arXiv subjects

Apostolos Pilaftsis

Publications and source records attributed to Apostolos Pilaftsis.

156 records · Page 9Linked to original sources

arXiv1996

Mixing and CP Violation: Status and Prospects

Brief overview of the present status on mixing and CP violation in kaons and B mesons is given by means of the unitarity triangle. Theoretical predictions on epsilon'/epsilon are confronted with experimental results. The prospects of detecting CP violation at high-energy p pbar, p p, e- e+ and mu- mu+ colliders are discussed in resonant scatterings involving top quarks and/or heavy scalars. The relevance of the latter for baryogenesis is briefly outlined.

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arXiv1996

Gauge-Invariant Resummation Formalism for Two-Point Correlation Functions

The consistent description of unstable particles, renormalons, or other Schwinger--Dyson-type of solutions within the framework of perturbative gauge field theories necessitates the definition and resummation of off-shell Green's functions, which must respect several crucial physical requirements. A formalism is presented for resummation of off-shell two-point correlation functions, which is mainly based on arguments of analyticity, unitarity, gauge invariance and renormalizability. The analytic results obtained with various methods, including the background field gauges and the pinch technique are confronted with the physical requirements imposed; to one-loop order the pinch technique approach satisfies all of them. Using renormalization group arguments, we discuss issues of uniqueness of the resummation procedure related to the latter method.

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arXiv1996

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 x 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.

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arXiv1995

W- and Z-Boson Interactions in Supersymmetric Models with Explicit R-Parity Violation

The minimal supersymmetric Standard Model with explicit R-parity nonconservation 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 presence of 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 nonuniversal 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.

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arXiv1995

A Gauge-Independent Approach to Resonant Transition Amplitudes

We present a new gauge-independent approach to resonant transition amplitudes with nonconserved external currents, based on the pinch technique method. In the context of $2\to 2$ and $2\to 3$ scattering processes, we show explicitly that the analytic results derived respect $U(1)_{em}$ gauge symmetry and do not depend on the choice of the $SU(2)_L$ gauge fixing. Our analytic approach treats, on equal footing, fermionic as well as bosonic contributions to the resummed gauge boson propagators, does not contain any residual space-like threshold terms, shows the correct high-energy unitarity behaviour, admits renormalization, and satisfies a number of other required properties, including the optical theorem. Even though our analysis has mainly focused on the Standard Model gauge bosons, our method can easily be extended to the top quark, and be directly applied to the study of unstable particles present in renormalizable models of new physics.

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arXiv1995

Gauge Invariance and Unstable Particles

A gauge-independent approach to resonant transition amplitudes with nonconserved external currents is presented, which is implemented by the pinch technique. The analytic expressions derived with this method are $U(1)_{em}$ invariant, independent of the choice of the gauge-fixing parameter, and satisfy a number of required theoretical properties, including unitarity. Although special attention is paid to resonant scatterings involving the $γWW$ and $ZWW$ vertices in the minimal Standard Model, our approach can be extended to the top quark or other unstable particles appearing in renormalizable models of new physics.

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arXiv1995

SEMILEPTONIC LEPTON-NUMBER/FLAVOUR-VIOLATING tau DECAYS IN MAJORANA NEUTRINO MODELS

Motivated by the recent investigation of neutrinoless $τ$-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 $τ$-lepton decays into an electron or muon and a pseudoscalar or vector meson can have branching ratios close to the experimental sensitivity. Numerical estimates show that the predominant decay modes of this kind are $τ^-\to e^-ϕ$, $τ^-\to e^-ρ^0$, and $τ^-\to e^-π^0$, with branching ratios of order $10^{-6}$.

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arXiv1995

SLC/LEP CONSTRAINTS ON UNIFIED MODELS

We examine the potential of constraining possible nondecoupling effects of heavy neutrinos and Higgs bosons at LEP and SLC that may show up in the nonoblique part of the $Zl_il_j$ couplings. We analyze this type of new-physics interactions within the context of low-energy scenarios motivated by unified theories, such as the Standard Model (SM) with neutral isosinglets, the left-right symmetric model, and the minimal supersymmetric SM. Our analysis comprises a complete set of physical quantities based on the nonobservation of flavour-violating $Z$-boson decays, lepton universality in the decays $Z\to l\bar{l}$, and universality of lepton asymmetries at the $Z$ peak. It is found that these quantities form a set of complementary observables and may hence constrain the parameter space of the theories. Non-SM contributions of new-physics interactions to $R_b=Γ(Z\to b\bar{b})/Γ(Z\to \mbox{hadrons})$ are briefly discussed within these models.

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arXiv1995

CONFRONTING LEFT-RIGHT SYMMETRIC MODELS WITH ELECTROWEAK PRECISION DATA AT THE Z PEAK

In view of the recent and future electroweak precision data accumulated at LEP and SLC, we systematically analyze possible new physics effects that may occur in the leptonic sector within the context of $SU(2)_R \times SU(2)_L \times U(1)_{B-L}$ theories. It is shown that nonobservation of flavour-violating $Z$-boson decays, lepton universality in the decays $Z\to l\bar{l}$, and universality of lepton asymmetries at the $Z$ peak form a set of complementary observables, yielding severe constraints on the parameter space of these theories. Contributions of new-physics effects to $R_b=Γ(Z\to b\bar{b})/Γ(Z\to \mbox{hadrons})$ are found to give interesting mass relations for the flavour-changing Higgs scalars present in these models.

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arXiv1994

Signatures of Higgs-Triplet Representations at Tev--e+e- Colliders

We investigate the potential of future TeV linear e^+e^- colliders to observe singly-charged Higgs bosons ($H^\pm$) via the coupling $H^\pm W^\mp Z$, 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 pro- ducing charged Higgs-triplet bosons that couple predominantly to $W$ and $Z$ bosons in 0.5--2 TeV-$e^+e^-$ colliders. The principal backgrounds are evaluated and the viability of the signal is discussed and illustrated.

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arXiv1994

Heavy-Neutrino Effects on tau-Lepton Decays

Minimal extensions of the Standard Model that are motivated by grand unified theories or superstring models with an E_6 symmetry can naturally predict heavy neutrinos of Dirac or Majorana nature. Such heavy neutral leptons violate the de- coupling theorem at the one-loop electroweak order and hence offer a unique chance for possible lepton-flavour decays of the tau lepton, e.g. tau -> eee or tau -> mu mu mu, to be seen in LEP experiments. We analyze such decays in models with three and four generations.

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arXiv1993

Astrophysical and Terrestrial Constraints on Singlet Majoron Models

The general Lagrangian containing the couplings of the Higgs scalars to Majorana neutrinos is presented in the context of singlet Majoron models with intergenerational mixings. The analytical expressions for the coupling of the Majoron field to fermions are derived within these models. Astrophysical considerations imply severe restrictions on the parameters of the model if the singlet Majoron model with three generations is assumed to be embedded in grand unified theories. Bounds that originate from analyzing possible charged lepton-violating decays in terrestrial experiments are also discussed. In particular, we find that experimental searches for muon decays by Majoron emission cannot generally be precluded by astrophysical requirements.

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