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 $SU(n_f)_L \times SU(n_f)_R$ breakdown ($n_f =$ 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 pp→γγ(750 GeV)pp\rightarrow \gamma\gamma(750\ {\rm GeV}) and h→τ±μ∓h \to \tau^\pm \mu^\mp

The two Higgs doublet model emerges as a minimal scenario in which to address, at the same time, the $\gamma\gamma$ excess at 750 GeV and the lepton flavour violating decay into $\tau^\pm \mu^\mp$ of the 125 GeV Higgs boson. The price to pay is additional matter to enhance the $\gamma\gamma$ 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