arXiv subjects
Publications and source records attributed to Giampiero Passarino.
Higgs Computed Axial Tomography, an excerpt. Taking a closer look at the camel-shaped tail of the light Higgs boson resonance and looking to the transformation of the (camel-shaped) signal into a square-root--shaped signal + interference with particular emphasis on residual theoretical uncertainties.
The processes involving a Higgs boson, a photon(gluon) and a fermion pair pose severe challenges to the experimental analysis. They represent rare decays and production mechanisms of the Higgs boson at LHC. However, they are not Yukawa suppressed at next-to-leading order opening a window for the correct definition of pseudo-observables, e.g. a definition of the Higgs decay width into Z + photon with universal inherent meaning, that are currently used in extracting information for the couplings of the newly discovered resonance at LHC. The impact of genuinely electroweak NLO corrections is discussed, as well as the comparison of the full-fledged cross-sections to their zero-width approximation.
The problem of estimating the effect of missing higher orders in perturbation theory is analyzed with emphasis in the application to Higgs production in gluon-gluon fusion. Well-known mathematical methods for an approximated completion of the perturbative series are applied with the goal to not truncate the series, but complete it in a well-defined way, so as to increase the accuracy - if not the precision - of theoretical predictions. The uncertainty arising from the use of the completion procedure is discussed and a recipe for constructing a corresponding probability distribution function is proposed.
Either late autumn this year or latest early next year LHC should have results with 2-3 times the current data which migth give first clues on the couplings of the light narrow resonance. A strategy for measuring deviations from the Standard Model can be based on using the "full" Standard Model, including all available QCD and electroweak higher-order corrections, and supplement it with d= 6 local operators. Their Wilson coefficients are assumed to be small enough that they can be treated at leading order. Examples of the connection of local operators with BSM Lagrangians are presented as well as a discussion of Lagrangians with/without decoupling of heavy degrees of freedom. The whole strategy is critically reviewed in the light of internal consistency.
In the Higgs search at the LHC, a light Higgs boson (115 GeV <~ M_H <~ 130 GeV) is not excluded by experimental data. In this mass range, the width of the Standard Model Higgs boson is more than four orders of magnitude smaller than its mass. The zero-width approximation is hence expected to be an excellent approximation. We show that this is not always the case. The inclusion of off-shell contributions is essential to obtain an accurate Higgs signal normalisation at the 1% precision level. For gg (-> H) -> VV, V= W,Z, O(10%) corrections occur due to an enhanced Higgs signal in the region M_VV > 2 M_V, where also sizable Higgs-continuum interference occurs. We discuss how experimental selection cuts can be used to exclude this region in search channels where the Higgs invariant mass cannot be reconstructed. We note that the H -> VV decay modes in weak boson fusion are similarly affected.
Interference between the Standard Model Higgs boson and continuum contributions is considered in the heavy-mass scenario. Results are available at leading order for the background. It is discussed how to combine the result with the next-to-next-to-leading order Higgs production cross-section and a proposal for estimating the associated theoretical uncertainty is presented.
The current searches for a heavy Higgs boson assume on-shell (stable) Higgs-boson production. The Higgs-boson production cross section is then sampled with a Breit-Wigner distribution (with fixed-width or running-width) and implemented in MonteCarlo simulations. Therefore the question remains of what is the limitation of the narrow Higgs-width approximation. The main focus of this work is on the description of the Standard Model Higgs--boson-lineshape in the heavy Higgs region, typically $\MH$ above $600\UGeV$. The framework discussed in this paper is general enough and can be used for all processes and for all kinematical regions. Numerical results are shown for the gluon-fusion process. Issues of gauge invariance and residual theoretical uncertainties are also discussed. Limitations due to a breakdown of the perturbative expansion are comprehensively discussed, including a discussion of th equivalence theorem for (off-shell) virtual vector-bosons. Analytic continuation in a theory with unstable particles is thoroughly discussed.
Complete electroweak two-loop corrections to the process $gg \to H$ are presented and discussed in a Standard Model with a fourth generation of heavy fermions. The latter is studied at the LHC to put exclusion limits on a fourth generation of heavy fermions. Therefore also a precise knowledge of the electroweak(EW) next-to-leading-order(NLO) corrections is important. The corrections due to the fourth generation are positive and large for a light Higgs boson, positive but relatively small around the $\bar{t}-t$ threshold and start to become negative for a Higgs boson mass around $M_H = 450 GeV$. Increasing further the value of the Higgs boson mass, the EW NLO effects tend to become huge and negative, ${\cal O}(-100%)$, around the heavy-fermion threshold, assumed at $1.2 TeV$, so that $gg$-fusion becomes non-perturbative. Above that threshold they start to grow again and become positive around $M_H= 1.75 TeV$. The behaviour at even larger values of $M_H$ shows a positive enhancement, ${\cal O}(+100%)$ at $M_H= 3 TeV$.
The relation between physical observables measured at LHC and Tevatron and standard model Higgs pseudo-observables (production cross section and partial decay widths) is revised by extensively using the notion of the Higgs complex pole on the second Riemann sheet of the $S $-matrix. The extension of their definition to higher orders is considered, confronting the problems that arise when QED(QCD) corrections are included in computing realistic observables. Numerical results are presented for pseudo-observables related to the standard model Higgs boson decay and production. The relevance of the result for exclusion plots of the standard model Higgs boson for high masses (up to $600 $GeV) is discussed. Furthermore, a recipe for the analytical continuation of Feynman loop integrals from real to complex internal masses and complex Mandelstam invariants is thoroughly discussed.
In this paper the complete two-loop corrections to the Higgs-boson decay, H -> gamma gamma, are presented. The evaluations of both QCD and electroweak corrections are based on a numerical approach. The results cover all kinematical regions, including the WW normal-threshold, by introducing complex masses in the relevant (gauge-invariant) parts of the LO and NLO amplitudes.
Recent developments in the evaluation of two-loop pseudo-observables and observables are briefly reviewed.
A comprehensive study is performed of two-loop Feynman diagrams with three external legs which, due to the exchange of massless gauge-bosons, give raise to infrared and collinear divergencies. Their relevance in assembling realistic computations of next-to-next-to-leading corrections to physical observables is emphasised. A classification of infrared singular configurations, based on solutions of Landau equations, is introduced. Algorithms for the numerical evaluation of the residues of the infrared poles and of the infrared finite parts of diagrams are introduced and discussed within the scheme of dimensional regularization. Integral representations of Feynman diagrams which form a generalization of Nielsen - Goncharov polylogarithms are introduced and their numerical evaluation discussed. Numerical results are shown for all different families of multi-scale, two-loop, three-point infrared divergent diagrams and successful comparisons with analytical results, whenever available, are performed. Part of these results has already been included in a recent evaluation of electroweak pseudo-observables at the two-loop level.
The atomic parity-violation (APV) parameter QW for a nucleus with `n' neutrons and `z' protons has been included in the list of pseudo-observables accessible with the codes TOPAZ0 and ZFITTER. In this way one can add the APV results in the LEP EWWG `global' electroweak fits, checking the corresponding effect when added to the existing precision measurements.
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.
The complete list of definitions for quantities relevant in the analysis of SLD/LEP-1 results around the Z-resonance is given. The common set of conventions adopted by the programs TOPAZ0 and ZFITTER, following the recommendations of the LEP electroweak working group, is reviewed. The relevance of precision calculations is discussed in detail both for pseudo-observables (PO) and for realistic observables (RO). The model-independent approach is also discussed. A critical assessment is given of the comparison between TOPAZ0 and ZFITTER.
The so-called CC20 processes have been extensively analyzed in the literature. They are a sensitive probe of anomalous electromagnetic couplings of the W boson and represent a background to searches for new physics beyond the standard model. Moreover, they represent a contribution to the WW total cross section, used to derive a value for the W boson mass. The issue of gauge invariance in the CC20 family has been solved by the introduction of the Fermion-Loop scheme but several subtleties remain, connected with the region of vanishing scattering angle of the electron and with the limit of massless final state fermions in a fully extrapolated setup. A satisfactory solution for computing the total cross section is given in the context of the equivalent photon or Weizsacker-Williams approximation which factorizes the flux of quasi-real photons emitted by the electron from the interaction rate between the positron and the photon assumed to be real. The correct kinematics for the inclusion of initial state QED radiation is established. QCD corrections to the process are discussed and numerical results are shown and commented.
A critical assessment is given of the comparison between the new versions of the programs TOPAZ0 40i and ZFITTER 510. The relevance for precision calculations around the $\zb$ resonance is briefly discussed. << more references added >>
A critical assessment is given of the theoretical uncertainty in the predicted cross-sections for large-angle Bhabha scattering at LEP 1, with or without t-channel subtraction. To this end a detailed comparison is presented of the results obtained with the programs ALIBABA and TOPAZ0. Differences in the implementation of the radiative corrections and the effect of missing higher-order terms are critically discussed.