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Fotis Koutroulis

Publications and source records attributed to Fotis Koutroulis.

17 recordsLinked to original sources
arXiv2026

Gravitational ultra-relativistic freeze-out during general reheating

We investigate ultrarelativistic freeze-out (UFO) in the context of generic reheating scenarios. While the standard WIMP dark matter paradigm has been extensively studied, UFO has so far only been analyzed within the specific reheating channel $\phi \rightarrow f\bar{f}$. Unlike in the standard WIMP mechanism, where dark matter can only be diluted after freeze-out at $T_\mathrm{FO} \sim m_\chi/\mathcal{O}(10)$, UFO dark matter can undergo freeze-in like phases following the initial freeze-out, driven by the non-trivial temperature evolution. The exact temperature evolution then needs to be accounted for, as a change in the temperature scaling can modify the IR/UV nature of UFO, impacting the relic abundance. We first generalize UFO to an arbitrary temperature profile $T \sim a^{-\xi}$, making explicit the UV and IR regimes for a thermally averaged cross section $\langle \sigma v \rangle \sim T^n / \Lambda^{n+2}$. Then, as a concrete example, we consider the minimal scenario in which gravitational particle production at the onset of reheating sources an initial radiation abundance, and show that this early hot bath changes the UFO parameter space. We refer to this effect as GUFO. Specializing to $n = 2$, we find that matter-like reheating ($V \sim \phi^2$) accommodates dark matter masses up to $10^7~\mathrm{GeV}$ for $\Lambda \lesssim 10^9~\mathrm{GeV}$ as thermalization becomes less stringent, while radiation-like reheating ($V \sim \phi^4$) is compatible with GUFO across all reheating channels only if gravitational processes are taken into account.

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arXiv2026

UV-complete and stable Quintom Dark Energy models in the light of DESI DR2

We propose that Quintom dark energy, the simplest framework allowing crossing of the cosmological-constant boundary, admits a natural UV completion in a 5D anisotropic orbifold lattice: the Non-Perturbative Gauge-Higgs Unification (NPGHU) model. In this setup, a bulk 5D SU(2) gauge field projects on the 4D boundary to a complex scalar and a U(1) gauge field, identified with the dynamical dark-energy sector, while the Standard Model and dark matter remain localized in four dimensions. At late times, bulk-induced dimension-6 higher-derivative operators generate both physical and phantom scalar and gauge degrees of freedom. We show that the resulting 4D effective action is a modified Quintom model whose background equation of state can naturally realize Quintom-B behavior. A crucial contribution arises from the massive gauge ghost, allowing an excellent fit to DESI data with negligible fine-tuning, unlike standard Quintom scenarios. We further show that the inherited properties of the NPGHU construction e.g. absence of fundamental ghost instabilities, absence of potential terms and a finite low-energy cutoff $\Lambda$ associated with approximate Lorentz invariance, play a central role in the consistency of the effective theory under linear perturbations and vacuum decay. For the most natural regime, $\Lambda \approx {\cal O}(10)H_0$, the model remains robust despite the presence of IR phantom modes. Our results provide a natural and predictive framework in which Quintom dark energy can be consistently embedded in a fundamental theory.

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arXiv2025

Quark, lepton and right-handed neutrino production via inflation

Inflationary expansion of space-time provides us with an efficient particle production mechanism in the Early Universe. The fermion production efficiency depends critically on the particle mass, which is generated via the Yukawa coupling and sensitive to the corresponding scalar field value. During inflation, scalar fields experience large quantum fluctuations driving the average field values to the Hubble scale and above. This applies, in particular, to the Higgs field, making the Standard Model fermions very heavy and facilitating their production. Using the Bogolyubov coefficient approach, we compute the corresponding fermion abundance taking into account time dependence of the mass term. We find that the Standard Model fermion and the right-handed neutrino production grows dramatically compared to the naive estimate based on the low energy masses. The inflationary production mechanism can be the leading source of the right handed neutrinos, if they gain a Majorana mass from the Yukawa coupling to a light scalar. We also find a lower bound on the mass of fermionic dark matter, which can be produced by inflation.

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arXiv2025

The thermal backreaction of a scalar field in de Sitter spacetime

We argue that a scalar field in de Sitter spacetime should feel explicit thermal effects associated with its curvature. Starting from the Bunch-Davies vacuum and a scalar field with small mass compared to the de Sitter curvature, we use the thermo-field dynamics formalism in order to expose these thermal effects. We compute the thermal Wightman function connecting two spacetime points and from it, via the point-splitting regularization technique, the renormalized thermal energy-momentum tensor. We then examine how these corrections affect the de Sitter geometry by solving for the semi-classical backreaction and find that their sign depends on the initial conditions. In order to place our results in context, we compare them to the corresponding 2-loop quantum gravity correction to the cosmological constant derived in [1].

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arXiv2024

Dark Branes for Dark Matter

We propose a setup for the origin of dark matter based on spacetime with a warped extra dimension and three branes: the Planck brane, the TeV brane, at a (few) TeV scale $\rho_T$, and a dark brane, at a (sub)-GeV scale $\rho_1\lesssim 100$ GeV $\ll\rho_T$. The Standard Model is localized in the TeV brane, thus solving the Higgs hierarchy problem, while the dark matter $\chi$, a Dirac fermion with mass $m_\chi<\rho_1$, is localized in the dark brane. The radion, with mass $m_r<m_\chi$, interacts strongly ($\sim m_\chi/\rho_1\sim\mathcal O(1)$) with dark matter and very weakly ($\sim m_{f}\rho_1/\rho_T^2\ll 1$) with the Standard Model matter $f$. The generic conflict between the bounds on its detection signatures and its proper relic abundance is avoided as dark matter annihilation is $p$-wave suppressed. The former is determined by its very weak interactions with the SM and the latter by its much stronger annihilation into radions. Therefore, there is a vast range in the Dark Matter's parameter space where the correct relic abundance is achieved consistently with the existing bounds. Moreover, for the dark brane with $\rho_1\lesssim 3$ GeV, a confinement/deconfinement first order phase transition, where the radion condensates, produces a stochastic gravitational waves background at the nanoHz frequencies, which can be identified with the signal detected by the Pulsar Timing Array (PTA) experiments. In the PTA window, for $0.15 \textrm{ GeV}\lesssim m_\chi\lesssim 2$ GeV the relic abundance is reproduced and all constraints are satisfied.

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arXiv2023

Gravitational production of sterile neutrinos

We consider gravitational production of singlet fermions such as sterile neutrinos during and after inflation. The production efficiency due to classical gravity is suppressed by the fermion mass. Quantum gravitational effects, on the other hand, are expected to break conformal invariance of the fermion sector by the Planck scale-suppressed operators irrespective of the mass. We find that such operators are very efficient in fermion production immediately after inflation, generating a significant background of stable or long-lived feebly interacting particles. This applies, in particular, to sterile neutrinos which can constitute cold non-thermal dark matter for a wide range of masses, including the keV scale.

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arXiv2023

Machine Learning Classification of Sphalerons and Black Holes at the LHC

In models with large extra dimensions, "miniature" black holes (BHs) might be produced in high-energy proton-proton collisions at the Large Hadron Collider (LHC). In the semi-classical regime, those BHs thermally decay, giving rise to large-multiplicity final states with jets and leptons. On the other hand, similar final states are also expected in the production of electroweak sphaleron/instanton-induced processes. We investigate whether one can discriminate these scenarios when BH or sphaleron-like events are observed in the LHC using machine learning (ML) methods. Classification among several BH scenarios with different numbers of extra dimensions and the minimal BH masses is also examined. In this study we consider three ML models: XGBoost algorithms with (1) high- and (2) low-level inputs, and (3) a Residual Convolutional Neural Network. In the latter case, the low-level detector information is converted into an input format of three-layer binned event images, where the value of each bin corresponds to the energy deposited in various detector subsystems. We demonstrate that only a small number of detected events are sufficient to effectively discriminate between the sphaleron and BH processes. Separation between BH scenarios with different minimal masses is possible with an order of 10 events passing the preselection. A sufficient number of events could be observed in combined Run-2 and -3 data, if the production cross section is not much smaller than the present limit ~ 0.1 fb. We find, however, that a large number of events is needed to discriminate between BH hypotheses with the same minimal BH mass, but different numbers of extra dimensions.

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arXiv2023

Phases of Pseudo-Nambu-Goldstone Bosons

We study the vacuum dynamics of pseudo-Nambu-Goldstone bosons (pNGBs) for $SO(N+1) \rightarrow SO(N)$ spontaneous and explicit symmetry breaking. We determine the magnitude of explicit symmetry breaking consistent with an EFT description of the effective potential at zero and finite temperatures. We expose and clarify novel additional vacuum transitions that can arise for generic pNGBs below the initial scale of $SO(N+1) \rightarrow SO(N)$ spontaneous symmetry breaking, which may have phenomenological relevance. In this respect, two phenomenological scenarios are analyzed: thermal and supercooled dark sector pNGBs. In the thermal scenario the vacuum transition is first-order but very weak. For a supercooled dark sector we find that, depending on the sign of the explicit symmetry breaking, one can have a symmetry-restoring vacuum transition $SO(N-1) \rightarrow SO(N)$ which can be strongly first-order, with a detectable stochastic gravitational wave background signal.

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arXiv2023

Flavour and Higgs physics in $Z_2$-symmetric 2HD models near the decoupling limit

With no evidence of any exotic particle detected so far beyond the Standard Model, the new physics may lie above the presently accessible energies at colliders and, at low-energies, can be accounted for via an effective description. The interplay of flavour and Higgs physics data allows setting stringent bounds on the parameters of the effective Lagrangian. In this paper, we focus on $Z_2$-symmetric two Higgs doublet models near the decoupling limit: the corresponding effective description relies on only a few parameters, thus predicting many interesting correlations between observables that work as tests of the theory. We present the results of a global fit to the existing data, updating and extending over the past literature. We comment on the triple Higgs coupling as a probe of an extended scalar sector and on the recent CDF II measurement of the $W$-mass.

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arXiv2022

Ising Cosmology

Using arguments from holography we propose that the deviation of the cosmological spectral index $n_S$ of scalar fluctuations from unity may be controlled almost entirely by the critical exponent $\eta$ of the $d = 3$ Ising model

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arXiv2022

Thermal effects in Ising Cosmology

We consider a real scalar field in de Sitter background and compute its thermal propagators. We propose that in a dS/CFT context, non-trivial thermal effects as seen by an 'out' observer can be encoded in the anomalous dimensions of the $d = 3$ Ising model. One of these anomalous dimensions, the critical exponent $\eta$, fixes completely a number of cosmological observables, which we compute

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arXiv2021

RG flows in Non-Perturbative Gauge-Higgs Unification II. Effective action for the Higgs phase near the quantum phase transition

We construct the zero temperature (no compact dimensions) effective action for an SU(2) Yang-Mills theory in five dimensions, with boundary conditions that reduce the symmetry on the four-dimensional boundary located at the origin to a U(1)-complex scalar system. In order to be sensitive to the Higgs phase, we need to include higher dimensional operators in the effective action, which can be naturally achieved by generating it by expanding the corresponding lattice construction in small lattice spacing, taking the naive continuum limit and then renormalizing. In addition, we build in the effective action non-perturbative information, related to a first order quantum phase transition known to exist. As a result, the effective action acquires a finite cut-off that is low and the fine tuning of the scalar mass is rather mild.

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arXiv2019

On RG flows in Generalized Effective Field Theory

Generalized Effective Field Theory (GEFT) is the non-renormalizable extension of an Effective Field Theory where the Wilson coefficients are endowed by their own, independent scale dependence. Such an effective theory can be constructed by quantizing a Lagrangian in the presence of all internal symmetry respecting operators of any mass dimension. The resulting theory may be practically useful in regimes of its phase diagram where the perturbative expansion and a truncation of the infinite tower of Higher Dimensional Operators (HDO) are valid. The massless limit of GEFT is especially interesting as the spontaneous breaking of internal symmetry and of scale invariance are correlated.

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arXiv2019

Symmetry breaking and RG flows with higher dimensional operators

We discuss the role of higher dimensional operators in the spontaneous breaking of internal symmetry and scale invariance, in the context of the Lorentz invariant scalar field theory. Using the $\varepsilon$-expansion we determine phase diagrams and demonstrate that (un)stable RG flows computed with a certain basis of dimension 6 operators in the Lagrangian, map to (un)stable RG flows of another basis related to the first by field redefinitions. Crucial is the presence of reparametrization ghosts if Ostrogradsky ghosts appear.

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arXiv2018

RG flows in Non-Perturbative Gauge-Higgs Unification I

We initiate the continuum description of a non-perturbative 5d lattice Yang-Mills model with 4d boundaries using the $\varepsilon$-expansion. In its simplest version classically the bulk has an $SU(2)$ gauge symmetry and on the boundary there is an Abelian-Higgs system with zero scalar potential. In part I we compute the Renormalization Group flows and related quantities in a limit where the boundary decouples from the bulk.

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arXiv2018

Renormalization of the Abelian-Higgs Model in the R-xi and Unitary gauges and the physicality of its scalar potential

We perform an old school, one-loop renormalization of the Abelian-Higgs model in the Unitary and $R_ξ$ gauges, focused on the scalar potential and the gauge boson mass. Our goal is to demonstrate in this simple context the validity of the Unitary gauge at the quantum level, which could open the way for an until now (mostly) avoided framework for loop computations. We indeed find that the Unitary gauge is consistent and equivalent to the $R_ξ$ gauge at the level of $β$-functions. Then we compare the renormalized, finite, one-loop Higgs potential in the two gauges and we again find equivalence. This equivalence needs not only a complete cancellation of the gauge fixing parameter $ξ$ from the $R_ξ$ gauge potential but also requires its $ξ$-independent part to be equal to the Unitary gauge result. We follow the quantum behaviour of the system by plotting Renormalization Group trajectories and Lines of Constant Physics, with the former the well known curves and with the latter, determined by the finite parts of the counter-terms, particularly well suited for a comparison with non-perturbative studies.

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