Search results for "quantum electrodynamics"
showing 10 items of 809 documents
On the Pauli-Villars regularisation scheme in the NJL model
1993
Abstract We consider the Nambu-Jona-Lasinio model with a Pauli-Villars cutoff. We point out that previous attempts to use this regularisation have not been consistent and it is shown that if used correctly this approach yields very reasonable values for the cutoff parameter and quark condensate when we demand that ƒπ is fixed to its experimental value. Finally it is argued that because of its covariance and smooth nature this regularisation scheme has significant advantages over other regulators on the market.
Nucleon matrix elements from lattice QCD with all-mode-averaging and a domain-decomposed solver: An exploratory study
2017
We study the performance of all-mode-averaging (AMA) when used in conjunction with a locally deflated SAP-preconditioned solver, determining how to optimize the local block sizes and number of deflation fields in order to minimize the computational cost for a given level of overall statistical accuracy. We find that AMA enables a reduction of the statistical error on nucleon charges by a factor of around two at the same cost when compared to the standard method. As a demonstration, we compute the axial, scalar and tensor charges of the nucleon in $N_f=2$ lattice QCD with non-perturbatively O(a)-improved Wilson quarks, using O(10,000) measurements to pursue the signal out to source-sink sepa…
Relativistic constituent quark model with infrared confinement
2009
We refine the relativistic constituent quark model developed in our previous papers to include the confinement of quarks. It is done, first, by introducing the scale integration in the space of alpha-parameters, and, second, by cutting this scale integration on the upper limit which corresponds to an infrared cutoff. In this manner one removes all possible thresholds presented in the initial quark diagram. The cutoff parameter is taken to be the same for all physical processes. We adjust other model parameters by fitting the calculated quantities of the basic physical processes to available experimental data. As an application, we calculate the electromagnetic form factors of the pion and t…
Fitting strategies to extract the axial charge of the nucleon from lattice QCD
2014
We report on a comparison of several fit methods used for the extraction of the nucleon axial charge gA from lattice QCD with two dynamical flavours of O(a) improved Wilson quarks. We use plateau fits, summed operator insertions (the summation method) and a new “midpoint” method to investigate contributions from excited states that affect the determination of gA. We also present a method to perform correlated fits when the standard estimator for the inverse of the covariance matrix becomes unstable.
Is the spectrum of highly excited mesons purely coulombian?
2008
We show that a static central potential may provide a precise description of highly excited light unflavoured mesons. Due to string breaking this potential becomes of chromoelectric type at sufficiently large quark-antiquark distances giving rise to a coulombian spectrum. The same conclusion can be inferred for any other meson sector through a straightforward extension of our analysis.
Abelian dominance and the dual Meissner effect in local unitary gauges in SU(2) gluodynamics
2007
Performing highly precise Monte-Carlo simulations of SU(2) gluodynamics, we observe for the first time Abelian dominance in the confining part of the static potential in local unitary gauges such as the F12 gauge. We also study the flux-tube profile between the quark and antiquark in these local unitary gauges and find a clear signal of the dual Meissner effect. The Abelian electric field is found to be squeezed into a flux tube by the monopole supercurrent. This feature is the same as that observed in the non-local maximally Abelian gauge. These results suggest that the Abelian confinement scenario is gauge independent. Observing the important role of space-like monopoles in the Polyakov g…
Laurent series expansion of a class of massive scalar one-loop integrals toO(ε2)
2005
We use dimensional regularization to calculate the O({epsilon}{sup 2}) expansion of all scalar one-loop one-, two-, three-, and four-point integrals that are needed in the calculation of hadronic heavy quark production. The Laurent series up to O({epsilon}{sup 2}) is needed as input to that part of the next-to-next-to-leading order corrections to heavy flavor production at hadron colliders where the one-loop integrals appear in the loop-by-loop contributions. The four-point integrals are the most complicated. The O({epsilon}{sup 2}) expansion of the three- and four-point integrals contains in general polylogarithms up to Li{sub 4} and functions related to multiple polylogarithms of maximal …
The errant life of a heavy quark in the quark–gluon plasma
2011
In the high-temperature phase of QCD, the heavy-quark momentum diffusion constant determines, via a fluctuation–dissipation relation, how fast a heavy quark kinetically equilibrates. This transport coefficient can be extracted from thermal correlators via a Kubo formula. We present a lattice calculation of the relevant Euclidean correlators in the gluon plasma, based on a recent formulation of the problem in heavy-quark effective field theory (HQET). We find a ≈20% enhancement of the Euclidean correlator at maximal time separation as the temperature is lowered from 6Tc to 2Tc, pointing to stronger interactions at lower temperatures. At the same time, the correlator becomes flatter from 6Tc …
QCD thermodynamics with two flavours of Wilson fermions on large lattices
2012
We explore the phase diagram of two flavour QCD at vanishing chemical potential using dynamical O(a)-improved Wilson quarks. In the approach to the chiral limit we use lattices with a temporal extent of N_t=16 and spatial extent L=32,48 and 64 to enable the extrapolation to the thermodynamic limit with small discretisation effects. In addition to an update on the scans at constant \kappa, reported earlier, we present first results from scans along lines of constant physics at a pion mass of 290 MeV. We probe the transition using the Polyakov loop and the chiral condensate, as well as spectroscopic observables such as screening masses.
Polar angle dependence of the longitudinal polarization of quarks produced in e + e - -annihilation
1997
We calculate one-loop radiative QCD corrections to the three polarized and unpolarized structure functions that determine the beam-quark polar angle dependence of the longitudinal polarization of light and heavy quarks produced in e+e−-annihilations. We present analytical and numerical results for the longitudinal polarization and its polar angle dependence. We discuss in some detail the zero-mass limit of our results and the role of the anomalous spin-flip contributions to the polarization observables in the zero-mass limit. Our discussion includes transverse and longitudinal beam polarization effects.