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Constantia Alexandrou

Publications and source records attributed to Constantia Alexandrou.

At least 127 records · Page 7Linked to original sources
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.

hep-lat↗
arXiv2017

Using a new analysis method to extract excited states in the scalar meson sector

We explore a method to extract energy eigenstates, called Athens Model Independent Analysis Scheme (AMIAS), which is an alternative to solving standard Generalized Eigenvalue Problems (GEVP). The method is based on statistically sampling the space of fit parameters according to the $χ^2$ value of the fit function. The method is particularly suited for correlators or correlation matrices with strong contributions from several energy eigenstates and for rather noisy data, e.g. for correlators with disconnected and partly disconnected diagrams. We apply the method to the analysis of the $J^P = 0^+$ channel in the context of our investigation of the $a_0(980)$ meson and point out advantages compared to the GEVP.

hep-lat↗
arXiv2017

Investigating efficient methods for computing four-quark correlation functions

We discuss and compare the efficiency of various methods, combinations of point-to-all propagators, stochastic timeslice-to-all propagators, the one-end trick and sequential propagators, to compute two-point correlation functions of two-quark and four-quark interpolating operators of different structure including quark-antiquark type, mesonic molecule type, diquark-antidiquark type and two-meson type. Although we illustrate our methods in the context of the $a_0(980)$, they can be applied for other multi-quark systems, where similar diagrams appear. Thus our results could provide helpful guidelines on the choice of methods for correlation function computation for future lattice QCD studies of meson-meson scattering and possibly existing tetraquark states.

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.

hep-lat↗
arXiv2016

Novel applications of Lattice QCD: Parton distribution functions, proton charge radius and neutron electric dipole moment

We briefly discuss the current status of lattice QCD simulations and review selective results on nucleon observables focusing on recent developments in the lattice QCD evaluation of the nucleon form factors and radii, parton distribution functions and their moments, and the neutron electric dipole moment. Nucleon charges and moments of parton distribution functions are presented using simulations generated at physical values of the quark masses, while exploratory studies are performed for the parton distribution functions and the neutron electric dipole moment at heavier than physical value of the pion mass.

hep-lat↗
arXiv2016

Nucleon tomography. What can we do better today than Rutherford 100 years ago?

A survey is presented on the current status of 3D nucleon tomography. Several research frontiers are addressed that dominate modern physics from theory to current and future experiments. We have now a much more detailed spatial image of the nucleon thanks to various theoretical concepts and methods to describe its charge distribution and spin decomposition which are highlighted here. The progress of lattice computations of these quantities is reported and the prospects of what we can come to expect in the near future are discussed. Multi-dimensional maps of the nucleon's partonic structure appear now within reach of forthcoming experiments.

hep-ph↗
arXiv2016

Importance of closed quark loops for lattice QCD studies of tetraquarks

To investigate the light scalar tetraquark candidate $a_0(980)$ (quantum numbers $J^P =0^+$), a correlation matrix including a variety of two- and four-quark interpolating operators has to be computed. We discuss efficient techniques to compute the elements of this correlation matrix, in particular diagrams with closed quark loops. Furthermore, we present evidence that such diagrams are not negligible given our precision, and their contribution is essential to obtain physically meaningful results. In particular, we find indications of the existence of an "additional" state around the two-particle thresholds of $K+ \bar K$ and $η+π$, which could correspond to the $a_0(980)$ meson.

hep-lat↗
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.

hep-lat↗
arXiv2016

Adaptive Aggregation-based Domain Decomposition Multigrid for Twisted Mass Fermions

The Adaptive Aggregation-based Domain Decomposition Multigrid method (arXiv:1303.1377) is extended for two degenerate flavors of twisted mass fermions. By fine-tuning the parameters we achieve a speed-up of the order of hundred times compared to the conjugate gradient algorithm for the physical value of the pion mass. A thorough analysis of the aggregation parameters is presented, which provides a novel insight into multigrid methods for lattice QCD independently of the fermion discretization.

hep-lat↗
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.

hep-lat↗
arXiv2016

A study of the radiative transition $ππ\to πγ^{*}$ with lattice QCD

Lattice QCD calculations of radiative transitions between hadrons have in the past been limited to processes of hadrons stable under the strong interaction. Recently developed methods for $1\to2$ transition matrix elements in a finite volume now enable the determination of radiative decay rates of strongly unstable particles. Our lattice QCD study focuses on the process $ππ\to πγ^{*}$, where the $ρ$ meson is present as an enhancement in the cross-section. We use $2+1$ flavors of clover fermions at a pion mass of approximately $320$ MeV and a lattice size of approximately $3.6$ fm. The required $2$-point and $3$-point correlation functions are constructed from a set of forward, sequential and stochastic light quark propagators. In addition to determining the $ρ$ meson resonance parameters via the Lüscher method, the scattering phase shift is used in conjunction with the $1\to2$ transition matrix element formalism of Briceño, Hansen and Walker-Loud to compute the $ππ\toπγ^{*}$ amplitude at several values of the momentum transfer and $ππ$ invariant mass.

hep-lat↗
arXiv2016

A Lattice Calculation of Parton Distributions

We present results for the $x$ dependence of the unpolarized, helicity, and transversity isovector quark distributions in the proton using lattice QCD, employing the method of quasi-distributions proposed by Ji in 2013. Compared to a previous calculation by us, the errors are reduced by a factor of about 2.5. Moreover, we present our first results for the polarized sector of the proton, which indicate an asymmetry in the proton sea in favor of the $u$ antiquarks for the case of helicity distributions, and an asymmetry in favor of the $d$ antiquarks for the case of transversity distributions.

hep-lat↗
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↗
arXiv2016

Study of decuplet baryon resonances from lattice QCD

A lattice QCD study of the strong decay width and coupling constant of decuplet baryons to an octet baryon - pion state is presented. The transfer matrix method is used to obtain the overlap of lattice states with decuplet baryon quantum numbers on the one hand and octet baryon-pion quantum numbers on the other as an approximation to the matrix element of the corresponding transition. By making use of leading order effective field theory, the coupling constants, as well as the widths for the various decay channels are determined. The transitions studied are $ Δ\to π\,N$, $Σ^* \to Λ\,π$, $Σ^* \to Σ\,π$ and $Ξ^* \to Ξ\,π$. We obtain results for two ensembles of $N_f=2+1$ dynamical fermion configurations, one using domain wall valence quarks on a staggered sea at a pion mass of $350\,\mathrm{MeV}$ and a box size of $3.4\,\mathrm{fm}$ and a second one using domain wall sea and valence quarks at pion mass $180\,\mathrm{MeV}$ and box size $4.5\,\mathrm{fm}$.

hep-lat↗
arXiv2016

Parton distribution functions from Lattice QCD

We present recent results on the first moments of parton distributions using gauge configurations generated with two degenerate flavors of light twisted mass quarks with pion mass fixed approximately to its physical value. We also present a first study of the vector parton distribution function using a twisted mass ensemble at pion mass of 373 MeV.

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

Calculation of the decay width of decuplet baryons

We calculate the coupling constant and decay width of the decuplet to octet baryon transitions in lattice QCD using the transfer matrix method. The transition amplitude is related to the coupling constant and via the Fermi's Golden Rule to the decay width. The method is applicable for near-degeneracy of the energy levels of initial and final states and, when this condition is fulfilled, yields a good estimate of the decay width. We present results using a hybrid action with domain wall valence quarks on a staggered sea with $350$ MeV pion mass as well as for a domain wall fermion action with $180$ MeV pion mass. We find $Γ\left( Δ\to π\,N \right) = 119\,( 8)\,( 8)$ MeV for the transition of Delta to pion-nucleon within the unitary domain wall setup. We also report values for the decay widths of the $Σ^*$ and $Ξ*$ baryons.

hep-lat↗