arXiv subjects

Thomas Hambye

Publications and source records attributed to Thomas Hambye.

At least 19 recordsLinked to original sources

arXiv2026

The KM3NeT event: a primordial high energy neutrino?

High energy neutrinos can be injected in the early Universe from the decay or annihilation of long lived primordial relics. We analyse the possibility that the ultrahigh energy neutrino event recently observed by the KM3NeT neutrino telescope could have such an origin. This possibility has the advantage of leading to a sharp spectral feature in a way that the neutrino flux can be small at all energies except at the KM3NeT event energy. Thus, along this scenario the tension with null results from other experiments is reduced with respect to the usual power law case analysed by the KM3NeT and IceCube experiments. At such energies and for an emission around the recombination time, interactions of these neutrinos with background neutrinos prove to be relevant and must be determined from the development of a dedicated code. These interactions, as well as final state radiation processes, modify the spectrum. Interestingly, it turns out that the scenario can also leave an imprint in the CMB that could be probed in the near future. Interestingly too, this scenario does not predict an associated $\gamma$-ray flux beyond observation. All in all we do find that the high energy neutrino could be a primordial high energy neutrino, provided it has been produced around the recombination time or later.

astro-ph.HE↗

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, $m_{H'}<580\,{\rm GeV}$.

hep-ph↗

arXiv2025

Primordial high energy neutrinos: theoretical/observational constraints and sharp spectral features

Among the few ways that allow or could allow us to probe the early Universe from the observation of a flux of primordial particles, there is one possibility which has been little studied: the observation today of high energy neutrinos which could have been emitted shortly after the Big Bang, from the decay or annihilation of early universe relics. We perform a general study of such a possibility. To this end, we first emphasise that these neutrinos could display various kinds of sharp spectral features, resulting from the primary energy spectrum at emission, and from how this spectrum is smoothed by redshift and radiative correction effects. Next, we determine the ranges of mass (from a fraction of eV all the way to the Planck scale) and lifetime of the source particles along which we do not/we do expect that the sharp spectral feature will be altered by interactions of the neutrinos on their way to the detector, mainly with the cosmic neutrino background or between themselves. We also study the theoretical (i.e. mainly BBN and CMB) and observational constraints which hold on such a possibility. This allows us to delineate the regions of parameter space (mass, lifetime and abundance) that are already excluded, hopeless for future observation or, instead, which could lead to the observation of such neutrinos in the near future.

hep-ph↗

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.

hep-ph↗

arXiv2025

Photo- and Hadrodisintegration constraints on massive relics decaying into neutrinos

We perform a detailed study of the cosmological constraints on the decay of a relic particle $\phi$ into neutrinos, $\phi \rightarrow \nu \bar{\nu}$, in particular those arising from the observed light-element abundances in the early Universe. We focus on the late-time disintegration of the light elements previously synthesised during BBN. Several processes are relevant, including final-state radiation associated with the decay, as well as subsequent interactions of the injected neutrinos with the thermal background neutrinos or between themselves. All processes generically contribute to the production of electromagnetic and often also hadronic material and may therefore induce late-time photodisintegration and hadrodisintegration reactions, i.e.~the destruction of light elements that have previously been formed during BBN. Here, we examine this scenario with a Monte-Carlo inspired probabilistic approach rather than Boltzmann techniques, taking into account all of these different reactions as well as their interplay. We find the resulting constraints to be very significant, covering a broad range of previously unexplored masses and lifetimes of the relic source particle.

hep-ph↗

arXiv2024

Dark matter bound-state formation in the Sun

The Sun may capture asymmetric dark matter (DM), which can subsequently form bound-states through the radiative emission of a sub-GeV scalar. This process enables generation of scalars without requiring DM annihilation. In addition to DM capture on nucleons, the DM-scalar coupling responsible for bound-state formation also induces capture from self-scatterings of ambient DM particles with DM particles already captured, as well as with DM bound-states formed in-situ within the Sun. This scenario is studied in detail by solving Boltzmann equations numerically and analytically. In particular, we take into consideration that the DM self-capture rates require a treatment beyond the conventional Born approximation. We show that, thanks to DM scatterings on bound-states, the number of DM particles captured increases exponentially, leading to enhanced emission of relativistic scalars through bound-state formation, whose final decay products could be observable. We explore phenomenological signatures with the example that the scalar mediator decays to neutrinos. We find that the neutrino flux emitted can be comparable to atmospheric neutrino fluxes within the range of energies below one hundred MeV. Future facilities like Hyper-K, and direct DM detection experiments can further test such scenario.

hep-ph↗

arXiv2023

Accidentally light scalars from large representations

In models with spontaneous symmetry breaking by scalar fields in large group representations, we observe that some of the scalar masses can be loop-suppressed with respect to the naive expectation from symmetry selection rules. We present minimal models -- the $\rm{SU(2)}$ five-plet and $\rm{SU(3)}$ ten-plet -- with such accidentally light scalars, featuring compact tree-level flat directions lifted by radiative corrections. We sketch some potential applications, from stable relics and slow roll in cosmology, to hierarchy and fine-tuning problems in particle physics.

hep-ph↗

arXiv2022

Dark matter from the centre of SU(N)

A dark sector with non-abelian gauge symmetry provides a sound framework to justify stable dark matter (DM) candidates. We consider scalar fields charged under a $SU(N)$ gauge group, and show that the centre of $SU(N)$, the discrete subgroup $Z_N$ also known as $N$-ality, can ensure the stability of scalar DM particles. We analyse in some details two minimal DM models of this class, based on $SU(2)$ and $SU(3)$, respectively. These models have non-trivial patterns of spontaneous symmetry breaking, leading to distinctive phenomenological implications. For the $SU(2)$ model these include a specific interplay of two DM states, with the same interactions but different masses, and several complementary DM annihilation regimes, either within the dark sector or through the Higgs portal. The $SU(3)$ model predicts dark radiation made of a pair of dark photons with a unique gauge coupling, as well as regimes where DM semi-annihilations become dominant and testable.

hep-ph↗

arXiv2021

Cosmological constraints on the decay of heavy relics into neutrinos

A massive particle decaying into neutrinos in the early Universe is known to be less constrained than if it was decaying into other standard model particles. However, even if the decay proceeds into neutrinos, the latter still inevitably emit secondary particles undergoing electromagnetic interactions that can be probed. We analyse in details how sensitive various cosmological probes are to such secondary particles, namely CMB anisotropies, CMB spectral distortions, and Big Bang Nucleosynthesis. For relics whose lifetime is shorter than the age of the Universe, this leads to original and stringent bounds on the particle's lifetime as a function of its abundance and mass.

hep-ph↗

arXiv2021

Dark matter electromagnetic dipoles: the WIMP expectation

We perform a systematic study of the electric and magnetic dipole moments of dark matter (DM) that are induced at the one-loop level when DM experiences four-fermion interactions with Standard Model (SM) charged fermions. Related to their loop nature these moments can largely depend on the UV completion at the origin of the four-fermion operators. We illustrate this property by considering explicitly two simple ways to generate these operators, from $t$- or $s$-channel tree-level exchange. Fixing the strength of these interactions from the DM relic density constraint, we obtain in particular a magnetic moment that, depending on the interaction considered, lies typically between $10^{-20}$ to $10^{-23}$ ecm or identically vanishes. These non-vanishing values induce, via photon exchange, DM-nucleus scattering cross sections that could be probed by current or near future direct detection experiments.

hep-ph↗

arXiv2021

The domain of thermal dark matter candidates

We consider, in general terms, the possible parameter space of thermal dark matter candidates. We assume that the dark matter particle is fundamental and was in thermal equilibrium in a hidden sector with a temperature $T'$, which may differ from that of the Standard Model temperature, $T$. The candidates lie in a region in the $T'/T$ vs. $m_{\rm dm}$ plane, which is bounded by both model-independent theoretical considerations and observational constraints. The former consists of limits from dark matter candidates that decoupled when relativistic (the relativistic floor) and from those that decoupled when non-relativistic with the largest annihilation cross section allowed by unitarity (the unitarity wall), while the latter concerns big bang nucleosynthesis ($N_{\rm eff}$ ceiling) and free streaming. We present three simplified dark matter scenarios, demonstrating concretely how each fits into the domain.

hep-ph↗

arXiv2021

Seesaw determination of the dark matter relic density

In this article, we show that in the usual type-I seesaw framework, augmented solely by a neutrino portal interaction, the dark matter (DM) relic density can be created through freeze-in in a manner fully determined by the seesaw interactions and the DM particle mass. This simple freeze-in scenario, where dark matter is not a seesaw state, proceeds through slow, seesaw-induced decays of Higgs, $W$ and $Z$ bosons. We identify two scenarios, one of which predicts the existence of an observable neutrino line.

hep-ph↗

arXiv2020

Neutrino lines from DM decay induced by high-scale seesaw interactions

If the stability of the dark matter (DM) particle is due to an accidental symmetry, nothing prevents UV physics from destabilising it by inducing DM decays suppressed by powers of the UV scale. The seesaw physics, presumably at the origin of neutrino mass, could induce such a decay. We show that if the seesaw scale lies around the usual Weinberg operator scale, the induced DM decay could generically lead to neutrino lines whose intensity is of the order of the present sensitivity of neutrino telescopes. We illustrate this possibility with models in which the DM is made of the gauge boson(s) of an abelian or non-abelian gauge symmetry.

hep-ph↗

arXiv2020

Dark matter as a heavy thermal hot relic

If, during the early Universe epoch, the dark matter particle thermalizes in a hidden sector which does not thermalize with the Standard Model thermal bath, its relativistic thermal decoupling can easily lead to the observed relic density, even if the dark matter particle mass is many orders of magnitude heavier than the usual $\sim$ eV hot relic mass scale. This straightforward scenario simply requires that the temperature of the hidden sector thermal bath is one to five orders of magnitude cooler than the temperature of the Standard Model thermal bath. In this way the resulting relic density turns out to be determined only by the dark matter mass scale and the ratio of the temperatures of both sectors. In a model independent way we determine that this can work for a dark matter mass all the way from $\sim 1$ keV to $\sim 30$ PeV. We also show how this scenario works explicitly in the framework of two illustrative models. One of them can lead to a PeV neutrino flux from dark matter decay of the order of the one needed to account for the high energy neutrinos observed by IceCube.

hep-ph↗

arXiv2019

Minimal self-interacting dark matter models with light mediator

The light mediator scenario of self-interacting dark matter is strongly constrained in many ways. After summarizing the various constraints, we discuss minimal options and models which allow to nevertheless satisfy all these constraints. One straightforward possibility arises if the dark matter and light mediator particles have a temperature sizably smaller than the SM particles. Another simple possibility arises if dark matter doesn't annihilate dominantly into a pair of light mediators but into heavier particles. Both possibilities are discussed with scalar as well as vector boson light mediators. Further possibilities, such as with a hierarchy of quartic scalar couplings, are also identified.

hep-ph↗

arXiv2019

Dark matter from dark photons: a taxonomy of dark matter production

We analyse how dark matter (DM) can be produced in the early universe, working in the framework of a hidden sector charged under a U(1)' gauge symmetry and interacting with the Standard Model through kinetic mixing. Depending on the masses of the dark matter particle and of the dark photon, as well as on the hidden U(1)' gauge coupling and the kinetic mixing parameter, we classify all the distinct regimes along which the observed dark matter relic density can be accounted for. We find that 9 regimes are potentially operative to produce the DM particles and this along 5 distinct dynamical mechanisms. Among these, 4 regimes are new and correspond to regimes in which the DM particles are produced by on-shell dark photons. One of them proceeds along a new dynamical mechanism, which we dub sequential freeze-in. We argue that such regimes and the associated dynamical mechanisms are characteristic of DM models for which, on top of the Standard Model and the dark sector, there are other massive, but relatively light particles -- akin to the dark photon -- that interact both with the SM and the DM sectors.

hep-ph↗

arXiv2018

New Analysis of Neutron Star Constraints on Asymmetric Dark Matter

Due to their extreme density and low temperature, neutron stars (NS) are efficient probes to unveil interactions between standard model and dark matter (DM) particles. From elastic scatterings on NS material, DM can get gravitationally trapped by the star. The cooling of DM through further collisions may lead to the formation of a dense core which could collapse into a black hole, thus destroying the whole NS. From the observation of old NS, such a scenario leads to very stringent constraints on the parameter space of asymmetric DM. In this work we reexamine this possibility in detail. This includes: (a) a new detailed determination of the number of DM particles captured, properly taking into account the fact that neutrons form a highly degenerate Fermi material; (b) the determination of the time evolution of the DM density and energy profiles inside the NS, which allows us to understand how, as a function of time, DM thermalizes with NS material; (c) the determination of the corresponding constraints which hold on the DM-neutron cross section, including for the case where a large fraction of DM particles have not thermalized; (d) the first determination of the stringent constraints which also hold in a similar way on the DM-muon cross section, particularly relevant for leptophilic DM models; and (e) the use of realistic NS equations of state in determining these constraints.

hep-ph↗

arXiv2018

SO(10) paths to dark matter

The grand-unification gauge group SO(10) contains matter parity as a discrete subgroup. This symmetry could be at the origin of dark matter stability. The properties of the dark matter candidates depend on the path along which SO(10) is broken, in particular through Pati-Salam or left-right symmetric subgroups. We systematically determine the non-supersymmetric dark matter scenarios that can be realized along the various paths. We emphasize that the dark matter candidates may have colored or electrically charged partners at low scale that belong to the same SO(10) multiplet. These states, which in many cases are important for co-annihilation, could be observed more easily than the dark matter particle. We determine the structure of the tree-level and loop-induced mass splittings between the dark matter candidate and their partners and discuss the possible phenomenological implications.

hep-ph↗