5,726 research outputs found
Lattice QCD data versus Chiral Perturbation Theory: the case of
I present a selection of recent lattice data by major collaborations for the
pseudo-Goldstone boson masses in full () QCD, where the valence quarks
are chosen exactly degenerate with the sea quarks. At least the more chiral
points should be consistent with Chiral Perturbation Theory for the latter to
be useful in extrapolating to physical masses. Perspectives to reliably
determine NLO Gasser-Leutwyler coefficients are discussed.Comment: 3 pages, 4 figures, ICHEP 2002, v2: one statement clarified, one ref.
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Integrating out the heaviest quark in N--flavour ChPT
We extend a known method to integrate out the strange quark in three flavour
chiral perturbation theory to the context of an arbitrary number of flavours.
As an application, we present the explicit formulae to one--loop accuracy for
the heavy quark mass dependency of the low energy constants after decreasing
the number of flavours by one while integrating out the heaviest quark in
N--flavour chiral perturbation theory.Comment: 18 pages, 1 figure. Text and references added. To appear in EPJ
On the pion decay constant
The pion decay constant f_pi plays a crucial role in many areas of low energy
particle physics. Its value may e.g. be deduced from experimental data on
leptonic pion decays. Here, we provide comments on several aspects of this
evaluation. In particular, we point out that at the present level of
experimental accuracy, the value of f_pi is sensitive to the value of the pion
mass chosen in its chiral expansion.Comment: 15 pages, 3 figures. Accepted for publication in Phys. Lett.
Pion mass dependence of the semileptonic scalar form factor within finite volume
We calculate the scalar semileptonic kaon decay in finite volume at the
momentum transfer , using chiral perturbation
theory. At first we obtain the hadronic matrix element to be calculated in
finite volume. We then evaluate the finite size effects for two volumes with and and find that the difference between the finite
volume corrections of the two volumes are larger than the difference as quoted
in \cite{Boyle2007a}. It appears then that the pion masses used for the scalar
form factor in ChPT are large which result in large finite volume corrections.
If appropriate values for pion mass are used, we believe that the finite size
effects estimated in this paper can be useful for Lattice data to extrapolate
at large lattice size.Comment: 19 pages, 5 figures, accepted for publication in EPJ
Thermal reaction of Pt film with 110 GaN epilayer
Backscattering spectrometry, x-ray diffractometry, and scanning electron microscopy have been used to study the reaction of a thin Pt film with an epilayer of [110] GaN on [110] sapphire upon annealing at 450, 550, 650, 750, and 800 degrees C for 30 min. A Ga concentration of 2 at. % is detected by MeV He-4(++) backscattering spectrometry in the Pt layer at 550 degrees C. By x-ray diffraction, structural changes are observed already at 450 degrees C. At 650 OC, textured Ga2Pt appears as reaction product. The surface morphology exhibits instabilities by the formation of blisters at 650 degrees C and voids at 800 degrees C
Solitons and kinks in a general car-following model
We study a car-following model of traffic flow which assumes only that a
car's acceleration depends on its own speed, the headway ahead of it, and the
rate of change of headway, with only minimal assumptions about the functional
form of that dependence. The velocity of uniform steady flow is found
implicitly from the acceleration function, and its linear stability criterion
can be expressed simply in terms of it. Crucially, unlike in previously
analyzed car-following models, the threshold of absolute stability does not
generally coincide with an inflection point in the steady velocity function.
The Burgers and KdV equations can be derived under the usual assumptions, but
the mKdV equation arises only when absolute stability does coincide with an
inflection point. Otherwise, the KdV equation applies near absolute stability,
while near the inflection point one obtains the mKdV equation plus an extra,
quadratic term. Corrections to the KdV equation "select" a single member of the
one-parameter set of soliton solutions. In previous models this has always
marked the threshold of a finite- amplitude instability of steady flow, but
here it can alternatively be a stable, small-amplitude jam. That is, there can
be a forward bifurcation from steady flow. The new, augmented mKdV equation
which holds near an inflection point admits a continuous family of kink
solutions, like the mKdV equation, and we derive the selection criterion
arising from the corrections to this equation.Comment: 25 page
Isospin Violation in Chiral Perturbation Theory and the Decays \eta \ra \pi \ell \nu and \tau \ra \eta \pi \nu
I discuss isospin breaking effects within the standard model. Chiral
perturbation theory presents the appropriate theoretical framework for such an
investigation in the low--energy range. Recent results on the electromagnetic
contributions to the masses of the pseudoscalar mesons and the
amplitudes are reported. Using the one--loop formulae for the
form factors, rather precise predictions for the decay rates of can be obtained. Finally, I present an estimate of
the \tau \ra \eta \pi \nu branching ratio derived from the dominant meson
resonance contributions to this decay.Comment: 10 pages, latex, one figure available upon reques
The role of strange sea quarks in chiral extrapolations on the lattice
Since the strange quark has a light mass of order Lambda_QCD, fluctuations of
sea s-s bar pairs may play a special role in the low-energy dynamics of QCD by
inducing significantly different patterns of chiral symmetry breaking in the
chiral limits N_f=2 (m_u=m_d=0, m_s physical) and N_f=3 (m_u=m_d=m_s=0). This
effect of vacuum fluctuations of s-s bar pairs is related to the violation of
the Zweig rule in the scalar sector, described through the two O(p^4)
low-energy constants L_4 and L_6 of the three-flavour strong chiral lagrangian.
In the case of significant vacuum fluctuations, three-flavour chiral expansions
might exhibit a numerical competition between leading- and
next-to-leading-order terms according to the chiral counting, and chiral
extrapolations should be handled with a special care. We investigate the impact
of the fluctuations of s-s bar pairs on chiral extrapolations in the case of
lattice simulations with three dynamical flavours in the isospin limit.
Information on the size of the vacuum fluctuations can be obtained from the
dependence of the masses and decay constants of pions and kaons on the light
quark masses. Even in the case of large fluctuations, corrections due to the
finite size of spatial dimensions can be kept under control for large enough
boxes (L around 2.5 fm).Comment: 31 pages, 9 figures. A few comments added and typos correcte
Chiral Perturbation Theory for the Quenched Approximation of QCD
[This version is a minor revision of a previously submitted preprint. Only
references have been changed.] We describe a technique for constructing the
effective chiral theory for quenched QCD. The effective theory which results is
a lagrangian one, with a graded symmetry group which mixes Goldstone bosons and
fermions, and with a definite (though slightly peculiar) set of Feynman rules.
The straightforward application of these rules gives automatic cancellation of
diagrams which would arise from virtual quark loops. The techniques are used to
calculate chiral logarithms in , , , and the ratio of
to . The leading
finite-volume corrections to these quantities are also computed. Problems for
future study are described.Comment: 14 page
Watson's theorem and electromagnetism in K -> pi pi decay
We consider what constraints unitarity and CPT invariance yield on the strong
and electromagnetic phases entering K -> pi pi decay. In particular, we show
that the relative size of the electromagnetically-induced changes in the I=0
and I=2 phase shifts in the two--pion final state do not depend on the explicit
coupling to the pi^+ pi^- gamma channel. This demonstrates that Watson's
theorem can be extended to include the presence of electromagnetism. We point
out the consequences for the general structure of the K -> pi pi decay
amplitudes in the presence of isospin violation.Comment: 7 pages, LaTeX, axodraw.st
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