arXiv subjects

Enrico Nardi

Publications and source records attributed to Enrico Nardi.

95 records · Page 6Linked to original sources

arXiv1994

New Neutral Gauge Bosons and New Heavy Fermions in the Light of the New LEP Data

We derive limits on a class of new physics effects that are naturally present in grand unified theories based on extended gauge groups, and in particular in $E_6$ and $SO(10)$ models. We concentrate on $i$) the effects of the mixing of new neutral gauge bosons with the standard $Z_0$; $ii$) the effects of a mixing of the known fermions with new heavy states. We perform a global analysis including all the LEP data on the $Z$ decay widths and asymmetries collected until 1993, the SLC measurement of the left--right asymmetry, the measurement of the $W$ boson mass, various charged current constraints, and the low energy neutral current experiments. We use a top mass value in the range announced by CDF. We derive limits on the $Z_0$--$Z_1$ mixing, which are always $\lsim 0.01$ and are at the level of a few {\it per mille} if some specific model is assumed. Model-dependent theoretical relations between the mixing and the mass of the new gauge boson in most cases require $M_{Z'} > 1\,$TeV. Limits on light--heavy fermion mixings are also largely improved with respect to previous analyses, and are particularly relevant for a class of models that we discuss.

hep-ph↗

arXiv1994

Neutrino Phenomenology from Unconventional E$_6$ models

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, $m_{ν_τ}\sim 0.1-10\,$eV is generated at one loop, $m_{ν_μ} \sim 10^{-3}\,$eV is generated at two loops and ${ν_e}$ remains massless.

hep-ph↗

arXiv1994

Identifying Unconventional E$_{\bf 6}$ Models at $e^+ e^-$ Colliders

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 $(Z_θ)$ 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 $e_L$, $μ_L$ and $τ_L$ and for the right-handed $d$-type quarks, as well as on the type of $Z_θ$ 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 $Z_θ$ masses as large as $\sim 1.5$ TeV.

hep-ph↗

arXiv1993

Review of constraints on fermion mixing

The constraints on the mixing angles of the standard fermions with new heavy particles that can appear in many extensions of the electroweak theory are reviewed. Some emphasis is put in distinguishing the effects of a mixing with new states which transform in a non-canonical way with respect to $SU(2)\times U(1)$ (left-handed singlets or right-handed doublets), from the effects of a mixing with new states with standard quantum number assignments. Constraints from flavor changing neutral current processes, as well as from flavor diagonal and charged current experimental data are considered. New limits for lepton flavor violating mixings are presented. By using the most recent results on the $τ$ mass, lifetime and branching ratios, updated limits on the mixing parameters of the $τ$ neutrino are derived. These limits are improved up to a factor of 3, and no signals of deviation from the standard model predictions are found. Limits on the mass of a $Z_1$ gauge boson from E$_6$, from the absence of muon number violating processes are briefly discussed as well.

hep-ph↗

arXiv1992

New flavor changing interactions in extended gauge models

A new class of flavor changing (FC) neutral interactions can arise in models based on extended gauge groups (rank $>$4) when new charged fermions are present together with new neutral gauge bosons. In some cases the FC couplings to a new $Z_1$ are expected to be sizeable, implying that the $Z_1$ mass must be large enough as to explain the observed suppression of FC transitions. Concentrating on E$_6$ models and assuming for the FC parameters a theoretically natural range of values, I show that in most cases the presence of a $Z_1$ much lighter than 1 TeV is unlikely.

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