292 research outputs found
Locality with staggered fermions
We address the locality problem arising in simulations, which take the square
root of the staggered fermion determinant as a Boltzmann weight to reduce the
number of dynamical quark tastes. A definition of such a theory necessitates an
underlying local fermion operator with the same determinant and the
corresponding Green's functions to establish causality and unitarity. We
illustrate this point by studying analytically and numerically the square root
of the staggered fermion operator. Although it has the correct weight, this
operator is non-local in the continuum limit. Our work serves as a warning that
fundamental properties of field theories might be violated when employing
blindly the square root trick. The question, whether a local operator
reproducing the square root of the staggered fermion determinant exists, is
left open.Comment: 24 pages, 7 figures, few remarks added for clarity, accepted for
publication in Nucl. Phys.
The locality problem for two tastes of staggered fermions
We address the locality problem arising in simulations, which take the square
root of the staggered fermion determinant as a Boltzmann weight to reduce the
number of dynamical quark tastes from four to two. We study analytically and
numerically the square root of the staggered fermion operator as a candidate to
define a two taste theory from first principles. Although it has the correct
weight, this operator is non-local in the continuum limit. Our work serves as a
warning that fundamental properties of field theories might be violated when
employing blindly the square root trick. The question, whether a local operator
reproducing the square root of the staggered fermion determinant exists, is
left open.Comment: Talk presented at Lattice2004(theory), Fermilab, June 21-26, 200
Non-perturbative renormalization of quark mass in Nf=2+1 QCD with the Schroedinger functional scheme
We present an evaluation of the quark mass renormalization factor for Nf=2+1
QCD. The Schroedinger functional scheme is employed as the intermediate scheme
to carry out non-perturbative running from the low energy region, where
renormalization of bare mass is performed on the lattice, to deep in the high
energy perturbative region, where the conversion to the renormalization group
invariant mass or the MS-bar scheme is safely carried out. For numerical
simulations we adopted the Iwasaki gauge action and non-perturbatively improved
Wilson fermion action with the clover term. Seven renormalization scales are
used to cover from low to high energy regions and three lattice spacings to
take the continuum limit at each scale. The regularization independent step
scaling function of the quark mass for the Nf=2+1 QCD is obtained in the
continuum limit. Renormalization factors for the pseudo scalar density and the
axial vector current are also evaluated for the same action and the bare
couplings as two recent large scale Nf=2+1 simulations; previous work of the
CP-PACS/JLQCD collaboration, which covered the up-down quark mass range heavier
than MeV and that of PACS-CS collaboration for much lighter
quark masses down to MeV. The quark mass renormalization factor is
used to renormalize bare PCAC masses in these simulations.Comment: 26 pages, 17 Postscript figures. Two tables are update
Non-perturbative renormalization of the static axial current in two-flavour QCD
We perform the non-perturbative renormalization of matrix elements of the
static-light axial current by a computation of its scale dependence in lattice
QCD with two flavours of massless O(a) improved Wilson quarks. The
regularization independent factor that relates any running renormalized matrix
element of the axial current in the static effective theory to the
renormalization group invariant one is evaluated in the Schroedinger functional
scheme, where in this case we find a significant deviation of the
non-perturbative running from the perturbative prediction. An important
technical ingredient to improve the precision of the results consists in the
use of modified discretizations of the static quark action introduced earlier
by our collaboration. As an illustration how to apply the renormalization of
the static axial current presented here, we connect the bare matrix element of
the current to the B_s-meson decay constant in the static approximation for one
value of the lattice spacing, a ~ 0.08 fm, employing large-volume N_f=2 data at
beta=5.3.Comment: 33 pages including figures and tables, latex2e, uses JHEP3.cls;
version published in JHEP, small additions, results unchange
Relative Permeability Determination for Steam Injection Processes: An Analytical Approach
Due to the nature of bitumen and the inability to flow at reservoir conditions, it becomes necessary to alter the original state by imparting heat into the reservoir, thus lowering the viscosity to produce the fluid. The heat introduction is achieved through the utilization of steam assisted gravity drainage (SAGD) as well as solvent steam assisted gravity drainage (S-SAGD). However, the presence of three distinct fluid phases (oil, steam, and water) in the reservoir during steam injection has implications for the effective modeling of the complex fluid dynamics. Inherent complexity as a result of introducing a third phase (steam) in conjunction with temperature dependencies results in the inability to generate a comprehensive three phase relative permeability capable of adequately representing flow in the reservoir at all domains.
In this study, experimental oil recovery data for two SAGD experiments, three S-SAGD experiments and one hot water flood are empirically modelled by manipulating relative permeabilities. Addition of a solvent during the steam injection (S-SAGD) takes advantage of a miscibility component with the bitumen in conjunction with the thermal mechanism of enhanced oil recovery which results in incremental recovery of bitumen along with a minimization of the environmental footprint. The two SAGD experiments are differentiated by different types of clay content while the three S-SAGD experiments vary due to the different solvent and injection strategy utilized during the experiment. The analytical approach implemented allows for the representation of fluid flow in the reservoir by achieving a pseudo-two phase relative permeability that results in comparable performance to the experiments. The assumption of a waterflood through the application of fractional flow theory allowed for the negation of the steam phase in the model and so two-phase flow was established. The sensitivity of the relative permeability curves to temperature change results in the inability to formulate a generic three-phase curve and so the pseudo-two phase curve is valuable for the analysis of the relative permeability behavior for different SAGD scenarios
Spectrum of quenched twisted mass lattice QCD at maximal twist
Hadron masses are computed from quenched twisted mass lattice QCD for a
degenerate doublet of up and down quarks with the twist angle set to pi/2,
since this maximally twisted theory is expected to be free of linear
discretization errors. Two separate definitions of the twist angle are used,
and the hadron masses for these two cases are compared. The flavor breaking,
that can arise due to twisting, is discussed in the context of mass splittings
within the Delta(1232) multiplet.Comment: 23 pages, 16 figures, added discussion of pion decay constan
HQET at order : II. Spectroscopy in the quenched approximation
Using Heavy Quark Effective Theory with non-perturbatively determined
parameters in a quenched lattice calculation, we evaluate the splittings
between the ground state and the first two radially excited states of the
system at static order. We also determine the splitting between first excited
and ground state, and between the and ground states to order
. The Generalized Eigenvalue Problem and the use of all-to-all
propagators are important ingredients of our approach.Comment: (1+18) pages, 3 figures (4 pdf files); pdflatex; v2: corrections to
table 1, results unaffecte
Light quarks with twisted mass fermions
We investigate Wilson twisted mass fermions in the quenched approximation
using different definitions of the critical bare quark mass m_c to realize
maximal twist and, correspondingly, automatic O(a) improvement for physical
observables. A particular definition of m_c is given by extrapolating the value
of m_c obtained from the PCAC relation at non-vanishing bare twisted quark mass
mu to mu=0. Employing this improved definition of the critical mass the Wilson
twisted mass formulation provides the possibility to perform reliable
simulations down to very small quark masses with correspondingly small pion
masses of m_pi \simeq 250 MeV, while keeping the cutoff effects of O(a^2) under
control.Comment: 10 pages, 8 figures, v2: version accepted for publication in PLB,
minor correction
Hadronic Vacuum Polarization Contribution to g-2 from the Lattice
We give a short description of the present situation of lattice QCD
simulations. We then focus on the computation of the anomalous magnetic moment
of the muon using lattice techniques. We demonstrate that by employing improved
observables for the muon anomalous magnetic moment, a significant reduction of
the lattice error can be obtained. This provides a promising scenario that the
accuracy of lattice calculations can match the experimental errors.Comment: Proceedings of the International Workshop on e+e- collisions from Phi
to Psi (PHIPSI11), Sep. 19-22, 2011, BINP, Novosibirsk, Russi
Spatial correlators in strongly coupled plasmas
We numerically calculate the spatial correlators of the scalar and
pseudoscalar operators and , in SU(3) Yang-Mills theory at
zero and finite-temperature on the lattice. We compare the results over the
distances to the free-field prediction, to the
operator-product expansion as well as to the strongly coupled large-
\sN=4 super-Yang-Mills theory, where results are obtained by AdS/CFT methods.
For , both channels exhibit stronger spatial correlations than
in the vacuum, and we give an explanation for this, using sum-rules and the
operator-product expansion. The AdS/CFT calculation provides a
semi-quantitatively successful description of the vacuum-subtracted
correlator, renormalized in the 3-loop scheme, in the
interval of temperatures , while the free-field prediction has
the wrong sign. The and correlators are predicted to have the
same functional form both at weak coupling and in the strongly coupled SYM
theory. The Yang-Mills plasma does not meet that expectation below .
Instead we find that strong fluctuations of are present at least up
to that temperature. We discuss the impact of our results on our understanding
of the quark-gluon plasma.Comment: 32 pages, 9 figures, 4 tables; added some references, more detailed
captions, conclusions unchange
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