Maria J. Herrero

Unitarized chiral perturbation theory for elastic pion-pion scattering

Abstract The low-energy two-pion elastic scattering is calculated within the framework of the one-loop chiral perturbation theory in combination with the Pade approximant method for the partial wave amplitudes. Elastic unitarity is satisfied to all orders. It is shown that the experimental data below 900 MeV can be described in terms of two independent parameters and that the chiral symmetry limit of massless pions gives a good description of the data above 350 MeV. With chiral symmetry breaking (finite pion mass) our calculation yields a ϱ width of 155 MeV and an S-wave I=0 scattering length of a0=0.27mπ−1, in good agreement with experimental data. These results demonstrate the validity of our method as applied to the strongly interacting Higgs sector.

Chiral Lagrangians as a tool to probe the symmetry breaking sector of the SM at LEP

Abstract An effective SU (2) L ×U(1) Y invariant chiral lagrangian can accomodate any symmetry breaking scheme in the standard model. It is shown that all process-independent quantities relevant to LEP precision measurements ( Δπ , Δκ and Δr ) depend only on one coefficient of the weak chiral lagrangian, taking different values depending on the precise symmetry breaking scheme. We remove any ambiguity due to the use of an effective theory by a careful choice of observables and make definite predictions that can be tested in the foreseeable future. Our analysis complements nicely that of longitudinal W and Z scattering.

Phenomenological lagrangian approach to the symmetry breaking sector of the standard model

Abstract A chiral lagrangian expansion is used in order to parametrize the unknown symmetry breaking dynamics of the standard model, in the hypothetical case of a strongly interacting symmetry breaking sector. To lowest order, O(s), the model is completely determined by the experimental fact that ϱ≅1. The dependence on the underlying theory shows up to order s2 where consistency requires one-loop effects to be included through two renormalized parameters. The unitarity properties and the domain of applicability of the model are studied. Some experimental prospects are also commented at the end.

Study of the strongly interacting Higgs sector

Abstract The most general structure of the strongly interacting Higgs sector has two unknown parameters and corresponds, in this unitarized one-loop calculation, to the possible existence of an I=0, J=0 Higgs scalar boson and an I=1, J=1 vector boson. The vector boson width is shown to satisfy the KSRF relation while the Higgs boson width is reduced by a factor of 2 3 from the standard tree value. As the calculation is completely unitary, no bound can be obtained for the Higgs boson and the vector voson masses.

Testing the Hypothesis of Strongly Interacting Longitudinal Weak Bosons in Electron - Positron Collisions at Tev Energies

Abstract In a previous paper, we presented a phenomenological model based on a chiral lagrangian expansion to order s 2 for the scattering amplitudes of the longitudinally polarized W L and Z L gauge bosons in the case of a generic strongly interacting symmetry breaking sector. Here we apply this model in order to study the viability of testing the strongly interacting hypothesis within the next generation e + e − colliders at TeV energies. The different patterns expected for strong V L −V L (V L =W L + , W L − and Z L ) scattering that come from different underlying theories describing the symmetry breaking sector are also examined. In particular we compare in detail the predictions for two different plausible models: Higgs-like models such as the standard model with a heavy Higgs, and QCD-like models such as technicolour models.

New constraints on general slepton flavor mixing

The work of S.H. was supported in part by CICYT (Grant No. FPA 2010- 22163-C02-01) and by the Spanish MICINN’s Consolider- Ingenio 2010 Program under Grant No. MultiDark CSD2009-00064.M. J. H. andM.A.-C. acknowledge partial support from the European Union FP7 ITN INVISIBLES (Marie Curie Actions, PITN-GA-2011-289442), from the CICYT through the Project No. FPA2009-09017 and from CM (Comunidad Autonoma de Madrid) through the Project No. HEPHACOS S2009/ESP-1473. The work is also supported in part by the Spanish Consolider-Ingenio 2010 Programme CPAN (CSD2007-00042)

Chiral lagrangians and precision tests of the symmetry breaking sector of the standard model

We consider extensions of the minimal Standard Model with various alternatives for the symmetry breaking sector. Different underlying theories are parametrized in terms of the coefficients of a chiral lagrangian. At present and future energies, assuming an exact custodial symmetry in the symmetry breaking sector, all relevant information is contained in just four coefficients. We analyze how to determine these coefficients in the minimal Standard Model with a large Higgs mass, where ordinary perturbation theory is not well suited, and in a class of vector-like technicolor models, but the extension to more general theories is straightforward. We study how to compute radiative corrections in an unambiguous manner and how to extract precise information even if the symmetry breaking sector is described by an effective theory. In doing that we clarify the relationship between the linear and non-linear theories. We apply our techniques to the processes e+e− → ff and e+e− → W+W−, introducing in the latter case an improved Born approximation with a number of phenomenological constants. We identify those processes where it will be possible to unambiguously probe the symmetry breaking sector and compute the full set of one-loop corrections to them in different underlying theories.

Updated Constraints on General Squark Flavor Mixing

The work of S. H. was supported by the Spanish MICINN’s Consolider-Ingenio 2010 Program under Grant No. MultiDark CSD2009-00064. The work of M. H. and M. A.-C. was partially supported by the European Union FP7 ITN INVISIBLES (Marie Curie Actions, Grant No. PITN-GA-2011-289442), by the CICYT through Project No. FPA2012-31880, by the Spanish Consolider-Ingenio 2010 Programme CPAN (CSD2007-00042) and by the Spanish MINECO’s “Centro de Excelencia Severo Ochoa” Program under Grant No. SEV-2012-0249.

The role of chiral Lagrangians in strongly interactingW L W L signals atpp supercolliders

In this paper we apply a simple phenomenological model to describe the scattering amplitudes of strongly interacting longitudinal gauge bosons at the futurepp colliders, LHC and SSC. The model is based on the use of the well-known techniques from chiral effective Lagrangians and chiral perturbation theory, supplemented by a unitarization method for the scattering amplitudes. The generality of the approach allows one to deal with various physical situations in the unknown symmetry breaking sector of the Standard Model. In particular we mimic the two typical scenarios for the symmetry breaking sector: one of Higgs-type and the other of QCD-type. Two different unitarization methods have been implemented for comparison: the Padé approximants method and theK-matrix method. The first one permits the incorporation of the possibility of dynamical resonances as it can develop poles for the various different (I, J)-channels. A systematic study of strongly interacting signals for both LHC and SSC is presented, and their rates and efficiencies are compared.

Effective Higgs-quark-quark couplings from a heavy supersymmetric spectrum

In this paper we study the Yukawa Higgs-quark-quark interactions that are generated from radiative corrections of squarks and gluinos, in the minimal supersymmetric standard model. We compute the corrections to the effective action for Higgs and quark fields that are produced by explicit integration in the path integral formalism of all the squarks and gluinos at the one-loop level and order