Theoretical developments on Quartic Gauge Boson Couplings at LEP

The search for quartic anomalous gauge couplings (QAGC) at LEP requires appropriate predictions for the radiative processes e+ e- \to \nu\bar\nu \gamma\gamma, e+ e- \to q\bar{q}\gamma\gamma and e+ e- \to 4 fermions+\gamma. The current knowledge on dimension-six operators giving rise to QAGC is briefly reviewed, together with their implementation in event generators. The accuracy of calculations based on real approximations (used up to now for the LEP experimental analysis) is examined by comparing them with the available exact matrix element calculations.Comment: LaTeX, JHEP style, 5 pag. Talk presented at EPS HEP2001, Budapest, Hungary, 12 - 18 July, 200

MCSTHAR++, a Monte Carlo code for the microcanonical hadronization

MCSTHAR++ is a new Monte Carlo code implementing the Statistical Hadronization Model. This model assumes that hadronization proceeds through the microcanonical decay of massive extended clusters. Unlike other hadronization models, in this approach very few free parameters are needed, as has been demonstrated in previous studies. The tuning of the model and the comparison with the data is ongoing.Comment: 3 pages; To appear in the proceedings of the conference IFAE 2010, Roma, Italy, 7-9 April 201

O(α2){\cal O} (\alpha^2) Next-to-Leading Photonic Corrections to Small-Angle Bhabha Scattering in the Structure Function Formalism

A general method for computing ${\cal O} (\alpha^2)$ and higher-order next-to-leading photonic corrections is presented and applied to the precision calculation of the small-angle Bhabha scattering cross section in the phase-space region of interest for the luminosity measurement at LEP. The formulation is based on a proper matching of exact $\cal O (\alpha)$ results with higher-order corrections in the Structure Function formalism. The results of the approach are analytically compared with theoretical calculations, both for $s$- and $t$-channel processes, available for simple Event Selections. Numerical predictions for realistic Event Selections are also provided and critically compared with the ones existing in the literature.Comment: 16 pages, gzip'ed uuencoded tar fil

H -> gamma gamma: a Comment on the Indeterminacy of Non-Gauge-Invariant Integrals

We reanalyze the recent computation of the amplitude of the Higgs boson decay into two photons presented by Gastmans et al.. The reasons for which this result cannot be the correct one have been discussed in some recent papers. We address here the general issue of the indeterminacy of integrals with four-dimensional gauge-breaking regulators and to which extent it might eventually be solved by imposing physical constraints. Imposing gauge invariance as the last step upon R_xi gauge calculations with four-dimensional gauge-breaking regulators, allows indeed to recover the well known H -> gamma gamma result. However we show that in the particular case of the unitary gauge, the indeterminacy cannot be tackled in this same way. The combination of unitary gauge with a cutoff regularization scheme turns out to be non-predictive.Comment: 17 pages, 6 figure

A Mechanism for Hadron Molecule Production in p pbar(p) Collisions

We propose a mechanism allowing the formation of loosely bound molecules of charmed mesons in high energy proton-(anti)proton collisions.Comment: 4 pages, 3 figure

Six-fermion production and Higgs boson physics at future e+e−e^+ e^- colliders

The six-fermion production processes $e^+e^-\to q\bar q l^+ l^-\nu\bar\nu$, with all the lepton flavours and $q=u,d,c,s$, relevant to the study of the intermediate-mass Higgs boson at future $e^+e^-$ linear colliders, are analysed. A Monte Carlo program, taking into account the whole set of tree-level scattering amplitudes and the relevant radiative effects, is developed to provide integrated cross sections and generation of unweighted events. The complete calculation is compared with the available results of real Higgs production, and the opportunities of precision studies with event generation are discussed, demonstrating the relevance of a full six-fermion calculation. Numerical results for integrated cross sections with various kinematical cuts and including radiative effects are given and commented. In the analysis of event samples, several distributions are studied and found to be sensitive to the presence and to the properties of the Higgs boson.Comment: LaTeX, 27 pages, 13 figures included. Minor changes, final version to appear in Eur. Phys. J.

Six-Fermion Calculation of Intermediate-mass Higgs Boson Production at Future e+e−e^+ e^- Colliders

The production of an intermediate-mass Higgs boson in processes of the kind $e^+ e^- \to 6$ fermions at the energies of future linear colliders is studied. The recently developed and fully automatic algorithm/code ALPHA is used to compute the tree-level scattering amplitudes for the reactions $e^+ e^- \to \mu^+ \mu^- \tau^- \bar\nu_{\tau} u \bar d, \mu^+ \mu^- e^- \bar\nu_{e} u \bar d$. The code has been interfaced with the Monte Carlo program HIGGSPV/WWGENPV, properly adapted to 6-fermion production, in order to provide realistic results, both in the form of cross sections and event samples at the partonic level. Phenomenological results, that incorporate the effects of initial-state radiation and beamstrahlung, are shown and commented, emphasizing the potentials of full six-fermion calculations for precise background evaluation as well as for detailed studies of the fundamental properties of the Higgs particle.Comment: 31 pages, LaTeX, 15 eps low-resolution figures include