arXiv subjects

Miguel Escudero

Publications and source records attributed to Miguel Escudero.

At least 19 recordsLinked to original sources

arXiv2026

Nucleosynthesis and CMB bounds on photophilic ALPs: a fresh look

We provide a fresh look at the cosmological constraints on axion-like particles (ALPs) that couple predominantly to photons, focusing on lifetimes $τ_{a} \lesssim 10^{4}\, {\rm s}$ and masses $m_a\lesssim 10\,{\rm GeV}$. We consider Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) bounds and explore how these limits depend upon the unknown reheating temperature of the Universe, $T_{\rm reh}$. Compared with some previous studies, we account for the rare decays of these ALPs into light hadrons and show that this leads to extended constraints for several reheating temperatures. Our limits are cast in a model-independent way, and we identify regions of parameter space where these ALPs could alleviate small tensions in the determinations of $N_{\rm eff}$ and the deuterium abundance.

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arXiv2026

What does it take to have $N_{\rm eff} < 3$ at CMB times?

The vast majority of extensions of the Standard Model affecting the number of effective relativistic neutrino species ($N_{\rm eff}$) do so additively, namely, they enhance this quantity with some light state contributing to dark radiation. In this work, we consider precisely the opposite case: new physics scenarios that can lead to $N_{\rm eff} < 3$ that are consistent with all known cosmological, astrophysical, and laboratory data. We are motivated by three main reasons: 1) a recent measurement from ACT and SPT in combination with Planck that leads to $N_{\rm eff} = 2.81\pm0.12$, 2) by a new and powerful measurement of the primordial helium abundance, which anchors $N_{\rm eff}$ to be very close to the Standard Model value one second after the Big Bang, 3) by the deployment of the Simons Observatory which will provide precise tests of the radiation content in the Universe and which may detect with a high significance cosmologies with $N_{\rm eff}<3$. We survey the main theoretical possibilities and find that only a few simple scenarios can consistently give $N_{\rm eff}=2.81\pm0.12$. One class consists of thermal electrophilic relics with masses $m\sim 8\!-\!13\,{\rm MeV}$. Another consists of out-of-equilibrium particles decaying to $e^+e^-$ or $γγ$, with a rather particular lifetime $0.05\,{\rm s}\lesssim τ\lesssim 3\,{\rm min}$, mass $250\,{\rm MeV}\lesssim m \lesssim 600\,{\rm MeV}$, and abundance $ρ/ρ_γ\sim 0.1$ at decay. Thermal electrophilic particles are especially interesting because they can account for the dark matter in the Universe and can be tested in experiments such as SENSEI, DAMIC-M, and Oscura, and their portals to the visible sector at experiments such as NA64 and LDMX. We conclude that if the Simons Observatory confirms that $N_{\rm eff} \simeq 2.8$, it will point to very specific extensions of the Standard Model.

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arXiv2025

A Limit on the Total Lepton Number in the Universe from BBN and the CMB

At temperatures below the QCD phase transition, any substantial lepton number in the Universe can only be present within the neutrino sector. In this work, we systematically explore the impact of a non-vanishing lepton number on Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB). Relying on our recently developed framework based on momentum averaged quantum kinetic equations for the neutrino density matrix, we solve the full BBN reaction network to obtain the abundances of primordial elements. We find that the maximal primordial total lepton number $L$ allowed by BBN and the CMB is $-0.12 \,(-0.10) \leq L \leq 0.13\,(0.12) $ for NH (IH), while specific flavor directions can be even more constrained. This bound is complementary to the limits obtained from avoiding baryon overproduction through sphaleron processes at the electroweak phase transition since, although numerically weaker, it applies at lower temperatures and is obtained completely independently. We publicly release the C++ code COFLASY-C on GitHub which solves for the evolution of the neutrino quantum kinetic equations numerically.

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arXiv2025

Keeping It Renormalizable: Minimal Baryogenesis induced Asymmetric Dark Matter

Many asymmetric dark matter scenarios have been proposed to date. Among them, perhaps the most motivated ones are those in which the dark matter asymmetry is induced from the baryon/lepton asymmetries via chemical equilibration without any new sources of CP violation. However, most of the models put forward along these lines have been excluded by now and/or are based on complicated setups. In this letter, we present a new, simple, and viable scenario. It assumes only two new fields: a scalar singlet and an inert scalar doublet, and is based only on renormalizable interactions, that slowly generate the dark matter asymmetry from the Standard Model Higgs asymmetry. The model allows for the direct detection of dark matter in the upcoming generation of experiments, and the inert doublet is predicted to be light enough to be potentially produced and observed at the LHC and future colliders, $m_{H'}<580\,{\rm GeV}$.

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arXiv2025

Lepton Flavor Asymmetries: from the early Universe to BBN

Large primordial lepton flavor asymmetries with almost vanishing total baryon-minus-lepton number can evade the usual BBN and CMB constraints if neutrino oscillations lead to perfect flavor equilibration. Solving the momentum averaged quantum kinetic equations (QKEs) describing neutrino oscillations and interactions, we perform the first systematic investigation of this scenario, uncovering a rich flavor structure in stark contradiction to the assumption of simple flavor equilibration. We find (i) a particular direction in flavor space, $Δn_e \simeq -2/3 \, (-1) Δn_μ$ for normal (inverted) neutrino mass hierarchy, in which the flavor equilibration is efficient and primordial asymmetries are essentially unconstrained, (ii) a minimal washout factor, $Δn_e^2|_\mathrm{BBN} \leq 0.03 \, (0.016) \sum_αΔn_α^2|_{\mathrm{ini}}$ yielding a conservative estimate for the allowed primordial asymmetries in a generic flavor direction, and (iii) particularly strong or weak washout if one of the initial flavor asymmetries vanishes due to non-adiabatic muon- or electron-driven MSW transitions. These results open up the possibility of a first-order QCD phase transition facilitated by large lepton asymmetries as well as baryogenesis from large and compensated $Δn_e =- Δn_μ$ asymmetries. Our first-principles approach of deriving momentum averaged QKEs includes collision terms beyond the damping approximation, energy transfer between the neutrino and electron-photon plasma, and provides a fast and reliable way to investigate the impact of primordial lepton asymmetries at the time of BBN. We publicly release the Mathematica code COFLASY-M on GitHub which solves the QKEs numerically.

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arXiv2025

The Simplest Dark Matter Model at the Edge of Perturbativity

Increasingly sensitive direct detection dark matter experiments are testing important regions of parameter space for WIMP dark matter and pushing many models to the multi-TeV regime. This brings into question the perturbativity of these models. In this context, and in light of the new limits from the LZ experiment, we investigate the status of the simplest thermal dark matter model: a singlet scalar, real or complex, coupled to the Higgs boson. We calculate the next-to-leading order (NLO) corrections to the direct detection rates as well as for the annihilations driving thermal freeze-out. For the complex case, we find that the entire perturbative region is excluded by direct detection. For the real case we find that the mass should be $\gtrsim 20\,{\rm TeV}$ at NLO, compared with the $\gtrsim 30\,{\rm TeV}$ LO limit. We highlight that a three-fold improvement on WIMP spin independent interactions can fully test the real scalar model in the perturbative regime. Finally, for both models, there is still an allowed (albeit narrow) region near the Higgs resonance where couplings are perturbative.

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arXiv2025

Neutrino Theory in the Precision Era

This document summarises discussions on future directions in theoretical neutrino physics, which are the outcome of a neutrino theory workshop held at CERN in February 2025. The starting point is the realisation that neutrino physics offers unique opportunities to address some of the most fundamental questions in physics. This motivates a vigorous experimental programme which the theory community fully supports. \textbf{A strong effort in theoretical neutrino physics is paramount to optimally take advantage of upcoming neutrino experiments and to explore the synergies with other areas of particle, astroparticle, and nuclear physics, as well as cosmology.} Progress on the theory side has the potential to significantly boost the physics reach of experiments, as well as go well beyond their original scope. Strong collaboration between theory and experiment is essential in the precision era. To foster such collaboration, \textbf{we propose to establish a CERN Neutrino Physics Centre.} Taking inspiration from the highly successful LHC Physics Center at Fermilab, the CERN Neutrino Physics Centre would be the European hub of the neutrino community, covering experimental and theoretical activities.

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arXiv2024

Living at the Edge: A Critical Look at the Cosmological Neutrino Mass Bound

Cosmological neutrino mass bounds are becoming increasingly stringent. The latest limit within $Λ$CDM from Planck 2018+ACT lensing+DESI is $\sum m_ν< 0.072\,{\rm eV}$ at 95\% CL, very close to the minimum possible sum of neutrino masses ($\sum m_ν> 0.06\,{\rm eV}$), hinting at vanishing or even ``negative'' cosmological neutrino masses. In this context, it is urgent to carefully evaluate the origin of these cosmological constraints. In this paper, we investigate the robustness of these results in three ways: i) we check the role of potential anomalies in Planck CMB and DESI BAO data; ii) we compare the results for frequentist and Bayesian techniques, as very close to physical boundaries subtleties in the derivation and interpretation of constraints can arise; iii) we investigate how deviations from $Λ$CDM, potentially alleviating these anomalies, can alter the constraints. From a profile likelihood analysis, we derive constraints in agreement at the $\sim 10\%$ level with Bayesian posteriors. We find that the weak preference for negative neutrino masses is mostly present for Planck 18 data, affected by the well-known `lensing anomaly'. It disappears when the new Planck 2020 HiLLiPoP is used, leading to significantly weaker constraints. Additionally, the pull towards negative masses in DESI data stems from the $z=0.7$ bin, which contains a BAO measurement in $\sim 3σ$ tension with Planck expectations. Without this bin, and in combination with HiLLiPoP, the bound relaxes to $\sum m_ν< 0.11\,{\rm eV}$ at 95\% CL. The recent preference for dynamical dark energy alleviates this tension and further weakens the bound. As we are at the dawn of a neutrino mass discovery from cosmology, it will be very exciting to see if this trend is confirmed by future data.

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arXiv2024

How large could CP violation in neutral $B$ meson mixing be? Implications for baryogenesis and upcoming searches

CP violation in neutral $B$ meson oscillations is an experimental observable that could be directly related to the baryon asymmetry of the Universe through the $B$-Mesogenesis mechanism. As this phenomenon is highly suppressed in the Standard Model, it could also be a sensitive probe for many new physics scenarios that modify neutral meson mixing. Motivated by these facts, and the timely $B$ physics program at the LHC and Belle II, we analyze how large CP violation in the mixing of neutral $B_d$ and $B_s$ meson systems could be. We answer this question, in light of current experimental data, within three different scenarios, namely: (i) generic heavy new physics only affecting the mass mixing $M_{12}^q$, (ii) vector-like quark extensions that introduce deviations of 3$\times$3 CKM unitarity, and (iii) light new physics modifying the decay mixing $Γ_{12}^q$. We find that enhancements of the semileptonic asymmetries, that measure the amount of CP violation in mixing, at the level of $10^{-3}$ for the $B_d$ system and $10^{-4}$ for the $B_s$ system can be achieved within scenarios (i) and (ii), while they are much more suppressed in realistic UV completions triggering scenario (iii). With respect to cosmology, the difficulty of finding large CP asymmetries in our analysis puts the $B$-Mesogenesis mechanism in tension. Finally, we conclude that upcoming experimental searches for CP violation in $B$ meson mixing at LHCb and Belle II are unlikely to detect a new physics signal for the most generic models.

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arXiv2024

A seesaw model for large neutrino masses in concordance with cosmology

Cosmological constraints on the sum of the neutrino masses can be relaxed if the number density of active neutrinos is reduced compared to the standard scenario, while at the same time keeping the effective number of neutrino species $N_{\rm eff}\approx 3$ by introducing a new component of dark radiation. We discuss a UV complete model to realise this idea, which simultaneously provides neutrino masses via the seesaw mechanism. It is based on a $U(1)$ symmetry in the dark sector, which can be either gauged or global. In addition to heavy seesaw neutrinos, we need to introduce $\mathcal{O}(10)$ generations of massless sterile neutrinos providing the dark radiation. Then we can accommodate active neutrino masses with $\sum m_ν\sim 1$ eV, in the sensitivity range of the KATRIN experiment. We discuss the phenomenology of the model and identify the allowed parameter space. We argue that the gauged version of the model is preferred, and in this case the typical energy scale of the model is in the 10 MeV to few GeV range.

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arXiv2024

The first limit on invisible decays of $B_s$ mesons comes from LEP

Motivated by the recent evidence for $B^+\to K^+\barν ν$ decays at Belle II, we point out that fully invisible $B_d$ and $B_s$ meson decays are strongly constrained by LEP. A reinterpretation of an old inclusive ALEPH search for $b$-hadron decays with large missing energy allows us to place the limits $\mathrm{Br}(B_d \rightarrow \mathrm{invisible}) < 1.4\times10^{-4}$ and $\mathrm{Br}(B_s \rightarrow \mathrm{invisible}) < 5.6\times10^{-4}$, both at $90\%$ CL. The $B_d$ limit is only a factor of 6 looser than the world-leading one provided by the BaBar collaboration, while the $B_s$ one is the first limit in the literature on this decay mode. These results are relevant in the context of new light states coupled to quarks and exemplify the power of a future Tera-$Z$ factory at FCC-ee to look for $B$ meson decays containing missing energy.

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arXiv2024

Soliton Merger Rates and Enhanced Axion Dark Matter Decay

Solitons are observed to form in simulations of dark matter (DM) halos consisting of bosonic fields. We use the extended Press-Schechter formalism to compute the mass function of solitons, assuming various forms for the relationship between halo mass and soliton mass. We further provide a new calculation of the rate of soliton major mergers. Solitons composed of axion DM are unstable above a critical mass, and decay to either relativistic axions or photons, depending on the values of the coupling constants. We use the computed soliton major merger rate to predict the enhanced DM decay rate due to soliton instability. For certain values of currently allowed axion parameters, the energy injection into the intergalactic medium from soliton decays to photons is comparable to or larger than the energy injection due to core collapse supernovae at $z>10$. A companion paper explores the phenomenology of such an energy injection.

astro-ph.CO↗

arXiv2024

Axion Star Explosions and the Reionization History of the Universe

Cosmological structure formation simulations of ultralight axion-like dark matter have shown that an axion star forms at the center of every dark matter halo in the Universe. These axion stars would then form in large numbers during the dark ages, $z \lesssim 70$. Axion stars would represent the densest axion environments in the Universe, and as such they can trigger collective processes that cannot otherwise occur for axions in vacuum. In particular, even though the lifetime of individual sub-eV axions decaying into a pair of photons is much larger than the age of the Universe, axion stars can decay into photons on very short time scales due to parametric resonance. In this talk, based on arXiv:2302.10206 and arXiv:2301.09769, I will discuss the cosmological implications of such decays. We show that massive enough axion stars will decay into a large number of radio photons which will in turn lead to heating and ionization during the dark ages which is strongly constrained by Planck. As a result, we find that couplings $10^{-14}\,{\rm GeV}^{-1} \lesssim g_{aγγ} \lesssim 10^{-10}\,{\rm GeV}^{-1}$ are excluded by Planck for $10^{-14}\,{\rm eV}\lesssim m_a\lesssim 10^{-8}\,{\rm eV}$ within our benchmark model of axion star abundance. We also highlight that future measurements of the 21 cm line can have sensitivity to couplings at least one order of magnitude smaller.

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arXiv2024

Axion Star Explosions: A New Source for Axion Indirect Detection

If dark matter is composed of axions, then axion stars form in the cores of dark matter halos. These stars are unstable above a critical mass, decaying to radio photons that heat the intergalactic medium, offering a new channel for axion indirect detection. We recently provided the first accurate calculation of the axion decay rate due to axion star mergers. In this work we show how existing data concerning the CMB optical depth leads to strong constraints on the axion photon coupling in the mass range $10^{-14}\,{\rm eV}\lesssim m_a\lesssim 10^{-8}\,{\rm eV}$. Axion star decays lead to efficient reionization of the intergalactic medium during the dark ages. By comparing this non-standard reionization with Planck legacy measurements of the Thompson optical width, we show that couplings in the range $10^{-14}\,{\rm GeV}^{-1} \lesssim g_{aγγ} \lesssim 10^{-10}\,{\rm GeV}^{-1}$ are excluded for our benchmark model of axion star abundance. Future measurements of the 21cm emission of neutral hydrogen at high redshift could improve this limit by an order of magnitude or more, providing complementary indirect constraints on axion dark matter in parameter space also targeted by direct detection haloscopes.

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arXiv2023

Synergy between cosmological and laboratory searches in neutrino physics

The intersection of the cosmic and neutrino frontiers is a rich field where much discovery space still remains. Neutrinos play a pivotal role in the hot big bang cosmology, influencing the dynamics of the universe over numerous decades in cosmological history. Recent studies have made tremendous progress in understanding some properties of cosmological neutrinos, primarily their energy density. Upcoming cosmological probes will measure the energy density of relativistic particles with higher precision, but could also start probing other properties of the neutrino spectra. When convolved with results from terrestrial experiments, cosmology can become even more acute at probing new physics related to neutrinos or even Beyond the Standard Model (BSM). Any discordance between laboratory and cosmological data sets may reveal new BSM physics and/or suggest alternative models of cosmology. We give examples of the intersection between terrestrial and cosmological probes in the neutrino sector, and briefly discuss the possibilities of what different laboratory experiments may see in conjunction with cosmological observatories.

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arXiv2023

Neff in the Standard Model at NLO is 3.043

The effective number of relativistic neutrino species is a fundamental probe of the early Universe and its measurement represents a key constraint on many scenarios beyond the Standard Model of Particle Physics. In light of this, an accurate prediction of $N_{\rm eff}$ in the Standard Model is of pivotal importance. In this work, we consider the last ingredient needed to accurately calculate $N_{\rm eff}^{\rm SM}$: standard zero and finite temperature QED corrections to $e^+e^- \leftrightarrow ν\barν$ interaction rates during neutrino decoupling at temperatures around $T\sim {\rm MeV}$. We find that this effect leads to a reduction of $-0.0007$ in $N_{\rm eff}^{\rm SM}$. This NLO QED correction to the interaction rates, together with finite temperature QED corrections to the electromagnetic density of the plasma, and the effect of neutrino oscillations, implies that $N_{\rm eff}^{\rm SM} = 3.043$ with a theoretical uncertainty that is much smaller than any projected observational sensitivity.

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arXiv2023

Precision CMB constraints on eV-scale bosons coupled to neutrinos

The cosmic microwave background (CMB) has proven to be an invaluable tool for studying the properties and interactions of neutrinos, providing insight not only into the sum of neutrino masses but also the free streaming nature of neutrinos prior to recombination. The CMB is a particularly powerful probe of new eV-scale bosons interacting with neutrinos, as these particles can thermalize with neutrinos via the inverse decay process, $ν\barν \rightarrow X$, and suppress neutrino free streaming near recombination -- even for couplings as small as $λ_ν\sim \mathcal{O}(10^{-13})$. Here, we revisit CMB constraints on such bosons, improving upon a number of approximations previously adopted in the literature and generalizing the constraints to a broader class of models. This includes scenarios in which the boson is either spin-$0$ or spin-$1$, the number of interacting neutrinos is either $N_{\rm int} = 1,2 $ or $3$, and the case in which a primordial abundance of the species is present. We apply these bounds to well-motivated models, such as the singlet majoron model or a light $U(1)_{L_μ-L_τ}$ gauge boson, and find that they represent the leading constraints for masses $m_X\sim 1\, {\rm eV}$. Finally, we revisit the extent to which neutrino-philic bosons can ameliorate the Hubble tension, and find that recent improvements in the understanding of how such bosons damp neutrino free streaming reduces the previously found success of this proposal.

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arXiv2023

New Physics Searches at Kaon and Hyperon Factories

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

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