937 research outputs found
Exploring the phase structure of lattice QCD with twisted mass quarks
The phase structure of zero temperature twisted mass lattice QCD is
investigated. We find strong metastabilities in the plaquette observable when
the untwisted quark mass sweeps across zero.Comment: Talks presented at Lattice2004(spectrum), 6 pages, 6 figure
Renormalisation of quark propagators from twisted-mass lattice QCD at =2
We present results concerning the non-perturbative evaluation of the
renormalisation constant for the quark field, , from lattice simulations
with twisted mass quarks and three values of the lattice spacing. We use the
RI'-MOM scheme. has very large lattice spacing artefacts; it is
considered here as a test bed to elaborate accurate methods which will be used
for other renormalisation constants. We recall and develop the non-perturbative
correction methods and propose tools to test the quality of the correction.
These tests are also applied to the perturbative correction method. We check
that the lattice spacing artefacts scale indeed as . We then study the
running of with particular attention to the non-perturbative effects,
presumably dominated by the dimension-two gluon condensate \VEV{A^2} in
Landau gauge. We show indeed that this effect is present, and not small. We
check its scaling in physical units confirming that it is a continuum effect.
It gives a contribution at 2 GeV. Different variants are used in
order to test the reliability of our result and estimate the systematic
uncertainties. Finally combining all our results and using the known Wilson
coefficient of \VEV{A^2} we find g^2(\mu^2) \VEV{A^2}_{\mu^2\; CM} =
2.01(11)(^{+0.61}_{- 0.73}) \;\mathrm {GeV}^2 at , in
fair agreement within uncertainties with the value indepently extracted from
the strong coupling constant.Comment: 38 pages, 8 tables, 8 figure
Twisted mass fermions: neutral pion masses from disconnected contributions
Twisted mass fermions allow light quarks to be explored but with the
consequence that there are mass splittings, such as between the neutral and
charged pion. Using a direct calculation of the connected neutral pion
correlator and stochastic methods to evaluate the disconnected correlations, we
determine the neutral pion mass. We explore the dependence on lattice spacing
and quark mass in quenched QCD. For dynamical QCD, we determine the sign of the
splitting which is linked, via chiral PT, to the nature of the phase transition
at small quark mass.Comment: 6 pages, poster (hadron spectrum and quark masses) at Lattice
2005,Dublin, July 25-3
Lattice Spacing Dependence of the First Order Phase Transition for Dynamical Twisted Mass Fermions
Lattice QCD with Wilson fermions generically shows the phenomenon of a first
order phase transition. We study the phase structure of lattice QCD using
Wilson twisted mass fermions and the Wilson plaquette gauge action are used in
a range of beta values where such a first order phase transition is observed.
In particular, we investigate the dependence of the first order phase
transition on the value of the lattice spacing. Using only data in one phase
and neglecting possible problems arising from the phase transition we are able
to perform a first scaling test for physical quantities using this action.Comment: 15 pages, 7 figures, typo corrected, web-list of authors correcte
O(a^2) cutoff effects in lattice Wilson fermion simulations
In this paper we propose to interpret the large discretization artifacts
affecting the neutral pion mass in maximally twisted lattice QCD simulations as
O(a^2) effects whose magnitude is roughly proportional to the modulus square of
the (continuum) matrix element of the pseudoscalar density operator between
vacuum and one-pion state. The numerical size of this quantity is determined by
the dynamical mechanism of spontaneous chiral symmetry breaking and turns out
to be substantially larger than its natural magnitude set by the value of
Lambda_QCD.Comment: 38 pages, 1 figure, 2 table
f_B and f_Bs with maximally twisted Wilson fermions
We present a lattice QCD calculation of the heavy-light decay constants f_B
and f_Bs performed with Nf=2 maximally twisted Wilson fermions, at four values
of the lattice spacing. The decay constants have been also computed in the
static limit and the results are used to interpolate the observables between
the charm and the infinite-mass sectors, thus obtaining the value of the decay
constants at the physical b quark mass. Our preliminary results are f_B=191(14)
MeV, f_Bs=243(14) MeV, f_Bs/f_B=1.27(5). They are in good agreement with those
obtained with a novel approach, recently proposed by our Collaboration (ETMC),
based on the use of suitable ratios having an exactly known static limit.Comment: Proceedings of the 27th International Symposium on Lattice Field
Theory (Lattice 2009), Beijing, China, 2009 July 26-31. 8 pages, 3 figure
A first look at maximally twisted mass lattice QCD calculations at the physical point
In this contribution, a first look at simulations using maximally twisted
mass Wilson fermions at the physical point is presented. A lattice action
including clover and twisted mass terms is presented and the Monte Carlo
histories of one run with two mass-degenerate flavours at a single lattice
spacing are shown. Measurements from the light and heavy-light pseudoscalar
sectors are compared to previous results and their phenomenological
values. Finally, the strategy for extending simulations to is
outlined.Comment: presented at the 31st International Symposium on Lattice Field Theory
(Lattice 2013), 29 July - 3 August 2013, Mainz, German
First Physics Results at the Physical Pion Mass from Wilson Twisted Mass Fermions at Maximal Twist
We present physics results from simulations of QCD using dynamical
Wilson twisted mass fermions at the physical value of the pion mass. These
simulations were enabled by the addition of the clover term to the twisted mass
quark action. We show evidence that compared to previous simulations without
this term, the pion mass splitting due to isospin breaking is almost completely
eliminated. Using this new action, we compute the masses and decay constants of
pseudoscalar mesons involving the dynamical up and down as well as valence
strange and charm quarks at one value of the lattice spacing,
fm. Further, we determine renormalized quark masses as well as their
scale-independent ratios, in excellent agreement with other lattice
determinations in the continuum limit. In the baryon sector, we show that the
nucleon mass is compatible with its physical value and that the masses of the
baryons do not show any sign of isospin breaking. Finally, we compute
the electron, muon and tau lepton anomalous magnetic moments and show the
results to be consistent with extrapolations of older ETMC data to the
continuum and physical pion mass limits. We mostly find remarkably good
agreement with phenomenology, even though we cannot take the continuum and
thermodynamic limits.Comment: 45 pages, 15 figure
High-Pressure Amorphous Nitrogen
The phase diagram and stability limits of diatomic solid nitrogen have been
explored in a wide pressure--temperature range by several optical spectroscopic
techniques. A newly characterized narrow-gap semiconducting phase has
been found to exist in a range of 80--270 GPa and 10--510 K. The vibrational
and optical properties of the phase produced under these conditions
indicate that it is largely amorphous and back transforms to a new molecular
phase. The band gap of the phase is found to decrease with pressure
indicating possible metallization by band overlap above 280 GPa.Comment: 5 pages, 4 figure
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