arXiv subjects

G. J. Gounaris

Publications and source records attributed to G. J. Gounaris.

At least 55 records · Page 3Linked to original sources

arXiv2001

The processes gamma gamma to Z H in SM and MSSM

The process gamma gamma to ZH first arises at the 1-loop level, and as such it provides remarkable tests of the structure of the electroweak Higgs sector. These tests are complementary to those in the gauge sector involving gamma gamma to gamma gamma, gamma Z, ZZ. We show that in the Standard Model (SM) where H=H_{SM}, as well as in the supersymmetric case where H=h^0, H^0 or A^0, observables exist (like e.g. the energy dependence, angular distribution, photon polarization dependence or final Z polarization) which present rather spectacular properties. Such properties involve strong threshold effects with steps, bumps or peaks, reflecting the type of Higgs and heavy quarks and chargino masses and couplings predicted by the SM and supersymmetric models.

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arXiv2001

The heavy neutral Higgs signature in the gamma gamma to Z Z process

If the Standard Model (SM) Higgs particle is sufficiently heavy, then its contribution to gamma gamma to Z Z should be largely imaginary, interfering with the also predominantly imaginary SM "background" generated by the W-loop. For standard model Higgs masses in the region 200 \lsim m_H \lsim 500 GeV, this interference is found to be constructive and increasing the Higgs signal. In the minimal SUSY case an interference effect should also appear for the contribution of the heavier CP-even neutral Higgs boson H^0, provided it is sufficiently heavy. The effect is somewhat reduced though, by the smallness of the H^0 width and the gamma gamma and ZZ branching ratios. The interference is again found to be constructive for part of the parameter space corresponding to sfermion masses at the TeV scale and maximal stop mixing. For both the SM and SUSY cases, regions of the parameter space exist though, where the interference may be destructive. It is therefore essential to take these effects into account when searching for possible scalar Higgs-like candidates. To this aim, we present the complete analytic expressions for both resonance and background amplitudes.

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arXiv2000

The $γγ\to A^0 A^0$ process at a $γγ$ Collider

The helicity amplitudes for the process $γγ\to A^0 A^0$ are studied to 1-loop order in the minimal SUSY (MSSM) model, where $A^0$ is the CP-odd Higgs particle. Simple exact analytic formulae are obtained, in terms of the $C_0$ and $D_0$ Passarino-Veltman functions; in spite of the fact that the loop-diagrams often involve different particles running along their sides. For a usual mSUGRA set of parameters, $σ(γγ\to A^0 A^0) \sim (0.1-0.2)\rm fb $ is expected. If SUSY is realized in Nature, these expressions should be useful for understanding the Higgs sector.

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arXiv2000

The process γγ-> γγ, γZ, ZZ

The Standard Model contributions to the processes $γγ\to γγ$, $γZ$ and ZZ at sufficiently high energies, are found to be helicity conserving and almost purely imaginary. This is due to the W-loop contribution, which is much bigger than the fermionic ones at such energies. Thus the structure of these amplitudes acquires an impressive simplicity at high energies. Nothing like this appears in other process like e.g. the the production of a pair of neutral Higgs bosons.

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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.

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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.

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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.

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arXiv1999

Light by Light Scattering at High Energy: a Tool to Reveal New Particles

We point out a few remarkable properties of the $γγ\toγγ$ process at high energy. They should allow to search for effects of new particles and interactions. We give illustrations with threshold effects due to pairs of new charged particles (charginos, charged Higgses, sfermions), resonance effects due to s-channel production of neutral scalars (standard or supersymmetric Higgs particles, technipions) and unitarity saturating amplitudes due to a strongly interacting sector. The use of polarized photon beams is also briefly discussed.

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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.

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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 $100 fb^{-1}$.

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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.

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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.

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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.

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arXiv1998

Search for New Physics Through $e^- e^+ \to t \bar t$

If all new particles are too heavy to be directly produced in the future Colliders, then the long sought New Physics (NP) could only appear in the form of new interactions, beyond those of the Standard Model (SM). Many of the processes that could be used to study these interactions, have already been discussed in the literature. Here, I first briefly discuss the list of all not yet excluded CP conserving such interactions, realized as $SU(3)\times SU(2) \times U(1)$ gauge invariant $dim=6$ operators affecting the Higgs and the quarks of the third family. Subsequently, I concentrate on the $γt\bar t$ and $Zt\bar t$ vertices generated by NP, and on the possibility to study them by using various spin asymmetries accessible in $e^-e^+ \to t \bar t$, for polarized and unpolarized beams. It is found that these asymmetries can fully determine the form of the $γt\bar t$ and $Zt\bar t$ couplings.

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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.

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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.

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arXiv1997

Studying Trilinear Gauge Couplings at Linear Collider Energies

We study the sensitivity of the processes `e+ e- -> lepton (l) neutrino (v) quark (u) antiquark (d)', where the lepton is an electron or a muon, on the non-standard trilinear gauge couplings (TGC), using the optimal observables method at Linear Collider energies. Our study is based on the four-fermion generator ERATO. Taking into account all possible correlations between the different trilinear gauge coupling parameters, we show that they can be measured with an accuracy of $10^{-3}$ to $10^{-4}$ for typical Linear Collider energies and luminosities.

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arXiv1997

Signatures of CP-violation in $γγ\to H$ using polarized beams

The possibility of observing CP-violation in the process $γγ\to H$ is investigated for masses of the Higgs particle in the interval $M_z \lsim m_H \lsim 2 m_t$, using a 0.5TeV tunable linear $e^{+} e^{-}$ collider through laser backscattering. The use of polarized beams allows the formation of two different asymmetries sensitive to CP-violating New Physics (NP) interactions among the gauge and Higgs bosons. It is shown that very low values of the corresponding NP couplings can be probed, for a large range of the Higgs mass.

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