Enrico Nardi

μ-e conversion in nuclei and Z′ physics

Together with the existence of new neutral gauge bosons, models based on extended gauge groups (rank > 4) often predict also new charged fermions. A mixing of the known fermions with new states with exotic weak-isospin assignments (left-handed singlets and right-handed doublets) will induce tree level flavour changing neutral interactions mediated by Z exchange, while if the mixing is only with new states with ordinary weak-isospin assignments, the flavour changing neutral Currents are mainly due to the exchange of the lightest new neutral gauge boson Z’. We show that the present experimental limits on js—e conversion in nuclei give a nuclear-model-independent bound on the Z—e—1i vertex which is twice as strong as that obtained from ~s—* eee. In the case of E6 models these limits provide quite stringent constraints on the Z’ mass and on the Z—Z’ mixing angle. We point out that the proposed experiments to search for ti—c conversion in nuclei have good chances to find evidence of lepton flavour violation, either in the case that new exotic fermions are present at the electroweak scale, or if a new neutral gauge boson Z’ of E6 origin lighter than a few TeV exists.

LIMITS ON NEUTRINO MIXING WITH NEW HEAVY PARTICLES

We study the effects induced by new neutral fetmions below their mass threshold, due to their possible mixing with the standard neutrinos. We use as experimental constraints the recent results on lepton universality, together with the measurement of the ,X decay rate and the updated LEP data. In particular, the inclusion in our data set of the most recent determinations of the r branching fractions, mass and lifetime implies that a previous indication of a non-vanishing mixing for r+ is no longer present. We obtain new stringent limits on the mixing parameters between v,, v~, V, and heavy neutral states of different weak isospin. If no assumption on the type of neutrinos involved in the mixing is made, we find sz, < 0.0071, S& < 0.0014 and sz < 0.033. 1. Introdllction Several extensions of the Standard Model (SM), such as the Left-Right, SO( 10) and E6 models, predict the existence of new neutral fermions. LEP results imply that these new particles, if they are not singlets under the SM group, should be heavier than N Mz/2. Even in the case in which these new particles are too heavy to be directly produced, their presence could still affect the physics at low energy, since in general they will be mixed with the SM neutrinos. Such a mixing will affect the neutral current (NC) and charged current (CC) couplings of the known neutral states, and hence can be constrained by the precision tests of the SM. Previous model-independent, comprehen

New neutral gauge bosons and new heavy fermions in the light of the new LEP data

Abstract 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 ≲ 0.01 and are at the level of a few 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.

Z', new fermions, and flavor-changing processes: Constraints on E6 models from micro-->eee.

We study a new class of flavor-changing interactions, which can arise in models based on extended gauge groups (rank > 4) when new charged fermions are present together with a new neutral gauge boson. We discuss the cases in which the flavor-changing couplings in the new neutral current coupled to the Z are theoretically expected to be large, implying that the observed suppression of neutral flavor-changing transitions must be provided by heavy Z masses together with small Z-Z mixing angles. Concentrating on E 6 models, we show how the tight experimental limit on μ → eee implies serious constraints on the Z mass and mixing angle

Unconventional superstring-derived E6 models and neutrino phenomenology.

Conventional superstring-derived E[sub 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[sub 6], leading to nonuniversal gauge interactions under the additional U(1)[prime] factors for the known fermions. We argue that models realizing 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 with the standard case, rank 6 models are not disfavored 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) [ital m][sub [nu][tau]][similar to]10 eV is generated at one loop, [ital m][sub [nu][mu]] is generated at two loops and lies in a range interesting for themorexa0» solar neutrino problem, and [nu][sub [ital e]] remains massless. In addition, since baryon and lepton number are conserved, there is no proton decay in the model. In order to illustrate the nonstandard 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 [nu][sub [mu]][r arrow][nu][sub [tau]] conversion of the upward-going atmospheric neutrinos could arise as well.«xa0less

Identifying unconventional E6 models at e+e- colliders.

Recently it was shown that, in the framework of superstring inspired

Signals of unconventional E6 models at e+e- colliders.

E

Identifying unconventional E[sub 6] models at [ital e][sup +][ital e][sup [minus]] colliders

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

New flavor changing interactions in extended gauge models

_6

Neutrino phenomenology from unconventional E(6) models

. In this scenario, these different embeddings of the conventional fermions imply that the left-handed charged leptons and the right-handed