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J. Layssac

Publications and source records attributed to J. Layssac.

At least 37 records · Page 2Linked to original sources
arXiv2000

New and Standard Physics contributions to anomalous Z and gamma self-couplings

We examine the Standard and the New Physics (NP) contributions to the ZZZ, ZZgamma and Zgammagamma neutral gauge couplings. At the one-loop level, if we assume that there is no CP violation contained in NP beyond the Standard Model one, we find that only CP conserving neutral gauge couplings are generated, either from the standard quarks and leptons, or from possible New Physics (NP) fermions. Bosonic one-loop diagrams never contribute to these couplings, while the aforementioned fermionic contributions satisfy hZ3=-fgamma5, hZ4=hgamma4=0. We also study examples of two-loop NP effects that could generate non vanishing h4 couplings. We compare quantitative estimates from SM, MSSM and some specific examples of NP contributions, and we discuss their observability at future colliders.

hep-ph↗
arXiv2000

Signatures of the anomalous $Zγ$ and ZZ production at the lepton and hadron Colliders

The possible form of New Physics (NP) interactions affecting the ZZZ, $ZZ γ$ and $Zγγ$ vertices, is critically examined. Their signatures and the possibilities to study them, through ZZ and $Zγ$ production, at the e^-e^+ Colliders LEP and LC and at the hadronic Colliders Tevatron and LHC, are investigated. Experimental limits obtained or expected on each coupling are collected. A simple theoretical model based on virtual effects due to some heavy fermions is used for acquiring some guidance on the plausible forms of these NP vertices. In such a case specific relations among the various neutral couplings are predicted, which can be experimentally tested and possibly used to constrain the form of the responsible NP structure.

hep-ph↗
arXiv1999

The gamma gamma to Z Z process and the search for virtual SUSY effects at a gamma gamma Collider

We study the helicity amplitudes of the process gamma gamma to ZZ in the Standard Model at high energy. These amplitudes receive contributions from the W and charged quark and lepton loops, analogous to those encountered in the gamma gamma to gamma gamma, gamma Z cases studied before. But gamma gamma to ZZ also receives contributions from the Higgs s-channel poles involving the effective Higgs-gamma gamma vertex. At energies of about 300 GeV, the amplitudes in all three processes are mainly helicity-conserving and almost purely imaginary; which renders them a very useful tool in searching for New Physics. As an example, a SUSY case is studied, and the signatures due to the virtual effects induced by a chargino-, charged slepton- or a lightest stop-loop in gamma gamma to ZZ, are explored. These signatures, combined with the analogous ones in gamma gamma to gamma gamma and gamma gamma to gamma Z, should help identifying the nature of possible New Physics particles.

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↗
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

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↗
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

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↗
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.

hep-ph↗
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.

hep-ph↗
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.

hep-ph↗
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.

hep-ph↗
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 $Γ(H\toγγ)/Γ(H\toγZ)$.

hep-ph↗
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.

hep-ph↗
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.

hep-ph↗
arXiv1995

Signatures of anomalous coupling in boson pair production through γγcollisions

We discuss possible New Physics (NP) effects on the processes $γγ\to W^+W^-$, $ZZ$, $Zγ$, $γγ$, $HH$ which are observable in $γγ$ collisions. Such collisions may be achieved through laser backscattering at a high energy $e^+e^-$ linear collider. To the extent that no new particles will be directly produced in the future colliders, it has already been emphasized that the new physics possibly hidden in the bosonic interactions, may be represented by the seven $dim=6$ operators $Ø_W$, $Ø_{BΦ}$, $Ø_{WΦ}$, $Ø_{UB}$, $Ø_{UW}$, $\olØ_{UB}$ and $\olØ_{UW}$ (the last two ones being CP-violating). In this paper, we show that the above processes are sensitive to NP scales at the several TeV range, and we subsequently discuss the possibility to disentangle the effects of the various operators.

hep-ph↗
arXiv1994

A Probe of New Resonant Structures with Four Fermion Processes at a 1 TeV $e^+e^-$ Collider

Oblique contributions from vector particles that are strongly coupled to the known gauge bosons are calculated for the case of various observables at a 1 TeV $e^+e^-$ collider. Constraints provided by LEP 1 results are taken into account in a model-independent way through a dispersion relation technique. Depending on the assumed theoretical properties, mass limits of several TeV should be observable.

hep-ph↗