Search

We study Lepton Flavour Violating hadron decays of the tau lepton within the Simplest Little Higgs model. Namely we consider $\tau \rightarrow \mu (P, V, PP)$ where $P$ and $V$ are short for a pseudoscalar and a vector meson. We find that, in the most positive scenarios, branching ratios for these processes are predicted to be, at least, four orders of magnitude smaller than present experimental bounds., Comment: 13 pages, 13 Figures

Instanton solutions of non-abelian Yang-Mills theories generate an effective action that may induce lepton and baryon number violations, namely $\Delta B = \Delta L = n_f$, being $n_f$ the number of families coupled to the gauge group. In this article we study instanton mediated processes in a $SU(2)_{\ell} \otimes SU(2)_h \otimes U(1)$ extension of the Standard Model that breaks universality by singularizing the third family. In the construction of the instanton Green functions we account systematically for the inter-family mixing. This allows us to use the experimental bounds on proton decay in order to constrain the gauge coupling of $SU(2)_h$. Tau lepton non-leptonic and radiative decays with $\Delta B = \Delta L = 1$ are also analysed., Comment: 33 pages, 3 figures. Further explanations about the instanton solutions for the G(221) model added in a new appendix, conclusions unchanged. Version published in JHEP

This report describes the scientific aims and potentials as well as the preliminary technical design of IRIDE, an innovative tool for multi-disciplinary investigations in a wide field of scientific, technological and industrial applications. IRIDE will be a high intensity 'particle factory', based on a combination of a high duty cycle radio-frequency superconducting electron linac and of high energy lasers. Conceived to provide unique research possibilities for particle physics, for condensed matter physics, chemistry and material science, for structural biology and industrial applications, IRIDE will open completely new research possibilities and advance our knowledge in many branches of science and technology. IRIDE will contribute to open new avenues of discoveries and to address most important riddles: What does matter consist of? What is the structure of proteins that have a fundamental role in life processes? What can we learn from protein structure to improve the treatment of diseases and to design more efficient drugs? But also how does an electronic chip behave under the effect of radiations? How can the heat flow in a large heat exchanger be optimized? The scientific potential of IRIDE is far reaching and justifies the construction of such a large facility in Italy in synergy with the national research institutes and companies and in the framework of the European and international research. It will impact also on R&D work for ILC, FEL, and will be complementarity to other large scale accelerator projects. IRIDE is also intended to be realized in subsequent stages of development depending on the assigned priorities., Comment: 270 pages

We study, at leading order in the large number of colours expansion and within the Resonance Chiral Theory framework, the odd-intrinsic-parity $e^+ e^- \rightarrow \pi^+ \pi^- (\pi^0, \eta)$ cross-sections in the energy regime populated by hadron resonances, namely $3 \, m_{\pi} \lsim E \lsim 2 \, \mbox{GeV}$. In addition we implement our results in the Monte Carlo generator PHOKHARA 7.0 and we simulate hadron production through the radiative return method., Comment: 39 pages, 5 figures

The existence of a new strong interacting sector around E ~ 1 TeV is a common feature of Higgsless electroweak theories but also of theories with a light Higgs, for instance, when this is not elementary. In those schemes, this new interaction could be at the origin of an extended spectra with, in particular, spin-1 resonances that could be hinted in elastic gauge boson scattering. Information on those resonances, if they exist, must be contained in the low-energy couplings of the electroweak chiral effective theory. Using the facts that: i) the scattering of longitudinal gauge bosons, W_L, Z_L, can be well described in the high-energy region (E >> M_W) by the scattering of the corresponding Goldstone bosons (equivalence theorem) and that ii) the zeros of the scattering amplitude carry the information on the heavier spectrum that has been integrated out; we employ the O(p^4) electroweak chiral Lagrangian, with or without a light Higgs state to identify the parameter space region of the low-energy couplings where vector resonances may arise., Comment: Talk at the 7th International Workshop on Chiral Dynamics, August 6-10, 2012, Thomas Jefferson National Accelerator Facility, 7 pages, 2 figures

A comprehensive overview of kaon decays is presented. The Standard Model predictions are discussed in detail, covering both the underlying short-distance electroweak dynamics and the important interplay of QCD at long distances. Chiral perturbation theory provides a universal framework for treating leptonic, semileptonic and nonleptonic decays including rare and radiative modes. All allowed decay modes with branching ratios of at least 10^(-11) are analyzed. Some decays with even smaller rates are also included. Decays that are strictly forbidden in the Standard Model are not considered in this review. The present experimental status and the prospects for future improvements are reviewed., Comment: 53 pages, 21 figures. Typos corrected, references updated, content matches the version published in Rev. Mod. Phys. 84 (2012) 399

We review the main components that have to be considered, within Resonance Chiral Theory, in the study of processes whose dynamics is dominated by hadron resonances. We show its application in the study of the tau -> (3 pion) nu_tau decay., Comment: 10 pages, 2 figures. To be published in the Proceedings of the Chiral10 International Workshop on Chiral Symmetry in Hadrons and Nuclei, Valencia (21st -24th June, 2010)

The tau -> pi pi pi nu_tau decay is driven by the hadronization of the axial-vector current. Within the resonance chiral theory, and considering the large-Nc expansion, this process has been studied in Ref.[1]. In the light of later developments we revise here this previous work by including a new off-shell width for the lightest a_1 resonance that provides a good description of the tau -> pi pi pi nu_tau spectrum and branching ratio. We also consider the role of the rho(1450) resonance in these observables. Thus we bring in an overall description of the tau -> pi pi pi nu_tau process in excellent agreement with our present experimental knowledge., Comment: 15 pages, 3 figures

We analyse the hadronization structure of both vector and axial-vector currents leading to tau -> KKpi nu_tau decays. At leading order in the 1/Nc expansion, and considering only the contribution of the lightest resonances, we work out, within the framework of the resonance chiral Lagrangian, the structure of the local vertices involved in those processes. The couplings in the resonance theory are constrained by imposing the asymptotic behaviour of vector and axial-vector spectral functions ruled by QCD. In this way we predict the hadron spectra and conclude that, contrarily to previous assertions, the vector contribution dominates by far over the axial-vector one in all KKpi charge channels., Comment: 32 pages, 7 figures

Here we study and compare the sensitivity to the Higgs sector of the SUSY-seesaw models via the LFV tau decays: tau-> 3 mu, tau->K^{+}K^{-}, tau->mu eta and tau-> mu f_{0}. We emphasize that, at present, the two later channels are the most efficient ones to test indirectly the Higgs particles., Comment: 4 pages, 3 figures, conference SUSY09 Boston (M.Herrero)

In this work we study the Lepton Flavour Violating semileptonic tau --> mu f0(980) decay within the context of SUSY-Seesaw Models, where the MSSM spectrum is extended by three right handed neutrinos and their SUSY partners, and where the seesaw mechanism is used to generate the neutrino masses. We estimate its decay rate when it proceeds via the Higgs mediated channel tau --> mu H* --> mu f0(980), where H refers to the CP-even MSSM Higgs bosons h^0 and H^0, and the Lepton Flavour Violating tau mu H vertex is radiatively generated via SUSY loops. In order to describe the f0(980) meson we follow the guidelines from chiral constraints. As an implication of our computation, we explore the sensitivity to the Higgs sector in this decay and compare it with other LFV tau decay channels. The confrontation of our predictions for BR(tau --> mu f0(980)) with its very competitive present experimental bound leads us to extract some interesting restrictions on the most relevant model parameters, particularly, tanbeta and m_{H^0}., Comment: 24 pages, 14 figures

In this paper we review our main results for Lepton Flavour Violating (LFV) semileptonic tau decays and muon-electron conversion in nuclei within the context of two Constrained SUSY-Seesaw Models, the CMSSM and the NUHM. The relevant spectrum is that of the Minimal Supersymmetric Standard Model extended by three right handed neutrinos, $\nu_{R_i}$ and their corresponding SUSY partners, ${\tilde \nu}_{R_i}$, ($i=1,2,3$). We use the seesaw mechanism for neutrino mass generation and choose a parameterisation of this mechanism that allows us to incorporate the neutrino data in our analysis of LFV processes. In addition to the full one-loop results for the rates of these processes, we will also review the set of simple formulas, valid at large $\tan \beta$, which are very useful to compare with present experimental bounds. The sensitivity to SUSY and Higgs sectors in these processes will also be discussed. This is a very short summary of the works in Refs. \cite{Arganda:2008jj} and \cite{Arganda:2007jw} to which we refer the reader for more details., Comment: 6 pages, 6 figures. To be published in the proceedings of the Tau08 Conference, Novosibirsk, Russia, 22-25 September 2008

The recent measurement of the tau -> Ks pion nu_tau spectrum by the Belle Collaboration provides the frame to set forth a theoretical description of the decay, which is based on the contributing vector F_+(s) and scalar F_0(s) form factors. We show that a good representation of data is obtained through the use of form factors calculated within resonance chiral theory and constrained by dispersion relations and short-distance QCD. Hence we obtain a determination of K*(892) parameters and the low-energy parameterization of F_+(s)., Comment: 4 pages, 2 figures

Here we review the main results of LFV in the semileptonic tau decays $\tau \to \mu PP$ ($PP = \pi^+ \pi^-, \pi^0 \pi^0, K^+ K^-, K^0 \bar{K}^0$), $\tau \to \mu P$ ($P = \pi, \eta, \eta^{\prime}$), and $\tau \to \mu V$ ($V = \rho, \phi$) as well as in $\mu-e$ conversion in nuclei within SUSY-seesaw scenarios, and compare our predictions with the present experimental bounds, Comment: Talk given by M. J. Herrero in SUSY08 conference, Seoul

In this work we study the Lepton Flavour Violating semileptonic $\tau$ decays: 1) $\tau \to \mu PP$ with $PP= \pi^+\pi^-, \pi^0\pi^0, K^+K^-, K^0 {\bar K}^0$; 2) $\tau \to \mu P$ with $P=\pi^0, \eta, \eta'$ and 3) $\tau \to \mu V$ with $V = \rho^0, \phi$. We work within the context of two constrained MSSM scenarios: the CMSSM and NUHM. A full one-(SUSY)loop computation is presented and the importance of the various contributions, the $\gamma$-, $Z$-, and Higgs bosons mediated ones are analysed. The hadronization of quark bilinears is performed within the chiral framework. Some discrepancies in the predicted rates for BR($\tau \to \mu \eta$), BR($\tau \to \mu \eta'$) and BR($\tau \to \mu K^+K^-$) are found with respect to previous estimates, which will be commented here. These three channels will be shown to be the most competitive ones to test simultaneously the Lepton Flavour Violation and the Higgs sector. We further present here a set of approximate formulas for all the semileptonic channels which we believe can be useful for further comparison with present and future data., Comment: 51 pages, 18 figures. Version published in JHEP06(2008)079

Chiral low-energy constants (LECs) carry the information of short-distance dynamics involving heavier degrees of freedom not present in the chiral Lagrangian. Our knowledge of the LECs is all-important at phenomenological level because their relevance in the prediction of hadronic observables at low-energies and on the other side because they provide hints on the construction of a dual theory of QCD in the low-energy regime. I review briefly the status of these important couplings., Comment: 7 pages. Talk given at the 31st International Conference of Theoretical Physics, 5-11 Sep 2007, Ustron (Katowice, Poland)

We perform an analysis of the form factors that rule the structure-dependent amplitude in the radiative pion decay. The resonance contributions to pion -> e nu_e gamma decays are computed through the proper construction of the vector and axial-vector form factors by setting the QCD driven asymptotic properties of the three-point Green functions VVP and VAP, and by demanding the smoothing of the form factors at high transfer of momentum. A comparison between theoretical and experimental determinations of the form factors is also carried out. We also consider and evaluate the role played by a non-standard tensor form factor. We conclude that, at present and due to the hadronic incertitudes, the search for New Physics in this process is not feasible., Comment: 14 pages, no figures. Typos corrected. Accepted for publication in The European Physical Journal C

Kl3 decays constitute, at present, the golden modes to extract |Vus| from experimental data. Its incertitude is dominated by the theoretical error in the determination of the vector form factor at zero transfer of momentum. I review the most relevant analytical approaches for the calculation of this parameter., Comment: 5 pages, 1 figure. Talk given at the 4th International Workshop on the CKM Unitarity Triangle, CKM2006, 12th-16th December 2006, Nagoya (Japan). References corrected. Conclusions unchanged

Chiral low-energy constants incorporate short-distance information from the dynamics involving heavier degrees of freedom not present in the chiral Lagrangian. We have studied the contribution of the lightest resonances to the chiral low-energy constants, up to O(p^6), within a systematic procedure guided by the large-Nc limit of QCD and also including short-distance asymptotic constraints., Comment: 8 pages, 1 figure. Talk given at the Final Euridice Meeting "Effective Theories of colours and flavours : from EURODAPHNE to EURIDICE", 24th-27th August 2006, Kazimierz (Poland)

The structure of the form factors stemmed from the hadronization of QCD currents in the energy region of the resonances can be explored through the analyses of exclusive hadronic decays of the tau lepton. I give a short review on the later theoretical progress achieved in the description of experimental data., Comment: 13 pages, 5 figures. Talk given at the 9th International Workshop on Tau Lepton Physics, TAU06, 19th-22nd September 2006, Pisa (Italy). Error in the text corrected. Settings and conclusions unchanged

With the newly available data sets on hadronic tau decays from the B-factories BABAR and BELLE, and future data from BESIII, precise information on the decay distributions will soon become available. This calls for an improvement of the decay spectra also on the theoretical side. In this work, the distribution function for the decay tau -> nu_tau K pi will be presented with the relevant K pi vector and scalar form factors being calculated in the framework of the resonance chiral theory, also taking into account additional constraints from dispersion relations and short-distances. As a by-product the slope and curvature of the vector form factor F_+^{K pi}(s) are determined to be lambda_+^' = 25.6 10^{-3} and lambda_+^{''} = 1.31 10^{-3} respectively. From our approach it appears that it should be possible to obtain information on the low lying scalar K_0^*(800) as well as the second vector K^*(1410) resonances from the tau decay data. In particular, the exclusive branching fraction of the scalar component is found to be B[tau -> nu_tau(K pi)_{S-wave}] = (3.88 +- 0.19) 10^{-4}., Comment: 14 pages, 2 figures

Guided by the large-Nc limit of QCD, we construct the most general chiral resonance Lagrangian that can generate chiral low-energy constants up to O(p^6). By integrating out the resonance fields, the low-energy constants are parametrized in terms of resonance masses and couplings. Information on those couplings and on the low-energy constants can be extracted by analysing QCD Green functions of currents both for large and small momenta. The chiral resonance theory generates Green functions that interpolate between QCD and chiral perturbation theory. As specific examples we consider the VAP and SPP Green functions., Comment: 48 pages, 3 figures. Enlarged explanation on the role of Proca fields. Conclusions and results unchanged. Version accepted for publication in Nuclear Physics B

We consider the Resonance Chiral Theory with one multiplet of scalar and pseudoscalar resonances, up to bilinear couplings in the resonance fields, and evaluate its beta-function at one-loop with the use of the background field method. Thus we also provide the full set of operators that renormalize the theory at one loop and render it finite., Comment: 37 pages, 2 figures

Tau decays into hadrons foresee the study of the hadronization of vector and axial-vector QCD currents, yielding relevant information on the dynamics of the resonances entering into the processes. We analyse tau -> (3 pion) nu_tau decays within the framework of the Resonance Chiral Theory, comparing this theoretical scheme with the experimental data, namely ALEPH spectral function and branching ratio. Hence we get values for the mass and on-shell width of the a_1(1260) resonance, and provide the structure functions that have been measured by OPAL and CLEO-II., Comment: 7 pages, 4 figures. Talk given at the International Workshop on Quantum Chromodynamics and Experiment (QCD@Work 2005), Conversano (Bari, Italy), 16th-20th June 2005. To appear in the Proceedings

Using the 1/N_C expansion scheme and truncating the hadronic spectrum to the lowest-lying multiplets of resonances we develop a procedure to implement QCD information on the non-perturbative regime populated by resonances through the use of three-point Green Functions of QCD currents. The process implies to enforce on these functions the constraints provided by their asymptotic behaviour ruled by QCD using their OPE or associated form factors. As a result we are able to determine several couplings of the O(p^6) Chiral Perturbation Theory Lagrangian and, in consequence, to address phenomenological issues such as the SU(3) breaking in Kl3 decays., Comment: 4 pages, no figures. Talk given at the High-Energy Physics International Conference on Quantum Chromodynamics, 2-9 July (2005), Montpellier (France).References added

Using the 1/N_C expansion scheme and truncating the hadronic spectrum to the lowest-lying resonances, we match a meromorphic approximation to the < S P P > Green function onto QCD by imposing the correct large-momentum falloff, both off-shell and on the relevant hadron mass shells. In this way we determine a number of chiral low-energy constants of O(p^6), in particular the ones governing SU(3) breaking in the K_{l3} vector form factor at zero momentum transfer. The main result of our matching procedure is that the known loop contributions largely dominate the corrections of O(p^6) to f_{+}(0). We discuss the implications of our final value f_{+}^{K^0 \pi^-}(0)=0.984 \pm 0.012 for the extraction of V_{us} from K_{l3} decays., Comment: 18 pages, published version: results and conclusions unchanged

Exclusive hadronic decays of the tau lepton provide an excellent framework to study the hadronization of QCD currents in a non-perturbative energy region populated by many resonances. I give a short review both on the main theoretical tools employed to analyse experimental data and on how Theory compares with Experiment., Comment: 13 pages, 4 figures. Invited talk given at the International Workshop on Tau Lepton Physics, TAU04 (14th-17th September 2004), Nara (Japan)

We analyse the three-point function of vector, axial-vector and pseudoscalar currents. In the spirit of large N_C, a resonance dominated Green function is confronted with the leading high-energy behaviour from the operator product expansion. The matching is shown to be fully compatible with a chiral resonance Lagrangian and it allows to determine some of the chiral low-energy constants of O(p^6)., Comment: 13 pages, 2 figures. Published version. Results and conclusions unchanged

We analyse tau -> (3 pions) nu_tau decays within the framework of the resonance chiral theory of QCD. We have worked out the relevant Lagrangian that describes the axial-vector current hadronization contributing to these processes, and the new coupling constants that arise have been constrained by imposing the asymptotic behaviour of the corresponding spectral function within QCD. Hence we compare the theoretical framework with the experimental data, obtaining a good quality fit from the ALEPH spectral function and branching ratio. We also get values for the mass and on-shell width of the a_1(1260) resonance, and provide the tau -> (3 pions) nu_tau structure functions that have been measured by OPAL and CLEO-II finding an excellent agreement., Comment: 5 pages, 4 figures

tau -> (3 pions) nu_tau decays are analysed within the framework of the resonance effective theory of QCD. We work out the relevant Lagrangian that describes the axial-vector current hadronization contributing to these processes, in particular the local a_1(1260)-rho(770)-Goldstone interactions. The new coupling constants are constrained by imposing the asymptotic behaviour of the corresponding spectral function within QCD. Hence we compare the theoretical framework with the experimental data, obtaining a good quality fit from the ALEPH spectral function and branching ratio. We also get values for the mass and on-shell width of the a_1(1260) resonance. In this way we are able to provide the structure functions that have been measured by OPAL and CLEO-II and we find an excellent agreement., Comment: 24 pages, 7 figures

We study the hadronic vacuum polarization in the resonance energy region, using the framework given by the Resonance Effective Theory of QCD. We consider the incorporation of vector-pseudoscalar meson loops that give, inclusively, three and four pseudoscalar meson cuts. After resummation we achieve a QCD-based inclusive parameterization of the correlator, hence of the hadronic cross-section in the energy region populated by resonances., Comment: 6 pages, 5 figures. To appear in the Proceedings of the Workshop on Hadronic Cross-Section at Low Energy (SIGHAD03), 8th-10th October 2003, Pisa (Italy)

We analyse the odd-intrinsic-parity effective Lagrangian of QCD valid for processes involving one pseudoscalar with two vector mesons described in terms of antisymmetric tensor fields. Substantial information on the odd-intrinsic-parity couplings is obtained by constructing the vector-vector-pseudoscalar Green's three-point function, at leading order in 1/N_C, and demanding that its short-distance behaviour matches the corresponding OPE result. The QCD constraints thus enforced allow us to predict the decay amplitude omega -> pi gamma, the O(p^6) corrections to pi -> gamma gamma and the slope parameter in pi -> gamma gamma^*., Comment: 4 pages, 1 figure. Talk given at QCD 03: High-Energy Physics International Conference in Quantum Chromodynamics, Montpellier, France, 2-8 Jul 2003

We analyse the most general odd-intrinsic-parity effective Lagrangian of QCD valid for processes involving one pseudoscalar with vector mesons described in terms of antisymmetric tensor fields. Substantial information on the odd-intrinsic-parity couplings is obtained by constructing the vector-vector-pseudoscalar Green's three-point function, at leading order in 1/Nc, and demanding that its short-distance behaviour matches the corresponding OPE result. The QCD constraints thus enforced allow us to predict the decay amplitude omega -> pion gamma, and the O(p^6) corrections to pion -> gamma gamma. Noteworthy consequences concerning the vector meson dominance assumption in the decay omega -> 3 pions are also extracted from the previous analysis., Comment: 20 pages, 4 figures

We study a model-independent parameterization of the vector pion form factor that arises from the constraints of analyticity and unitarity. Our description should be suitable up to s^(1/2) ~ 1.2 GeV and allows a model-independent determination of the mass of the rho(770) resonance. We analyse the experimental data on tau^- -> pion^- pion^0 nu_tau and e^+ e^- -> pion^+ pion^- in this framework, and its consequences on the low-energy observables worked out by chiral perturbation theory. An evaluation of the two pion contribution to the anomalous magnetic moment of the muon, a_{mu}, and to the fine structure constant, alpha(M_Z^2), is also performed., Comment: 5 pages, 2 figures. To appear in the proceedings of the High-Energy Physics International Conference on Quantum Chromodynamics QCD02, Montpellier (France), 2-9 July (2002)

We evaluate the two-photon box contribution to heavy fermion production in electron positron annihilation, that provides O(alpha^2) electromagnetic corrections to the Born cross section. The study of its non-relativistic expansion, relevant at energies close to the threshold of production, is also performed. We also verify that the threshold expansion of the one-loop integrals correctly reproduces our results, thus extending the applicability of this technique to heavy fermion production diagrams., Comment: 18 pages, 2 figures

We analyse new contributions to the theoretical input in heavy quark sum rules and we show that the general theory of singularities of perturbation theory amplitudes yields the method to handle these specific features. In particular we study the inclusion of heavy quark radiation by light quarks at O(alpha_s^2) and non-symmetric correlators at O(alpha_s^3). Closely related, we also propose a solution to the construction of moments of the spectral densities at O(alpha_s^3) where the presence of massless contributions invalidates the standard approach. We circumvent this problem through a new definition of the moments, providing an infrared safe and consistent procedure., Comment: 15 pages, 3 figures. Version accepted for publication in The European Journal of Physics C; several new comments and references added, conclusions unchanged

Within the framework of phenomenological Lagrangians we construct the effective action of QCD relevant for the study of semileptonic decays of charmed mesons. Hence we evaluate the form factors of D -> P(0^-) l^+ nu_l at leading order in the 1/N_C expansion and, by demanding their QCD-ruled asymptotic behaviour, we constrain the couplings of the Lagrangian. The features of the model-independent parameterization of form factors provided and their relevance for the analysis of experimental data are pointed out., Comment: 1+24 pages, 1 figure. Updated version. Conclusions unchanged. Accepted for publication in The European Physical Journal C

Recently Groote and Pivovarov have given notice of a possible fault in the use of sum rules involving two-point correlation functions to extract information on heavy quark parameters, due to the presence of massless contributions that invalidate the construction of moments of the spectral densities. Here we show how to circumvent this problem through a new definition of the moments, providing an infrared safe and consistent procedure., Comment: 1+9 pages, 3 figures. Discussion on QCD sum rules applications added. Conclusions unchanged. Version to be published in Journal of Physics G

We study a model-independent parameterization of the vector pion form factor that arises from the constraints of analyticity and unitarity. Our description should be suitable up to sqrt(s) ~ 1.2 GeV and allows a model-independent determination of the mass of the rho(770) resonance, M(rho) = (775.1 +- 0.5) MeV. We analyse the experimental data on tau(-) -> pion(-) pion(0) neutrino(tau), in this framework, and its consequences on the low-energy observables worked out by chiral perturbation theory. An evaluation of the two-pion contribution to the anomalous magnetic moment of the muon, a_muon, and to the fine structure constant, alpha(M(Z)^2), is also performed., Comment: 1+15 pages, 2 figures

Until present the study of the form factors associated to the vector and axial-vector components of the hadronic weak current has been carried out with a plethora of modelizations of the underlying strong interactions. While of importance to get an understanding of the dynamics involved, the amount and quality of the experimental data start to show some discrepancies with the analysed models. Moreover, and from a theoretical point of view, most of these models are not consistent with quantum chromodynamics (QCD). We propose a QCD-based model-independent procedure to analyse those decays through the use of the resonance chiral theory, the low-energy effective action of QCD in the relevant resonance region. Within this framework we study the hadronic off-shell width of the Rho meson and the vector form factor of the pion in Tau(-) -> Pi(-) Pi(0) nu_tau. We also comment on the Tau(-) -> (Pi Pi Pi)(-) nu_tau decay., Comment: 8 pages, 4 figures. To appear in the proceedings of the 6th International Workshop on Tau Lepton Physics, 18-21 September (2000), Victoria (Canada)

Within the resonance chiral effective theory we study the dressed propagators of the spin-1 fields, as arise from a Dyson-Schwinger resummation perturbatively constructed from loop diagrams with absorptive contributions in the s-channel. We apply the procedure to the vector pion form factor and the elastic pion-pion scattering to obtain the off-shell width of the rho_0 meson. We adopt a definition of the off-shell width of spin-1 meson resonances that satisfies the requirements of analyticity, unitarity, chiral symmetry and asymptotic behaviour ruled by QCD. To fulfil these constraints the resummation procedure cannot consist only of self-energy diagrams. Our width definition is shown to be independent of the formulation used to describe the spin-1 meson resonances., Comment: 1+12 pages, 4 figures. Minor changes in text. Conclusions unchanged. Version accepted for publication in Phys. Rev. D

We analyze the supersymmetric contributions to direct-CP-violating observables in $K \to \pi\pi\gamma$ decays induced by gluino-mediated magnetic-penguin operators. We find that $\epsp_{+-\gamma}$ and the differential width asymmetry of $K^\pm \to \pi^\pm \pi^0 \gamma$ decays could be substantially enhanced with respect to their Standard Model values, especially in the scenario where $\epsp/\eps$ is dominated by supersymmetric contributions. These observables could therefore provide a useful tool to search for New Physics effects in $|\Delta S|=1$ transitions, complementary to $\epsp/\eps$ and rare decays., Comment: 14 pages, Latex. Numerical error in Eq.(36) corrected, conclusions unchanged; version to appear in Phys. Lett. B

We present a model-independent analysis of $K^+\to\pi^+\ell^+\ell^-$ and $K_S\to\pi^0\ell^+\ell^-$ decays, including $K\to 3 \pi$ unitarity corrections and a general decomposition of the dispersive amplitude. From the existing data on $K^+\to\pi^+ e^+ e^-$ we predict the ratio $R= B(K^+\to\pi^+\mu^+\mu^-)/B(K^+\to\pi^+ e^+ e^-)$ to be larger than 0.23, in slight disagreement with the recent measurement $R = 0.167 \pm 0.036$. Consequences for the $K^\pm \to\pi^\pm e^+ e^-$ charge asymmetries and for the $K_L\to\pi^0 e^+ e^-$ mode are also discussed., Comment: 18 pages, latex

A new analysis of the long-distance two-photon dispersive amplitude of K_L -> mu^+ mu^- is presented. We introduce a phenomenological parameterization of the K_L -> gamma^* gamma^* form factor, constrained at low energies by K_L -> gamma l^+ l^- (l = e,mu) data and at high energies by perturbative QCD. Using this form factor we provide a reliable estimate of magnitude and relative uncertainty of the two-photon dispersive contribution to K_L -> mu^+ mu^-. We finally discuss the implications of this analysis for the extraction of short-distance information from B(K_L -> mu^+ mu^-)., Comment: 12 pages, LaTeX, 3 figures included as PS files

The D^0-D^0(bar) mixing is analyzed in a weak gauged U(4)_L x U(4)_R chiral lagrangian model where the electroweak interaction is introduced as a gauge theory over the meson degrees of freedom. This model allows a particular realization of the G.I.M. mechanism and then could be useful in the study of processes where G.I.M. suppression is effective. As a test of the model we have also analyzed the K^0-K^0(bar) mixing. We find \Delta m_K in good agreement with the experimental result and we show that the D^0-D^0(bar) mixing is very much suppressed in agreement with previous estimates in the Heavy Quark expansion framework., Comment: 14 pages, 2 figures, Latex, minor changes in the writing, final version to be published in Phys. Lett. B

A way of testing the $\pi \pi$ predictions of Chiral Perturbation Theory against experimental data is to use dispersion relations to continue experimental information into the subthreshold region where the theory should unambiguously apply. Chell and Olsson have proposed a test of the subthreshold behaviour of chiral expansions which highlights potential differences between the Standard and the Generalized forms of the theory. We illustrate how, with current experimental uncertainties, data cannot distinguish between these particular {\em discriminatory} coefficients despite their sensitivity. Nevertheless, the Chell--Olsson test does provide a consistency check of the chiral expansion, requiring that the ${\cal O}(p^6)$ corrections to the {\em discriminatory} coefficients in the Standard theory must be $\sim 100%$. Indeed, some of these have been deduced from the new ${\cal O}(p^6)$ computations and found to give such large corrections. One can then check that the ${\cal O}(p^8)$ corrections must be much smaller. We conclude that this test, like others, cannot distinguish between the different forms of Chiral Symmetry Breaking embodied in the alternative versions of Chiral Perturbation Theory without much more precise experimental information near threshold., Comment: 23 pages, plain LaTeX file, 1 figure included as several PS files, uses epsf.sty, submitted to Phys. Rev. D

We compute the one-loop unitarity corrections O(p^6) from K^+ -> pi^+ pi^+ pi^- to K^+ -> pi^+ gamma gamma and we find they are relevant, increasing the leading order prediction for the width in a 30-40%. The contributions of local O(p^6) amplitudes, generated by vector meson exchange, are discussed in several models and we conclude that the vector resonance contribution should be negligible compared to the unitarity corrections., Comment: 15 pages, LaTex, 4 figures included as PS files, uuencoded with uufiles, submitted to Phys. Lett. B

The all--important consequence of Chiral Dynamics for $\pi \pi$ scattering is the Adler zero, which forces $\pi \pi$ amplitudes to grow asymptotically. The continuation of this subthreshold zero into the physical regions requires a $P$--wave resonance, to be identified with the $\rho$. It is a feature of $\pi \pi$ scattering that convergent dispersive integrals for the $I=1$ channel are essentially saturated by the $\rho$--resonance and are much larger than those with $I=2$ quantum numbers. These facts predict the parameters $\overline{\ell}_1$, $\overline{\ell}_2$ of the Gasser--Leutwyler Chiral Lagrangian, as well as reproducing the well--known KSFR relation and self-consistently generating the $\rho-$resonance., Comment: Plain LaTeX file, 13 pages, 3 figures not included (available from the Authors under request). A comparison of our procedure with Chiral Perturbation Theory has been included. Results have not changed. Accepted for publication in Phys. Lett. B

The polarizabilities of the pion have been predicted in several different theoretical frameworks. The status of these is reviewed., Comment: 19 pages, no figures, DTP-94/52. To be published in the Second DAPHNE Physics Handbook, ed. G. Pancheri and N. Paver. (Slightly extended version)

Non--resonant decays of charmed mesons into three pseudoscalars are analyzed in a weak gauged $U(4)_L \otimes U(4)_R$ chiral lagrangian model. The calculation is free of unknown parameters and only requires the masses of pseudoscalar mesons as hadronic inputs. When comparison with experimental data is possible we find that in some processes we have good agreement and in others we are an order of magnitude below. This may be due to the absence in our calculation of final state interactions, presumably important here, and the manifest difficulty in extracting the non--resonant contribution from an experimental point of view., Comment: 20 pages, LaTeX (no macros needed) , two figures included (feynman.tex file needed for the figures appended at the end of the file). Version accepted for publication in Phys. Lett. B