Chiral Dynamics in the electroweak sector

Using a chiral Lagrangian we show that strongly interacting models of electroweak symmetry breaking are not in conflict with precision data. Such models, like Technicolor, need not lead to a heavy Higgs-like signal. Furthermore, the allowed values for the low-energy constants in the effective Lagrangian, derived from bounds on the oblique correction parameters S,T,U, are not unnatural. Finally, we point out that there are some problems with gauge invariance, if one tries to relate the oblique parameters to the low-energy constants in the ordinary chiral Lagrangian for QCD of Gasser and Leutwyler. In particular, S cannot be identified with l_5^{GL}.Comment: 2 pages, Latex (sprocl.sty); Talk held at the Workshop Chiral Dynamics 2000, Newport News, USA, 17-22 July 200

Rho meson properties in nuclear matter from QCD sum rules

We study the properties of rho mesons in nuclear matter by means of QCD sum rules at finite density. For increased sensitivity, we subtract out the vacuum contributions. With the spectral function as estimated in the literature, these subtracted sum rules are found to be not well satisfied. We suppose that Landau singularities from higher resonance states in the nearby region in this channel are the cause for this failure.Comment: 5 pages including 1 figure, Latex, uses sprocl.sty, psfig; Presented at the Conference on Strong and Electroweak Matter (SEWM 2000), Marseille, France, 14-17 June 200

Hadronic light-by-light scattering contribution to the muon g-2 in large-N_C QCD

We present our recent semi-analytical evaluation of the pion-pole contribution to hadronic light-by-light scattering, using a description of the pion-photon-photon form factor based on large-N_C and short-distance properties of QCD. Inclusion of all light pseudoscalar states leads to a_{mu}(LbyL;PS) = + 8.3 (1.2) x 10^{-10}. We also sketch an effective field theory approach to hadronic light-by-light scattering. It yields the leading logarithmic terms that are enhanced by a factor N_C and shows that the sign of a_{mu}(LbyL;had) cannot be inferred from a constituent quark-loop. In view of several problematic issues that still remain to be clarified, our estimate for the full hadronic light-by-light scattering contribution is a_{mu}(LbyL;had) = + 8 (4) x 10^{-10}.Comment: 10 pages including 3 figures, Latex, uses ws-p8-50x6-00.cls; Talk presented at the Workshop on the Phenomenology of Large N_C QCD, Tempe, Arizona, USA, 9-11 January 200

Hadronic light-by-light scattering in the muon g-2: impact of proposed measurements of the (pi^0 -> gamma gamma) decay width and the (gamma^* gamma -> pi^0) transition form factor with the KLOE-2 experiment

We discuss, how planned measurements at KLOE-2 of the (pi^0 -> gamma gamma) decay width and the (gamma^* gamma -> pi^0) transition form factor can improve estimates for the numerically dominant pion-exchange contribution to hadronic light-by-light scattering in the muon g-2 and what are the limitations related to the modelling of the off-shellness of the pion.Comment: 4 pages, talk presented at MESON 2012 - 12th International Workshop on Meson Production, Properties and Interaction, Krakow, Poland, 31 May - 5 June 201

The electroweak chiral Lagrangian revisited

Using a manifestly gauge-invariant approach we show that the set of low-energy constants in the electroweak chiral Lagrangian currently used in the literature is redundant. In particular, by employing the equations of motion for the gauge fields, one can choose to remove two low-energy constants which contribute to the self-energies of the gauge bosons. The relation of this result to the experimentally determined values for the oblique parameters S,T and U is discussed. We then evaluate the matching relation between gauge-invariant Green's functions in the full and the effective theory for the case of the Standard Model with a heavy Higgs boson and compare the results for the independent low-energy constants with those for a simple Technicolor model. Since the pattern of the low-energy constants is very different in these two models it may be misleading to mimic any strongly interacting symmetry breaking sector by a heavy Higgs boson. From our investigation we conclude that current electroweak precision data do not really rule out such strongly interacting models.Comment: 10 pages, Latex. Based on talks presented at the Linear Collider Workshop, Oxford, 20-23 March 1999 and at the International Symposium Ahrenshoop, Buckow, Germany, 23-28 August 1999. To appear in the Proceedings of the 2nd ECFA/DESY Study on Physics Studies for a Future Linear Collider. v2: one sign in Table 1 and some typos corrected; some comments added in Section 5; Conclusions unchange

Status of hadronic light-by-light scattering in the muon g-2

We give an update on the status of the hadronic light-by-light scattering contribution to the muon g-2. We review recent work by various groups, list some of the open problems and give an outlook on how to better control the uncertainty of this contribution. This is necessary in order to fully profit from planned future muon g-2 experiments to test the Standard Model. Despite some recent developments, we think that the estimate a_{mu}^{HLbL} = (116 \pm 40) x 10^{-11} still gives a fair description of the current situation.Comment: 9 pages, 2 figures, based on presentations at PHIPSI13, Rome; Working Group on Radiative Corrections and Monte Carlo Generators for Low Energies, Frascati; LC13, Trento, September 201

Resonance estimates of O(p^6) low-energy constants and QCD short-distance constraints

Starting from the study of the low-energy and high-energy behaviours of the QCD three-point functions , and , several O(p^6) low-energy constants of the chiral Lagrangian are evaluated within the framework of the lowest meson dominance (LMD) approximation to the large-N_C limit of QCD. In certain cases, values that differ substantially from estimates based on a resonance Lagrangian are obtained. It is pointed out that the differences arise through the fact that QCD short-distance constraints are in general not correctly taken into account in the approaches using resonance Lagrangians. We discuss the implications of our results for the O(p^6) counterterm contributions to the vector form factor of the pion and to the decay \pi -> e \nu_e \gamma, and for the pion-photon-photon transition form factor.Comment: 39 pages, Latex. v2: some typos corrected, references updated, version to appear in Eur. Phys. J.