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 Physics , 1994, DOI: 10.1103/PhysRevD.50.203 Abstract: Recently it was shown that, in the framework of superstring inspired $\E$ models, the presence of generation dependent discrete symmetries allows us to construct a phenomenologically viable class of models in which the three generations of fermions do not have the same embedding within the fundamental {\bf 27} dimensional representation of E$_6$. In this scenario, these different embeddings of the conventional fermions imply that the left-handed charged leptons and the right-handed $d$-type quarks are coupled in a non--universal way to the new neutral gauge bosons $(Z_\theta)$ present in these models. It was also shown that a unique signature for this scenario, would be a deviation from unity for the ratio of cross sections for the production of two different lepton species in $e^+e^-$ annihilation. However, several different scenarios are possible, depending on the particular assignment chosen for $e_L$, $\mu_L$ and $\tau_L$ and for the right-handed $d$-type quarks, as well as on the type of $Z_\theta$ boson. Such scenarios can not be disentangled from one another by means of cross section measurements alone. In this paper we examine the possibility of identifying the pattern of embeddings through measurements of polarized and unpolarized asymmetries for fermion pair-production at the 500 GeV $e^+e^-$ Next Linear Collider (NLC). We show that it will be possible to identify the different patterns of unconventional assignments for the left-handed leptons and for the $b_R$ quark, for $Z_\theta$ masses as large as $\sim 1.5$ TeV.
 A. Djouadi Physics , 1995, Abstract: We summarize the potential of high--energy $\ee$ linear colliders for discovering, and in case of discovery, for studying the signals of extended gauge models. We will mainly focus on the virtual signals of new neutral gauge bosons and on the production of new heavy leptons. [Invited talk given at the Workshop on Physics and Experiments with Linear Colliders, Morioka-Appi, Japan, September 8-12 1995.]
 Physics , 1993, DOI: 10.1007/BF01411025 Abstract: We discuss the production, at high--energy e$^+$e$^-$ linear colliders, of new heavy fermions predicted by extensions of the Standard Model. We analyze in great details the various signals and the corresponding backgrounds for both pair production and single production in association with ordinary fermions. Concentrating on new leptons, we use a model detector for e$^+$e$^-$ collisions at a center of mass energy of 500 GeV, to illustrate the discovery potential of the Next Linear Colliders.
 Physics , 2002, Abstract: We examine the signatures for noncommutative QED at $e^+e^-$ colliders with center of mass energies in excess of 1 TeV such as CLIC. For integrated luminosities of 1 ab$^{-1}$ or more, sensitivities to the associated mass scales greater than $\sqrt s$ are possible.
 Physics , 1999, DOI: 10.1103/PhysRevD.60.112003 Abstract: Gravity can become strong at the TeV scale in the theory of extra dimensions. An effective Lagrangian can be used to describe the gravitational interactions below a cut-off scale. In this work, we study the associated production of the gravitons with a $Z$ boson or a photon at $e^+ e^-$ colliders of energies of LEPII to the Next Linear Colliders (NLC) ($\sqrt{s}=0.25-1.5$ TeV) and calculate the sensitivity to the new interactions. We also obtain the limit on the cut-off scale using the present data from LEPII.
 Physics , 1997, Abstract: We study pair production of leptoquarks in e+e- annihilation at linear colliders. A detailed simulation including beamstrahlung and initial state radiation, leptoquark decay and hadronization, as well as detector smearing, is performed. Discovery limits are estimated for center-of-mass energies of 500 and 800 GeV. The prospects for determining masses and couplings of leptoquarks are also investigated.
 Enrico Nardi Physics , 1994, Abstract: Superstring derived $\E$ models can accommodate small neutrino masses if a discrete symmetry is imposed which forbids tree level Dirac neutrino masses but allows for radiative mass generation. The only possible symmetries of this kind are known to be generation dependent. We explore the possibility that, as a consequence of such a symmetry, the three sets of light states in each generation do not have the same assignments with respect to the {\bf 27} of $E_6$, implying that the gauge interactions under the additional $U(1)^\pr$ factors are non universal. Models realising such a scenario are found to be viable, and by requiring the number of light neutral states to be minimal, an almost unique pattern of neutrino masses and mixings arises. We briefly discuss a model in which, with a natural choice of the parameters, $m_{\nu_\tau}\sim 0.1-10\,$eV is generated at one loop, $m_{\nu_\mu} \sim 10^{-3}\,$eV is generated at two loops and ${\nu_e}$ remains massless.
 Enrico Nardi Physics , 1993, DOI: 10.1103/PhysRevD.48.3277 Abstract: Conventional superstring derived E$_6$ models can accommodate small neutrino masses if a discrete symmetry is imposed which forbids tree level Dirac neutrino masses but allows for radiative mass generation. Since the only possible symmetries of this kind are known to be generation dependent, we explore the possibility that the three sets of light states in each generation do not have the same assignments with respect to the 27 of $E_6$, leading to non universal gauge interactions under the additional $U(1)'$ factors for the known fermions. We argue that models realising such a scenario are viable, with their structure being constrained mainly by the requirement of the absence of flavor changing neutral currents in the Higgs sector. Moreover, in contrast to the standard case, rank 6 models are not disfavoured with respect to rank 5. By requiring the number of light neutral states to be minimal, these models have an almost unique pattern of neutrino masses and mixings. We construct a model based on the unconventional assignment scenario in which (with a natural choice of the parameters) $m_{\nut}\sim O(10)$eV is generated at one loop, $m_{\num}$ is generated at two loops and lies in a range interesting for the solar neutrino problem, and $\nue$ remains massless. In addition, since baryon and lepton number are conserved, there is no proton decay in the model. To illustrate the non-standard phenomenology implied by our scheme we also discuss a second scenario in which an attempt for solving the solar neutrino puzzle with matter enhanced oscillations and practically massless neutrinos can be formulated, and in which peculiar effects for the $\num$ --> $\nut$ conversion of the upward-going atmospheric neutrinos could arise as well.
 Physics , 1997, DOI: 10.1016/S0550-3213(97)00825-0 Abstract: If the gravitino G is very light and all the other supersymmetric particles are above threshold, supersymmetry may still be found at colliders, by looking at processes with only gravitinos and ordinary particles in the final state. We compute here the cross-section for the process e^+ e^- ==> G G gamma, whose final state can give rise to a distinctive photon plus missing energy signal at present and future e^+ e^- colliders. We describe how the present LEP data can be used to establish a lower bound on the gravitino mass of order 10^{-5} eV. We conclude with a critical discussion of our results, comparing them with related ones and outlining possible generalizations.
 Physics , 2002, DOI: 10.1103/PhysRevD.66.015003 Abstract: We study lepton flavor violating signals at a future e+ e- linear collider within the general MSSM, allowing for the most general flavor structure. We demonstrate that there is a large region in parameter space with large signals, while being consistent with present experimental bounds on rare lepton decays such as mu -> e gamma. In our analysis, we include all possible signals from charged slepton and sneutrino production and their decays as well as from the decays of neutralinos and charginos. We also consider the background from the Standard Model and the MSSM. We find that in general the signature e tau missing energy is the most pronounced one. We demonstrate that even for an integrated luminosity of 100 inverse fb the signal can be large. At a high luminosity linear collider, precision experiments will allow one to determine the lepton flavor structure of the MSSM.
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