arXiv subjects

F. M. Renard

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

At least 109 records · Page 6Linked to original sources

arXiv1997

General description of New Physics in t, b and boson Interactions and its Unitarity Constraints

We list all possible dim=6 CP conserving and SU(3)* SU(2)*U(1) gauge invariant interactions, which could be generated in case no new particles would be reachable in the future Colliders, and the only observable New Physics would be in the form of new interactions affecting the scalar sector and the quarks of the third family. These interactions are described by operators involving the standard model scalar field, the quarks of the third family and the gauge bosons. Subsequently, we identify those operators which do not contribute to LEP1 (and lower energy) observables at tree level and are not purely gluonic. Since present measurements do not strongly constrain the couplings of these operators, we derive here the unitarity bounds on them. Finally, in order to get a feeling on the possible physical meaning of the appearance of any of these operators, we identify the operators generated in a class of renormalizable dynamical models which at the TeV scale, are fully described by the SU(3)*SU(2)*U(1) gauge group.

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arXiv1996

A Model-independent Description of New Physics effects in e+e- to t tbar

We study the potential of a future $e^+e^-$ collider for the search of anomalous gamma t t bar and Z t t bar couplings, assuming that CP-invariance holds. This is done in a model-independent way, considering that all six possible couplings do appear. Two experimental situations are envisaged, with and without electron beam polarization. Observability limits in the form of domains in the 6-dimensional parameter space are established. Illustrations for specific constrained models are also presented and implications for new physics searches are discussed.

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arXiv1996

Dynamical Scenarios for Anomalous Interactions involving t, b quarks and bosons

In the present paper we explore various dynamical scenarios for New Physics (NP) which could be generated by very heavy particles and observed in the form of non-standard "low energy" interactions affecting the scalar sector, the gauge bosons and the quarks of the third family. Such interactions have been previously expressed in terms of 48 CP-conserving gauge invariant operators. We show that each scenario is characterized by a specific subset of such operators. Thus, if some traces of NP are ever found in the experimental data, then the comparison with the predictions of the various scenarios, should be able to guide the search towards the underlying dynamics generating it.

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arXiv1996

Longitudinal Polarization at future $e^+e^-$ Colliders and Virtual New Physics Effects

The theoretical merits of longitudinal polarization asymmetries of electron-positron annihilation into two final fermions at future colliders are examined, using a recently proposed theoretical description. A number of interesting features, valid for searches of virtual effects of new physics, is underlined, that is reminiscent of analogous properties valid on top of $Z$ resonance. As an application to a concrete example, we consider the case of a model with triple anomalous gauge couplings and show that the additional information provided by these asymmetries would lead to a drastic reduction of the allowed domain of the relevant parameters.

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arXiv1996

Observability at LEP2 hadronic channels of a Z' with small lepton couplings

We consider the effects of a number of models with one extra $Z$, with enhanced couplings to quarks, in the final \underline{hadronic} channels at LEP2. We show that, for a number of representative cases, visible effects could be produced even for very low values of the \underline{lepton} couplings, much smaller than the existing LEP1/SLC and the future LEP2 (lepton channel) bounds.

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arXiv1996

Effective lagrangian description of top production and decay

We propose a rather general description of residual New Physics (NP) effects on the top quark couplings. These effects are described in terms of 20 gauge invariant $dim=6$ operators involving gauge and Higgs bosons as well as quarks of the third family. We compute their implications for the $γt\bar t$, $Z t \bar t$ and $tbW$ vertices and study their observability in the process $e^-e^+ \to t\bar t$ with $t\to bW \to b\ell^+ν_\ell$. We present results for the integrated cross section, the angular distribution and various decay distribution and polarization asymmetries for NLC energies of $0.5-2~TeV$. Observability limits are discussed and interpreted in terms of the NP scales associated to each operator through the unitarity constraints. The general landscape of the residual NP effects in the heavy quark and bosonic sectors is also presented.

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arXiv1996

Searches for Clean Anomalous Gauge Couplings effects at present and future $e^+e^-$ colliders

We consider the virtual effects of a general type of Anomalous (triple) Gauge Couplings on various experimental observables in the process of electron-positron annihilation into a final fermion-antifermion state. We show that the use of a recently proposed "$Z$-peak subtracted" theoretical description of the process allows to reduce substantially the number of relevant parameters of the model, so that a calculation of observability limits can be performed in a rather simple way. As an illustration of our approach, we discuss the cases of future measurements at LEP2 and at a new 500 GeV linear collider.

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arXiv1996

Z' Physics

The limits on extra neutral gauge bosons, which could be reached at LEP2, are reviewed. Exclusion and discovery limits are discussed for f\bar f and WW production.

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arXiv1996

Z' Reservation at LC2000

We consider the possibility that one extra $Z\equiv Z'$ exists with a mass of more then two TeV and fermion couplings that do not violate (charged) lepton universality. We show that, in such a situation, a functional relationship is generated between the \underline{deviations} from the SM values of three leptonic observables of two-fermion production at LC 2000 that is completely independent of the values of the $Z'$ mass and couplings. This selects a certain region in the 3-d space of the deviations ($Z'$ "reservation") that is \underline{characteristic} of the model and that would have no observable intersection with analogous regions corresponding to other interesting models of new physics.

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arXiv1996

Anomalous $Z'$ effects in the WW channel at $LC 2000$

We consider the virtual signals of a $Z'$ of very general type in the process $e^+e^-\to W^+W^-$ at a future linear $e^+e^-$ collider of 2000 GeV c.m. energy ($LC2000$). We show that possible deviations from the SM predictions in this channel are related to similar deviations in the purely leptonic one in a way that is only characteristic of this $Z'$ model, and not in general of possible competitor models with anomalous gauge couplings.

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arXiv1996

Residual New Physics Effects in $e^+e^-$ and $γγ$ Collisions at NLC

We report on a set of studies concerning the description of New Physics (NP) effects characterized by a scale much higher than the electroweak scale. We concentrate on the residual effects described by an effective lagrangian involving only bosonic fields and restrict to the case that the Higgs field is described linearly, so that the Higgs particle exists. It has already been emphasized that this lagrangian may be represented by eight $dim=6$ operators; namely the blind CP-conserving operators $Ø_W$, $Ø_{BΦ}$, $Ø_{WΦ}$, $Ø_{UB}$, $Ø_{UW}$, the ''superblind'' ${\cal O}_{Φ2}$ and the two CP-violating ones $\olØ_{UB}$ and $\olØ_{UW}$. To each operator and for any given value of its coupling, we associate an unambiguous NP scale determined by the energy where unitarity is saturated. Our study of the possible NP tests realizable at a future linear $e^+e^-$ collider concentrates on $(e^+e^-\to HZ ~,~ Hγ)$, and on $γγ$ collisions producing boson pairs ($γγ\to W^+W^-$, $ZZ$, $Zγ$, $γγ$, $HH$), or a single Higgs ($γγ\to H$). The sensitivities to the various operators involved in the effective lagrangian are estimated. We find that the testable NP scales vary from a few tens of TeV in the boson pair processes, up to 65 TeV in single H production. Ways to disentangle the various operators are also proposed. In this respect a comparison of the NP effects in the various boson pair processes, like \eg\@ a comparison of $e^+e^-\to HZ$ versus $e^+e^-\to Hγ$, should be very fruitful. This is applied also to the study of the Higgs branching ratios, and especially the ratio $Γ(H\toγγ)/Γ(H\toγZ)$.

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arXiv1996

Hadrophilic Z':a bridge from LEP1, SLC and CDF to LEP2 anomalies

In order to explain possible departures from the Standard Model predictions for $b\bar b$ and $c\bar c$ production at $Z$ peak, we propose the existence of a $Z'$ vector boson with enhanced couplings to quarks. We first show that this proposal is perfectly consistent with the full set of LEP1/SLC results. In particular, $Z-Z'$ mixing effects naturally explain the fact that $Γ_b$ and $Γ_c$ deviate from the SM in opposite directions. We then show that there is a predicted range for enhanced $Z'q\bar q$ couplings which explains, for a precise and interesting range of $Z'$ masses, the excess of dijet events seen at CDF. A $Z'$ with such couplings and mass would produce clean observable effects in $b\bar b$ and in total hadronic production at LEP2.

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arXiv1996

Triple Gauge Boson Couplings

We present the results obtained by the "Triple Gauge Couplings" working group during the LEP2 Workshop (1994-1995). The report concentrates on the measurement of $WWγ$ and $WWZ$ couplings in $e^-e^+\to W^-W^+$ or, more generally, four-fermion production at LEP2. In addition the detection of new interactions in the bosonic sector via other production channels is discussed.

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arXiv1995

Z' Reservation at LEP2

We consider the possibility that one extra $Z\equiv Z'$ exists with arbitrary mass and fermion couplings that do not violate (charged) lepton universality. We show that, in such a situation, a functional relationship is generated between the \underline{deviations} from the SM values of three leptonic observables of two-fermion production at future $e^+e^-$ colliders that is completely independent of the values of the $Z'$ mass and couplings. This selects a certain region in the 3-d space of the deviations that is \underline{characteristic} of the model ($Z'$ "reservation"). As a specific and relevant example, we show the picture that would emerge at LEP2 under realistic experimental conditions.

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arXiv1995

Tests of Anomalous Higgs Boson Couplings through $e^-e^+ \to HZ$ and $Hγ$

We show how the processes $e^- e^+ \to ZH, ~ γH$, which will be studied at LEP2 and at high energy colliders, could be used to search for types of New Physics (NP) characterized by an effective NP scale in the few TeV range and affecting the scalar sector only. In particular, for $e^- e^+ \to Z H$, we propose the measurement of suitable azimuthal asymmetries determined from the angular distribution of the $Z$-decay plane with respect to the $Z$-production one. Such asymmetries, together with the $ZH$ angular distribution, may allow a complete disentangling of the five $dim=6$ operators $ {\cal O }_{UB}$, $ {\cal O }_{UW}$, $\overline{\cal O }_{UB}$, $\overline{\cal O }_{UW}$, ${\cal O }_{\Phi2}$, describing the residual NP effects to the Higgs couplings. We note that the first four of these operators contribute to processes relevant to LEP1 precision measurements only at the 1-loop level, while the last operator remains completely unconstrained at this level. The process $e^- e^+ \to γH$ is also very sensitive to NP and should bring important independent information.

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arXiv1995

Universal and Non Universal New Physics Effects in a General Four-fermion Process: a Z-peak Subtracted Approach

We calculate, using a Z-peak subtracted representation of four-fermion processes previously illustrated for the case of electron-positron annihilation into charged lepton-antilepton, the corresponding expressions of the new physics contributions for the case of final quark-antiquark states, allowing the possibility of both universal and non universal effects. We show that, in each case, the main result obtained for the final lepton channel can be generalized, so that every experimentally measurable quantity can be expressed in terms of input parameters \underline{measured} on Z resonance, of $α(0)$ and of a small number of subtracted one loop expressions. Some examples of models are considered for several c.m. energy values, showing that remarkable simplifications are often introduced by our approach. In particular, for the case of a dimension-six lagrangian with anomalous gauge couplings, the same reduced number of parameters that would affect the observables of final leptonic states are essentially retained when one moves to final hadronic states. This leads to great simplifications in the elaboration of constraints and, as a gratifying byproduct, to the possibility of making the signal from these models clearly distinguishable from those from other (both universal and non universal) competitors.

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arXiv1995

Anomalous $Z'$ effects in the WW channel at NLC

We consider the virtual signals of a $Z'$ of very general type in the process $e^+e^-\to W^+W^-$ at a future linear collider (NLC). We show that possible deviations from the SM predictions in this channel are related to similar deviations in the purely leptonic one in a way that is only characteristic of this $Z'$ model, and not in general of possible competitor models with anomalous gauge couplings.

hep-ph↗

arXiv1995

Residual New Physics Effects in $e^+e^-$ and $γγ$ Collisions

We present a set of studies concerning the description of New Physics (NP) effects caracterized by a scale much higher than the electroweak scale. We show that both present experimental results and several types of theretical considerations about the Standard Model (SM) and NP suggest a hierarchy among the sectors in which NP manifestations should appear. We concentrate on residual effects described by effective lagrangians involving bosonic and/or heavy quark fields. For each operator we propose an unambiguous definition of the NP scale given by the energy at which unitarity is saturated. We also consider the possible existence of Higher Vector Bosons. We then study the tests realizable at present and future colliders. We start from the analysis of the high precision tests at Z peak, discussing separately the constraints obtained from the light fermionic sector and those due to the $b\bar b$ sector. We then consider the process $e^+e^-\to f\bar f$ at higher energies (LEP2 and NLC) and we propose the "Z peak subtracted representation" which allows to automatically take into account Z peak constraints and to describe several types of NP effects at any other energy. Applications to various NP effects (Higher Vector Bosons, Technicolour resonances, Anomalous Gauge Boson couplings) are given. We then concentrate on several bosonic processes, $e^+e^-\to W^+W^-$, $HZ$, $Hγ$, and $γγ$ collisions producing boson pairs or a single Higgs. We study the sensitivities to the various operators involved in the effective lagrangian and we propose ways to disentangle them. The NP scales which can be felt vary from a few TeV at LEP2 up to 200 TeV in single H production at NLC.

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