Apostolos Pilaftsis

Flavour-violating charged lepton decays in seesaw-type models

Analytic expressions of lepton-flavour- and lepton-number-violating decays of charged leptons are derived in the context of general SU(2)L ⊗ U(1)Y seesaw scenarios that are motivated by grand unified theories (GUTs) or superstring models, in which left-handed and/or right-handed neutral singlets are present. Possible constraints imposed by cosmology and low-energy data are briefly discussed. The violation of the decoupling theorem in flavour-dependent graphs due to the presence of heavy neutral leptons of Dirac or Majorana nature is emphasized. Numerical estimates reveal that the decays τ− → e− e− e+or τ− → e− μ− μ+ can be as large as ∼ 10−6, which may be observed in LEP experiments or other τ factories.

Probing lepton number violation via Majorana neutrinos at hadron supercolliders.

The possibility of discovering heavy Majorana neutrinos and lepton number violation via the like-sign dilepton signal at hadron supercolliders is investigated. The cross sections for the production of these neutrinos singly as well as in pairs are computed both in three- and four-generation scenarios within the framework of the gauge group SU(2)[sub [ital L]][direct product]U(1)[sub [ital Y]] and the dominant processes are identified. The suppression of the standard model background by suitable kinematical cuts is also discussed.

W- and Z-boson interactions in supersymmetric models with explicit R-parity violation

Abstract The minimal supersymmetric Standard Model with explicit R -parity non-conservation contains bilinear terms involving the left-handed lepton superfields and the Higgs chiral multiplet with hypercharge Y = +1, which cannot in general be rotated away in the presenceof soft-supersymmetry-breaking interactions. These bilinear lepton-number-violating terms are found to give rise to non-zero vacuum expectation values of the scalar neutrinos. This leads to non-universal and flavour-violating tree-level couplings of the W and Z bosons to charged leptons and neutrinos. The parameter space of this novel scenario is systematically analyzed and further restricted by a number of laboratory, astrophysical, and cosmological constraints. The possibility that our minimal model can account for the KARMEN anomaly is examined.

Mixing renormalization in Majorana neutrino theories

The renormalization of general theories with inter-family mixing of Dirac and/or Majorana fermions is studied at the one-loop electroweak order. The phenomenological significance of the mixing-matrix renormalization is discussed, within the context of models based on the SU(2)L⊗U(1)Y gauge group. The effect of radiative neutrino masses present in these models is naturally taken into account in this formulation. As an example, charged-lepton universality in pion decays is investigated in the heavy-neutrino limit. Non-decoupling heavy-neutrino effects induced by mixing renormalization are found to considerably affect the predictions in these new-physics scenarios.

Generalized pinch technique and the background field method in general gauges

Abstract It is shown that Cornwalls pinch technique can be extended in a consistent diagrammatic way, so as to describe general background field gauges in Yang-Mills theories. The resulting one-loop Green functions are found to obey Ward identities identical to those derived from the classical action at the tree level. This generalization of the pinch technique may hence be related to the background field method implemented with novel gauge-fixing conditions invariant under background field gauge transformations. To one loop, the connection between the generalized pinch technique and the background field method in covariant and in non-covariant gauges is explicitly demonstrated.

Resonant CP violating scalar - pseudoscalar transitions at mu+ mu- colliders

A mu+ mu- collider is an appealing machine to probe resonant CP-violating transitions of a CP-even Higgs particle into the Z boson or into another CP-odd Higgs scalar. These phenomena are studied within a manifestly gauge-invariant approach implemented by the pinch technique. The CP invariance of an extended Higgs sector motivated by supersymmetric E_6 models is assumed to be broken radiatively by the presence of heavy Majorana fermions. CP violation originating from Higgs-Z mixing is found to be very modest, whereas CP-number violating transitions involving Higgs scalars only can be resonantly enhanced up to order of unity.

Signatures of Higgs-triplet representations at TeV e+e- colliders.

We investigate the potential for future TeV linear {ital e}{sup +}{ital e}{sup {minus}} colliders to observe singly charged Higgs bosons ({ital H}{sup {plus_minus}}) via the coupling {ital H}{sup {plus_minus}}{ital W}{sup {minus_plus}}{ital Z}, which would signal the existence of exotic Higgs representations. In the context of a Higgs-triplet model compatible with the electroweak oblique parameters, we estimate the cross section for producing charged Higgs-triplet bosons that couple predominantly to {ital W} and {ital Z} bosons in 0.5--2 TeV {ital e}{sup +}{ital e}{sup {minus}} colliders. The principal backgrounds are evaluated and the viability of the signal is discussed and illustrated.

Semileptonic lepton number / flavor violating tau decays in Majorana neutrino models

Motivated by the recent investigation of neutrinoless

Confronting left-right symmetric models with electroweak precision data at the Z peak

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HEAVY-NEUTRINO EFFECTS ON τ-LEPTON DECAYS

-lepton decays by the CLEO collaboration, we perform a systematic analysis of such decays in a possible new-physics scenario with heavy Dirac/Majorana neutrinos, including heavy-neutrino nondecoupling effects, finite quark masses, and quark as well as meson mixings. We find that