Chiral fermions and gauge-fixing in five-dimensional theories

We study in detail the issue of gauge-fixing in theories with one universal extra dimension, i.e. theories where both bosons and fermions display Kaluza-Klein (KK) excitations. The extra dimension is compactified using the standard orbifold construction for a massless chiral fermion. We carry out the gauge-fixing procedure at the level of the five-dimensional theory and determine the tree-level propagators and interaction vertices needed for performing perturbative calculations with the effective four-dimensional theory resulting after the compactification. The gauge-independence of the tree-level S-matrix involving massive KK modes is verified using specific examples. In order to obtain massive fermionic zero modes one has to enlarge the theory by introducing a set of mirror fermions, a construction which is carried out in detail. Finally, the gauge-independence of the tree-level S-matrix involving the resulting new mass-eigenstates is proved by resorting to generalized current conservation equations.Comment: 10 pages, 5 figures, revtex and axodra

"Secret" neutrino interactions

We review the information about a potentially strong non-standard four-neutrino interaction that can be obtained from available experimental data. By using LEP results and nucleosynthesis data we find that a contact four-fermion neutrino interaction that involve only left-handed neutrinos or both left-handed and right-handed neutrinos cannot be stronger than the standard weak interactions. A much stronger interaction involving only right-handed neutrinos is still allowed.Comment: 12 pages, 2 figures, latex with ws-p8-50x6-00.cls, Talk presented in "Neutrino Mixing", in honour of Samoil Bilenky's 70th Birthday, Torino, March 199

The hyperchargeless triplet Majoron model

We study the general conditions to maintain the scale of the lepton-number-breaking vacuum expectation value at the electroweak scale. It is shown that the only possibilities are if the main component of the resulting Majoron is a hyperchargeless complex triplet or a neutral singlet. Models with a hyperchargeless triplet, even though phenomenologically more interesting, seem to be very difficult to build because they like to break charge conservation. However, we have found a particular extension, by adding an additional neutral singlet, that solves this problem. The model can give a Majorana mass to the neutrinos in the eV range, μ→eγ can proceed with branching ratios at the verge of the present experimental limit and there are no additional decay modes of the Z0 into invisible particles

μ-e conversion in nuclei versus μ→e γ: an effective field theory point of view

Using an effective lagrangian description we analyze possible new physics contributions to the most relevant muon number violating processes: μ→eγ and μ-e conversion in nuclei. We identify a general class of models in which those processes are generated at one loop level and in which μ-e conversion is enhanced with respect to μ→eγ by a large ln(m2μ/Λ2), where Λ is the scale responsible for the new physics. For this wide class of models bounds on μ-e conversion constrain the scale of new physics more stringently than μ→eγ already at present and, with the expected improvements in μ-e conversion experiments, will push it upwards by about one order of magnitude more. To illustrate this general result we give an explicit model containing a doubly charged scalar and derive new bounds on its couplings to the leptons

Bounding effective operators at the one-loop level: the case of four-fermion neutrino interactions

The contributions of non-standard four-neutrino contact interactions to electroweak observables are considered at the one-loop level by using the effective quantum field theory language. The analysis is done in terms of three unknown parameters: the strength of the non-standard neutrino interactions, F , an additional derivative coupling needed to renormalize the divergent contributions that appear when the four-neutrino interactions are used at the loop level and a non-standard non-derivative Z-v¯v coupling. Then, the precise measurements of the invisible width of the Z-boson at LEP and the data on the neutrino deep-inelastic scattering yield the result F = (−100±140)GF. Assuming that there are no unnatural cancellations between the contributions of the three effective couplings a much stronger bound is obtained: |F | ≲ 2GF, which is a factor 200 better than the one obtained in previous analyses based on tree level calculations

Composite Higgs bosons from neutrino condensates in an inverted seesaw scenario

We present a realization of the idea that the Higgs boson is mainly a bound state of neutrinos induced by strong four-fermion interactions. The conflicts of this idea with the measured values of the top quark and Higgs boson masses are overcome by introducing, in addition to the right-handed neutrino, a new fermion singlet, which, at low energies, implements the inverse seesaw mechanism. The singlet fermions also develop a scalar bound state that mixes with the Higgs boson. This allows us to obtain a small Higgs boson mass even if the couplings are large, as required in composite scalar scenarios. The model gives the correct masses for the top quark and Higgs boson for compositeness scales below the Planck scale and masses of the new particles above the electroweak scale, so that we obtain naturally a low-scale seesaw scenario for neutrino masses. The theory contains additional scalar particles coupled to the neutral fermions, which could be tested in present and near future experiments