arXiv subjects

Stefano Pozzorini

Publications and source records attributed to Stefano Pozzorini.

49 records · Page 3Linked to original sources

arXiv2010

NLO QCD corrections to top anti-top bottom anti-bottom production at the LHC

The next-to-leading order (NLO) QCD corrections to top anti-top bottom anti-bottom (tTbB) production at the LHC reveal that the scale choice adopted in previous lowest-order simulations underestimates the tTbB cross section by a factor two. We discuss a new dynamical scale that stabilizes the perturbative predictions and describe the impact of the corrections on the shape of distributions. We also account for the techniques employed to compute the six-particle one-loop amplitudes with high CPU efficiency.

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arXiv2010

NLO QCD corrections to top anti-top bottom anti-bottom production at the LHC: 2. full hadronic results

We present predictions for top anti-top bottom anti-bottom production at the LHC in next-to-leading order QCD. The precise description of this background process is a prerequisite to observe associated top anti-top Higgs production in the Higgs -> bottom anti-bottom decay channel and to directly measure the top-quark Yukawa coupling at the LHC. The leading-order cross section is extremely sensitive to scale variations. We observe that the traditional scale choice adopted in ATLAS simulations underestimates the top anti-top bottom anti-bottom background by a factor two and introduce a new dynamical scale that stabilizes the perturbative predictions. We study various kinematic distributions and observe that the corrections have little impact on their shapes if standard cuts are applied. In the regime of highly boosted Higgs bosons, which offers better perspectives to observe the top anti-top Higgs signal, we find significant distortions of the kinematic distributions. The one-loop amplitudes are computed using process-independent algebraic manipulations of Feynman diagrams and numerical tensor reduction. We find that this approach provides very high numerical stability and CPU efficiency.

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arXiv2009

Flavour-dependent radiative correction to neutrino-neutrino refraction

In the framework of the Standard Model we calculate the flavour non-universal correction for neutrino refraction in a neutrino background and verify a similar previous result for the case of ordinary-matter background. The dominant term arises at loop level and involves tau leptons circulating in the loop. These O(G_F m_tau^2) corrections to the tree-level potential provide the dominant refractive difference between nu_mu and nu_tau unless the medium contains mu or tau leptons. Our results affect the flavour evolution of dense neutrino gases and may be of interest for collective three-flavour oscillations of supernova neutrinos. We spell out explicitly how these non-universal neutrino-neutrino interactions enter the flavour oscillation equations.

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arXiv2008

Two-loop electroweak next-to-leading logarithms for processes involving heavy quarks

We derive logarithmically enhanced two-loop virtual electroweak corrections for arbitrary fermion-scattering processes at the TeV scale. This extends results previously obtained for massless fermion scattering to processes that involve also bottom and top quarks. The contributions resulting from soft, collinear, and ultraviolet singularities in the complete electroweak Standard Model are explicitly extracted from two-loop diagrams to next-to-leading-logarithmic accuracy including all effects associated with symmetry breaking and Yukawa interactions.

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arXiv2008

NLO QCD corrections to pp -> ttbb+X via quark anti-quark annihilation

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arXiv2008

NLO QCD corrections to top anti-top bottom anti-bottom production at the LHC: 1. quark-antiquark annihilation

The process pp -> top anti-top bottom anti-bottom + X represents a very important background reaction to searches at the LHC, in particular to top anti-top H production where the Higgs boson decays into a bottom anti-bottom pair. A successful analysis of top anti-top H at the LHC requires the knowledge of direct top anti-top bottom anti-bottom production at next-to-leading order in QCD. We take the first step in this direction upon calculating the next-to-leading-order QCD corrections to the subprocess initiated by quark anti-quark annihilation. We devote an appendix to the general issue of rational terms resulting from ultraviolet or infrared (soft or collinear) singularities within dimensional regularization. There we show that, for arbitrary processes, in the Feynman gauge, rational terms of infrared origin cancel in truncated one-loop diagrams and result only from trivial self-energy corrections.

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arXiv2002

Two-loop electroweak corrections at high energies

We discuss two-loop leading and angular-dependent next-to-leading logarithmic electroweak virtual corrections to arbitrary processes at energies above the electroweak scale. The relevant Feynman diagrams involving soft-collinear gauge bosons gamma, Z, W, have been evaluated in eikonal approximation. We present results obtained from the analytic evaluation of massive loop integrals. To isolate mass singularities we used the Sudakov method and alternatively the sector decomposition method in the Feynman-parameter representation.

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arXiv2002

Electroweak-correction effects in gauge-boson pair production at the LHC

We have studied the effect of one-loop logarithmic electroweak radiative corrections on WZ and $Wγ$ production processes at the LHC. We present analytical results for the leading-logarithmic electroweak corrections to the corresponding partonic processes du -> WZ, Wgamma. Using the leading-pole approximation we implement these corrections into Monte Carlo programs for $pp\to lν_l l'\bar l', lν_lγ$. We find that electroweak corrections lower the predictions by 5-20% in the physically interesting region of large transverse momentum and small rapidity separation of the gauge bosons.

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arXiv2002

Electroweak radiative corrections at high energies

This PhD thesis is concerned with one-loop virtual electroweak corrections to arbitrary processes in the high-energy limit. Complete results are presented for the leading and subleading logarithms of large ratios of energy scales to mass scales. These results include the logarithmic dependence on the photon and weak-boson masses, on the light-fermion masses, as well as on the Higgs- and top-masses in the heavy-Higgs and heavy-top limit. All sources of electroweak logarithmic corrections are taken into account, including the exchange of soft and/or collinear electroweak gauge bosons as well as the renormalization-group running of the gauge, scalar and Yukawa couplings. The logarithmic corrections are derived in a process-independent way, resulting in simple analytic formulas that apply to arbitrary electroweak processes that are not mass-suppressed in the high-energy limit. We also present analytical and numerical applications for the processes: e^+e^- \to f\bar{f}, e^+ e^- \to W^+W^-, ZZ, Zγ, γγand \bar{d}u \to W^+Z, W^+γ.

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arXiv2001

Leading electroweak logarithms at one loop

We summarize results for the complete one-loop electroweak logarithmic corrections for general processes at high energies and fixed angles. Our results are applicable to arbitrary matrix elements that are not mass-suppressed. We give explicit results for W-boson-pair production in e^+e^- annihilation.

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arXiv2001

One-loop leading logarithms in electroweak radiative corrections, I. Results

We present results for the complete one-loop electroweak logarithmic corrections for general processes at high energies and fixed angles. Our results are applicable to arbitrary matrix elements that are not mass-suppressed. We give explicit results for 4-fermion processes and gauge-boson-pair production in electron-positron annihilation.

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arXiv2001

One-loop leading logarithms in electroweak radiative corrections: II. Factorization of collinear singularities

We discuss the evaluation of the collinear single-logarithmic contributions to virtual electroweak corrections at high energies. More precisely, we proof the factorization of the mass singularities originating from loop diagrams involving collinear virtual gauge bosons coupled to external legs. We discuss, in particular, processes involving external longitudinal gauge bosons, which are treated using the Goldstone-boson equivalence theorem. The proof of factorization is performed within the 't Hooft--Feynman gauge at one-loop order and applies to arbitrary electroweak processes that are not mass-suppressed at high energies. As basic ingredient we use Ward identities for Green functions with arbitrary external particles involving a gauge boson collinear to one of these. The Ward identities are derived from the BRS invariance of the spontaneously broken electroweak gauge theory.

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arXiv1998

Electroweak radiative corrections to polarized Moeller scattering at high energies

The cross section for $e^-e^- \to e^-e^-$ with arbitrary electron polarizations is calculated within the Electroweak Standard Model for energies large compared to the electron mass, including the complete virtual and soft-photonic $O(α)$ radiative corrections. The relevant analytical results are listed, and a numerical evaluation is presented for the unpolarized and polarized cross sections as well as for polarization asymmetries. The relative weak corrections are typically of the order of 10%. At low energies, the bulk of the corrections is due to the running of the electromagnetic coupling constant. For left-handed electrons, at high energies the vertex and box corrections involving virtual W bosons become very important. The polarization asymmetry is considerably reduced by the weak radiative corrections.

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