521 research outputs found
On the spectral density of the Wilson operator
We summarize our recent determination [1] of the spectral density of the
Wilson operator in the p-regime of Wilson chiral perturbation theory. We
discuss the range of validity of our formula and a possible extension to our
computation in order to better understand the behaviour of the spectral density
in a finite volume close to the threshold.Comment: 7 pages, 2 figures, Proceedings for the XXIX International Symposium
on Lattice Field Theor
Corrections to the Banks-Casher relation with Wilson quarks
The Banks-Casher relation links the spectral density of the Dirac operator
with the existence of a chiral condensate and spontaneous breaking of chiral
symmetry. This relation receives corrections from a finite value of the quark
mass, a finite space-time volume and, if evaluated on a discrete lattice, from
the finite value of the lattice spacing a. We present a status report of a
determination of these corrections for Wilson quarks.Comment: 6 pages, 4 figures, Chiral Dynamics 2012 proceedin
The epsilon regime with twisted mass Wilson fermions
We investigate the leading lattice spacing effects in mesonic two-point
correlators computed with twisted mass Wilson fermions in the epsilon-regime.
By generalizing the procedure already introduced for the untwisted Wilson
chiral effective theory, we extend the continuum chiral epsilon expansion to
twisted mass WChPT. We define different regimes, depending on the relative
power counting for the quark masses and the lattice spacing. We explicitly
compute, for arbitrary twist angle, the leading O(a^2) corrections appearing at
NLO in the so-called GSM^* regime. As in untwisted WChPT, we find that in this
situation the impact of explicit chiral symmetry breaking due to lattice
artefacts is strongly suppressed. Of particular interest is the case of maximal
twist, which corresponds to the setup usually adopted in lattice simulations
with twisted mass Wilson fermions. The formulae we obtain can be matched to
lattice data to extract physical low energy couplings, and to estimate
systematic uncertainties coming from discretization errors.Comment: 26 pages, 3 figure
Beyond-the-Standard-Model matrix elements with the gradient flow
At the Forschungszentrum Juelich (FZJ) we have started a long-term program
that aims to determine beyond-the-Standard-Model (BSM) matrix elements using
the gradient flow, and to understand the impact of BSM physics in nucleon and
nuclear observables. Using the gradient flow, we propose to calculate the QCD
component of key beyond the Standard Model (BSM) matrix elements related to
quark and strong theta CP violation and the strange content within the nucleon.
The former set of matrix elements impacts our understanding of Electric Dipole
Moments (EDMs) of nucleons and nuclei (a key signature of BSM physics), while
the latter contributes to elastic recoil of Dark Matter particles off nucleons
and nuclei. If successful, these results will lay the foundation for extraction
of BSM observables from future low-energy, high-intensity and high-accuracy
experimental measurements.Comment: 7 pages, 2 figures, presented at the 32nd International Symposium on
Lattice Field Theory (Lattice 2014). Correct version of proceedings.
Different wording of few paragraphs and different notation on few formulas.
Added 1 referenc
HMC algorithm with multiple time scale integration and mass preconditioning
We describe a new HMC algorithm variant we have recently introduced and
extend the published results by preliminary results of a simulation with a
pseudo scalar mass value of about 300 MeV. This new run confirms our
expectation that simulations with such pseudo scalar mass values become
feasible and affordable with our HMC variant. In addition we discuss
simulations from hot and cold starts at a pseudo scalar mass value of about 300
MeV, which we performed in order to test for possible meta-stabilities.Comment: 6 pages, Talk presented at Lattice 2005 (machines and algorithms
Renormalization constants of local operators within the Schr\"odinger functional scheme
We define, within the Schr\"odinger functional (SF) scheme, the matrix
elements of the twist-2 operators corresponding to the first two moments of
non-singlet parton density, and the first moment of singlet parton densities.
We perform a lattice one-loop calculation that fixes the relation between the
SF scheme and other common schemes and shows the main source of lattice
artefacts. Few remarks on the improvement case are added.Comment: Presented at LATTICE99, 3 page
Scaling test of quenched Wilson twisted mass QCD at maximal twist
We present the results of an extended scaling test of quenched Wilson twisted
mass QCD. We fix the twist angle by using two definitions of the critical mass,
the first obtained by requiring the vanishing of the pseudoscalar meson mass
m_PS for standard Wilson fermions and the second by requiring restoration of
parity at non-zero value of the twisted mass mu and subsequently extrapolating
to mu=0. Depending on the choice of the critical mass we simulate at values of
beta in [5.7,6.45], for a range of pseudoscalar meson masses 250 MeV < m_PS < 1
GeV and we perform the continuum limit for the pseudoscalar meson decay
constant f_PS and various hadron masses (vector meson m_V, baryon octet m_oct
and baryon decuplet m_dec) at fixed value of r_0 m_PS. For both definitions of
the critical mass, lattice artifacts are consistent with O(a) improvement.
However, with the second definition, large O(a^2) discretization errors present
at small quark mass with the first definition are strongly suppressed. The
results in the continuum limit are in very good agreement with those from the
Alpha and CP-PACS Collaborations.Comment: 6 pages, Talk presented at Lattice 2005, Dublin, 25-30 July 200
Spectral density of the Hermitean Wilson Dirac operator: a NLO computation in chiral perturbation theory
We compute the lattice spacing corrections to the spectral density of the
Hermitean Wilson Dirac operator using Wilson Chiral Perturbation Theory at NLO.
We consider a regime where the quark mass and the lattice spacing obey
the relative power counting : in this situation
discretisation effects can be treated as perturbation of the continuum
behaviour. While this framework fails to describe lattice spectral density
close to the threshold, it allows nevertheless to investigate important
properties of the spectrum of the Wilson Dirac operator. We discuss the range
of validity of our results and the possible implications in understanding the
phase diagram of Wilson fermions.Comment: 27 pages, 4 figures; few typos corrected, added footnote, published
versio
Expansion coefficient of the pseudo-scalar density using the gradient flow in lattice QCD
We use the Yang-Mills gradient flow to calculate the pseudo-scalar expansion
coefficient . This quantity is a key ingredient to obtaining the
chiral condensate and strange quark content of the nucleon using the Lattice
QCD formulation, which can ultimately determine the spin independent (SI)
elastic cross section of dark matter models involving WIMP-nucleon
interactions. The goal, using the gradient flow, is to renormalize the chiral
condensate and the strange content of the nucleon without a power divergent
subtraction. Using Chiral symmetry and the small flow time expansion of the
gradient flow, the scalar density at zero flow time can be related to the
pseudo-scalar density at non zero flow time. By computing the flow time
dependance of the pseudo-scalar density over multiple lattices box sizes,
lattice spacings and pion masses, we can obtain the scalar density of the
nucleon. Our lattice ensembles are , PCAC-CS gauge field
configurations, varying over ~MeV at
~fm, with additional ensembles that vary ~fm at ~MeV
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