Insights into Q2Qˉ2Q^2 \bar{Q}^2 states from an effective perspective

We discuss the two photon coupling of the lightest scalar meson on the basis of an extension of $\chi$PT. Using low energy data on the pion form-factor and the $\gamma\gamma\to \pi^+\pi^-(\pi^0\pi^0)$ cross-sections as inputs, we find $\Gamma(\sigma\to\gamma\gamma) \cong 0.126~\rm{keV}$. The smallness of the result and the relative weight between its components, ${\Gamma_{\gamma\gamma\to S_1}\over \Gamma{ \gamma\gamma\to\pi\pi\to S_1}} < 1$, suggests that the scalar $0^{++}$ meson is mainly a $Q^2\bar{Q}^2$ state.Comment: 14 pages, 6 figures. Part of the paper rewritten and enlarged, including a discussion on pi-pi phase shifts. Added reference

QCD corrections to forward-backward charge asymmetries in l- l+ j production at hadron colliders

The large cross sections for gauge boson production at the Fermilab Tevatron and the CERN Large Hadron Collider (LHC) might give a chance to determine the electroweak parameters with high precision. We calculated two different forward-backward charge asymmetries (A^CS_FB and A^j_FB) of lepton pairs in events with a large transverse momentum jet p p (pbar) -> Z, gamma* + j -> e- e+ j at next-to-leading order (NLO), O(alpha_s) corrections, making use of the Monte Carlo programs MCFM and ALPGEN. These observables could provide a new determination of the weak mixing angle sin^2 theta^lept_eff (M_Z^2) with a statistical precision for each lepton flavour of 10^{-3} (7 x 10^{-3}) at LHC (Tevatron). If b jets are identified, a new asymmetry with respect to the b quark (A^b_FB) can also be measured with a statistical precision of 2 x 10^{-3} (4 x 10^{-2}) at LHC (Tevatron). Finally, we comment on the dependence of our results on various sources of uncertainties and compare, in the case of A^b_FB, the exact result with an approximation that might be more suitable when performing a realistic experimental analysis.Comment: 5 pages, 1 figure, talk presentad by R. Pittau at the International Europhysics Conference on High Energy Physics, July 21st - 27th 2005, Lisbon, Portuga

Semileptonic π\pi and KK Decays and the Chiral Anomaly at One-loop

We calculate the $O(p^6)$ corrections to the anomalous form factors appearing in $\pi^+$, $K^+ \to e^+ \nu \gamma,\ \mu^+\nu\gamma$ and $K_{l4}$ decays in Chiral Perturbation Theory. The relevant dimension $6$ terms of the lagrangian are evaluated assuming their saturation by the vector meson contribution.Comment: 10 pages of LaTeX (+3 figures available on request

Neural network analysis for gamma gamma -> 3pi at Daphne

We consider the possibility of using neural networks in experimental data analysis in Daphne. We analyze the process $\gamma\gamma\to \pi^+ \pi^- \pi^0$ and its backgrounds using neural networks and we compare their performances with traditional methods of applying cuts on several kinematical variables. We find that the neural networks are more efficient and can be of great help for processes with small number of produced events.Comment: 11 pages, latex, 2 figures

Chiral-loop and vector-meson contributions to eta -> pi pi gamma gamma decays

The process eta -> pi0 pi0 gamma gamma is discussed in Chiral Perturbation Theory (ChPT) extending two recent analyses. Special attention is devoted to one-loop corrections, eta-eta' mixing effects and vector-meson dominance of ChPT counter-terms. The less interesting eta -> pi^+ pi^- gamma gamma transition is briefly discussed too.Comment: 15 pages, 3 Postscript figures, uses epsfig.st

Discriminating signal from background using neural networks. Application to top-quark search at the Fermilab Tevatron

The application of Neural Networks in High Energy Physics to the separation of signal from background events is studied. A variety of problems usually encountered in this sort of analyses, from variable selection to systematic errors, are presented. The top--quark search is used as an example to illustrate the problems and proposed solutions.Comment: 11 pages, 3 figures, psfi

Vector-like fermion and standard Higgs production at hadron colliders

Vector-like fermions are characterized by large neutral current decay rates, in particular into Higgs bosons. If they exist, their clear signals at hadron colliders open a window to Higgs detection, especially to the intermediate Higgs mass region. We discuss in some detail rates and signatures for simple cases.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28619/1/0000431.pd

Enhancing the top signal at Tevatron using Neural Nets

We show that Neural Nets can be useful for top analysis at Tevatron. The main features of $t\bar t$ and background events on a mixed sample are projected in a single output, which controls the efficiency and purity of the $t\bar t$ signal.Comment: 11 pages, 6 figures (not included and available from the authors), Latex, UB-ECM-PF 94/1