5,421 research outputs found
How To Fix Non-Perturbatively A Parameter Dependent Covariant Gauge On The Lattice
We describe how to overcome some problems that usually prevent from obtaining
an efficient algorithm to fix a generic covariant gauge on the lattice. This
gauge is the lattice equivalent of the generic gauge usually adopted in
perturbative calculations. It depends on a parameter whose value can be varied
in order to check the gauge dependence of measured matrix elements.Comment: 3 pages, 2 eps figures, LATTICE 9
Quark and Gluon Propagators in Covariant Gauges
We present data for the gluon and quark propagators computed in the standard
lattice Landau's gauge and for three values of the covariant gauge-fixing
parameter lambda=0,8,16. Our results are obtained using the SU(3) Wilson action
in the quenched approximation at beta=6.0 and volume=16^3x32.Comment: Lattice2001(theorydevelop
Numerical Exploration of the RI/MOM Scheme Gauge Dependence
The gauge dependence of some fermion bilinear RI/MOM renormalization
constants is studied by comparing data which have been gauge-fixed in two
different realizations of the Landau gauge and in a generic covariant gauge.
The very good agreement between the various sets of results and the theory
indicates that the numerical uncertainty induced by the lattice gauge-fixing
procedure is below the statistical errors of our data sample which is of the
order of (1-1.5)%.Comment: 3 pages, 2 figures, Lattice2002(theoretical
Electromagnetic proton-neutron knockout off 16O: new achievements in theory
Results for the cross sections of the exclusive 16O(e,e'pn)14N and
16O(gamma,pn)14N knockout reactions are presented and discussed in different
kinematics. In comparison with earlier work, a complete treatment of the
center-of-mass (CM) effects in the nuclear one-body current is considered in
connection with the problem of the lack of orthogonality between initial bound
and final scattering states. The effects due to CM and orthogonalization are
investigated in combination with different treatments of correlations in the
two-nucleon overlap function and for different parametrizations of the two-body
currents. The CM effects lead in super-parallel kinematics to a dramatic
increase of the 16O(e,e'pn) cross section to the 1_2^+ excited state (3.95 MeV)
of 14N. In all the situations considered the results are very sensitive to the
treatment of correlations. A crucial role is played by tensor correlations, but
also the contribution of long-range correlations is important.Comment: 13 pages, 10 figure
On the Definition of Gauge Field Operators in Lattice Gauge-Fixed Theories
We address the problem of defining the gauge four-potential on the lattice,
in terms of the natural link variables. Different regularized definitions are
shown, through non perturbative numerical computation, to converge towards the
same continuum renormalized limit.Comment: 8 pages, LaTeX2e/LaTeX209, 3 eps figure
Charge-Exchange and multi-scattering effects in (e,e'n) knockout
Final-state interactions in (e,e'n) knockout reactions in the quasi-free
region are studied by considering the multistep direct scattering of the
ejectile nucleon. Primary and multiple particle emission are included within
the same model and are found to become important with increasing excitation
energy. Charge-exchange effects taken into account through the two-step
(e,e'p)(p,n) and three-step (e,e'p)(p,N)(N,n) processes are also found to
increase with energy. A comparison with the results obtained with an
isospin-dependent optical potential at small excitation energies is presented.Comment: 12 pages, 4 Postscript figures. A new section on multiple particle
emission added together with 2 new figures including primary and multiple
emission cross section
NN final-state interaction in two-nucleon knockout from
The influence of the mutual interaction between the two outgoing nucleons
(NN-FSI) in electro- and photoinduced two-nucleon knockout from has
been investigated perturbatively. It turns out that the effect of NN-FSI
depends on the kinematics and on the type of reaction considered. The effect is
generally larger in pp- than in pn-knockout and in electron induced than in
photoinduced reactions.
In superparallel kinematics NN-FSI leads in the channel to a
strong increase of the cross section, that is mainly due to a strong
enhancement of the -current contribution. In pn-emission, however, this
effect is partially cancelled by a destructive interference with the seagull
current. For photoreactions NN-FSI is considerably reduced in superparallel
kinematics and can be practically negligible in specific kinematics.Comment: 16 pages, 9 figure
The light-quark contribution to the leading HVP term of the muon from twisted-mass fermions
We present a lattice calculation of the leading Hadronic Vacuum Polarization
(HVP) contribution of the light u- and d-quarks to the anomalous magnetic
moment of the muon, , adopting the gauge configurations
generated by the European Twisted Mass Collaboration with
dynamical quarks at three values of the lattice spacing with pion masses in the
range 210 - 450 MeV. Thanks to several lattices at fixed values of the
light-quark mass and scale but with different sizes we perform a careful
investigation of finite-volume effects (FVEs). In order to remove FVEs we
develop an analytic representation of the vector correlator, which describes
the lattice data for time distances larger than fm. The
representation is based on quark-hadron duality at small and intermediate time
distances and on the two-pion contributions in a finite box at larger time
distances. After extrapolation to the physical pion point and to the continuum
limit we obtain . Adding the
contribution of strange and charm quarks, obtained by ETMC, and an estimate of
the isospin-breaking corrections and quark-disconnected diagrams from the
literature we get , which is
consistent with recent results based on dispersive analyses of the experimental
cross section data for annihilation into hadrons. Using our analytic
representation of the vector correlator, taken at the physical pion mass in the
continuum and infinite volume limits, we provide the first eleven moments of
the polarization function and we compare them with recent results of the
dispersive analysis of the channels. We estimate also the
light-quark contribution to the missing part of not covered
in the MUonE experiment.Comment: 34 pages, 20 figures, 7 tables. Version to appear in PR
Proton recoil polarization in exclusive (e,e'pp) reactions
The general formalism of nucleon recoil polarization in the () reaction is given. Numerical predictions are presented for the
components of the outgoing proton polarization and of the polarization transfer
coefficient in the specific case of the exclusive O()C knockout reaction leading to discrete states in the residual
nucleus. Reaction calculations are performed in a direct knockout framework
where final-state interactions and one-body and two-body currents are included.
The two-nucleon overlap integrals are obtained from a calculation of the
two-proton spectral function of O where long-range and short-range
correlations are consistently included. The comparison of results obtained in
different kinematics confirms that resolution of different final states in the
O()C reaction may act as a filter to
disentangle and separately investigate the reaction processes due to
short-range correlations and two-body currents and indicates that measurements
of the components of the outgoing proton polarization may offer good
opportunities to study short-range correlations.Comment: 12 pages, 6 figure
- …