arXiv subjects

Joaquin Masias

Publications and source records attributed to Joaquin Masias.

13 recordsLinked to original sources

arXiv2026

A LooKK at the Higuchi Bound

Massive spin 2 fields on de Sitter must satisfy the Higuchi bound, $m^2\geq 2H^2$. Compactifications of higher dimensional gravity to $\mathrm{dS}_4$ come with a tower of Kaluza-Klein gravitons whose masses are fixed by the internal geometry, so the bound becomes a constraint on the compactification. We propose that the KK gravitons of a consistent compactification never violate it, and test this in a few examples. On a circle stabilized by Casimir energy the bound holds unless the circle shrinks below the species scale, and on a warped interval with negative tension branes the tower is gapped at $m^2\geq \tfrac{9}{4}H^2$ for any length of the interval. On group manifolds, we argue that if the internal curvature is not parametrically larger than the Hubble scale, the bound can apparently be violated by smooth deformations. The violation disappears once one promotes these deformations to moduli, which are immediately extremized when solving the 10d equations of motion.

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arXiv2026

Black Hole-Tower Correspondence: Species backreaction in minimal black hole limits

The Black Hole-String Correspondence identifies the microstates of a minimal black hole with those of a highly excited string. In decompactification limits, where the light states are Kaluza--Klein modes, its proposed counterpart is the Black Hole-Tower Correspondence at the species scale. We extend it in three directions. First, we expand the free thermodynamic analysis to include towers with string-oscillators and Kaluza--Klein modes, and clarify whether the ensemble connects to a $d$-dimensional black hole or a wrapped black string. Second, we include gravitational backreaction in the pure Kaluza--Klein tower transition. With no winding-tachyon analogue, the interpolating configuration is the one-loop Kaluza--Klein gas, which reorganizes into higher-dimensional radiation and yields self-gravitating solutions whose instability points to the black string and whose thermodynamics match it at the correspondence point, $S\sim N_{\rm sp}$, where the size approaches the species length. This shows that the matching is robust under self-interactions. Third, we ask whether the two phases are continuously connected, revisiting and extending the worldsheet and topological analyses of the Black Hole-String case to include also Kaluza--Klein towers. Linear sigma model families interpolate smoothly in the heterotic string, while in type II the transition is obstructed. The cobordism classes of the two saddles agree in every structure we check, but their brane-charge lattices, given by bordism groups, differ, which is the source of the type II obstruction. This obstruction is thus perturbative, showing that the transition can only proceed through the non-perturbative, charge-violating processes of the kind predicted by the Cobordism Conjecture.

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arXiv2026

Inflationary Particle Production and the Swampland

We investigate the impact of particle production during inflation in scenarios where an infinite tower of states features a mass scale that decreases exponentially along the inflationary trajectory. Such couplings naturally arise in string effective field theories and are in fact motivated by the Swampland Distance Conjecture (SDC). We show that the corrections to inflationary observables sourced by the tower scale as $(H/Λ_{\text{sp}})^{2+p}$, with $H$ being the Hubble scale, $Λ_{\text{sp}}$ being the species scale, that is the quantum gravity cut-off, and $p\geq 1$ characterizes the density of states in the tower. As a result, in gravitationally weakly coupled cosmological effective theories, the tower-induced contributions are suppressed relative to the standard single-field predictions, leaving the inflationary phenomenology essentially unchanged. We demonstrate this explicitly across a set of well-motivated inflationary potentials, and we compare the resulting predictions with the most recent observational constraints, including those from the Atacama Cosmology Telescope.

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arXiv2026

Quintessential $α$-attractors fit DESI

We study quintessence in $α$-attractor models in light of recent DESI indications for dynamical dark energy. We show that the \emph{knee} of the attractor potential provides an excellent approximation to the axion-like quintessence model used as a DESI benchmark. This leads to a simple relation between the axion decay constant $f_a$ and the attractor parameter $α$, allowing the experimental constraints to be translated into a preference for $α=\mathcal{O}(1)$, in agreement with string-motivated expectations. We solve the background dynamics numerically and find good agreement with the DESI-preferred evolution of $w(z)$ up to $z\sim\mathcal{O}(1)$. More generally, we point out that the agreement between axion-like and attractor potentials reflects a common requirement imposed by the data: today's potential energy and slope are both of order the Hubble scale in Planck units. We finally comment on the origin of the required initial conditions, which can naturally arise in multifield attractor scenarios.

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arXiv2025

Higher Curvature Inflation and the Species Scale

We study the scalar potentials that arise from higher curvature corrections in general $f(R)$ theories of gravity and their connection to a dynamical species scale. Starting from general considerations in arbitrary dimensions, we show that at large field values, the scalar potential generated by an infinite series of curvature terms and the field dependent species scale arising from circle compactification both decay exponentially, in complementary ways. We identify conditions under which these two effects precisely balance out, giving rise to exponentially flat, plateau-like potentials. We additionally find a precise embedding of Starobinsky inflation consistent with the Swampland program, and we discuss possible implications the mechanism proposed could have for M and string theory.

hep-th↗

arXiv2025

Dark bubble cosmology and the equivalence principle

The main goal of string phenomenology is to find realistic models of particle physics and cosmology within string theory. Dark bubble cosmology is an alternative to string compactifications, where our universe lives on a bubble expanding in a higher-dimensional spacetime. This construction is inherently non-supersymmetric and can yield a four-dimensional realistic cosmology, where radiation behaves as expected due to its coupling to higher-dimensional fields. We study the coupling of the electroweak and strong sectors to the induced braneworld gravity via the same mechanism. While the electroweak sector is unaffected, the gravitational and inertial masses of the proton differ significantly, severely violating measurements of the equivalence principle.

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arXiv2025

Bulk/boundary Modular Quintessence and DESI

The latest DESI DR2 results, when combined with other independent cosmological data on the Cosmic Microwave Background and supernovas, suggest a preference for dynamical dark energy. We propose a novel cosmological scenario, which features two distinct scalar fields. One governs the magnitude of the present-day dark energy density and is related to the size of extra-dimensions. Accounting for the observed smallness of this energy density requires the scalar to reside near the boundary of field space. The second field, responsible for the time evolution of dark energy and associated with the string coupling, must instead lie in the bulk to remain consistent with the non-observation of light string states. We show that a natural candidate for such dark energy dynamics is a quintessence modular-invariant potential, in which the second scalar field rolls down a negatively curved slope, starting from a self-dual critical point. We find that this scenario is in good agreement with the latest findings by DESI.

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arXiv2025

A short overview on the Black Hole-Tower Correspondence and Species Thermodynamics

The breakdown of gravitational effective field theories is intimately connected to the emergence of infinite towers of light states near infinite-distance limits in field space. In string theory, up to duality frame, such towers arise from Kaluza-Klein or weakly-coupled critical string oscillator modes. Motivated by the Black Hole-String Correspondence, we review a broader mechanism whereby black holes undergo a transition into a tower of light states, governed by the Quantum Gravity cutoff -- known as the Species Scale. Building on these developments, the Black Hole-Tower correspondence aims to provide a unified thermodynamic framework that describes black hole entropy in terms of the spectrum of the lightest degrees of freedom across various perturbative regimes of quantum gravity theories. In those regimes, thermodynamic consistency of such transition imposes stringent constraints on the spectrum, in agreement with string theory predictions. This defines the basis of the so-called Species Thermodynamics. In this review, we emphasize these recent advances and synthesize their implications, offering an overview of how the outlined correspondence, the species scale and related thermodynamic principles enhance our understanding of black hole entropy within the effective field theory framework.

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arXiv2025

On the Origin of Species Thermodynamics and the Black Hole - Tower Correspondence

Species thermodynamics has been proposed in analogy to black hole thermodynamics. The entropy scales like an area and is given by the mere counting of the number of the species. In this work, we $\textit{derive}$ the constitutive relations of species thermodynamics and explain how those $\textit{originate}$ from standard thermodynamics. We consider configurations of species in thermal equilibrium inside a box of size $L$, and show that the temperature $T$ of the system, which plays a crucial role, is always upper bounded above by the species scale $Λ_{\rm sp}$. We highlight three relevant regimes: (i) when $L^{-1}< T<Λ_{\rm sp}$, and gravitational collapse is avoided, the system exhibits standard thermodynamics features, for example, with the entropy scaling like the volume of the box; (ii) in the limit $L^{-1}\simeq T\rightarrow Λ_{\rm sp}$ we recover the rules of species thermodynamics with the entropy scaling like the area of the box; (iii) an intermediate regime with $ L^{-1}\simeq T< Λ_{\rm sp}$ that avoids gravitational collapse and fulfills the Covariant Entropy Bound; this interpolates between the previous two regimes and its entropy is given simply in terms of the counting of the species contributing to the thermodynamic ensemble. This study also allows us to find a novel and independent bottom-up rationale for the Emergent String Conjecture. Finally, we present the $\textit{Black Hole - Tower Correspondence}$ as a generalization of the celebrated Black Hole - String Correspondence. This provides us with a robust framework to interpret the results of our thermodynamic investigation. Moreover, it allows us to qualitatively account for the entropy of black holes in terms of the degrees of freedom of the weakly coupled species in the tower.

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arXiv2025

Dynamical dark energy in 0'B braneworlds

We build a novel realization of dark bubble cosmology in non-supersymmetric string theory. Among the simplest models in ten dimensions, the type 0'B orientifold is the unique option which yields a scale-separated construction. The resulting setting produces a logarithmically varying dynamical dark energy, reflecting its holographic counterpart in terms of running gauge couplings. We analyze in detail the phenomenological consequences of the model for particle physics, inflation and late-time cosmology. We find that, although particle physics may be consistently realized, neither early-time nor late-time are observationally viable.

hep-th↗

arXiv2024

Starobinsky Inflation in the Swampland

We argue that the Starobinsky model of inflation, realised via an $R^2$ term in the Lagrangian, can originate from quantum effects due to a tower of light species. By means of two separate arguments, we show how this implies that the scale of the $R^2$ term must be of order of the species scale $Λ_s$, namely the energy at which gravity becomes strongly coupled. We discuss the implications and challenges of this scenario for inflation, inflationary reheating, and string theory embeddings. In this context, we collect strong evidence to conclude that Starobinsky inflation lies in the Swampland.

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arXiv2021

Resolving a challenging supersymmetric low-scale seesaw scenario at the ILC

We investigate a scenario inspired by natural supersymmetry, where neutrino data is explained within a low-scale seesaw scenario. For this the Minimal Supersymmetric Standard Model is extended by adding light right-handed neutrinos and their superpartners, the R-sneutrinos. Moreover, we consider the lightest electroweakinos to be higgsino-like. We demonstrate that a prospective international $e^+ e^-$ linear collider with a center of mass energy of 1 TeV will be able to discover sleptons in scenarios which can be difficult for current colliders. Moreover, we also show that a measurement of the spectrum will be possible within 1-3 per-cent accuracy.

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arXiv2019

Using a portable muon detector for radioactive source measurements and identification

We have re-purposed portable plastic scintillator muon detectors, designed by the CosmicWatch project, for the measurement of electrons emitted by the decay of radioactive sources. For the latter purpose we have first calibrated the detectors using the local atmospheric muon flux, performing angular distribution and attenuation measurements. In addition, we have simulated the detector using Geant4 in a detailed fashion for a cross-check and better understanding of the device. Then, we have developed a method to evaluate the activity of $β$-sources and to discriminate different $β$-sources by looking into their respective voltage spectrum output.

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