arXiv subjects

Michael J. Baker

Publications and source records attributed to Michael J. Baker.

At least 19 recordsLinked to original sources

arXiv2026

Stability in Axion Inflation with Strong Backreaction from a Massive Vector Boson

We study a modification of the model of axion inflation coupled to a $U(1)$ gauge field where the vector field is massive. In the conventional scenario with a massless gauge field, the onset of the regime where the gauge field strongly backreacts on the inflaton displays an instability whose nonlinear evolution and endpoint remain poorly understood. We argue that, if the gauge field is massive enough, the transition to the strong backreaction regime instead occurs smoothly, avoiding this instability. This observation suggests that axion inflation with massive gauge fields admits a controllable strong backreaction regime, so that a phenomenologically viable realization of inflation might be possible in this class of models.

astro-ph.CO↗

arXiv2026

Heavy Vector Triplets at a Muon Collider

Heavy spin-one particles are well-motivated new physics candidates that can have their origin in weakly coupled extensions of the Standard Model gauge group or in strongly coupled Composite Higgs models. Due to the variety of production and decay modes, heavy vector triplets are a useful benchmark for the study and comparison of future colliders. Here we perform a detailed collider analysis of a variety of $2 \to 2$ and $2 \to 3$ processes at a proposed future muon collider. We focus on decays into leptons and Standard Model gauge bosons, and find that heavy vector triplets could be probed up to masses of around $12\,$TeV for almost any (perturbative) value of the coupling. We compare the direct reach of a muon collider to the LHC and to updated projections for the HL-LHC, HE-LHC and FCC-hh, and include indirect limits from future measurements of electroweak precision observables. We find that a muon collider offers projected sensitivities that are competitive with future hadron colliders, exceeding those of the HE-LHC in the scenarios considered though not reaching the projected sensitivity of the FCC-hh.

hep-ph↗

arXiv2026

Explaining the PeV Neutrino Fluxes at KM3NeT and IceCube with Quasi-Extremal Primordial Black Holes

The KM3NeT experiment has recently observed a neutrino with an energy around 100PeV, and IceCube has detected five neutrinos with energies above 1PeV. While there are no known astrophysical sources, exploding primordial black holes could have produced these high-energy neutrinos. For Schwarzschild black holes this interpretation results in tensions between the burst rates inferred from the KM3NeT and IceCube observations, with indirect constraints from the extragalactic gamma ray background and with the non-observation of an associated gamma ray signal at LHAASO. In this letter we show that if there is a population of primordial black holes charged under a new dark $u(1)$ symmetry which spend most of their time in a quasi-extremal state, the neutrino emission at 1PeV may be more suppressed than at 100PeV. The burst rates implied by the KM3NeT and IceCube observations and the indirect constraints can then all be consistent at $1σ$, and no associated gamma-ray signal was expected at LHAASO. Furthermore, these black holes could constitute all of the observed dark matter in the universe.

hep-ph↗

arXiv2025

Probing Dark Sectors with Exploding Black Holes: Gamma Rays

The Hawking radiation from the explosion of a black hole would provide definitive information on the particle spectrum of nature. Here we quantify the potential of current and future gamma ray telescopes to probe new dark sectors. We improve on the analysis used in previous work by making careful use of the experimental response functions, deriving a more realistic estimate of the backgrounds and optimizing the statistical analysis. We compute the sensitivity of the current experiments (HAWC and LHAASO) and estimate the reach of the future experiments (SWGO and CTA North and South), for various sky positions of the explosion. We find that for a black hole exploding at $0.01\,\text{pc}$ the gamma ray signal observed by HAWC could probe dark sectors with 10-20 (or more) new Dirac fermions up to masses around $10^5\,\text{GeV}$, while CTA will be able to probe 2-15 new Dirac fermions with masses up to $10^6\,\text{GeV}$. CTA North and South will have sensitivity to 10 dark fermions up to a distance of 0.1 pc and 50 up to a distance of 0.6 pc.

hep-ph↗

arXiv2025

Could We Observe an Exploding Black Hole in the Near Future?

Observation of an exploding black hole would provide the first direct evidence of primordial black holes, the first direct evidence of Hawking radiation, and definitive information on the particles present in nature. However, indirect constraints suggest that direct observation of an exploding Schwarzschild black hole is implausible. We introduce a dark-QED toy model consisting of a dark photon and a heavy dark electron. In this scenario a population of light primordial black holes charged under the dark $u(1)$ symmetry can become quasi-extremal, so they survive much longer than if they were uncharged, before discharging and exhibiting a Schwarzschild-like final explosion. We show that the answer is "yes", in this scenario the probability of observing an exploding black hole over the next $10$ years could potentially be over $90\%$.

hep-ph↗

arXiv2025

t-channel dark matter at the LHC -- a whitepaper

This report, summarising work achieved in the context of the LHC Dark Matter Working Group, investigates the phenomenology of $t$-channel dark matter models, spanning minimal setups with a single dark matter candidate and mediator to more complex constructions closer to UV-complete models. For each considered class of models, we examine collider, cosmological and astrophysical implications. In addition, we explore scenarios with either promptly decaying or long-lived particles, as well as featuring diverse dark matter production mechanisms in the early universe. By providing a unified analysis framework, numerical tools and guidelines, this work aims to support future experimental and theoretical efforts in exploring $t$-channel dark matter models at colliders and in cosmology.

hep-ph↗

arXiv2025

Primordial Black Holes from First-Order Cosmological Phase Transitions

We discuss the possibility of forming primordial black holes during a first-order phase transition in the early Universe. As is well known, such a phase transition proceeds through the formation of true-vacuum bubbles in a Universe that is still in a false vacuum. When there is a particle species whose mass increases significantly during the phase transition, transmission of the corresponding particles through the advancing bubble walls is suppressed. Consequently, an overdensity can build up in front of the walls and become sufficiently large to trigger primordial black hole formation. We track this process quantitatively by solving a Boltzmann equation, and we delineate the phase transition properties required for our mechanism to yield an appreciable abundance of primordial black holes.

astro-ph.CO↗

arXiv2025

Detailed Calculation of Primordial Black Hole Formation During First-Order Cosmological Phase Transitions

Primordial black holes could potentially form during a first-order cosmological phase transition due to a build-up of particles which are predominantly reflected from the advancing bubble walls. After discussing the general mechanism, we examine the criteria that need to be satisfied for a black hole to form. We then set out the Boltzmann equation that describes the evolution of the relevant phase space distribution function, carefully describing our treatment of the Liouville operator and the collision term. Assuming a spherical false vacuum pocket of sufficient size and a constant wall velocity, we find that black holes can form in a range of different scenarios.

astro-ph.CO↗

arXiv2025

Hot Leptogenesis

We investigate a class of leptogenesis scenarios in which the sector containing the lightest right-handed neutrino establishes kinetic equilibrium at a temperature $T_{N_1} > T_\text{SM}$, where $T_\text{SM}$ is the temperature of the Standard Model sector. We study the reheating processes which realise this "hot leptogenesis" and the conditions under which kinetic and chemical equilibrium can be maintained. We derive and solve two sets of evolution equations, depending on the presence of chemical equilibrium within the hot sector, and numerically solve these for benchmark scenarios. We compare the viable parameter space of this model with standard leptogenesis scenarios with a thermal initial condition and find that hot leptogenesis resolves the neutrino and Higgs mass fine-tuning problems present in the standard scenario.

hep-ph↗

arXiv2025

A Simplified Model of Heavy Vector Singlets for the LHC and Future Colliders

We study a simplified model of two colourless heavy vector resonances in the singlet representation of $SU(2)_{L}$, with zero and unit hypercharge. We discuss mixing with the Standard Model gauge bosons due to electroweak symmetry breaking, semi-analytic formulae for production at proton colliders, requirements to obey the narrow width approximation and selected low energy constraints. We show current LHC constraints and sensitivity projections for the HL-LHC, HE-LHC, SPPC and FCC-hh on the charged and neutral heavy vectors. The utility of the simplified model Lagrangian is demonstrated by matching these results onto three explicit models: a weakly coupled abelian extension of the Standard Model gauge group, a weakly coupled non-abelian extension and a strongly coupled minimal composite Higgs model. All our results are presented in terms of physical resonance masses, using expressions which are accurate even at vector masses near the electroweak scale due to a parameter inversion we derive. We discuss the importance of this inversion and point out that its effect, and the effects of electroweak symmetry breaking, can remain important up to resonance masses of several TeV. Finally, we clarify the relation between this simplified model and the Heavy Vector Triplet (HVT) model, a simplified model for heavy $SU(2)_{L}$ triplets with zero hypercharge, and provide exact and approximate matching relations.

hep-ph↗

arXiv2023

The Role of Vector Boson Fusion in the Production of Heavy Vector Triplets at the LHC and HL-LHC

We clarify the role of vector boson fusion (VBF) in the production of heavy vector triplets at the LHC and the HL-LHC. We point out that the presence of VBF production leads to an unavoidable rate of Drell-Yan (DY) production and highlight the subtle interplay between the falling parton luminosities and the increasing importance of VBF production as the heavy vector mass increases. We discuss current LHC searches and HL-LHC projections in di-boson and di-lepton final states and demonstrate that VBF production outperforms DY production for resonance masses above 1 TeV in certain regions of the parameter space. We define two benchmark parameter points which provide competitive production rates in vector boson fusion.

hep-ph↗

arXiv2022

Report of the Topical Group on Physics Beyond the Standard Model at Energy Frontier for Snowmass 2021

This is the Snowmass2021 Energy Frontier (EF) Beyond the Standard Model (BSM) report. It combines the EF topical group reports of EF08 (Model-specific explorations), EF09 (More general explorations), and EF10 (Dark Matter at Colliders). The report includes a general introduction to BSM motivations and the comparative prospects for proposed future experiments for a broad range of potential BSM models and signatures, including compositeness, SUSY, leptoquarks, more general new bosons and fermions, long-lived particles, dark matter, charged-lepton flavor violation, and anomaly detection.

hep-ph↗

arXiv2022

Black Hole Evaporation Beyond the Standard Model of Particle Physics

The observation of an evaporating black hole would provide definitive information on the elementary particles present in nature. In particular, it could discover or exclude particles beyond those present in the standard model of particle physics. We consider a wide range of motivated scenarios beyond the standard model and identify those which would be best probed in the event of an observation. For those models we define representative benchmark parameters and characterise the photon spectra as a function of time. For the supersymmetric benchmark model, where most of the new particles produce secondary photons, we provide secondary spectra and discuss the subtle interplay between faster black hole evaporation and an increased flux of secondary photons. Finally, we discuss the impact of these models on future experimental analysis strategies.

hep-ph↗

arXiv2022

Probing the Particle Spectrum of Nature with Evaporating Black Holes

Photons radiated from an evaporating black hole in principle provide complete information on the particle spectrum of nature up to the Planck scale. If an evaporating black hole were to be observed, it would open a unique window onto models beyond the Standard Model of particle physics. To demonstrate this, we compute the limits that could be placed on the size of a dark sector. We find that observation of an evaporating black hole at a distance of 0.01 parsecs could probe dark sector models containing one or more copies of the Standard Model particles, with any mass scale up to 100 TeV.

hep-ph↗

arXiv2022

Filtered Baryogenesis

We propose a new mechanism to simultaneously explain the observed dark matter abundance and the baryon asymmetry of the Universe. The mechanism is based on the Filtered Dark Matter scenario, where dark matter particles acquire a large mass during a first-order phase transition. This implies that only a small fraction of them are energetic enough to enter the advancing true vacuum bubbles and survive until today, while the rest are reflected and annihilate away quickly. We supplement this scenario with a CP-violating interaction, which creates a chiral asymmetry in the population of dark matter particles. In the false vacuum phase, a portal interaction quickly converts the dark sector chiral asymmetry into a Standard Model lepton asymmetry. The lepton asymmetry is then partially converted to a baryon asymmetry by standard electroweak sphaleron processes. We discuss the dependence of the generated asymmetry on the parameters of the model for two different portal interactions and demonstrate successful baryogenesis for both. For one of the portals, it is also possible to simultaneously explain the observed dark matter abundance, over many orders of magnitude in the dark matter mass.

hep-ph↗

arXiv2021

Collider Signatures of Coannihilating Dark Matter in Light of the B-Physics Anomalies

Motivated by UV explanations of the $B$-physics anomalies, we study a dark sector containing a Majorana dark matter candidate and a coloured coannihilation partner, connected to the Standard Model predominantly via a $U_1$ vector leptoquark. A TeV scale $U_1$ leptoquark, which couples mostly to third generation fermions, is the only successful single-mediator description of the $B$-physics anomalies. After calculating the dark matter relic surface, we focus on the most promising experimental avenue: LHC searches for the coloured coannihilation partner. We find that the coloured partner hadronizes and forms meson-like bound states leading to resonant signatures at colliders reminiscent of the quarkonia decay modes in the Standard Model. By recasting existing dilepton and monojet searches we exclude coannihilation partner masses less than 280 GeV and 400 GeV, respectively. Since other existing collider searches do not significantly probe the parameter space, we propose a new dedicated search strategy for pair production of the coloured partner decaying into $bbττ$ final states and dark matter particles. This search is expected to probe the model up to dark matter masses around 600 GeV with current luminosity.

hep-ph↗

arXiv2021

Radiative Muon Mass Models and $(g-2)_μ$

Recent measurements of the Higgs-muon coupling are directly probing muon mass generation for the first time. We classify minimal models with a one-loop radiative mass mechanism and show that benchmark models are consistent with current experimental results. We find that these models are best probed by measurements of $(g-2)_μ$, even when taking into account the precision of Higgs measurements expected at future colliders. The current $(g-2)_μ$ anomaly, if confirmed, could therefore be a first hint that the muon mass has a radiative origin.

hep-ph↗

arXiv2021

Has the Origin of the Third-Family Fermion Masses been Determined?

Precision measurements of the Higgs couplings are, for the first time, directly probing the mechanism of fermion mass generation. The purpose of this work is to determine to what extent these measurements can distinguish between the tree-level mechanism of the Standard Model and the theoretically motivated alternative of radiative mass generation. Focusing on the third-family, we classify the minimal one-loop models and find that they fall into two general classes. By exploring several benchmark models in detail, we demonstrate that a radiative origin for the tau-lepton and bottom-quark masses is consistent with current observations. While future colliders will not be able to rule out a radiative origin, they can probe interesting regions of parameter space.

hep-ph↗