Fernando Cornet

Two-pion production in photon-photon collisions

Abstract We calculate within chiral perturbation theory the cross section for γγ → π 0 π 0 and γγ → π + π − to next-to-leading order. The first process only depends on the one-loop contributions and forms a test of chiral perturbation theory. We also calculate the pion electromagnetic form factor and compare it to recent data.

Three Pseudoscalar Photon Interactions in Chiral Perturbation Theory

Abstract We have calculated the next to leading corrections to the processes γπ − → π 0 π − and η → π + π − γ in chiral perturbation theory. The loop diagrams are divergent, requiring the contributions from the O( p 6 ) lagrangian. The finite parts of the coefficients of the O( p 6 ) terms are calculated assuming their saturation by the vector meson contribution. These corrections improve the agreement between the theoretical predictions and the experimental data.

Chiral perturbation theory for η→π0γγ

The η→π0γγ decay amplitude is discussed in the general context of chiral perturbation theory assuming that the low-energy constants (counterterms) are saturated by the known meson resonances. The suppression of the lowest order contributions is understood in physical terms. A reasonable agreement with the experimental decay width is achieved.

Chiral perturbation theory for anomalous processes

A general expression containing all the divergences in Chiral Perturbation Theory at one loop for anomalous processes has been calculated. The nonrenormalization of some amplitudes (Adler-Bardeen theorem) is observed. The different types of corrections are identified. The contributions from vector meson exchange are determined for all anomalous processes. Explicit cancellation of divergences is shown for π0, η and η′ decays to γγ*. The dependence on the γ* mass squared is predicted to be essentially the same for all the three processes. It proceeds mainly through the VMD estimate of the finite parts of the next order Lagrangian and compares reasonably well with the available experimental data.

New 5 flavor next-to-leading-order analysis and parametrizations of parton distributions of the real photon

New, radiatively generated, NLO quark (u,d,s,c,b) and gluon densities in a real, unpolarized photon are presented. We perform three global fits, based on the NLO DGLAP evolution equations for Q^2>1 GeV^2, to all the available structure function F_2^gamma(x,Q^2) data. As in our previous LO analysis we utilize two theoretical approaches. Two models, denoted as FFNS_{CJK}1&2 NLO, adopt the so-called Fixed Flavour-Number Scheme for calculation of the heavy-quark contributions to F_2^gamma(x,Q^2), the CJK NLO model applies the ACOT(chi) scheme. We examine the results of our fits by a comparison with the LEP data for the Q^2 dependence of the F_2^gamma, averaged over various x-regions, and the F_2,c^gamma. Grid parametrizations of the parton densities for all fits are provided.

Semileptonic π and K decays and the chiral anomaly at one-loop

We calculate the O(p6) corrections to the anomalous form factors appearing in π+, K+ → e+νγ, μ+νγ and K14 decays in chiral perturbation theory. The relevant dimension 6 terms of the lagrangian are evaluated assuming their saturation by the vector meson contribution.

New 5-flavor LO analysis and parametrization of parton distributions in the real photon

New, radiatively generated, LO quark (u,d,s,c,b) and gluon densities in a real, unpolarized photon are presented. We perform a global 3-parameter fit, based on LO DGLAP evolution equations, to all available data for the structure function F2^gamma(x,Q^2). We adopt a new theoretical approach called ACOT(chi), originally introduced for the proton, to deal with the heavy-quark thresholds. This defines our basic model (CJKL model), which gives a very good description of the experimental data on F2^gamma(x,Q^2), for both Q^2 and x dependences. For comparison we perform a standard fit using the Fixed Flavour-Number Scheme (FFNS_CJKL model), updated with respect to the previous fits of this type. We show the superiority of the CJKL fit over the FFNS_CJKL one and other LO fits to the F2^gamma(x,Q^2) data. The CJKL model gives also the best description of the LEP data on the Q^2 dependence of the F2^gamma, averaged over various x-regions, and the F_2,c^gamma, which were not used directly in the fit. Finally, a simple analytic parametrization of the resulting parton densities obtained with the CJKL model is given.

Four-fermion contact terms in charged current processes and large extra dimensions

We study the bounds that can be obtained on four-fermion contact terms from the experimental data for

Pair production of Two Higgs Doublet Model light Higgs bosons in γγ collisions

{e}^{+}\stackrel{\ensuremath{\rightarrow}}{p}\overline{\ensuremath{\nu}}X

tau -pair production via photon-photon collisions at the CERN e+e- collider LEP.

obtained at DESY HERA and