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Ref Arxiv: 2509.21924
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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.

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DOI: 10.1063/5.0274757
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Brillouin spectroscopy is a non-invasive method technique for visualizing the mechanical properties of biological samples with sub-micrometer resolution. However, to obtain 2D or even 3D maps, a high-contrast spectrometer is required to effectively reject spurious signals and isolate the signal of interest. In this work, we present an optimized two-stage virtually imaged phased array-based Brillouin spectrometer capable of achieving a contrast of at least 80 dB thanks to a new single diffraction mask, and thus with an unprecedentedly high overall transmission. Numerical simulations of the electric field propagation through the spectrometer and dedicated experiments validate the increase in contrast. We have used the newly developed Brillouin spectrometer coupled to a confocal microscope to obtain 1 h high-resolution 2D images of about 104 spectra of transparent dental pulp stem cells near a MgF2 interface as well as of the much less transparent porcine dentin material, demonstrating its overall stability and reliability.

Ref HAL: hal-05240961_v1
DOI: 10.48550/arXiv.2509.03293
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Instanton contributions in 2d string theory are known to include subtle numerical factors $ζ_n$ closely related to a contour prescription in multi-instanton string amplitudes. Both ingredients appear to be ambiguous due to a degeneracy between $(1,n)$-ZZ instantons and $n$ (1,1)-ZZ instantons in the linear dilaton background. We resolve this ambiguity using insights from the dual matrix quantum mechanics where the multipliers $ζ_n$ can be derived from an integral representation of the scattering phase and follow from the median resummation prescribed by resurgence theory. We evaluate multi-instanton string amplitudes in the theory compactified on a circle of finite radius for arbitrary number of instantons and show that they reproduce the matrix model predictions provided the Lorentzian contour prescription is used for their evaluation. We also show that the non-perturbative free energy matches the structure of the D-instanton induced string field theory effective action, which suggests the vanishing of contributions from worldsheet topologies of negative Euler number.

Ref HAL: hal-05184942_v1
Ref Arxiv: 2507.16908
Ref INSPIRE: 2953431
DOI: 10.48550/arXiv.2507.16908
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Refined BPS indices give rise to a quantum Riemann-Hilbert problem that is inherently related to a non-commutative deformation of moduli spaces arising in gauge and string theory compactifications. We reformulate this problem in terms of a non-commutative deformation of a TBA-like equation and obtain its formal solution as an expansion in refined indices. As an application of this construction, we derive a generating function of solutions of the TBA equation in the unrefined case.

Ref HAL: hal-05136598_v1
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The normalization of multi-instanton effects in two-dimensional string theory involves numerical factors that are hard to determine. I'll show how they can be found using a chiral representation of the dual matrix quantum mechanics.The result is confirmed by a string field theory calculation of annuli amplitudes and nicely agrees with a median resummation of the Gamma function.

Ref HAL: hal-05060199_v1
Ref Arxiv: 2505.02572
DOI: 10.3390/e27070719
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Mock modular forms, first invented by Ramanujan, provide a beautiful generalization of the usual modular forms. In recent years, it was found that they capture generating functions of the number of microstates of BPS black holes appearing in compactifications of string theory with 8 and 16 supercharges. This review describes these results and their applications which range from the actual computation of these generating functions for both compact and non-compact compactification manifolds (encoding, respectively, Donaldson-Thomas and Vafa-Witten topological invariants) to the construction of new non-commutative structures on moduli spaces of Calabi-Yau threefolds.