150 research outputs found
A new determination of from Renormalization Group Optimized Perturbation
A new version of the so-called optimized perturbation (OPT), implementing
consistently renormalization group properties, is used to calculate the
nonperturbative ratio of the pion decay constant and
the basic QCD scale in the scheme. Using the experimental
input value it provides a new determination of for
and , and of the QCD coupling constant at
various scales once combined with a standard perturbative evolution. The
stability and empirical convergence properties of the RGOPT modified series is
demonstrated up to the third order. We examine the difference sources of
theoretical uncertainties and obtain , where the first errors are
estimates of the intrinsic theoretical uncertainties of our method, and the
second errors come from present uncertainties in , where is
in the exact chiral limit.Comment: 5 pages, talk given at EPS-HEP, Stockholm, Sweden 18-24 July, 201
Renormalization Group Improved Optimized Perturbation Theory: Revisiting the Mass Gap of the O(2N) Gross-Neveu Model
We introduce an extension of a variationally optimized perturbation method,
by combining it with renormalization group properties in a straightforward
(perturbative) form. This leads to a very transparent and efficient procedure,
with a clear improvement of the non-perturbative results with respect to
previous similar variational approaches. This is illustrated here by deriving
optimized results for the mass gap of the O(2N) Gross-Neveu model, compared
with the exactly know results for arbitrary N. At large N, the exact result is
reproduced already at the very first order of the modified perturbation using
this procedure. For arbitrary values of N, using the original perturbative
information only known at two-loop order, we obtain a controllable percent
accuracy or less, for any N value, as compared with the exactly known result
for the mass gap from the thermodynamical Bethe Ansatz. The procedure is very
general and can be extended straightforwardly to any renormalizable Lagrangian
model, being systematically improvable provided that a knowledge of enough
perturbative orders of the relevant quantities is available.Comment: 18 pages, 1 figure, v2: Eq. (4.5) corrected, comments adde
Optimized-Variational Quark Mass Expansion and Dynamical Chiral Symmetry Breakdown
A recently proposed variational mass expansion approach to dynamical chiral
symmetry breakdown is reviewed. We briefly explain how a specific integral
ansatz over the analytically continued, arbitrary Lagrangian mass parameter,
resums the usual variational mass expansion (``delta-expansion".) The
construction is then generalized to obtain non-perturbative expressions for the
order parameters of the breakdown ( 2 or
3), in QCD. Emphasis is put on some general aspects as well as possible
limitations of this approach.Comment: 5 pages, LaTex, using stwol.sty, no figures. Talk given at the 28th
ICHEP, 25-31 July 1996, Warsaw, Poland, to appear in the proceeding
Two Higgs doublets to explain the excesses and
The two Higgs doublet model emerges as a minimal scenario in which to
address, at the same time, the excess at 750 GeV and the lepton
flavour violating decay into of the 125 GeV Higgs boson.
The price to pay is additional matter to enhance the rate, and a
peculiar pattern for the lepton Yukawa couplings. We add TeV scale vector-like
fermions and find parameter space consistent with both excesses, as well as
with Higgs and electroweak precision observables.Comment: 15 pages, 1 figure; v2: discussion of tau-->mu gamma added, leading
to an additional constraint. v3: references added, figure 1 recovered and
figure 2 adde
- …