arXiv subjects

Théodore Fischer

Publications and source records attributed to Théodore Fischer.

3 recordsLinked to original sources

arXiv2026

No room for minimal monopole dark matter

The magnetic monopole of a dark sector has been advocated as an appealing dark matter candidate. We revisit the computation of the monopole abundance $Ω_M$, generated by a thermal phase transition in the minimal 't Hooft-Polyakov model. We explore the three regimes where the phase transition is second order, weakly first order, or supercooled, identifying the parameter space regions where $Ω_M$ can match the observed dark matter abundance. However, the dark sector necessarily contains a stable electrically-charged particle, namely a massive vector boson, with a calculable abundance $Ω_{W'}$. We show that, under minimal assumptions, $Ω_{W'}$ is always far larger than $Ω_M$: dark monopoles cannot constitute a sizeable fraction of dark matter.

hep-ph↗

arXiv2026

The price for monopole dark matter

We construct an explicit model where dark matter consists of 't Hooft-Polyakov monopoles. The dark sector is in thermal contact with the Standard Model, and dark monopoles are created by a thermal phase transition in the early Universe. Generically, the abundance of monopoles is negligible with respect to that of stable dark elementary particles. We show how to avoid this by taking the lightest stable particle, a dark fermion, sufficiently light for its abundance to be suppressed, yet heavy enough to satisfy constraints on dark radiation. In this specific window of parameters, dark matter is composed of monopoles with a mass of about $10^8$ GeV or larger, depending on the nature of the phase transition. This candidate lies beyond the reach of present, conventional dark-matter detection experiments. However, the model necessarily predicts dark radiation, with $ΔN_{\rm eff}$ close to present-day bounds. In addition, if the dark phase transition is strongly first order, we find that the corresponding gravitational wave spectrum lies close to the region probed by future interferometers.

hep-ph↗

arXiv2025

Multichannel Dyson equations for even- and odd-order Green's functions: application to double excitations

We extend the concept of the multichannel Dyson equation that we have recently derived to model photoemission spectra by coupling the one- and the three-body Green's functions, to higher-order Green's functions and to other spectroscopies. We show the general structure of the equations and how one can systematically approximate the corresponding multichannel self-energy. As a particular case, we focus on the coupling of the two-body and the four-body Green's functions in the electron-hole channel to describe neutral excitations. This formulation allows for the description of important many-body effects, such biexcitons, in a natural way. We illustrate our approach by applying it to a two-level model system, which, in a one-particle picture, exhibits single and double excitations. Our method can correctly describe both kinds of excitation, unlike standard approaches, and in good agreement with the exact results.

cond-mat.str-el↗