Electroweak Sudakov form factors and nonfactorizable soft QED effects at NLC energies

We study the leading log infrared behavior of electroweak corrections at TeV scale energies, that will be reached by next generation of linear colliders (NLC). We show that, contrary to what happens at typical LEP energies, it is not anymore possible to disentangle ``pure electroweak'' from ``photonic'' corrections. This means that soft QED effects do not factorize and therefore cannot be treated in the usual ``naive'' way they were accounted for in the LEP-era. The nonfactorizable effects come up first at the two loop LL level, that we calculate explicitly for a fermion source that is neutral under the SU(2)$\otimes$U(1) gauge group (explicitly, a Z' decay into two fermions). The basic formalism we set up can be used to calculate LL effects at any order of perturbation theory. The results of this paper might be important for future calculations of electroweak corrections at NLC energies.Comment: LaTeX, 8 pages, 3 figure

A Way to Dynamically Overcome the Cosmological Constant Problem

The Cosmological Constant problem can be solved once we require that the full standard Einstein Hilbert lagrangian, gravity plus matter, is multiplied by a total derivative. We analyze such a picture writing the total derivative as the covariant gradient of a new vector field (b_mu). The dynamics of this b_mu field can play a key role in the explanation of the present cosmological acceleration of the Universe.Comment: 5 page

New limits on the mass of neutral Higgses in General Models

In general electroweak models with weakly coupled (and otherwise arbitrary) Higgs sector there always exists in the spectrum a scalar state with mass controlled by the electroweak scale. A new and simple recipe to compute an analytical tree-level upper bound on the mass of this light scalar is given. We compare this new bound with similar ones existing in the literature and show how to extract extra information on heavier neutral scalars in the spectrum from the interplay of independent bounds. Production of these states at future colliders is addressed and the implications for the decoupling limit in which only one Higgs is expected to remain light are discussed.Comment: 14 pages, LaTe

Particle Currents in a Space-Time dependent and CP-violating Higgs Background: a Field Theory Approach

Motivated by cosmological applications like electroweak baryogenesis, we develop a field theoretic approach to the computation of particle currents on a space-time dependent and CP-violating Higgs background. We consider the Standard Model model with two Higgs doublets and CP violation in the scalar sector, and compute both fermionic and Higgs currents by means of an expansion in the background fields. We discuss the gauge dependence of the results and the renormalization of the current operators, showing that in the limit of local equilibrium, no extra renormalization conditions are needed in order to specify the system completely.Comment: 21 pages, LaTeX file, uses epsf.sty. 4 figures available as a compressed .ep

FRW Cosmological Perturbations in Massive Bigravity

Cosmological perturbations of FRW solutions in ghost free massive bigravity, including also a second matter sector, are studied in detail. At early time, we find that sub horizon exponential instabilities are unavoidable and they lead to a premature departure from the perturbative regime of cosmological perturbations.Comment: latex, 14 page

The role of the top mass in b-production at future lepton colliders

We compute the one loop contribution coming from vertex and box diagrams, where virtual top quarks are exchanged, to the asymptotic energy behaviour of $b\bar b$ pair production at future lepton colliders. We find that the effect of the top mass is an extra linear logarithmic term of Sudakov type that is not present in the case of (u,d,s,c) production. This appears to be particularly relevant in the case of the $b\bar{b}$ cross section.Comment: 9 pages and 3 figures; version submitted to Phys.ReV.D,Rapid. e-mail: [email protected]

Logarithmic expansion of electroweak corrections to four-fermion processes in the TeV region

Starting from a theoretical representation of the electroweak component of four-fermion neutral current processes that uses as theoretical input the experimental measurements at the Z peak, we consider the asymptotic high energy behaviour in the Standard Model at one loop of those gauge-invariant combinations of self-energies, vertices and boxes that contribute all the different observables. We find that the logarithmic contribution due to the renormalization group running of the various couplings is numerically overwhelmed by single and double logarithmic terms of purely electroweak (Sudakov-type) origin, whose separate relative effects grow with energy, reaching the 10% size at about one TeV. We then propose a simple "effective" parametrization that aims at describing the various observables in the TeV region, and discuss its validity both beyond and below 1 TeV, in particular in the expected energy range of future linear electron-positron (LC) and muon-muon colliders.Comment: 23 pages and 9 figures; version submitted to Phys.Rev.D. e-mail: [email protected]

Degrees of Freedom in Massive Gravity

We study in a systematic way a generic nonderivative (massive) deformation of general relativity using the Hamiltonian formalism. The number of propagating degrees of freedom is analyzed in a nonperturbative and background independent way. We show that the condition of having only five propagating degrees of freedom can be cast in a set of differential equations for the deforming potential. Though the conditions are rather restrictive, many solutions can be found.Comment: 5 pages. Final version published in PRD rapid communication