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P. E. L. Rakow

Publications and source records attributed to P. E. L. Rakow.

At least 163 records · Page 9Linked to original sources
arXiv2005

Stochastic Perturbation Theory and the Gluon Condensate

On the lattice searching for the gluon condensate is difficult because a large perturbative contribution to the expectation value of the action has to be subtracted before looking for a small contribution from a possible gluon condensate. The perturbative calculation therefore has to be very precise. We use a modified version of stochastic perturbation theory to calculate a perturbative series in a boosted coupling, which converges more rapidly than the series with the usual lattice coupling, reducing the uncertainties in our results. We do not see any condensate of dimension two, as suggested by some earlier lattice studies, but we do find a contribution from a dimension four condensate. The value of this condensate is approximately 0.04(1) GeV^4, but with large uncertainties.

hep-lat↗
arXiv2005

Meson decay constants from Nf=2 clover fermions

We present recent results for meson decay constants calculated on configurations with two flavours of O(a)-improved Wilson fermions. Non-perturbative renormalisation is applied and quark mass dependencies as well as finite volume and discretisation effects are investigated. In this work we also present a computation of the coupling of the light vector mesons to the tensor current using dynamical fermions.

hep-lat↗
arXiv2005

Perturbative renormalisation of quark bilinear operators for overlap fermions with and without stout links and improved gauge action

We calculate lattice renormalisation constants of local and one-link quark operators for overlap fermions and improved gauge actions in one-loop perturbation theory. For the local operators we stout smear the SU(3) links in the fermionic action. Using the popular tadpole improved Lüscher-Weisz actions at $β=8.45$ and $β=8.0$ we present numerical values for the Z factors in the $\bar{MS}$ scheme (partly as function of the stout smearing strength). We compare various levels of mean field (tadpole) improvement which have been applied to our results.

hep-lat↗
arXiv2005

A determination of the strange quark mass for unquenched clover fermions using the AWI

Using the O(a) Symanzik improved action an estimate is given for the strange quark mass for unquenched (nf=2) QCD. The determination is via the axial Ward identity (AWI) and includes a non-perturbative evaluation of the renormalisation constant. Numerical results have been obtained at several lattice spacings, enabling the continuum limit to be taken. Results indicate a value for the strange quark mass (in the MSbar-scheme at a scale of 2GeV) in the range 100 - 130MeV.

hep-lat↗
arXiv2005

Structure of the Pion from Full Lattice QCD

Moments of generalised parton distributions can be related to off-forward matrix elements of local operators. We calculate a few of the leading twist matrix elements for the pion on the lattice. The simulations are performed using two flavours of dynamical fermions and a range of pion masses from 550 to 1090 MeV. Our lattice spacings and spatial volumes lie in the range 0.07-0.12 fm and (1.6-2.2 fm)^3, respectively. Key features of our investigation are the use of O(a) improved Wilson fermions and non-perturbative renormalisation. We present first results for the two lowest moments of the generalised parton distributions of the pion and compare the pion electromagnetic form factor F_pi to experimental data. Good agreement is found between lattice data and experiment.

hep-lat↗
arXiv2005

Investigation of the Second Moment of the Nucleon's g1 and g2 Structure Functions in Two-Flavor Lattice QCD

The reduced matrix elements a_2 and d_2 are computed in lattice QCD with N_f=2 flavors of light dynamical (sea) quarks. For proton and neutron targets we obtain as our best estimates d_2^(p)=0.004(5) and d_2^(n)=-0.001(3), respectively, in the MSbar scheme at Q^2=5 GeV^2, while for a_2 we find a_2^(p)=0.077(12) and a_2^(n)=-0.005(5), where the errors are purely statistical.

hep-lat↗
arXiv2005

Quark helicity flip generalized parton distributions from two-flavor lattice QCD

We present an initiatory study of quark helicity flip generalized parton distributions (GPDs) in n_f=2 lattice QCD, based on clover-improved Wilson fermions for a large number of coupling constants and pion masses. Quark helicity flip GPDs yield essential information on the transverse spin structure of the nucleon. In this work, we show first results on their lowest moments and dipole masses and study the corresponding chiral and continuum extrapolations.

hep-lat↗
arXiv2005

Renormalisation of one-link quark operators for overlap fermions with Lüscher-Weisz gauge action

We compute lattice renormalisation constants of one-link quark operators ({\it i.e.} operators with one covariant derivative) for overlap fermions and Lüscher-Weisz gauge action in one-loop perturbation theory. Among others, such operators enter the calculation of moments of polarised and unpolarised hadron structure functions. Results are given for β=8.45, β=8.0 and mass parameter ρ=1.4, which are commonly used in numerical simulations. We apply mean field (tadpole) improvement to our results.

hep-lat↗
arXiv2005

A Determination of the Lambda Parameter from Full Lattice QCD

We present a determination of the QCD parameter Lambda in the quenched approximation (n_f=0) and for two flavours (n_f=2) of light dynamical quarks. The calculations are performed on the lattice using O(a) improved Wilson fermions and include taking the continuum limit. We find Lambda_{n_f=0} = 259(1)(20) MeV and Lambda_{n_f=2} = 261(17)(26) MeV}, using r_0 = 0.467 fm to set the scale. Extrapolating our results to five flavours, we obtain for the running coupling constant at the mass of the Z boson alpha_s(m_Z) = 0.112(1)(2). All numbers refer to the MSbar scheme.

hep-ph↗
arXiv2005

One-loop renormalisation of quark bilinears for overlap fermions with improved gauge actions

We compute lattice renormalisation constants of local bilinear quark operators for overlap fermions and improved gauge actions. Among the actions we consider are the Symanzik, Lüscher-Weisz, Iwasaki and DBW2 gauge actions. The results are given for a variety of $ρ$ parameters. We show how to apply mean field (tadpole) improvement to overlap fermions. The question, what is a good gauge action, is discussed from the perturbative point of view. Finally, we show analytically that the gauge dependent part of the self-energy and the amputated Green functions are independent of the lattice fermion representation, using either Wilson or overlap fermions.

hep-lat↗
arXiv2004

Generalized Parton Distributions in Full Lattice QCD

We present recent results on generalized parton distributions from dynamical lattice QCD calculations. Our set of twelve different combinations of couplings and quark masses allows for a preliminary study of the pion mass dependence of the transverse nucleon structure.

hep-lat↗
arXiv2004

Perturbative Renormalisation of the Second Moment of Generalised Parton Distributions

We calculate the non-forward quark matrix elements of operators with two covariant derivatives needed for the renormalisation of the second moment of generalised parton distributions in one-loop lattice perturbation theory using Wilson fermions. For some representations of the hypercubic group commonly used in simulations we determine the sets of all possible mixing operators. For those representations the one-loop mixing matrices of renormalisation factors are found. Due to non-vanishing contributions of operators with external ordinary derivatives the number of contributing operators increases compared to forward matrix elements.

hep-lat↗