arXiv subjects

G. Passarino

Publications and source records attributed to G. Passarino.

At least 37 records · Page 2Linked to original sources

arXiv2002

Project {\tt SANC} (former {\tt CalcPHEP}): Support of Analytic and Numeric calculations for experiments at Colliders

The project, aimed at the theoretical support of experiments at modern and future accelerators -- TEVATRON, LHC, electron Linear Colliders (TESLA, NLC, CLIC) and muon factories, is presented. Within this project a four-level computer system is being created, which must automatically calculate, at the one-loop precision level the pseudo- and realistic observables (decay rates and event distributions) for more and more complicated processes of elementary particle interaction, using the principle of knowledge storing. It was already used for a recalculation of the EW radiative corrections for Atomic Parity Violation [1] and complete one-loop corrections for the process $e^+ e^-\to t\bar{t}$ [2-4]; for the latter an, agreement up to 11 digits with FeynArts and the other results is found. The version of {\tt SANC} that we describe here is capable of automatically computing the decay rates and the distributions for the decays $Z(H,W)\to f\bar{f}$ in the one-loop approximation.

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arXiv2002

All-Purpose Numerical Evaluation of One-Loop Multi-Leg Feynman Diagrams

A detailed investigation is presented of a set of algorithms which form the basis for a fast and reliable numerical integration of one-loop multi-leg (up to six) Feynman diagrams, with special attention to the behavior around (possibly) singular points in phase space. No particular restriction is imposed on kinematics, and complex masses (poles) are allowed.

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arXiv2002

Project "CalcPHEP: Calculus for Precision High Energy Physics"

In this paper we describe the present status and our plans for the realization of next phases of the CalcPHEP project aimed at the theoretical support of experiments at modern and future accelerators: TEVATRON, LHC, electron Linear Colliders (LC's) i.e. TESLA, NLC, CLIC, and muon factories. Within this project, we are creating a four-level computer system which eventually must automatically calculate pseudo- and realistic observables for more and more complicated processes of elementary particle interactions, using the principle of knowledge storing. The first phase of the CalcPHEP system was realized in the site http://brg.jinr.ru/ in 2000--2001.

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arXiv2001

Algebraic-Numerical Evaluation of Feynman Diagrams: Two-Loop Self-Energies

A recently proposed scheme for numerical evaluation of Feynman diagrams is extended to cover all two-loop two-point functions with arbitrary internal and external masses. The adopted algorithm is a modification of the one proposed by F. V. Tkachov and it is based on the so-called generalized Bernstein functional relation. On-shell derivatives of self-energies are also considered and their infrared properties analyzed to prove that the method which is aimed to a numerical evaluation of massive diagrams can handle the infrared problem within the scheme of dimensional regularization. Particular care is devoted to study the general massive diagrams around their leading and non-leading Landau singularities.

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arXiv2001

Are Constants Constant?

The prospect of a time-dependent Higgs vacuum expectation value is examined within the standard model of electroweak interactions. It is shown that the classical equation of motion for the Higgs field admits a solution that is a doubly-periodic function of time. The corresponding Dirac equation for the electron field is equivalent to a second order differential equation with doubly-periodic coefficients. In the limit of very large primitive period of the Higgs background this equation can be solved in WKBJ approximation, showing plane-wave solutions with a time-dependent distortion factor which can be made arbitrarily small.

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arXiv2001

An Approach Toward the Numerical Evaluation of Multi-Loop Feynman Diagrams

A scheme for systematically achieving accurate numerical evaluation of multi-loop Feynman diagrams is developed. This shows the feasibility of a project aimed to produce a complete calculation for two-loop predictions in the Standard Model. As a first step an algorithm, proposed by F. V. Tkachov and based on the so-called generalized Bernstein functional relation, is applied to one-loop multi-leg diagrams with particular emphasis to the presence of infrared singularities, to the problem of tensorial reduction and to the classification of all singularities of a given diagram. Successively, the extension of the algorithm to two-loop diagrams is examined. The proposed solution consists in applying the functional relation to the one-loop sub-diagram which has the largest number of internal lines. In this way the integrand can be made smooth, a part from a factor which is a polynomial in the vector of Feynman parameters needed for the complementary sub-diagram with the smallest number of internal lines. Since the procedure does not introduce new singularities one can distort the integration hyper-contour into the complex hyper-plane, thus achieving numerical stability. The algorithm is then modified to deal with numerical evaluation around normal thresholds. Concise and practical formulas are assembled and presented, numerical results and comparisons with the available literature are shown and discussed for the so-called sunset topology.

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arXiv2001

A Practical Approach for Exponentiation of QED Corrections in Arbitrary Processes

It is a well-known fact that, among the electroweak corrections, QED radiation gives the largest contribution and the needed precision requires a re-summation of the large logarithms which show up in perturbation theory. For two fermion annihilation processes initial state radiation is a definable, gauge-invariant, concept and one has general tools to deal with it; the structure function approach and also the parton-shower method. However, when one tries to apply the algorithm to four-fermion processes that include non-annihilation channels a problem is faced: is it still possible to include QED corrections by making use of the standard tools? A systematization is attempted of several, recently proposed, algorithms. In particular, it is shown that starting from the exponentiation of soft photons one can still derive a description of QED radiation in terms of structure functions, i.e. the kernel for the hard scattering is convoluted with generalized structure functions where each of them is no longer function of one scale.

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arXiv2001

Precision Physics Near LEP Shutdown and Evolutionary Developments

The most celebrated aspects of precision physics are briefly summarized. New ideas are also introduced that may lead to further developments in the field of radiative corrections, with special emphasis to multi-loop numerical evaluation and to a correct treatment of QED radiation for arbitrary processes.

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arXiv2001

Non-Annihilation Processes, Fermion-Loop and QED Radiation

The bulk of large radiative corrections to any process can be obtained by promoting coupling constants to be running ones and by including QED radiation at the leading logarithmic level via structure functions evoluted at some scale. The problem of fixing the proper scale in running coupling constants and in structure functions for non-annihilation processes is briefly addressed and the general solution is analyzed.

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arXiv2000

Recent Theoretical Developments in LEP 2 Physics

Recent theoretical developments in electron-positron annihilation into fermion pairs are summarized. In particular, two-fermion production, DPA for WW signal, single-W production and ZZ signal

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arXiv2000

Single-W Production and Fermion-Loop Scheme: Numerical Results

The single-W production mechanism is synonymous to the electron-positron annihilation into electron, neutrino and a W boson with the outgoing electron lost in a small cone around the beam direction. It requires a Renormalization Scheme that preserves gauge invariance and fermion masses cannot be neglected in the calculation. A recently proposed generalization of the so-called Fermion-Loop scheme is applied to the evaluation of observables at LEP 2 energies. Single-W processes are dominated by a regime of low momentum transfer of the outgoing electron and any high-energy Renormalization Scheme fails to give the correct description of the scale. The Fermion-Loop scheme automatically converts all couplings of the theory into couplings that are running at the appropriate scale. Therefore, in addition to represent the only scheme fully justified on a field-theoretical basis, the Fermion-Loop is the best starting point to include radiative corrections into single-W production. Numerical results are presented, showing a decrease in the predictions that can be sizeable. There is no naive and overall rescaling of the e.m. coupling constant, in any pragmatic scheme, that can reproduce the Fermion-Loop results, at the requested accuracy, for all configurations and for all kinematical cuts.

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arXiv1999

Unstable Particles and Non-Conserved Currents: a Generalization of the Fermion-Loop Scheme

A generalization of the Fermion-Loop scheme is introduced to account for external, non-conserved, currents. Complete Dyson re-summed transitions are introduced, including the contributions from the Higgs-Kibble ghosts in the 't Hooft-Feynman gauge. Running vector boson masses are introduced and their relation with the corresponding complex poles are investigated. It is shown that any S-matrix element takes a very simple form when written in terms of these running masses. A special example of Ward identity, the U(1) Ward identity for single-W, is derived in a situation where all currents are non-conserved and where the top quark mass is not neglected inside loops.

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arXiv1999

The Twofold Way, a Short Disquisition of LEP Physics

A condensed survey of various aspects of LEP 1 physics is presented with a critical examination of the ingredients that are available for theoretical predictions. A prototype of comparisons for 2f calculations can be found in hep-ph/9902452.

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arXiv1998

TOPAZ0 4.0 - A new version of a computer program for evaluation of de-convoluted and realistic observables at LEP 1 and LEP 2

The program TOPAZ0 was developed for computing a variety of physical observables which are related to the e+e- annihilation into fermion pairs and to the large angle Bhabha scattering around the Z resonance. Among them, the Z parameters or pseudo-observables, the de-convoluted cross sections and those dressed with QED radiation, and finally the forward-backward asymmetries. The calculations are performed both for a completely inclusive experimental set-up and for a realistic one, i.e. a set-up with cuts on the acollinearity angle, on the energy of the outgoing fermions or on their invariant mass and angular acceptance. The new version, 4.0, includes several innovative features. First of all, the most important new capabilities since previous versions are recently computed electroweak and QCD correction factors that are relevant at the Z resonance in the light of the present experimental accuracy. Among them, the effect of the next-to-leading O(alpha^2 m_t^2) corrections and those radiative corrections to the hadronic decay of the Z which provide complete corrections of O(alpha alpha_s) to Gamma(Z --> q\bar{q}) with q=u,d,s,c and b. Secondly, the program has been upgraded to cover two-fermion final states at LEP 2 energies, where some of the assumptions made for earlier versions are no longer valid. In particular, to this aim all the electroweak radiative corrections that are negligible at the Z peak, but relevant far from it, have been added for s-channel processes, e.g. purely weak boxes, next-to-leading O(alpha^2) and leading O(alpha^3) QED corrections.

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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.

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arXiv1997

Electroweak Working Group Report

The report summarizes the results of the activities of the Working Group on Precision Calculations for the Z Resonance at CERN during 1994.

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arXiv1996

The fermion-loop scheme for finite-width effects in e^+ e^- annihilation into four fermions

We describe the gauge-invariant treatment of the finite-width effects of W and Z bosons in the fermion-loop scheme and its application to the six-fermion (LEP2) processes e^- e^+ -> four fermions, with massless external fermions. The fermion-loop scheme consists in including all fermionic one-loop corrections in tree-level amplitudes and resumming the self-energies. We give explicit results for the unrenormalized fermionic one-loop contributions to the gauge-boson self-energies and the triple gauge-boson vertices, and perform the renormalization in a gauge-invariant way by introducing complex pole positions and running couplings. A simple effective Born prescription is presented, which allows for a relatively straightforward implementation of the fermion-loop scheme in LEP1 and LEP2 processes. We apply this prescription to typical LEP2 processes, i.e., e^- e^+ -> μ^- \barν_μu \bar{d}, e^- e^+ -> s \bar{c} u \bar{d}, and e^- e^+ -> e^- \barν_e u \bar{d}, and give numerical comparisons with other gauge-invariance-preserving schemes in the energy range of LEP2, NLC and beyond.

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arXiv1996

WW Cross-sections and Distributions

We present the results obtained by the "WW Cross-sections and Distributions" working group during the CERN Workshop "Physics at LEP2" (1994/1995)

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