arXiv subjects

M. Moretti

Publications and source records attributed to M. Moretti.

52 records · Page 3Linked to original sources

arXiv1999

Top-quark physics in six-quark final states at the Next Linear Collider

The processes of six-quark production with one $b\bar b$ pair are studied by means of a complete tree-level electroweak calculation. The top-quark signal is examined: the importance of electroweak backgrounds, of the order of 10% above the $t\bar t$ threshold and of about 30% of the purely electroweak signal at threshold, is further stressed by studying the dependence of the cross-section at threshold on the Higgs mass in the range between 100 GeV and 185 GeV, and finding variations of the order of 10%. In the study of some event-shape variables, a strong effect of initial-state radiation is found, in particular for the thrust distribution, which is studied for several centre-of-mass energies at the TeV scale. The effectiveness of cuts on the thrust for isolating QCD backgrounds, as pointed out by some authors, is confirmed also in the presence of electroweak backgrounds and initial-state radiation.

hep-ph↗

arXiv1999

Light-Pair Corrections to Small-Angle Bhabha Scattering in a Realistic Set-up at LEP

Light-pair corrections to small-angle Bhabha scattering have been computed in a realistic set-up for luminosity measurements at LEP. The effect of acollinearity and acoplanarity rejection criteria has been carefully analysed for typical calorimetric event selections. The magnitude of the correction, depending on the details of the considered set-up, is comparable with the present experimental error.

hep-ph↗

arXiv1999

The relevance of the vertex bremsstrahlung photon detection in the electron-neutrino (antineutrino) electron scattering experiments at low energy

We discuss the size of the electron-antineutrino electron into electron-antineutrino electron cross section reduction due to the rejection of the events with a vertex bremsstrahlung photon above a certain energy in the final state. In particular we analyze the effect in experiments designed to detect the low energy electron-antineutrino and electron-neutrino from a nuclear reactor and from the Sun. We find that such reduction has to be considered in a relatively high statistic reactor experiment, while it is negligible for pp and 7Be solar neutrino detection.

hep-ph↗

arXiv1999

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

The six-fermion production processes $e^+e^-\to q\bar q l^+ l^-ν\barν$, 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.

hep-ph↗

arXiv1998

Light Pair Correction to Bhabha Scattering at Small Angle

This work deals with the computation of electron pair correction to small angle Bhabha scattering, in order to contribute to the improvement of luminometry precision at LEP/SLC below 0.1% theoretical accuracy. The exact QED four-fermion matrix element for $e^+e^-\to e^+e^-e^+e^-$, including all diagrams and mass terms, is computed and different Feynman graph topologies are studied to quantify the error of approximate calculations present in the literature. Several numerical results, obtained by a Monte Carlo program with full matrix element, initial-state radiation via collinear structure functions, and realistic event selections, are shown and critically compared with the existing ones. The present calculation, together with recent progress in the sector of $O(α^2)$ purely photonic corrections, contributes to achieve a total theoretical error in luminometry at the 0.05% level, close to the current experimental precision and important in view of the final analysis of the electroweak precision data.

hep-ph↗

arXiv1998

A new approach to multi-jet calculations in hadron collisions

We present an algorithm to evaluate the exact, tree-level matrix elements for multi-parton processes in QCD. We tested this technique, based on the recursive evaluation of the S-matrix, on processes such as gg -> n gluons and q qbar -> n gluons, with n up to 9. The summation over colour configurations is designed to allow the construction of parton-level event generators suitable to interfacing with a parton-shower evolution including the effects of colour-coherence. This leads to a fully exclusive, hadron-level description of multi-jet final states, accurately incorporating the dynamics of large jet-jet separation angles. Explicit results for the total rates and differential distributions of processes with 8 final-state partons are given.

hep-ph↗

arXiv1998

Single- and multi-photon final states with missing energy at e+e- colliders

The search for new physics in single- and multi-photon final states with large missing energy at LEP and future e^+ e^- colliders requires precise predictions for the Standard Model irreducible background. While at LEP1 the theoretical situation is under control, going to LEP2 (and beyond) some improvements are necessary. To approach the aimed $O(1%)$ theoretical accuracy, the tree-level matrix elements for the processes $e^+ e^- \to ν\barνnγ$, with n=1,2,3, are exactly computed in the Standard Model, including the possibility of anomalous couplings for single-photon production. Due to the presence of observed photons in the final state, particular attention is paid to the treatment of higher-order QED corrections. Comparisons with existing calculations are shown and commented. An improved version of the event generator NUNUGPV is presented.

hep-ph↗

arXiv1998

The anisotropy of inverse beta decay and antineutrino detection

The anisotropy of the positrons emitted in the reaction $\barν_{e}+p\to n+e^{+}$ has to be taken into account for extracting an antineutrino signal in Superkamiokande. For the Sun, this effect allows a sensitivity to $ν_{e}\to\barν_{e}$ transition probability at the 3% level already with the statistics collected in the first hundred days. For a supernova in the Galaxy, the effect is crucial for extracting the correct ratio of $ν-e$ to $\barν_{e}-p$ events.

astro-ph↗

arXiv1997

Event Generators for WW Physics

The report summarizes the results of the activities of the Working Group on Event Generators for WW Physics at CERN during 1995.

hep-ph↗

arXiv1997

Superkamiokande and solar antineutrinos

We propose to exploit the angular distribution of the positrons emitted in the inverse beta decay to extract a possible antineutrino signal from the Superkamiokande background. From the statistics collected in just 101.9 days one obtains a model independent upper bound on the antineutrino flux (for energy greater than 8.3 MeV) Phi < 9*10^4 cm^-2 s^-1 at the 95% C.L. By assuming the same energy spectrum as for the 8B neutrinos, the 95% C.L. bound is Phi < 6*10^4 cm^-2 s^-1. Within three years of data taking, the sensitivity to neutrino-antineutrino transition probability will reach the 1% level, thus providing a stringent test of hybrid oscillation models.

astro-ph↗

arXiv1997

Six-Fermion Calculation of Intermediate-mass Higgs Boson Production at Future $e^+ e^-$ 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 $e^+ e^- \to μ^+ μ^- τ^- \barν_τ u \bar d, μ^+ μ^- e^- \barν_{e} u \bar d$. 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.

hep-ph↗

arXiv1996

$e^+ \ e^-$ into 4 fermions + $γ$ with ALPHA

We apply the iterative numerical algorithm ALPHA, which automatically generates the N-point Green functions (as recently suggested \cite{alpha}) to the computation of the production rate of four fermions and four fermions plus photon in electron-positron annihilation. The discussion of the physical results is preceded by an introduction on the algorithm.

hep-ph↗

arXiv1996

Four Fermions Productions at a $γγ$ Collider

Using the recently proposed ALPHA algorithm (and the resulting code) I compute the rate (at tree level) for the process $γγ\rightarrow\barν_e e^- u \bar d$. The bulk of the contribution is due to W pair production and decay. However a non negligible ($\sim10 \%$) contribution comes from other channels, mainly the production and decay of a W and a collinear charged fermion. Requiring that the reconstructed invariant $u \bar d$ mass lies in the intervals $M_W\pm 5 $ GeV and $M_W\pm 20 $ GeV one obtains a rate which is lower, by 25 \% and 4 \% respectively, than the rate obtained in the $narrow$ $width$ approximation, thus demonstrating the relevance of the finite W width.

hep-ph↗

arXiv1995

An Algorithm to Compute Born Scattering Amplitudes without Feynman Graphs

In this paper we suggest an {\it iterative} algorithm to compute automatically the scattering matrix elements of any given effective lagrangian, $Γ$. By exploiting the relation between $Γ$ and the connected Green function generator, $Z$, we provide a formula which does not require the use of the Feynman graphs and it is suitable to implement a numerical routine. By means of this algorithm we have built a relatively simple and fast fortran code which we have used to calculate, at the tree level, the rate of four fermion production at LEP I\negthinspace{I} (finding a very good agreement with previous calculation) with and without the emission of one observable photon.

hep-ph↗

arXiv1994

Soft Supersymmetry Breaking from Gaugino Condensation

We study the structure of soft breaking terms in the context of a gaugino condensation scenario. Assuming that the Supergravity Lagrangian is the correct quantum field theory limit, at some momentum scale $μ_{UV}$, of a more fundamental one, we demonstrate that the correct result is obtained simply by substituting, in the tree level Supergravity Lagrangian, $λλ$ (the gaugino condensate) by its vacuum expectation value $Λ^3$. In string inspired scenarios this implies, in particular, that the scalar masses are vanishing at the string tree-level and receive a contribution, at the one loop level, which is proportional to the Green Schwarz coefficient $δ_{GS}$. Our results do not agree with the ones obtained in the effective Lagrangian approach. We study in detail the origin of this discrepancy, and we argue that the use of the supertrace anomaly to determine the effective theory for the condensate does not fix its gravitational interactions, leaving the soft breaking terms and the vacua of the theory unspecified.

hep-ph↗

arXiv1993

Phenomenological Implications of Supersymmetry Breaking by the Dilaton

We investigate the low energy properties of string vacua with spontaneously broken $N=1$ supersymmetry by a dilaton $F$-term. As a consequence of the universal couplings of the dilaton, the supersymmetric mass spectrum is determined in terms of only three independent parameters and more constrained than in the minimal supersymmetric Standard Model. For a $μ$-term induced by the \K\ potential the parameter space becomes two-dimensional; in the allowed regions of this parameter space we find that most supersymmetric particles are determined solely by the gluino mass. The Higgs is rather light and the top-quark mass always lower than 180 GeV.

hep-ph↗