twisted mass lattice calculation of the flavor non-singlet momentum, helicity and transversity distributions of the nucleon. Furthermore, we apply a newly proposed momentum improved smearing, which has the potential to reach higher nucleon momenta as required for a safe matching procedure to the physical distribution functions.

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arXiv2016

New Physics Searches from Nucleon Matrix Elements in Lattice QCD

In this paper we review recent progress in hadron structure using lattice QCD simulations, with main focus in the evaluation of nucleon matrix elements. We highlight developments that may guide new Physics searches, such as the scalar and tensor charges, as well as, the neutron electric dipole moment.

hep-lat↗
arXiv2016

Women in physics in Cyprus: A first report

This paper reviews the status of women in science, physics in particular, in Cyprus. We describe the development of physics in the country, focusing on the contributions and participation of women. We present statistical data for the last several years, reviewing the percentage of women who are pursuing physics as a subject of study or as a profession. We report the gender ratios at different career stages and find that while women are well represented in undergraduate studies, female physicists are underrepresented in senior positions. We discuss factors that might affect the career evolution of women in physics in Cyprus.

physics.ed-ph↗
arXiv2016

Disconnected diagrams with twisted-mass fermions

The latest results from the Twisted-Mass collaboration on disconnected diagrams at the physical value of the pion mass are presented. In particular, we focus on the sigma terms, the axial charges and the momentum fraction, all of them for the nucleon. A detailed error analysis for each observable follows, showing the strengths and weaknesses of the one-end trick. Alternatives are discussed.

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arXiv2016

Position space method for the nucleon magnetic moment in lattice QCD

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 a position space method that allows us to remove the momentum prefactor in the form factor decomposition and hence compute the magnetic form factor directly at zero momentum without the need to assume a functional form for its momentum dependence. The method is explored on one ensemble using $N_f=2+1+1$ Wilson twisted mass fermions with a light quark mass corresponding to $M_π\approx373\mathrm{GeV}$ and a lattice spacing of $a\approx0.082\mathrm{fm}$. We obtain results for the isovector magnetic moment and for the proton and neutron magnetic moments. The value we find for the isovector magnetic moment is larger as compared to fitting the form factor at the discrete values of the lattice momentum transfers using a dipole Ansatz, bringing it closer to the experimental value.

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arXiv2016

Recent results for the proton spin decomposition from lattice QCD

The exact decomposition of the proton spin has been a much debated topic, on the experimental as well as the theoretical side. In this talk we would like to report on recent non-perturbative results and ongoing efforts to explore the proton spin from lattice QCD. We present results for the relevant generalized form factors from gauge field ensembles that feature a physical value of the pion mass. These generalized form factors can be used to determine the total spin and angular momentum carried by the quarks. In addition we present first results for our ongoing effort to compute the angular momentum of the gluons in the proton.

hep-lat↗
arXiv2015

Recent progress in hadron structure from Lattice QCD

We review recent progress in hadron structure using lattice QCD simulations, with main focus in the evaluation of nucleon quantities such as the axial and tensor charges, and the spin con- tent of the nucleon, using simulations at pion masses close to the physical value. We highlight developments on the evaluation of the gluon moment, a new direct approach to compute quark parton distributions functions on the lattice, as well as, the neutron electric dipole moment. A discussion of the systematic uncertainties and the computation of the disconnected contributions using dynamical simulations is also included.

hep-lat↗
arXiv2015

The electric dipole moment of the neutron from $N_f=2+1+1$ twisted mass fermions

We extract the neutron electric dipole moment (nEDM) $ \vert \vec{d}_n \vert $ on configurations produced with $N_f=2+1+1$ twisted mass fermions with lattice spacing of $a \simeq 0.082$ fm and a light quark mass that corresponds to $M_π \simeq 373$ MeV. We do so by evaluating the $CP$-odd form factor $F_3$ for small values of the $CP$-violation parameter $θ$ in the limit of zero momentum transfer. This limit is extracted using the usual parametrization but in addition position space methods. The topological charge is computed via cooling and gradient flow using the Wilson, Symanzik tree-level improved and Iwasaki actions for smoothing. We obtain consistent results for all choices of smoothing procedures and methods to extract $F_3$ at zero momentum transfer. For the ensemble analyzed we find a value of nEDM of $\vert \vec{d}_n \vert / θ= -0.045(6)(1) {\rm e} \cdot {\rm fm}$.

hep-lat↗
arXiv2015

Renormalization functions for Nf=2 and Nf=4 Twisted Mass fermions

We present results on the renormalization functions of the quark field and fermion bilinears with up to one covariant derivative. For the fermion part of the action we employ the twisted mass formulation with $N_f{=}2$ and $N_f{=}4$ degenerate dynamical quarks, while in the gluon sector we use the Iwasaki improved action. The simulations for $N_f{=}4$ have been performed for pion masses in the range of 390MeV - 760MeV and at three values of the lattice spacing, $a$, corresponding to $β{=}1.90,\,1.95,\,2.10$. The $N_f{=}2$ action includes a clover term with $c_{\rm sw}{=}1.57551$ at $β{=}2.10$, and three ensembles at different values of $m_π$. The evaluation of the renormalization functions is carried out in the RI$'$ scheme using a momentum source. The non-perturbartive evaluation is complemented with a perturbative computation, which is carried out at one-loop level and to all orders in the lattice spacing, $a$. For each renormalization function computed non-perturbatively we subtract the corresponding lattice artifacts to all orders in $a$, so that a large part of the cut-off effects is eliminated. The renormalization functions are converted to the ${\overline{\rm MS}}$ scheme at a reference energy scale of $μ{=}2$ GeV after taking the chiral limit.

hep-lat↗
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.

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