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R. Pittau

Publications and source records attributed to R. Pittau.

At least 55 records · Page 3Linked to original sources
arXiv2005

Recursive numerical calculus of one-loop tensor integrals

A numerical approach to compute tensor integrals in one-loop calculations is presented. The algorithm is based on a recursion relation which allows to express high rank tensor integrals as a function of lower rank ones. At each level of iteration only inverse square roots of Gram determinants appear. For the phase-space regions where Gram determinants are so small that numerical problems are expected, we give general prescriptions on how to construct reliable approximations to the exact result without performing Taylor expansions. Working in 4+epsilon dimensions does not require an analytic separation of ultraviolet and infrared/collinear divergences, and, apart from trivial integrals that we compute explicitly, no additional ones besides the standard set of scalar one-loop integrals are needed.

hep-ph↗
arXiv2004

Beyond the Standard Model Higgs Boson self-couplings at the LHC

A study of two-Higgs-doublet models (2HDM) in the decoupling limit reveals the existence of parameter configurations with a large triple-Higgs self-coupling as the only low-energy trace of a departure from a Standard Model (SM) Higgs sector. This observation encourages attempts to search for double Higgs production at the Large Hadron Collider (LHC) and its luminosity upgrade (SLHC) even in mass regions which have been shown to be very hard to probe in the context of SM-like Higgs self-couplings. In a scenario where only an Intermediate Mass Higgs (IMH) boson, with 120 GeV <= m_h <= 140 GeV, is discovered at the LHC, with measured couplings to fermions and gauge bosons compatible with their SM values, we show that Higgs-pair production (with each Higgs state decaying in two b bbar pairs) through Weak Boson Fusion (WBF) could open a window on physics beyond the SM in the Higgs sector.

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arXiv2003

Towards an effective-action approach to fermion-loop corrections

We present a study of the effective action approach to incorporate higher-order effects in e^+e^- -> n fermions. In its minimal version, the effective action approach is found to exhibit problems with unitarity and high-energy behaviour. We identify the origin of these problems by investigating the zero-mode solutions of the Ward Identities. A numerical analysis of the importance of the zero-mode solutions is presented for four-fermion production processes.

hep-ph↗
arXiv2003

ALPGEN, a generator for hard multiparton processes in hadronic collisions

This paper presents a new event generator, ALPGEN, dedicated to the study of multiparton hard processes in hadronic collisions. The code performs, at the leading order in QCD and EW interactions, the calculation of the exact matrix elements for a large set of parton-level processes of interest in the study of the Tevatron and LHC data. The current version of the code describes the following final states: (W -> ffbar') QQbar+ N jets (Q being a heavy quark, and f=l,q), with N<=4; (Z/gamma* -> f fbar)+QQbar+Njets (f=l,nu), with N<=4; (W -> ffbar') + charm + N jets (f=l,q), N<= 5; (W -> f fbar') + N jets (f=l,q) and (Z/gamma* -> f fbar)+ N jets (f=l,nu), with N<=6; nW+mZ+lH+N jets, with n+m+l+N<=8 and N<=3 including all 2-fermion decay modes of W and Z bosons, with spin correlations; Q Qbar+N jets (N<=6) with t -> b f fbar' decays and relative spin correlations included if Q=t; Q Qbar Q' Qbar'+N jets, with Q and Q' heavy quarks (possibly equal) and N<=4; H Q Qbar+N jets (N<=4) with t -> b f fbar' decays and relative spin correlations included if Q=t; N jets, with N<=6. Parton-level events are generated, providing full information on their colour and flavour structure, enabling the evolution of the partons into fully hadronised final states.

hep-ph↗
arXiv2003

b-bbar final states in Higgs production via weak boson fusion at the LHC

We present a study of the Higgs production at the LHC via Weak Boson Fusion, with the Higgs boson decaying into a b-quark pair. A detailed partonic LO calculation of all the potential backgrounds is performed. We conclude that this channel for Higgs production can be extracted from the backgrounds, and present our estimates of the accuracy in the determination of the Hbb Yukawa coupling.

hep-ph↗
arXiv2001

Theoretical issues at LEP2 and LC

After 12 years of glorious data taking LEP has been shut down. During the past two years an incredible effort has been devoted to get more accurate predictions and estimates of the related theoretical uncertainties. Many of the theoretical questions driven by LEP are also relevant at the LC, where predictions will be needed with even higher accuracy. This is particularly true for 4-fermion Physics. In this contribution, I review some of the most important theoretical achievements at LEP in understanding W and Z pair production, 4-fermion + 1 visible photon signatures and in solving problems related with gauge invariance. Issues on single W Physics are covered elsewhere. Part of the presented work is in progress, the final LEP2 analysis still being under way.

hep-ph↗
arXiv2000

Four-Fermion Production in Electron-Positron Collisions

This report summarises the results of the four-fermion working group of the LEP2-MC workshop, held at CERN from 1999 to 2000. Recent developments in the calculation of four-fermion processes in electron-positron collisions at LEP-2 centre-of-mass energies are presented, concentrating on predictions for four main reactions: W-pair production, visible photons in four-fermion events, single-W production and Z-pair production. Based on a comparison of results derived within different approaches, theoretical uncertainties on these predictions are established.

hep-ph↗
arXiv2000

Four-fermion production in electron-positron collisions with NEXTCALIBUR

We introduce a new, fully massive, Monte Carlo program to compute all four-fermion processes in electron-positron collisions, including Higgs production. We outline our strategy for the matrix element evaluation, the phase space generation and the implementation of the leading higher order effects, and show, where available, comparisons with existing results.

hep-ph↗
arXiv1998

Inelastic photon-neutrino interactions using an effective Lagrangian

We justify the feasibility of substituting a photon leg by a neutrino current in the Euler-Heisenberg Lagrangian to obtain an effective Lagrangian for the process $γν\toγγν$ and its crossed reactions. We establish the link between these processes and the four-photon scattering in both the Standard Model and the effective theory. As an application, we compute the processes $γν\toγγν$ and $γγ\toγν\barν$, give their polarized cross sections, and show how to use the $γγ\toγγ$ results as a check. We settle the question about the disagreement between two computations in the literature concerning the reaction $γγ\to γν\barν$.

hep-ph↗
arXiv1998

Five-leg photon-neutrino interactions

In a first part, we justify the feasibility of substituting a photon leg by a neutrino current in the Euler-Heisenberg Lagrangian to obtain an effective Lagrangian for the process $γν\toγγν$ and its crossed reactions. We establish the link between these processes and the four-photon scattering in both the Standard Model and the effective theory. As an application, we compute in this effective theory the processes $γν\toγγν$ and $γγ\toγν\barν$ and show how to use the $γγ\toγγ$ results as a check. We settle the question of the disagreement between two computations in the literature concerning the reaction $γγ\to γν\barν$. In the second part, we present results of the direct computation of the photon-neutrino five-leg processes in the Standard Model, discuss possible astrophysical implications of our results, and provide simple fits to the exact expressions.

hep-ph↗
arXiv1998

Direct computation of inelastic photon-neutrino processes in the Standard Model

In this paper, we compute the Standard Model polarized amplitudes and cross sections of the processes $γν\toγγν$, $γγ\to γν\barν$ and $ν\barν\to γγγ$, for centre-of-mass energies $ω$ within the range of validity of the Fermi theory. By using a large electron-mass expansion of the exact result, we also derive the first correction term to the effective, low-energy ($ω< m_e$) formula for $γν\toγγν$. Finally, we discuss possible astrophysical implications of our results and provide simple fits to the exact expressions.

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↗