Search

We present the contributions to the imaginary part of the $W^+W^-$ elastic scattering from top and bottom quark one-loop diagrams. The computation is performed within the context of HEFT, where we compare them with the boson-loop corrections. We argue that the often neglected (top and bottom) fermion contributions may in fact be relevant., Comment: 4 pages, 4 figures, contribution to the proceedings of QCD22 - 25th International Conference in Quantum Chromodynamics (4-7/07/2022, Montpellier - France)

This article presents a number of technical tools and results that may be instrumental to discern the nature of the Higgs particle. In scenarios where an additional strongly interacting sector is present in the electroweak theory, resulting in a composite Higgs and longitudinal components of the massive gauge bosons, unitarity, analyticity and related techniques will be crucial to understand the properties of such a sector. The situation today may be reminiscent of the pre-QCD days: a strongly interacting theory governs the short-distances, but we have access only to long-distance experiment involving Nambu-Goldstone or pseudo Nambu-Goldstone bosons. Like in those days we can only rely on symmetry and general properties of field theory. Luckily, unlike in the pre-QCD days, we have now a much clearer idea of what we may be seeking. After presenting a classification of the various types of effective theories, we establish the criteria as to under which conditions different representations are possible and their equivalence. We discuss in detail the implications of analyticity, causality and unitarity; describe various unitarization methods and establish the properties of dynamically generated resonances and form factors. The relation to effective Lagrangians with explicit resonances is explained in the context of beyond the Standard Model (BSM) physics and the Higgs effective field theory (HEFT). We discuss how various BSM models may be reduced to the HEFT as well as implications from holography and lattice studies in establishing BSM phenomenology. The methods presented are then applied to various processes relevant to disentangle the existence and nature of an extended electroweak symmetry breaking sector visible in VV fusion: two-Higgs production, vector resonances, gamma gamma physics and top-antitop production. (C) 2020 Elsevier B.V. All rights reserved.

We examine the production of Wh and Zh pairs at the LHC in the context of a Strongly Interacting Symmetry Breaking Sector of the Standard Model. Our description is based on a non-linear Higgs Effective Theory, including only the Standard Model particles. We focus on its scalar sector (Higgs boson h and electroweak Goldstones associated to $W_L^\pm$ and $Z_L$), which is expected to give the strongest beyond Standard Model rescattering effects. The range of the effective theory is extended with dispersion-relation based unitarization, and compared to the alternative extension with explicit axial-vector resonances. We estimate the Wh and Zh production cross-section, where an intermediate axial-vector resonance is generated for certain values of the chiral couplings. We exemplify our analysis with a benchmark axial-vector with $M_A=3$ TeV. Interestingly enough, these different approaches provide essentially the same prediction. Finally we discuss the sensitivity of ATLAS and CMS to such resonances., 28 pages, 7 eps figures

In the present work we study the production of vector resonances at the LHC by means of the vector boson scattering $WZ \to WZ$ and explore the sensitivities to these resonances for the expected future LHC luminosities. We are assuming that these vector resonances are generated dynamically from the self interactions of the longitudinal gauge bosons, $W_L$ and $Z_L$, and work under the framework of the electroweak chiral Lagrangian to describe in a model independent way the supposedly strong dynamics of these modes. The properties of the vector resonances, mass, width and couplings to the $W$ and $Z$ gauge bosons are derived from the inverse amplitude method approach. We implement all these features into a single model, the IAM-MC, adapted for MonteCarlo, built in a Lagrangian language in terms of the electroweak chiral Lagrangian and a chiral Lagrangian for the vector resonances, which mimics the resonant behavior of the IAM and provides unitary amplitudes. The model has been implemented in MadGraph, allowing us to perform a realistic study of the signal versus background events at the LHC. In particular, we have focused our study on the $pp\to WZjj$ type of events, discussing first on the potential of the hadronic and semileptonic channels of the final $WZ$, and next exploring in more detail the clearest signals. These are provided by the leptonic decays of the gauge bosons, leading to a final state with $\ell_1^+\ell_1^-\ell_2^+\nu jj$, $\ell=e,\mu$, having a very distinctive signature, and showing clearly the emergence of the resonances with masses in the range of 1.5-2.5 TeV, which we have explored., Comment: Revised version accepted for publication in JHEP. Enlarged analysis. References added. 44 pages, 23 figures, 3 tables

We point out that in the early universe, for temperatures in the approximate interval 175-80 MeV (after the quark-gluon plasma), pions carried a large share of the entropy and supported the largest inhomogeneities. Thus, we examine the production of entropy in a pion gas, particularizing to inhomogeneities of the temperature, for which we benefit from the known thermal conductivity. We finally put that entropy produced in relaxing such thermal inhomogeneities in the broad context of this relatively unexplored phase of early-universe cosmology., Comment: 10 pages, 10 figures

If the electroweak symmetry breaking sector turns out to be strongly interacting, the actively investigated effective theory for longitudinal gauge bosons plus Higgs can be efficiently extended to cover the regime of saturation of unitarity (where the perturbative expansion breaks down). This is achieved by dispersion relations, whose subtraction constants and left cut contribution can be approximately obtained in different ways, giving rise to different unitarization procedures. We illustrate the ideas with the inverse amplitude method, one version of the N/D method, and another improved version of the K matrix. In the three cases we get partial waves which are unitary, analytical with the proper left and right cuts, and in some cases poles in the second Riemann sheet that can be understood as dynamically generated resonances. In addition, they reproduce at next to leading order the perturbative expansion for the five partial waves not vanishing (up to J = 2), and they are renormalization scale (μ) independent. Also the unitarization formalisms are extended to the coupled channel case. Then we apply the results to the elastic scattering amplitude for the longitudinal components of the gauge bosons V = W, Z at high energy. We also compute hh -> hh and the inelastic process VV -> hh which are coupled to the elastic VV channel for custodial isospin I = 0. We numerically compare the three methods for various values of the low-energy couplings and explain the reasons for the differences found in the I = J = 1 partial wave. Then we study the resonances appearing in the different elastic and coupled channels in terms of the effective Lagrangian parameters.

In this work we study the γγ → W + W − and γγ → Z L Z L scattering processes within the effective chiral Lagrangian approach, including a light Higgs-like scalar as a dynamical field together with the would-be-Goldstone bosons w ± and z associated to the electroweak symmetry breaking. This approach is inspired by the possibility that the Higgs-like boson be a composite particle behaving as another Goldstone boson, and assumes the existence of a mass gap between m h , m W , m Z and the potential new emergent resonances, setting an intermediate energy region (above m h,W,Z and below the resonance masses) where the use of these effective chiral Lagrangians are the most appropriate tools to compute the relevant observables. We analyse in detail the proper chiral counting rules for the present case of photon-photon scattering and provide the computation of the one-loop γγ → W + W − and γγ → Z L Z L scattering amplitudes within this Effective Chiral Lagrangian approach and the Equivalence Theorem, including a discussion on the involved renormalization procedure. We also propose here a joint analysis of our results for the twophoton scattering amplitudes together with other photonic processes and electroweak (EW) precision observables for a future comparison with data. This could help to disentangle the nature of the light Higgs-like particle.

By including the recently discovered Higgs-like scalar $\varphi$ in the Electroweak Chiral Lagrangian, and using the Equivalence Theorem, we carry out the complete one-loop computation of the elastic scattering amplitude for the longitudinal components of the gauge bosons $V=W, Z$ at high energy. We also compute $\varphi\varphi \rightarrow \varphi\varphi$ and the inelastic process $VV\rightarrow \varphi\varphi$, and identify the counterterms needed to cancel the divergences, namely the well known $a_4$ and $a_5$ chiral parameters plus three additional ones only superficially treated in the literature because of their dimension 8. Finally we compute all the partial waves and discuss the limitations of the one-loop computation due to only approximate unitarity., Comment: 28 pages, 19 plots, 9 Feynman-diagram sets This version revised and accepted in JHEP

We study the possibility of testing some generic properties of Brane-World scenarios at the LHC. In particular, we pay attention to KK-graviton and branon production. Both signals can be dominant depending on the value of the brane tension. We analyze the differences between these two signatures. Finally, we use recent data in the single photon channel from the ATLAS collaboration to constraint the parameter space of both phenomenologies., 26 pages, 3 figures

The claimed finding of a light Higgs boson makes the minimal Standard Model unitary. Yet we recall that the general low-energy dynamics for the minimal electroweak symmetry breaking sector with three Goldstone bosons and one light scalar is not so. We construct the effective Lagrangian for these four particles and their scattering amplitudes, that can be extracted from LHC experiments when longitudinal W, Z modes be properly isolated for E>>M_W (Equivalence Theorem). We then observe the known increase in interaction strength with energy and explore various unitarization methods in the literature in the absence of other new physics (as LHC experiments fail to report anything up to 600 GeV). Our generic conclusion is that for most of parameter space the high energy scattering of the longitudinal W's is strongly interacting (with the Minimal Standard Model a remarkable exception). We find and study a second sigma-like scalar pole of the W_L W_L amplitude., 32 pages, 24 plots, to be published in JPG, revised version including experimental bounds

In this work we present the results of a complete calculation of the $\gamma \gamma\rightarrow \pi^0\pi^0$ amplitude to leading order in the large $N$ approximation ($N$ being the number of Goldstone bosons) up to order $m^2_{\pi}/F^2$. The amplitude turns to be proportional to that of $\pi^+\pi^-\rightarrow \pi^0\pi^0$. In spite of the fact that this factorization property cannot hold in general (as it was recently pointed out by Morgan and Penington), it appears here since in the large $N$ limit only the $I=J=0$ channel contributes to the $\gamma \gamma\rightarrow \pi^0\pi^0$ reaction. Moreover it seems to be a reasonable approximation in this case since it is possible to reproduce, as a prediction, the experimental data starting from a one-parameter fit of the $\pi\pi$ scattering data.

In this work we deal with the critical behavior of the bulk viscosity in the linear sigma model (LSM) as an example of a system which can be treated by using different techniques. Starting from the Boltzmann-Uehling-Uhlenbeck equation we compute the bulk viscosity over entropy density of the LSM in the large-N limit. We search for a possible maximum of the bulk viscosity over entropy density at the critical temperature of the chiral phase transition. The information about this critical temperature, as well as the effective masses, is obtained from the effective potential. We find that the expected maximum (as a measure of the conformality loss) is absent in the large N in agreement with other models in the same limit. However, this maximum appears when, instead of the large-N limit, the Hartree approximation within the Cornwall-Jackiw-Tomboulis (CJT) formalism is used. Nevertheless, this last approach to the LSM does not give rise to the Goldstone theorem and also predicts a first order phase transition instead of the expected second order one. Therefore both, the large-N limit and the CJT-Hartree approximations, should be considered as complementary for the study of the critical behavior of the bulk viscosity in the LSM., 27 pages, 22 figures. v2: further clarifications and minor changes. Version to be published in Phys. Rev. D

In general relativity, there is a maximum mass allowed for neutron stars that, if exceeded, entails collapse into a black hole. Its precise value depends on details of the nuclear matter equation of state, a subject where much progress has been accomplished thanks to low energy effective theories. The discovery of a two-solar-mass neutron star, near that maximum mass, when analyzed with modern equations of state, implies that Newton's gravitational constant in the star cannot exceed its value on Earth by more than 12% at the 95% confidence level. This significantly extends the gravitational field intensity at which the constant has been constrained at the 10% level.

In this work we derive the version of the Equivalence Theorem that applies when the symmetry breaking sector of the Standard Model is described by a general chiral lagrangian. The demonstration is valid for renormalized fields for any value of the gauge parameter (in $R_{\xi}$ gauges) and any parametrization of the coset space. It is based in the absence of gauge anomalies which makes it possible to build an (anti)-BRS invariant chiral lagrangian in terms of the renormalized fields and therefore to use the corresponding Ward identities to obtain the theorem., Comment: 18 pages, LaTeX, FT/UCM/2/94 (We have corrected some errata)

We analyze the finite temperature (T) extension of the Wess-Zumino- Witten functional, discussed in a previous work, to one loop in chiral perturbation theory. As a phenomenological application, we calculate finite temperature corrections to the amplitude of the $��^0$ decay into two photons. This calculation is performed in three limits : i)$T/M_��<>1$ ($M_��$ being the pion mass). The $T$-corrections tend to vanish in the chiral limit, where only the kaon contribution remains (although it is exponentially suppressed)., Latex, 13 pages and 3 figures avalaible upon request

We study the bulk viscosity of a pion gas in unitarized Chiral Perturbation Theory at low and moderate temperatures, below any phase transition to a quark-gluon plasma phase. We argue that inelastic processes are irrelevant and exponentially suppressed at low temperatures. Since the system falls out of chemical equilibrium upon expansion,a pion chemical potential must be introduced, so we extend the existing theory to include it. We control the zero modes of the collision operator and Landau's conditions of fit when solving the Boltzmann equation with the elastic collision kernel. The dependence of the bulk viscosity with temperature is reminiscent of the findings of Fernandez-Fraile and Gomez Nicola, while the numerical value is closer to that of Davesne. In the zero-temperature limit we correctly recover the vanishing viscosity associated to a non-relativistic monoatomic gas., 8 pages, 8 figures

If the present dark matter in the Universe annihilates into Standard Model particles, it must contribute to the fluxes of cosmic rays that are detected on the Earth, and in particular, to the observed gamma ray fluxes. The magnitude of such contribution depends on the particular dark matter candidate, but certain features of the produced photon spectra may be analyzed in a rather model-independent fashion. In this work we provide the complete photon spectra coming from WIMP annihilation into Standard Model particle-antiparticle pairs obtained by extensive Monte Carlo simulations. We present results for each individual annihilation channel and provide analytical fitting formulae for the different spectra for a wide range of WIMP masses., Comment: 23 pages, 37 figures and 23 tables

In the general framework of chiral lagrangians we compute the leading log contribution to the three boson coupling in the standard model coming from eventual new physics. We apply the results, together with a complete analysis of the background and the statistical significances, to the LHC and the SSC and set bounds on the size of the new coupling to be detected in both accelerators.

In this letter we show how it is possible to obtain the large N(f) limit of chiral perturbation theory (chi-PT) in the chiral limit. In particular, we compute explicitly the Nambu-Goldstone boson (NGB) elastic scattering amplitude to leading order in 1/N (N being the number of NGB fields) to all powers in the external momenta. This amplitude is unitary to order 1/N and it can incorporate resonant behavior. The price to pay is the introduction of an infinite set of renormalized coupling constants that can be summarized in a single generating function. Playing with these constants, it is possible to simulate the linear sigma model and many others in the same large N limit.

We reexamine the possibility of employing the viscosity over entropy density ratio as a diagnostic tool to identify a phase transition in hadron physics to the strongly coupled quark-gluon plasma and other circumstances where direct measurement of the order parameter or the free energy may be difficult. It has been conjectured that the minimum of $\ensuremath{\eta}/s$ does indeed occur at the phase transition. We now make a careful assessment in a controlled theoretical framework, the linear sigma model at large $N$, and indeed find that the minimum of $\ensuremath{\eta}/s$ occurs near the second-order phase transition of the model due to the rapid variation of the order parameter (here the $\ensuremath{\sigma}$ vacuum expectation value) at a temperature slightly smaller than the critical one.

In the context of $f(R)$ theories of gravity, we address the problem of finding static and spherically symmetric black hole solutions. Several aspects of constant curvature solutions with and without electric charge are discussed. We also study the general case (without imposing constant curvature). Following a perturbative approach around the Einstein-Hilbert action, it is found that only solutions of the Schwarzschild-(anti) de Sitter type are present up to second order in perturbations. Explicit expressions for the effective cosmological constant are obtained in terms of the $f(R)$ function. Finally, we have considered the thermodynamics of black holes in anti-de Sitter space-time and found that this kind of solutions can only exist provided the theory satisfies $R_0+f(R_0), 19 pages, 9 figures. New comments and figures included. Accepted for publication in Phys. Rev. D

We present a calculation of η/s for the meson gas (zero baryon number), with the viscosity computed within unitarized next-to-leading-order chiral perturbation theory, and confirm the observation that η/s decreases towards the possible phase transition to a quark-gluon plasma/liquid. The value is somewhat higher than previously estimated in leading-order ΧPT. We also examine the case of atomic Argon gas to check the discontinuity of η/s across a first-order phase transition. Our results suggest employing this dimensionless number, sometimes called KSS number (in analogy with other ratios in fluid mechanics such as Reynolds number or Prandtl number) to pin down the phase transition and critical end point to a crossover in strongly interacting nuclear matter between the hadron gas and quark and gluon plasma/ liquid.

An effective SU(2)L X U(1)Y invariant chiral lagrangian can accommodate any symmetry breaking scheme in the standard model. It is shown that all process-independent quantities relevant to LEP precision measurements (DELTA-rho, DELTA-K and DELTA-r) 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.

In the context of f(R) theories of gravity, we study the evolution of scalar cosmological perturbations in the metric formalism. Using a completely general procedure, we find the exact fourth-order differential equation for the matter density perturbations in the longitudinal gauge. In the case of sub-Hubble modes, the expression reduces to a second-order equation which is compared with the standard (quasistatic) equation used in the literature. We show that for general f(R) functions the quasistatic approximation is not justified. However, for those functions adequately describing the present phase of accelerated expansion and satisfying local gravity tests, it provides a correct description for the evolution of perturbations.

The one-loop corrections to the skyrmion mass are computed by the use of a non-local effective lagrangian. A skyrmion mass of 1.4 GeV is obtained for the values of the parameters that fit the ππ elastic scattering data in the context of ChPT. Strong indications of the stability of the skyrmion when one-loop corrections are included are also presented.

In this work we compute the contributions to the Higgs effective potential coming from the fermion and gauge boson sectors at the one-loop level in the context of the SU(5)/SO(5) Littlest Higgs (LH) model using a cutoff Lambda and including all finite parts. We consider both, the (SU(2)xU(1))_{1} x (SU(2)xU(1))_{2} and the (SU(2)xU(1))_{1} x (SU(2)xU(1)) gauge group versions of the LH model. We also show that the Goldstone bosons present in the model do not contribute to the effective potential at the one-loop level. Finally, by neglecting the contribution of higher dimensional operators, we discuss the restrictions that the new one-loop contributions set on the parameter space of the LH model and the need to include higher loop corrections to the Higgs potential., Comment: LaTeX, 13 pages, 6 figures. Discussion and references added. New figures included. Published version in Phys.Rev.D

We study the possibility to explain the non-baryonic dark matter abundance and improve the present fits on the muon anomalous magnetic moment through the same new physics. The only viable way to solve simultaneously both problems which is known to date is by using supersymmetric theories. However in this work we show that massive brane fluctuations (branons) in large extra-dimensions models can provide a more economical alternative to supersymmetry. This is so because the low-energy branon physics depends effectively on only three parameters. Next collider experiments, such as LHC or ILC, will be sensitive to branon phenomenology in the natural parameter region where the theory is able to account for the two effects., Comment: 4 pages, 3 figures, REVTeX 4

In the context of brane-world scenarios, we study the effects produced by the exchange of virtual massive branons. A one-loop calculation is performed which generates higher-dimensional operators involving SM fields suppressed by powers of the brane tension scale. We discuss constraints on this scenario from colliders such as HERA, LEP and Tevatron and prospects for future detections at LHC or ILC. The most interesting phenomenology comes from new four-particles vertices induced by branon radiative corrections, mainly from four fermion interactions. The presence of flexible branes modifies also the muon anomalous magnetic moment and the electroweak precision observables., 23 pages, 4 figures, LaTeX

In this work we consider the possibility of describing the current evolution of the universe, without the introduction of any cosmological constant or dark energy (DE), by modifying the Einstein-Hilbert (EH) action. In the context of the f(R) gravities within the metric formalism, we show that it is possible to find an action without cosmological constant which exactly reproduces the behavior of the EH action with cosmological constant. In addition the f(R) action is analytical at the origin having Minkowski and Schwarzschild as vacuum solutions. The found f(R) action is highly non-trivial and must be written in terms of hypergeometric functions but, in spite of looking somewhat artificial, it shows that the cosmological constant, or more generally the DE, is not a logical necessity., Comment: 5 pages, no figures

In the context of the brane-world scenarios with compactified extra dimensions, we study the production of brane fluctuations (branons) in hadron colliders ($p \bar p$, $pp$ and $e^\pm p$) in terms of the brane tension parameter $f$, the branon mass $M$ and the number of branons $N$. From the absence of monojets events at HERA and Tevatron (run I), we set bounds on these parameters and we also study how such bounds could be improved at Tevatron (run II) and the future LHC. The single photon channel is also analyzed for the two last colliders., Comment: 17 pages, 10 figures, LaTeX. New comments and figures included. Final version to appear in Phys. Rev. D

We report a calculation of the shear viscosity in a relativistic multicomponent meson gas as a function of temperature and chemical potentials. We approximately solve the Uehling-Uhlenbeck transport equation of kinetic theory, appropriate for a boson gas, with relativistic kinematics. Since at low temperatures the gas can be taken as mostly composed of pions, with a fraction of kaons and etas, we explore the region where binary elastic collisions with at least one pion are the dominant scattering processes. Our input meson scattering phase shifts are fits to the experimental data obtained from chiral perturbation theory and the inverse amplitude method. Our results take the correct nonrelativistic limit (viscosity proportional to the square root of the temperature), show a viscosity of the order of the cube of the pion mass up to temperatures somewhat below that mass, and then a large increase due to kaons and etas. Our approximation may break down at even higher temperatures, where the viscosity follows a temperature power law with an exponent near 3.

In the context of the brane-world scenarios with compactified large extra dimensions, we study the production of the possible massive brane oscillations (branons) in electron-positron colliders. We compute their contribution to the electroweak gauge bosons decay width and to the single-photon and single-Z processes. With LEP-I results and assuming non observation at LEP-II, we present exclusion plots for the brane tension $\tau = f^4$ and the branon mass $M$. Prospects for the next generation of electron-positron colliders are also considered., Comment: LaTeX, 38 pages, 7 figures. Minor changes, matches published version

We consider a general brane-world model parametrized by the brane tension scale $f$ and the branon mass $M$. For low tension compared to the fundamental gravitational scale, we calculate the relic branon abundance and its contribution to the cosmological dark matter. We compare this result with the current observational limits on the total and hot dark matter energy densities and derive the corresponding bounds on $f$ and $M$. Using the nucleosynthesis bounds on the number of relativistic species, we also set a limit on the number of light branons in terms of the brane tension. Finally, we estimate the bounds coming from the energy loss rate in supernovae explosions due to massive branon emission., 26 pages, 6 figures, LaTeX. Final version with minor corrections. To appear in Phys. Rev. D

We investigate the decoupling properties of the Higgs-sector-induced one-loop corrections in the lightest Higgs-boson self-couplings, in the framework of the minimal supersymmetric standard model (MSSM). The renormalized n-point vertex functions with external Higgs particles in the MSSM and in the SM are derived to the one-loop level and compared in the M(A)(0)much greater thanM(Z) limit. The computation has been done in a general R-xi gauge and the on-shell renormalization scheme is chosen. By a comparison of the renormalized lightest Higgs-boson h(0) vertex functions with respect to the corresponding SM ones, we find that the differences between the predictions of both models are summarized in the lightest Higgs-boson mass correction DeltaM(h)(0). Consequently, the radiative corrections are absorbed in the Higgs-boson mass, and the trilinear and quartic h(0) self-couplings acquire the same structure as the couplings of the SM Higgs boson. Therefore, decoupling of the heavy MSSM Higgs bosons occurs and the MSSM h(0) self-interactions converge to the SM ones in the M(A)(0)much greater thanM(Z) limit.

We study the temperature evolution of the rho and sigma mass and width, using a unitary chiral approach. The one-loop pipi scattering amplitude in chiral perturbation theory at Tnot equal0 is unitarized via the inverse amplitude method. Our results predict a clear increase with T of both the rho and sigma widths. The masses decrease slightly for high T, while the rhopipi coupling increases. The rho behavior seems to be favored by experimental results. In the sigma case, it signals chiral symmetry restoration.

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 alpha(s). In addition, we consider the limit of nearly degenerate heavy squarks and gluinos, with masses much larger than the electroweak scale, and we derive the effective Lagrangian containing all the relevant new local Higgs-quark-quark interactions. We show that these new interactions do remain nonvanishing, even in the case of infinitely heavy supersymmetric particles and, therefore, we demonstrate explicitly the nondecoupling behavior of squarks and gluinos in Higgs bosons physics. We present the set of new Yukawa couplings and finally derive the corresponding one-loop, order alpha(s), corrections to the Higgs bosons partial decay widths into quarks.

Within chiral perturbation theory, we study elastic pion scattering in the $I=0,J=2,$ channel, whose main features are the $f(1270)$ resonance and the vanishing of the lowest order. By means of a chiral model that includes an explicit resonance coupled to pions, we describe the data and calculate the resonance contribution to the ${O(p}^{4})$ and ${O(p}^{6})$ chiral parameters. We also generalize the inverse amplitude method to higher orders, which allows us to study channels with vanishing lowest order. In particular, we apply it to the $I=0,J=2$ case, finding a good description of the ${f}_{2}(1270)$ resonance, as a pole in the second Riemann sheet.

By using Chiral Perturbation Theory and the Uehling-Uhlenbeck equation we compute the viscosity of a pion gas, in the low temperature and low density regime, in terms of the temperature, and the pion fugacity. The viscosity turns out to be proportional to the squared root of the temperature over the pion mass. Next to leading corrections are proportional to the temperature over the pion mass to the 3/2., 15 pages, 4 figures. RevTeX 4

We study, within the recently proposed brane-world scenario, the effective action for the low-energy brane excitations. These modes are understood as Goldstone bosons associated to the spontaneous isometry breaking, induced on the bulk space by the presence of the brane. Starting from the Nambu-Goto action for the brane, we obtain a non-linear sigma model describing the low-energy interactions of the Goldstone bosons and extend the calculation up to O(p^4). We also discuss the Higgs-like mechanism in which the Kaluza-Klein gauge fields absorb the Goldstone bosons and acquire mass. Finally, we present the explicit form of the effective action describing the low-energy interactions between the three-brane Goldstone bosons and the particles present in the Standard Model., 16 pages, LaTeX. Final version with minor corrections and new references included, to appear in Nucl.Phys.B

We present a unified analysis of the two main production processes of vector boson pairs at the LHC, VV-fusion and qqbar annihilation, in a minimal strongly interacting electroweak symmetry breaking sector. Using a unitarized electroweak chiral Lagrangian formalism and modeling the final V_L V_L strong rescattering effects by a form factor, we describe qqbar annihilation processes in terms of the two chiral parameters that govern elastic V_L V_L scattering. Depending on the values of these two chiral parameters, the unitarized amplitudes may present resonant enhancements in different angular momentum-isospin channels. Scanning this two parameter space, we generate the general resonance spectrum of a minimal strongly interacting electroweak symmetry breaking sector and determine the regions that can be probed at the LHC., Final version to appear in Phys. Rev. D, including a more detailed exposition and a few more references. Conclusions and results unchanged. 14 pages, 5 figures

In this paper we show how the nonlocal effective action for gravity, obtained after integrating out the matter fields, can be used to compute particle production and spectra for different space-time metrics. Applying this technique to several examples, we find that the perturbative calculation of the effective action up to second order in curvatures yields exactly the same results for the total number of particles as the Bogolyubov transformations method, in the case of masless scalar fields propagating in a Robertson-Walker space-time. Using an adiabatic approximation we also obtain the corresponding spectra and compare the results with the traditional WKB approximation., Comment: 22 pages, LaTeX, no figures. Corrected version with new comments and results. To appear in Phys. Rev. D

We propose a method to derive the low-energy efective action of QCD assuming that the long-distance properties of strong interactions can be described by a string theory. We bypass the usual problems related to the existence of the tachyon and absence of the adequate Adler zero by using a sigma model approach where excitations above the correct (chirally non-invariant) QCD vacuum are included. Two-dimensional conformal invariance then implies the vanishing of the O(p^4) effective lagrangian coefficients. We discuss ways to go beyond this limit., Comment: 13 pages, 5 figures. Some modifications in the discussion section

In this work we study the thermodynamic properties of the pion gas starting from the realistic elastic scattering phase-shifts obtained from Chiral Perturbation Theory and using the virial expansion. In particular we study the state equation and the behavior of the quark condensate as a function of the temperature and the density., 16 Pages, 12 figures

In this work we study the applicability of the Equivalence Theorem, either for unitary models or within an effective lagrangian approach. There are two types of limitations: the existence of a validity energy window and the use of the lowest order in the electroweak constants. For the first kind, we consider some methods, based on dispersion theory or the large $N$ limit, that allow us to extend the applicability. For the second, we have obtained numerical estimates of the effect of neglecting higher orders in the perturbative expansion., Comment: Final version to appear in Phys. Rev. D. Power counting and energy range estimates have been refined, improved referencing. 4 postscript figures, uses revtex. FT-UCM 1/97

In this note we provide a simple explanation of the recent finding of anisotropy in electromagnetic (EM) propagation claimed by Nodland and Ralston (astro-ph/9704196). We consider, as a possible origin of such effect, the effective coupling between EM fields and some tiny background torsion field. The coupling is obtained after integrating out charged fermions, it is gauge invariant and does not require the introduction of any new physics., 8 pages, LaTeX, one figure, enlarged version with minor corrections

In this paper we study the large N limit of the Standard Model Higgs sector with $N\lambda$, $Ng^2$ and $Ng'^2$ constant and $N$ being the number of would-be Goldstone bosons. Despite the simplicity of this method at leading order, its results satisfy simultaneously important requirements such as unitarity and the low-energy theorems in contrast with other more conventional approaches. Moreover, it is fully compatible with the Equivalence Theorem and it yields a consistent description of the Higgs boson mass and width. Finally we have also included a phenomenological discussion concerning the applications of this method to the LHC., Comment: 15 pages, 5 Figures, 3 .EPS files. Available at http://www-physics.lbl.gov/www/theorypapers/preprints.html

Based on a dispersive approach, we apply the inverse amplitude method to unitarize one-loop SU(2) and SU(3) Chiral Perturbation Theory. Numerically, we find that this unitarization technique yields the correct complex analytic structure in terms of cuts and poles. Indeed, using the chiral parameter estimates obtained from low energy experiments we obtain the poles associated with the rho(770) and K^*(982) resonances. Just by fixing their actual masses we obtain a parametrization of the pi-pi and pi-K phase shifts in eight different channels. With this fit we have then calculated several low-energy phenomenological parameters estimating their errors. Among others, we have obtained the chiral parameters and scattering lengths, which can be relevant for future experiments., Comment: 31 pages. 25 eps figures. Uses epsf.tex. Final reduced version to appear in Phys. Rev. D Some numerical errors corrected. Comments and clarifications added

Using the Fujikawa and the heat-kernel methods we make a complete and detailed computation of the global, gauge and gravitational anomalies present in the Standard Model defined on a curved space time with torsion. We find new contributions coming from curvature and torsion terms to the leptonic number anomaly (so that $B-L$ is not conserved any more), to the $U(1)_Y$ gauge and to the mixed $U(1)_Y$-gravitational anomalies, but the gauge anomaly cancellation conditions on the hypercharges remain the same. We also find that the condition, usually related to the cancellation of the mixed $U(1)_Y$-gravitational anomaly, can be reobtained in the context of the Standard Model in flat space-time by requiring the cancellation of the global Lorentz anomaly without any reference to gravitation., Comment: 44 pages, latex, no figures

In the present work we study the predictions for $WZ$ and $ZZ$ production at LHC with the Electroweak Chiral Lagrangian (EChL) approach. Our analysis will be focused on the less favored case from the experimental point of view, in which the predictions for the gauge bosons scattering amplitudes are considered in the low energy range where, by construction of the low energy approach, they reveal no resonant behavior. The study includes the complete set of amplitudes for all the polarization states of the initial and/or final gauge bosons and makes no use of the Equivalence Theorem. We express the results in terms of the range of values of the chiral parameters that will be accessible at LHC., Comment: 18 pages, Latex, 4 figures available on request