arXiv subjects

Apostolos Pilaftsis

Publications and source records attributed to Apostolos Pilaftsis.

At least 19 recordsLinked to original sources

arXiv2026

Renormalisation Group Invariants from Scaling and Non-overlapping Symmetries

We show how the synergy of scaling and non-overlapping global symmetries can lead to Renormalisation Group Invariants (RGIs) among the parameters of potentials with multiple scalars. The instrumental role of scale-invariant field directions in the identification and construction of RGIs for bilinear field operators to all loops is demonstrated. We present a few illustrative examples to showcase our constructive spurion-field approach, which is applied to simple non-supersymmetric models as well as to scenarios reported recently in the literature that include the two-Higgs doublet model. The problematic issues of RGIs to address the gauge-hierarchy problem beyond supersymmetry are discussed.

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arXiv2026

Trajectories of Critical Unstable Qubits in and on the Bloch Sphere

We extend previous studies on a novel class of unstable two-level systems which were called Critical Unstable Qubits (CUQs). In an appropriately defined co-decaying frame, the CUQs exhibit striking phenomena of indefinite anharmonic oscillations between two states and coherence-decoherence oscillations of mixed states. These features are distinct from the usual Rabi oscillations observed in the Hermitian counterpart of two-level systems, which are harmonic and preserve the coherence of the quantum state. We employ the density matrix formalism to study these phenomena for mixed states and delve into the nature of the trajectory traversed by these states in the Bloch sphere by studying the time evolution of the Bloch vector that describes the quantum state of the unstable qubit. In particular, we provide for the first time explicit geometric constructions to obtain trajectories of both pure and mixed CUQs in and on the Bloch sphere. This enables us to identify the stationary points of CUQs, at which the states do not evolve in time in the co-decaying frame. The potential implications of our findings for particle cosmology and quantum simulations of non-Hermitian Hamiltonians are discussed.

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arXiv2026

Dominant Thermal Resonant Mechanism for Low-Scale Leptogenesis

We explicitly demonstrate the importance of a new thermal resonant channel in the context of low-scale leptogenesis, which goes beyond the well-known mixing and oscillation of massive singlet neutrinos. This new channel is always present when considering the thermally-induced Higgs decay to leptons and relativistic singlet neutrinos, and can become dominant thanks to thermally-generated resonant lepton-doublet flavour coherences. This mechanism, which we call Thermal Resonant Leptogenesis (TRL), can yield the observed baryon asymmetry in our universe, even if there is no resonant enhancement from quasi-degenerate sterile neutrinos. The required active-to-sterile neutrino mixing for TRL differs from other known low-scale leptogenesis scenarios and can be probed in fixed-target and long-lived particle experiments, and by displaced vertex searches at high-energy colliders.

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arXiv2026

Low-Scale Leptogenesis from Resonant Thermal Lepton Flavour Coherences

Resonant heavy-neutrino mixing and sterile neutrino oscillations are two prominent mechanisms to realize low-scale leptogenesis, with singlet neutrino masses below TeV energies that could be probed in current and future laboratory experiments. In their minimal settings, both mechanisms require a significant degree of degeneracy in the singlet neutrino masses to compensate for the suppression that results from the small neutrino Yukawa couplings. After further developing the flavour-covariant Kadanoff-Baym formalism, we study in detail a novel dominant mechanism for low-scale leptogenesis which becomes greatly enhanced by resonant thermal lepton-flavour coherences at the two-loop level. This mechanism works successfully for both Dirac and Majorana singlet neutrinos, and it does not rely on whether these singlet neutrinos are quasi-degenerate or not. In particular, it implies that successful low-scale leptogenesis in the type-I seesaw framework can be naturally realised with heavy neutrino masses that could be as low as GeV.

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arXiv2026

Critical Unstable Qubits in Particle Physics

We study in detail the dynamics of unstable two-level quantum systems by adopting the Bloch-vector representation. We identify a novel class of critical scenarios in which the so-called energy-level and decay-width vectors, ${\bf E}$ and ${\bfΓ}$, are orthogonal to one another, and the parameter $r = |{\bf Γ}|/(2|{\bf E}|)$ is less than~1. Most remarkably, we find that critical unstable qubit systems exhibit atypical behaviours like coherence--decoherence oscillations when analysed in an appropriately defined co-decaying frame of the system. By making use of a Fourier series decomposition, we define anharmonicity observables that quantify the degree of non-sinusoidal oscillation of a CUQ. We apply the results of our formalism to the neutral-meson systems and derive generic upper limits on these new observables. In particular, we provide a compilation table of all well-explored meson--antimeson two-level systems in terms of Bloch-sphere parameters.

quant-ph↗

arXiv2025

Critical Unstable Qubits: an Application to $B^0\bar{B}^0$-Meson System

We extend our previous work on a novel class of unstable qubits which we have identified recently and called them Critical Unstable Qubits (CUQs). The characteristic property of CUQs is that the energy-level and decay-width vectors, ${\bf E}$ and ${\bf Γ}$, are orthogonal to one another, and the key parameter $r = |{\bf Γ}|/|2{\bf E}|$ is less than 1. Most remarkably, CUQs exhibit two atypical behaviours: (i) they display coherence-decoherence oscillations in a co-decaying frame of the system described by a unit Bloch vector ${\bf b}$, and (ii) the unit Bloch vector ${\bf b}$ describing a pure CUQ sweeps out unequal areas during equal intervals of time, while rotating about the vector ${\bf E}$. The latter anharmonic phenomenon emerges beyond the usual oscillatory pattern due to the energy-level difference of the two-level quantum system, which governs an ordinary qubit. By making use of a Fourier series decomposition, we define anharmonicity observables that quantify the degree of non-sinusoidal oscillation of a CUQ. We apply the results of our formalism to the $B^0\bar{B}^0$-meson system and derive, for the first time, generic upper limits on these new observables.

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arXiv2025

Leptogenesis and the relativistic degrees of freedom of the plasma

We investigate the impact of the temperature dependence of the relativistic degrees of freedom (dofs) of the plasma on lepton and baryon asymmetry. Motivated by the significant effect of the varying dofs on the tri-resonant leptogenesis particle model in low-scale leptogenesis, we show how this effect impacts the evolution of the lepton asymmetry in a simplified setup. We provide analytical approximations as well as numerical results showing that the simplified setup exhibits similar behavior as the concrete model. As the dofs enter the transport equations via the expansion rate of the Universe and the temperature of the plasma, we argue that any analysis must take these effects into account in order to be consistent.

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arXiv2025

Mixed CP Violation and Natural Alignment in 2HDMs

We present a new form of CP violation (CPV) that can be realised in Two-Higgs Doublet Models (2HDMs) and was studied recently in [1]. By examining the vacuum manifold of a generic (convex) 2HDM potential, we identify scenarios that exhibit Mixed Spontaneous and Explicit CP Violation (MCPV), in which at least two non-degenerate CP-violating local minima coexist. We illustrate how this identification is achieved at the tree level by determining the magnitude and phase of a novel complex parameter, which we call $r_{\rm CP}$. Since explicit CP Violation vanishes in 2HDMs where SM Higgs alignment is enforced through global continuous symmetries, we investigate how to maximise CPV in such scenarios by introducing soft or explicit breaking of the relevant symmetries. In doing so, we derive upper bounds on key CP-violating parameters that characterise misalignment with the SM, subject to constraints from the non-observation of the electron electric dipole moment. Finally, we delineate the region of the CP-violating parameter space in such constrained 2HDMs that can be further tested at the CERN Large Hadron Collider.

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arXiv2024

Dirac Algebra Formalism for Two Higgs Doublet Models: the One-Loop Effective Potential

We present a novel covariant bilinear formalism for the Two Higgs Doublet Model (2HDM) which utilises the Dirac algebra associated with the SL(2,C) group that acts on the scalar doublet field space. This Dirac-algebra approach enables us to obtain a fully O(1,3)-covariant and IR-safe expression for the one-loop effective potential. We illustrate how the formalism can be used to evaluate the breaking of global symmetries of the 2HDM potential by loop effects, in a field-reparameterisation invariant manner.

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arXiv2024

Supergeometric Quantum Effective Action

Supergeometric Quantum Field Theories (SG-QFTs) are theories that go beyond the standard supersymmetric framework, since they allow for general scalar-fermion field transformations on the configuration space of a supermanifold, without requiring an equality between bosonic and fermionic degrees of freedom. After revisiting previous considerations, we extend them by calculating the one-loop effective action of minimal SG-QFTs that feature non-zero fermionic curvature in two and four spacetime dimensions. By employing an intuitive approach to the Schwinger-DeWitt heat-kernel technique and a novel field-space generalised Clifford algebra, we derive the ultra-violet structure of characteristic effective-field-theory (EFT) operators up to four spacetime derivatives that emerge at the one-loop order and are of physical interest. Upon minimising the impact of potential ambiguities due to the so-called multiplicative anomalies, we find that the EFT interactions resulting from the one-loop supergeometric effective action are manifestly diffeomorphically invariant in configuration space. The extension of our approach to evaluating higher-loops of the supergeometric quantum effective action is described. The emerging landscape of theoretical and phenomenological directions for further research of SG-QFTs is discussed.

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arXiv2024

Maximising CP Violation in Naturally Aligned Two-Higgs Doublet Models

The Two-Higgs Doublet Model (2HDM) is a well-motivated theoretical framework that provides additional sources of CP Violation (CPV) beyond the Standard Model (SM). After studying the vacuum topology of a general (convex) 2HDM potential, we unambiguously identify three origins of CPV: (I) Spontaneous CPV (SCPV), where the vacuum manifold has at least two degenerate CPV minima disconnected by domain walls, (ii) Explicit CPV (ECPV) with one single CPV ground state, and (iii) Mixed Spontaneous and Explicit CPV (MCPV), where the theory possesses more than one $non$-degenerate CPV local minimum. Most importantly, we define a novel complex parameter $r_{\rm CP}$ whose norm and phase control the three different realisations of CPV, at least at the tree level. In all these scenarios, only two CPV phases can be made independent, as any third CPV parameter will always be constrained via the CP-odd tadpole condition. Since ECPV vanishes in 2HDMs where SM Higgs alignment is achieved naturally through accidental continuous symmetries, we analyse the possibility of maximising CPV through soft and explicit breaking of these symmetries. We derive upper limits on key CPV parameters that quantify the degree of SM misalignment from constraints due to the non-observation of an electron Electric Dipole Moment (EDM). Finally, we delineate the CP-violating parameter space of the so-constrained naturally aligned 2HDMs that can further be probed at the CERN Large Hadron Collider (LHC).

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arXiv2024

Supergeometric Approach to Quantum Field Theory

We review a recently proposed SuperGeometric (SG) approach to Quantum Field Theories (QFTs) that allow for scalar-fermion field transformations in a manifestly reparameterisation covariant manner. By adopting natural choices for the field-space supermetric, we show how a non-trivial field-space Riemannian curvature can be introduced in the fermionic sector, beyond the usual one that may have its origin in the scalar part of the theory. We present a minimal SG-QFT model with proper non-zero fermionic curvature both in two and four spacetime dimensions. Possible future directions in further developing SG-QFTs are discussed.

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arXiv2024

Quantum Coherence of Critical Unstable Two-Level Systems

We study in detail the dynamics of unstable two-level quantum systems by adopting the Bloch-sphere formalism of qubits. By employing the Bloch-vector representation for such unstable qubit systems, we identify a novel class of critical scenarios in which the so-called energy-level and decay-width vectors, ${\bf E}$ and ${\bfΓ}$, are orthogonal to one another, and the parameter $r = |{\bf Γ}|/(2|{\bf E}|)$ is less than 1. Most remarkably, we find that critical unstable qubit systems exhibit atypical behaviours like coherence--decoherence oscillations when analysed in an appropriately defined co-decaying frame of the system. In the same frame, a unit Bloch vector ${\bf b}$ describing a pure critical qubit will sweep out unequal areas during equal intervals of time, while rotating about the vector ${\bf E}$. These phenomena emerge beyond the usual oscillatory pattern due to the energy-level difference of the two-level quantum system. Interestingly enough, we observe that these new features will persist even for quasi-critical scenarios, in which the vectors ${\bf E}$ and ${\bfΓ}$ are not perfectly orthogonal to each other. Applications of our results to quantum information and to unstable meson--antimeson and other systems are discussed.

hep-ph↗

arXiv2024

Varying Entropy Degrees of Freedom Effects in Low-Scale Leptogenesis

We analyse in detail the effect of varying entropy degrees of freedom on low-scale leptogenesis models. As an archetypal model, we consider the Tri-Resonant Leptogensis${}$ (TRL) scenario introduced recently by the authors, where the neutrino-Yukawa coupling matrix is dictated by an approximate $\mathbb{Z}_n$ discrete symmetry (with $n=3,6$). TRL models exhibit no preferred direction in the leptonic flavour space and have the remarkable feature that leptogenesis can successfully take place even if all light neutrinos are strictly massless up to one-loop order. Most interestingly, for TRL scenarios with heavy Majorana neutrinos lighter than 100 GeV, temperature varying degrees of freedom associated with the entropy of the plasma have a dramatic impact on the predictions of the Baryon Asymmetry in the Universe (BAU), and may sensitively depend on the freeze-out sphaleron temperature $T_{\rm sph}$. We find that this is a generic feature of most freeze-out low-scale leptogenesis models discussed in the literature. In the same context, we consider heavy-neutrino scenarios realising dynamics related to critical unstable qudits in the thermal plasma and assess their significance in generating the BAU. The phenomenological implications of TRL scenarios at the intensity and high-energy frontiers are analysed.

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arXiv2023

Towards a Localised S-Matrix Theory

We formulate an S-matrix theory in which localisation effects of the particle interactions involved in a scattering process are consistently taken into account. In the limit of an infinite spread of all interactions, the S-matrix assumes its standard form. To better understand the significance of the emerging quantum phenomena in this formalism, we consider a solvable field-theoretic model with spatial Gaussian spreads at the interaction vertices. This solvable model, which was previously introduced in the literature, enables accurate descriptions of detection regions that are either close to or far from the source. In close analogy with light diffraction in classical optics, we call these two regions near-field and far-field zones, or the Fresnel and Fraunhofer regions. We revisit the question whether mixed mediators produce an oscillating pattern if their detection occurs in the Fresnel region. Besides corroborating certain earlier findings of the S-matrix amplitude in the forward Fresnel and Fraunhofer regimes, we observe several novel features with respect to its angular dependence which have not been accounted before in the literature. In particular, we obtain a ``quantum obliquity factor'' that suppresses particle propagation in the backwards direction, thereby providing an explicit quantum field-theoretic description for its origin in diffractive optics. Present and future colliders, as well as both short and long baseline neutrino experiments, would greatly benefit from the many predictions that can be offered from such a holistic localised S-matrix theory.

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arXiv2023

Minimal Supergeometric Quantum Field Theories

We formulate minimal SuperGeometric Quantum Field Theories (SG-QFTs) that allow for scalar-fermion field transformations in a manifestly reparameterisation covariant manner. First, we discuss the issue of uniqueness in defining the field-space supermetric of the underlying supermanifold, and clarify the fact that different supermetric definitions can lead to distinct theories in the off-shell kinematic region. By adopting natural choices for the field-space supermetric, we~then show that scalar fields alone cannot induce a non-trivial field-space Riemannian curvature in the fermionic sector, beyond the one originating from the scalar part of the theory. We~present for the first time minimal SG-QFT models that feature non-zero fermionic curvature both in two and four spacetime dimensions. Physical applications of SG-QFTs are discussed.

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arXiv2022

Frictionless UV-finite Instantons in Curved Spacetime

We identify a new class of UV-complete instanton solutions that describe the false vacuum\- decay of a real scalar field in a particular curved spacetime background. To this end, we consider a simple scalar theory with a Coleman potential and calculate the Euclidean action $S_{\text{E}}$ by assuming an O(4)-symmetric curved spacetime. The function $a(r)$ dictating the geometry of spacetime may consistently be chosen to be a constant, thereby eliminating the drag forces from the equations of motion and ensuring that the gravitational backgrounds of both the false vacuum and bounce solutions are identical. By employing standard WKB and Gelfand-Yaglom methods, we compute the corresponding prefactor due to quantum fluctuations around this frictionless bounce solution which becomes UV finite after renormalization. The possible consequences of such frictionless UV-finite instantons are discussed.

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arXiv2022

Geometrising the Micro-Cosmos on a Supermanifold

For more than half a century, covariant and differential geometric methods have been playing a central role in the development of Quantum Field Theory (QFT). After a brief historic overview of the major scientific achievements using these methods, we will focus on the covariant and differential geometric formalism originally proposed by Vilkovisky and DeWitt (VDW). We discuss recent developments made in addressing the uniqueness of the path-integral measure of the VDW effective action, and so address the problem of quantum frame dependence in cosmologically relevant scalar-tensor theories beyond the classical approximation. Particular attention will be drawn to a long-standing problem concerning the obstacles that the VDW formalism was facing from its original conception in describing generic QFTs that include fermions. We show how in addition to bosons the VDW effective action can be extended to supermanifolds to include fermions. The so-extended formulation appears to be very promising for a complete geometrisation of realistic theories of micro-cosmos, such as the Standard Model and its gravitational sector.

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