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F. M. Renard

Publications and source records attributed to F. M. Renard.

At least 91 records · Page 5Linked to original sources
arXiv1999

Precision Tests in e+e- and gamma-gamma processes and Virtual New Physics Effects

Contribution to LP99, International Symposium on Lepton-Photon Interactions, Stanford, Aug.1999. Summary of works done in collaboration with M. Beccaria, P. Ciafaloni, D. Comelli, G.J. Gounaris, J. Layssac, P. Porfyriadis, S. Spagnolo, C. Verzegnassi. We report on several works dealing with precison tests of the Standard Model (SM) and beyond, in the processes $e^+e^-\to f\bar f$ (f= lepton or quarks) and $γγ\to γγ, γZ$. We first point out a set of remarkable properties of the SM amplitudes and observables at high energies, at tree level and at 1-loop, like "Sudakov" behaviour or the dominance of purely imaginary non-flip amplitudes due to the WWWW box. We then consider various types of virtual NP contributions due to supersymmetry, anomalous gauge boson couplings, technicolour and extended gauge structures. Effects at LEP2 and LC (in particular with a laser backscattering $γγ$ mode) are discussed. We point out specific features like clear threshold enhancements.

hep-ph↗
arXiv1999

The gamma gamma to gamma Z process at high energies and the search for virtual SUSY effects

We study the helicity amplitudes and the observables of the process $γγ\to γZ$ at high energy. As in the case of the $γγ\to γγ$ process studied before, the relevant diagrams in the standard model (SM) involve $W$, charged quark and lepton loops; while in SUSY we also have contributions from chargino, and charged sfermion or Higgs loop diagrams. Above 250 GeV, the dominant SM amplitudes are themselves dominated by the $W$ loop, and as for $γγ\to γγ$, they are helicity conserving and almost purely imaginary. We discuss the complementary information provided by $γγ\to γZ$ for the identification of possible non standard effects. This process, together with $γγ\to γγ$, should provide very useful information on the nature of possible new physics particles, above the threshold of their direct production.

hep-ph↗
arXiv1999

Susy Virtual effects at LEP2 Boundary

We examine the possibility that SUSY particles are light, i.e. have a mass just beyond the final kinematical reach of LEP2. In this case, even if light particles are not directly detected, their virtual effects are enhanced by a ``close to threshold'' resonance in the s-channel. We find that this resonant effect is absent in the case of light sfermions, while it is enhanced in the case of light gauginos, since neutralinos and charginos add coherently in some regions of the allowed parameter space. We discuss this ``virtual-alliance'' in detail and try to examine the possibilities of its experimental verification.

hep-ph↗
arXiv1999

Tests of Higgs Boson Couplings at a mu+mu- Collider

We investigate the potential of a muon collider for testing the presence of anomalous Higgs boson couplings. We consider the case of a light (less than $160 GeV$) Higgs boson and study the effects on the Higgs branching ratios and total width, which could be induced by the non standard couplings created by a class of dim=6 $SU(3)\times SU(2)\times U(1)$ gauge invariant operators satisfying the constraints imposed by the present and future hadronic and $e^-e^+$ colliders. For each operator we give the minimal value of the $μ^+μ^-$ integrated luminosity needed for the muon collider ($μC$) to improve these constraints. Depending on the operator and the Higgs mass, this minimal $μC$ luminosity lies between $0.1 fb^{-1}$ and $100 fb^{-1}$.

hep-ph↗
arXiv1999

The gamma gamma to gamma gamma process in the Standard and SUSY models at high energies

We study the helicity amplitudes of the process γγ\to γγat high energy, which in the standard and SUSY models first arise at the one-loop order. In the standard model (SM), the diagrams involve W and charged quark and lepton loops, while in SUSY we also have contributions from chargino, charged sfermion and Higgs loop diagrams. The SUSY contributions are most important in the region above the threshold for producing the supersymmetric partners; since there, they interfere most effectively with the primarily imaginary SM amplitudes. Simple expressions for the relevant 1-loop functions are given, which provide a direct overview of the behaviour of the helicity amplitudes in the whole parameter space at high energies. The various characteristics of a large set of observables are studied in detail.

hep-ph↗
arXiv1998

Bounds on anomalous gauge couplings from past and near future experiments: the role of the different measurements

We analyze the bounds on the set of four "non-blind" anomalous gauge couplings that will be derived, in absence of deviations from the Standard Model predictions, from near future measurements at LEP2 and at the TEVATRON. The bounds are obtained by combining these negative results with those already available from LEP1 and from atomic parity violation experiments. In this process, the information coming from LEP2 is treated using a recently proposed ("Z-peak subtracted") theoretical approach. This makes it easier to identify the specific role that the different experiments play in this determination, in the spirit of a recent previous investigation for the set of three "blind" couplings.

hep-ph↗
arXiv1998

Glue constraining asymmetries in W, $γ$ or Z production at CERN LHC

We propose a class of forward-backward asymmetries with respect to the subprocess c.m. scattering angle in $V$+jet production at hadron colliders, with V being any of (W, Z, $γ$, H), which are directly proportional to the gluon distribution $g(x)$. The informations that these asymmetries can provide are complementary of those reachable from measurements of the transverse momentum and/or the rapidity distributions of a $V$ in kinematical regimes where gluon scattering subprocesses dominate. The accuracy which can be reached in the W, Z and $γ$ cases, at the upgraded Tevatron and at LHC, should allow a considerable improvement of our knowledge of the gluon distribution function, especially at large x.

hep-ph↗
arXiv1998

Rising Bosonic Electroweak Virtual Effects at High Energy e+e- Colliders

The virtual massive gauge boson effects on several observables in the process of charged fermion pairs production at $e^+e^-$ colliders are computed up to the TeV energy range in the Standard Model, working in the ``$Z$-peak subtracted'' representation. It is shown that these effects increase regularly with energy, approaching the typical ten percent relative size. A careful numerical evaluation is proposed, where the conditions dictated by gauge invariance are consistently implemented.

hep-ph↗
arXiv1998

Testing the Higgs boson gluonic couplings at LHC

We study Higgs + jet production at hadron colliders in order to look for new physics residual effects possibly described by the $dim=6$ operators $Ø_{GG}$ and ${\widetildeØ}_{GG}$ which induce anomalous $Hgg$ and $Hggg$ couplings. Two ways for constraining these operators at LHC may be ~useful. The first is based on the total Higgs boson production rate induced by gluon-gluon fusion, in which the main cause of limitations are due to theoretical uncertainties leading to sensitivities of $|d_G|\simeq 3.\times 10^{-4}$ and $|\widetilde{d}_G|\simeq 1.4\times 10^{-3}$ for the corresponding anomalous couplings, in the mass range $100 GeV \lsim \mh \lsim 2~00 GeV$. These results imply sensitivity to new physics scales of 51 and 24 TeV respectively. The second way investigated here concerns the shape of the Higgs transverse momentum; for which the theoretical uncertainties are less severe and the limitations are mainly induced by statistics. A simple analysis, based on the ratio of the number of events at large and low $p_T$ at LHC, leads to similar sensitivities, if only the $H\to γγ$ decay mode is used. But the sensitivities can now be improved by a factor 2 to 10, depending on the Higgs mass, if the Higgs decay modes to $WW^*$, $ZZ^*$, $WW$, $ZZ$ are also used.

hep-ph↗
arXiv1998

New Physics Signatures in Dijets at Hadron Colliders

We show how to detect and disentangle at the upgraded Tevatron and at LHC, the effects of the three purely gluonic $dim=6$ $SU(3)\times SU(2) \times U(1)$ CP-conserving and CP-violating gauge invariant operators $\olØ_{DG}$, $Ø_G$ and $\wtilØ_{G}$. These operators are inevitably generated by New Physics (NP), if the heavy particles responsible for it are coloured. We establish the relations between their coupling constants and the corresponding NP scales defined through the unitarity relations. We then study the sensitivity and limits obtainable through production processes involving one or two jets, and express these limits in terms of the NP scales implied by unitarity. A detailed comparison with the results of the studies of the analogous electroweak operators, is also made.

hep-ph↗
arXiv1997

Signals for Neutralino Box Effects at LEP2

We have computed the contribution to the observables of the final two fermion channel at LEP2, at the limiting energy $\sqrt{q^2}=200 GeV$, coming from boxes with two neutralinos of purely gaugino type, of mass $M=100 GeV$. We find a potentially visible effect only for the muon channel, in the cross section and, to a lesser extent, in the forward-backward asymmetry. Analogous effects coming from the chargino box are also briefly discussed.

hep-ph↗
arXiv1997

General constraints on light resonances in a strongly coupled symmetry breaking sector

In this paper we consider the information that can be obtained about a strongly-interacting symmetry breaking sector from precision measurements of four-fermion processes at LEP2 or a (500 GeV) NLC . Using a ``Z-peak'' subtracted approach to describe four-fermion processes, we show that measurements of the cross section for muon production, the related forward-backward asymmetry, and the total cross section for the production of hadrons (except $t$'s) can place constraints on (or measure the effectsof) the lightest vector or axial resonances present in a strong symmetry breaking sector. We estimate that such effects will be visible at LEP2 for resonances of masses up to approximately 350 GeV, and at a 500 GeV NLC for resonances of masses up to approximately 800 GeV. Multiscale models, for example, predict the presence of light vector and axial mesons in this mass range and their effects could be probed by these measurements.

hep-ph↗
arXiv1997

Tests of possible non standard properties of the heavy quarks and the Higgs boson

We discuss the sensitivity of the processes e+e- to b b(bar), e+e- to t t(bar) and gamma gamma to H to certain residual New Physics interactions affecting the heavy quark and Higgs boson sector. With a linear collider of 500 GeV, one should be able to detect and identify such effects with a characteristic scale up to 50 TeV.

hep-ph↗
arXiv1997

Constraints on anomalous gauge couplings from present LEP1 and future LEP2, BNL data

We analyze, in a rather general model where anomalous triple gauge couplings are present, the visible effects in R$_b$ (measured at LEP1), in W pair production (to be measured at LEP2) and in the muon anomalous magnetic moment (to be measured at BNL). From the combination of the three experiments a remarkable improvement on the pure LEP2 constraints is obtained.

hep-ph↗
arXiv1997

Using e-e+ to b b(bar) to test properties of new interactions at LEP2 and higher energies

We show that in e-e+ colliders at energies above the Z-peak, the process e-e+ to b b(bar) becomes very sensitive to the presence of residual New Physics (NP) effects described by the dim=6 SU(3)*SU(2)*U(1) gauge invariant operators O(qW), O(qB) and O(bB). This observation should be combined with the already known great sensitivity of the light fermion production through e-e+ annihilation above the Z-peak, to the bosonic operators O(DW) and O(DB). It is important to emphasize that the effects of all these operators are largely hidden at the Z-peak; while they are enhanced above it through the use the "Z-peak subtracted representation". The observability limits for detecting these operators at LEP2 and NLC, through such light fermion production processes, are also established.

hep-ph↗