arXiv subjects

Apostolos Pilaftsis

Publications and source records attributed to Apostolos Pilaftsis.

At least 145 records · Page 8Linked to original sources

arXiv1999

CP Violation in the Higgs Sector of the MSSM

Recently, it has been found that the tree-level CP invariance of the Higgs potential in the MSSM can be sizeably broken by loop effects due to soft-CP-violating trilinear interactions involving third generation scalar quarks. These soft-CP-violating couplings may be constrained by considering new two-loop contributions to the electron and neutron EDMs. The phenomenological consequences of such a minimal supersymmetric scenario of explicit CP violation at present and future colliders are briefly discussed.

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arXiv1999

Higgs Bosons in the Minimal Supersymmetric Standard Model with Explicit CP Violation

We study the Higgs-boson mass spectrum of the minimal supersymmetric standard model, in which the tree-level CP invariance of the Higgs potential is broken explicitly by loop effects of soft-CP-violating Yukawa interactions related to scalar quarks of the third generation. The analysis is performed by considering the CP-non-invariant renormalization-group improved effective potential through next-to-leading order that includes leading logarithms due to two-loop Yukawa and QCD corrections. We find that the three neutral Higgs particles predicted by the theory may strongly mix with one another, thereby significantly modifying their tree-level couplings to fermions and to the $W^\pm$ and Z bosons. We analyze the phenomenological consequences of such a minimal supersymmetric scenario of explicit CP violation on the production rates of the lightest Higgs particle, and discuss strategies for its potential discovery at high-energy colliders.

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arXiv1999

Leptogenesis in Theories with Large Extra Dimensions

We study the scenario of baryogenesis through leptogenesis in higher-dimensional theories, in which the scale of quantum gravity is many orders of magnitude smaller than the usual Planck mass. The minimal realization of these theories includes an isosinglet neutrino which feels the presence of large compact dimensions, whereas all the SM particles are localized on a $(1+3)$-dimensional subspace. In the formulation of minimal leptogenesis models, we pay particular attention to the existence of Majorana spinors in higher dimensions. After compactification of the extra dimensions, we obtain a tower of Majorana Kaluza-Klein excitations which act as an infinite series of CP-violating resonators, and derive the necessary conditions for their constructive interference. Based on this CP-violating mechanism, we find that the decays of the heavy Majorana excitations can produce a leptonic asymmetry which is reprocessed into the observed baryonic asymmetry of the Universe by means of out-of-equilibrium sphaleron interactions, provided the reheat temperature is above 5 GeV.

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arXiv1999

Heavy Majorana Neutrinos and Baryogenesis

The scenario of baryogenesis through leptogenesis is reviewed in models involving heavy Majorana neutrinos. The various mechanisms of CP violation occurring in the out-of-equilibrium lepton-number-violating decays of heavy Majorana neutrinos are studied within a resummation approach to unstable-particle mixing. It is explicitly demonstrated how the resummation approach preserves crucial field-theoretic properties such as unitarity and CPT invariance. Predictions of representative scenarios are presented after solving numerically the Boltzmann equations describing the thermodynamic evolution of the Universe. The phenomenological consequences of loop effects of heavy Majorana neutrinos on low-energy observables, such as lepton-flavour and/or lepton-number non-conservation in $τ$ and $Z$-boson decays and electron electric dipole moment, are discussed.

hep-ph↗

arXiv1999

Radiatively Induced Neutrino Masses and Large Higgs-Neutrino Couplings in the Standard Model with Majorana Fields

The Higgs sector of the Standard Model with one right-handed neutrino per family is systematically analyzed. In a model with intergenerational independent mixings between families, we can account for very light neutrinos acquiring Majorana masses radiatively at the first electroweak loop level. We also find that in such a scenario the Higgs coupling to the light-heavy neutrinos and to the heavy-heavy ones may be remarkably enhanced with significant implications for the production of these heavy neutrinos at high energy colliders.

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arXiv1998

Neutrino oscillations in space within a solvable model

We study neutrino oscillations in space within a realistic model in which both the source and the target are considered to be stationary having Gaussian-form localizations. The model admits an exact analytic solution in field theory which may be expressed in terms of complementary error functions, thereby allowing for a quantitative discussion of quantum-mechanical (coherent) versus statistical (incoherent) uncertainties. The solvable model provides an insightful framework in addressing questions related to propagation and oscillation of neutrinos that may not be attainable by the existing approaches. We find a novel form of plane-wave behaviour of neutrino oscillations if the localization spread of the source and target states due to quantum mechanics is of macroscopic size but much smaller than neutrinos' oscillation length. Finally, we discuss the limits on the coherence length of neutrino oscillations and find that they mainly arise from uncertainties of statistical origin.

hep-ph↗

arXiv1998

Baryogenesis through mixing of heavy Majorana neutrinos

We review the scenario of baryogenesis through leptogenesis induced by the out-of-equilibrium decays of heavy neutrinos. We pay special attention to the resonant phenomenon of CP violation through mixing of two nearly degenerate heavy Majorana neutrinos and show how unitarity and CPT invariance is maintained within the resummation approach. An important consequence of this is that the leptogenesis scale may be as low as 1 TeV, even for models with universal Yukawa couplings. We briefly discuss the impact of finite temperature effects and low-energy constraints to the afore-mentioned mechanism of CP violation.

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arXiv1998

Higgs Scalar-Pseudoscalar Mixing in the Minimal Supersymmetric Standard Model

In the minimal supersymmetric Standard Model, the heaviest CP-even Higgs boson H and the CP-odd Higgs scalar A are predicted to be almost degenerate in mass at the tree level, for the wide kinematic range M_A > 2M_Z and \tanβ\ge 2. However, if large soft-CP-violating Yukawa interactions involving scalar quarks of the third generation are present in the theory, then the CP invariance of the Higgs potential can be maximally broken beyond the Born approximation, and the high degree of mass degeneracy between H and A may be lifted through a sizeable HA mixing. After taking the CP-odd tadpole renormalization of the A boson into account, we find that the small mass difference M_H - M_A, which is about 1% of the A-boson mass at the tree level, can be substantially enhanced to the 25% level at the one-loop order. We also find that the loop-induced mixing between the lightest CP-even Higgs boson h and the A boson may be of comparable size to M_h. We briefly discuss the main phenomenological implications of the predicted hA and HA mixings for the general Higgs-boson mass spectrum and for CP-violating observables at collider and lower energies.

hep-ph↗

arXiv1998

CP-Odd Tadpole Renormalization of Higgs Scalar-Pseudoscalar Mixing

We consider an Abelian model with a CP-conserving Higgs potential spanned by two complex Higgs fields. The CP invariance of the Higgs potential is then broken explicitly beyond the Born approximation by introducing soft-CP-violating Yukawa interactions. Based on the non-renormalization theorem, we derive the consistency conditions under which a CP-odd counterterm exists and, at the same time, renders the one-loop-induced mixing of a CP-even Higgs boson with a CP-odd Higgs scalar ultra-violet finite. The novel CP-odd tadpole renormalization may then be determined from the minimization constraints on the Higgs potential. Finally, we discuss the phenomenological consequences of the so-generated CP-violating scalar-pseudoscalar mixing for the electric dipole moments of neutron, electron and muon.

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arXiv1997

Higgs-Boson Low-Energy Theorem and Compatible Gauge-Fixing Conditions

Conventional gauge-fixing schemes such as R$_ξ$ gauges may lead to a violation of the Higgs-boson low-energy theorem beyond the tree level. To elucidate this fact, we study a simple model whose U(1) gauge symmetry is spontaneously broken, and show how the Higgs-boson low-energy theorem can consistently be extended to the gauge and Higgs sectors of the model. In this formulation, any gauge-fixing condition must comply with the requirement that it should be independent of the vacuum expectation value of the Higgs field in the symmetric limit of the theory. We give a diagrammatic proof of the Higgs-boson low-energy theorem to all orders in perturbation theory, within the context of a judiciously modified R$_ξ$ gauge compatible with the above constraint. The dependence of the kinematic parameters on the Higgs tadpole is found to be very important for the proof of the theorem.

hep-ph↗

arXiv1997

Gauge- and Renormalization-Group- Invariant Formulation of the Higgs-Boson Resonance

A gauge- and renormalization-group- invariant approach implemented by the pinch technique is formulated for resonant transitions involving the Higgs boson. The lineshape of the Higgs boson is shown to consist of two distinct and physically meaningful contributions: a process-independent resonant part and a process-dependent non-resonant background, which are separately gauge independent, invariant under the renormalization group, satisfy naive, tree-level Ward identities, and respect the optical and equivalence theorem individually. The former process-independent quantity serves as the natural extension of the concept of the effective charge to the case of the Higgs scalar, and constitutes a common ingredient of every Born-improved amplitude. The difference in the phenomenological predictions obtained within our approach and those found with other methods is briefly discussed.

hep-ph↗

arXiv1997

Effective Charge of the Higgs Boson

The Higgs-boson lineshape is studied within the pinch technique resummation formalism. It is shown that any resonant Higgs-boson amplitude contains a universal part which is gauge independent, renormalization-group invariant, satisfies the optical and equivalence theorems, and constitutes the natural extension of the QED effective charge to the case of the Higgs scalar.

hep-ph↗

arXiv1997

Gauge-Independent Analysis of K_L --> e μin Left-Right Models

The lepton-flavour-violating decay K_L --> e μis studied in detail within the context of SU(2)_R x SU(2)_L x U(1)_(B-L) models, which include heavy Majorana neutrinos. Particular attention is paid to the gauge independence of this decay process to one loop. In analogy with earlier studies on the K^0\bar{K}^0 mixing, it is explicitly shown how restoration of gauge invariance occurs in the decay amplitude containing the box diagrams, when the relevant Higgs-dependent self-energy and vertex graphs are taken into account in the on-shell skeleton renormalization scheme. Based on the analytic expressions so derived, we find that the branching ratio B(K_L --> e μ) can be considerably enhanced due to the presence of left- and right-handed currents in the loop, and can reach values close to or even larger than the present experimental limit 3.3 x 10^{-11} in the manifest left-right symmetric model. Constraints on the parameter space of typical left-right models are derived from the possible decay K_L --> e μand a global analysis of other low-energy data.

hep-ph↗

arXiv1997

Resonant CP violation induced by particle mixing in transition amplitudes

We analyze CP violation in resonant transitions involving scalar as well as fermionic intermediate states, using a gauge-invariant resummation approach implemented by the pinch technique. We derive the necessary conditions for resonantly enhanced CP violation induced by particle mixing, by paying special attention to CPT invariance, and apply the results of our analysis to two representative new-physics scenarios: (i) the indirect mixing of a CP-even Higgs scalar with a CP-odd Higgs particle in two-Higgs doublet models and (ii) the CP-asymmetric mixing between the top quark and a new heavy up-type quark, $t'$, in mirror fermion models. Furthermore, we explicitly demonstrate the equivalence of our scattering-amplitude formalism with that based on the effective Hamiltonian and discuss the possibility of maximal CP violation in the limiting case of two degenerate particles.

hep-ph↗

arXiv1997

CP Violation and Baryogenesis due to Heavy Majorana Neutrinos

We analyze the scenario of baryogenesis through leptogenesis induced by the out-of-equilibrium decays of heavy Majorana neutrinos and pay special attention to CP violation. Extending a recently proposed resummation formalism for two-fermion mixing to decay amplitudes, we calculate the resonant phenomenon of CP violation due to the mixing of two nearly degenerate heavy Majorana neutrinos. Solving numerically the relevant Boltzmann equations, we find that the isosinglet Majorana mass may range from 1 TeV up to the grand unification scale, depending on the mechanism of CP violation and/or the flavour structure of the neutrino mass matrix assumed. Finite temperature effects and possible constraints from the electric dipole moment of electron and other low-energy experiments are briefly discussed.

hep-ph↗

arXiv1997

Resonant CP Violation in Higgs Production, Mixing and Decay

CP violation induced by resonant transitions of a scalar to a pseudoscalar intermediate state is analyzed within a gauge-invariant resummation approach based on the pinch technique. Necessary conditions governing the resonant enhancement of CP violation in transition amplitudes are derived. The results of this study are then applied to describe the indirect mixing of a CP-even Higgs scalar, $H$, with a CP-odd Higgs particle, $A$, in two-Higgs doublet models. CP violation through $HA$ mixing in high-energy $pp$, $e^+e^-$ and $μ^+μ^-$ scatterings are estimated to be fairly large for a natural choice of kinematic parameters. Low-energy constraints originating from experimental upper bounds on the neutron electric dipole model are implemented in this analysis.

hep-ph↗

arXiv1996

Generalized Pinch Technique and the Background Field Method in General Gauges

It is shown that Cornwall's 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's 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.

hep-ph↗

arXiv1996

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.

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