30 research outputs found
Investigation of new methods for numerical stochastic perturbation theory in Ï4 theory
Numerical stochastic perturbation theory is a powerful tool for estimating
high-order perturbative expansions in lattice field theory. The standard
algorithms based on the Langevin equation, however, suffer from several
limitations which in practice restrict the potential of this technique. In this
work we investigate some alternative methods which could in principle improve
on the standard approach. In particular, we present a study of the recently
proposed Instantaneous Stochastic Perturbation Theory, as well as a formulation
of numerical stochastic perturbation theory based on Generalized Hybrid
Molecular Dynamics algorithms. The viability of these methods is investigated
in theory.Comment: 45 pages, 12 figures. Added new section on cost comparison with
Langevin NSPT. Matches published versio
Towards a new determination of the QCD Lambda parameter from running couplings in the three-flavour theory
We review our new strategy and current status towards a high precision
computation of the Lambda parameter from three-flavour simulations in QCD. To
reach this goal we combine specific advantages of the Schr\"odinger functional
and gradient flow couplings.Comment: 7 pages, 3 figures; Proceedings of the 32nd International Symposium
on Lattice Field Theory; 23-28 June, 2014, Columbia University, New Yor
The -parameter in 3-flavour QCD and by the ALPHA collaboration
We present results by the ALPHA collaboration for the -parameter in
3-flavour QCD and the strong coupling constant at the electroweak scale,
, in terms of hadronic quantities computed on the CLS gauge
configurations. The first part of this proceedings contribution contains a
review of published material \cite{Brida:2016flw,DallaBrida:2016kgh} and yields
the -parameter in units of a low energy scale, . We
then discuss how to determine this scale in physical units from experimental
data for the pion and kaon decay constants. We obtain MeV which translates to
using perturbation theory to match between 3-, 4- and 5-flavour QCD.Comment: 21 pages. Collects contributions of A. Ramos, S. Sint and R. Sommer
to the 34th annual International Symposium on Lattice Field Theory; LaTeX
input encoding problem fixe
Stochastic locality and master-field simulations of very large lattices
In lattice QCD and other field theories with a mass gap, the field variables
in distant regions of a physically large lattice are only weakly correlated.
Accurate stochastic estimates of the expectation values of local observables
may therefore be obtained from a single representative field. Such master-field
simulations potentially allow very large lattices to be simulated, but require
various conceptual and technical issues to be addressed. In this talk, an
introduction to the subject is provided and some encouraging results of
master-field simulations of the SU(3) gauge theory are reported.Comment: Talk given at the 35th International Symposium on Lattice Field
Theory, 18-24 June 2017, Granada, Spain; LaTeX source with 6 figure
Large-order NSPT for lattice gauge theories with fermions:the plaquette in massless QCD
Numerical Stochastic Perturbation Theory (NSPT) allows for perturbative
computations in quantum field theory. We present an implementation of NSPT that
yields results for high orders in the perturbative expansion of lattice gauge
theories coupled to fermions. The zero-momentum mode is removed by imposing
twisted boundary conditions; in turn, twisted boundary conditions require us to
introduce a smell degree of freedom in order to include fermions in the
fundamental representation. As a first application, we compute the critical
mass of two flavours of Wilson fermions up to order in a
gauge theory. We also implement, for the first time,
staggered fermions in NSPT. The residual chiral symmetry of staggered fermions
protects the theory from an additive mass renormalisation. We compute the
perturbative expansion of the plaquette with two flavours of massless staggered
fermions up to order in a gauge theory, and
investigate the renormalon behaviour of such series. We are able to subtract
the power divergence in the Operator Product Expansion (OPE) for the plaquette
and estimate the gluon condensate in massless QCD. Our results confirm that
NSPT provides a viable way to probe systematically the asymptotic behaviour of
perturbative series in QCD and, eventually, gauge theories with fermions in
higher representations.Comment: 49 pages, 28 figures. Revised version, to be published in EPJC. Some
references added, typos corrected, and improved discussion on finite-volume
effect
The chirally rotated Schrödinger functional: theoretical expectations and perturbative tests
SF near the electroweak scale
We employ the chirally rotated Schr"odinger functional (SF) to study
two-point fermion bilinear correlation functions used in the determination of
on a series of well-tuned ensembles. The gauge configurations,
which span renormalisation scales from 4 to 70~GeV, are generated with massless flavors and Schr"odinger Functional (SF) boundary conditions.
Valence quarks are computed with SF boundary conditions. We show
preliminary results on the tuning of the SF Symanzik coefficient
and the scaling of the axial current normalization . Moreover we
carry out a detailed comparison with the expectations from one-loop
perturbation theory. Finally we outline how automatically
-improved matrix elements, including BSM
contributions, can be computed in a SF renormalization scheme