arXiv subjects

Zachary S. C. Picker

Publications and source records attributed to Zachary S. C. Picker.

18 recordsLinked to original sources

arXiv2026

Mineral Detection of Neutrinos and Dark Matter 2026 Proceedings

The fourth "Mineral Detection of Neutrinos and Dark Matter" (MDvDM'26) meeting was held April 14-17, 2026 in Karlsruhe, Germany, hosted by the Institute for Astroparticle Physics (IAP) at Karlsruhe Institute of Technology (KIT). These proceedings detail the contributions that were presented during MDvDM'26, illustrating the unprecedented progress in theoretical, computational and experimental studies towards the realization of the concept of mineral detectors. Mineral detectors represent an emerging particle detection concept that has risen in prominence in recent years due to the advent of modern computational and high-resolution microscopy techniques. Natural and synthetic crystals are capable of retaining microscopic damage features induced by nuclear recoils, which could be then read out with a variety of micrometer and nanometer resolution microscopy techniques. On laboratory time scales mineral detectors could be employed for reactor neutrino monitoring and dark matter detection, with the potential to measure the directions as well as the energies of the induced nuclear recoils. Uniquely, ancient natural crystals (so-called paleo-detectors) that have been recording nuclear recoils over geological timescales could be used for studying astrophysical neutrinos, cosmic rays, dark matter and heavy exotic particles, as well as the variation of their fluxes over our Galaxy's lifetime. In recent years the international MDvDM community has been successfully tackling the challenges associated with realizing the concept of mineral detectors, opening the pathway towards a fully fledged experimental program and potential future discoveries.

physics.ins-det↗

arXiv2026

Macroscopic dark matter constraints for extended mass functions

We compile and recompute a subset of the landscape of constraints for macroscopic dark matter (macros), explicitly including the dependence on the fraction of dark matter in macros $f_{\rm dm}$. This allows us to map these constraints to scenarios with extended mass functions for the macros, which is a generic outcome for many of the assembly processes relevant to macro formation. We compute a number of examples of constraints for extended mass functions, finding that in many cases, wider mass functions can be severely constrained by just the interactions of the tails of the distributions. Finally, we present the code and data required for computing these limits and generating these plots in a publicly accessible repository.

hep-ph↗

arXiv2026

Neutrino masses, matter-antimatter asymmetry, dark matter, and supermassive black hole formation explained with Majorons

The spontaneous breaking of a global lepton number symmetry can result in a (pseudo) Nambu-Goldstone boson known as the Majoron. We study a singlet Majoron model that couples to two Higgs doublets in which the lepton number current develops an electromagnetic anomaly, allowing the decay of Majorons into photons. We focus on Majorons at the eV scale with an enhanced anomaly and show that it serves as a dark matter candidate whose decay signals can be probed by space telescope observations. Furthermore, if the decay produces Lyman-Werner photons, heavy black hole seeds can be generated via the direct collapse mechanism and evolve into the active galactic nuclei we observe at high redshifts. Our framework thus simultaneously addresses the origin of neutrino masses, the baryon asymmetry of the Universe, the nature of dark matter, and the formation of high redshift supermassive black holes.

hep-ph↗

arXiv2025

When Tiny Halos Stir Spacetime: Gravitational Waves from Fifth-Force Mergers

Dark matter fermions interacting via attractive fifth forces mediated by a light mediator can form dark matter halos in the very early universe. We show that bound systems composed of these halos are capable of generating gravitational wave (GW) signals detectable today, even when the individual halos are very light. The Yukawa force dominates the dynamics of these halo binaries, rather than gravity. As a result, large GW signals can be produced at initially extremely high frequencies, which are then redshifted to frequency bands accessible to current or future GW observatories. In addition, the resulting GW signals carry distinctive features that enable future observations to distinguish them from conventional ones. Notably, even if only a tiny fraction of dark matter experiences strong fifth-force interactions, such effects provide a new avenue to discover self-interacting dark matter through GW observations.

astro-ph.CO↗

arXiv2025

A dark matter hail: Detecting macroscopic dark matter with asteroids, planetary rings, and craters

Dark matter could be composed of macroscopic objects with large masses and geometric cross-sections spanning many decades. We investigate the potential interaction of such `stuff-sized' dark matter by considering its interactions with asteroids, planetary rings, and terrestrial bodies. This hail of dark matter could catastrophically destroy these Solar System objects, evaporate them from their orbits, or cause substantial cratering. We estimate these effects and use them to place competitive bounds on a wide, previously-unconstrained swathe of the dark matter parameter space.

astro-ph.CO↗

arXiv2025

Black Holes from Fermi Ball Collapse

Fermi balls are non-topological solitons that can naturally form in an early universe containing a dark sector with heavy fermions and an attractive interaction mediated by a light scalar field. We compute the Fermi ball mass and radius scaling relations when the potential of the scalar field $φ$ has a non-negligible quartic coupling $λφ^4$. The resulting Fermi balls reach `saturation' very rapidly, even when their radius is much smaller than the effective Yukawa force range. These objects can therefore grow by mergers or by accretion of ambient dark fermions, until they become so dense that they fall within their Schwarzschild radius and collapse to black holes. This setup, therefore, provides an example of a rather natural and economical dark sector scenario for the formation of primordial black holes.

astro-ph.CO↗

arXiv2024

Direct-collapse supermassive black holes from relic particle decay

We investigate the formation of high-redshift supermassive black holes (SMBHs) via the direct collapse of baryonic clouds, where the unwanted formation of molecular hydrogen is successfully suppressed by a Lyman-Werner (LW) photon background from relic particle decay. We improve on existing studies by dynamically simulating the collapse, accounting for the adiabatic contraction of the DM halo, as well as the in-situ production of the LW photons within the cloud which reduce the impact of the cloud's shielding. We find a viable parameter space where the decay of either some of the dark matter or all of a subdominant decaying species successfully allows direct collapse of the cloud to a SMBH.

astro-ph.GA↗

arXiv2024

Feeding plankton to whales: high-redshift supermassive black holes from tiny black hole explosions

Recent observations of the high-redshift universe have uncovered a significant number of active galactic nuclei, implying that supermassive black holes (SMBHs) would have to have been formed at much earlier times than expected. Direct collapse of metal-free gas clouds to SMBHs after recombination could help explain the early formation of SMBHs, but this scenario is stymied by the fragmentation of the clouds due to efficient molecular hydrogen cooling. We show that a subdominant population of tiny, evaporating primordial black holes, with significant clustering in some gas clouds, can heat the gas sufficiently so that molecular hydrogen is not formed, and direct collapse to to black holes is possible even at high redshifts.

astro-ph.GA↗

arXiv2023

Explaining the GeV excess with exploding black holes

Black holes may form in present-day collapse of microscopic structures of dark matter. We show that, if microstructure black holes (MSBH) with mass $m\sim 10^{13}~g$ are produced, the spectrum of gamma rays from their evaporation agrees remarkably well with the GeV excess observed by Fermi Gamma-ray Space Telescope, while still avoiding all observational constraints. We also discuss the generic requirements for MSBHs to explain the GeV excess.

astro-ph.HE↗

arXiv2023

Constraints on late-forming exploding black holes

Black holes can be produced in collapse of small-scale dark matter structures, which can happen at any time from the early to present-day universe. Microstructure black holes (MSBHs) can have a wide range of masses. Small MSBHs evaporate via Hawking radiation with lifetimes shorter than the age of the universe, but they are not subject to the usual early-universe bounds on the abundance of small primordial black holes. We investigate the possible signal of such a population of exploding, late-forming black holes, constraining their abundance with observations from diffuse extragalactic gamma- and x-ray sources, the galactic center, and dwarf spheroidal galaxies.

astro-ph.CO↗

arXiv2022

Effects of Hawking evaporation on PBH distributions

Primordial black holes (PBHs) may lose mass by Hawking evaporation. For sufficiently small PBHs, they may lose a large portion of their formation mass by today, or evaporate completely if they form with mass $M<M_\mathrm{crit}\sim5\times10^{14}~\mathrm{g}$. We investigate the effect of this mass loss on extended PBH distributions, showing that the shape of the distribution is significantly changed between formation and today. We reconsider the $γ$-ray constraints on PBH dark matter in the Milky Way center with a correctly `evolved' lognormal distribution, and derive a semi-analytic time-dependent distribution which can be used to accurately project monochromatic constraints to extended distribution constraints. We also derive the rate of black hole explosions in the Milky Way per year, finding that although there can be a significant number, it is extremely unlikely to find one close enough to Earth to observe. Along with a more careful argument for why monochromatic PBH distributions are unlikely to source an exploding PBH population today, we (unfortunately) conclude that we are unlikely to witness any PBH explosions.

astro-ph.HE↗

arXiv2021

Apparent horizons of the Thakurta spacetime and the description of cosmological black holes

We discuss the validity of the Thakurta metric to describe cosmological black holes by analysing the nature of its horizon. By adopting the preferred foliation of the Thakurta spacetime associated with the Kodama time, we demonstrate that the Thakurta horizon is indeed a future outer trapping horizon. Therefore, the respective observers see it as a cosmological black hole, contrary to some claims in the literature.

gr-qc↗

arXiv2021

Cosmology of the companion-axion model: dark matter, gravitational waves, and primordial black holes

The companion-axion model introduces a second QCD axion to rescue the Peccei-Quinn solution to the strong-CP problem from the effects of colored gravitational instantons. As in single-axion models, the two axions predicted by the companion-axion model are attractive candidates for dark matter. The model is defined by two free parameters, the scales of the two axions' symmetry breaking, so we can classify production scenarios in terms of the relative sizes of these two scales with respect to the scale of inflation. We study the cosmological production of companion-axion dark matter in order to predict windows of preferred axion masses, and calculate the relative abundances of the two particles. Additionally, we show that the presence of a second axion solves the cosmological domain wall problem automatically in the scenarios in which one or both of the axions are post-inflationary. We also suggest unique cosmological signatures of the companion-axion model, such as the production of a $\sim$10 nHz gravitational wave background, and $\sim 100\,M_\odot$ primordial black holes.

hep-ph↗

arXiv2021

Phenomenology of the companion-axion model: photon couplings

We study the phenomenology of the 'companion-axion model' consisting of two coupled QCD axions. The second axion is required to rescue the Peccei-Quinn solution to the strong-CP problem from the effects of colored gravitational instantons. We investigate here the combined phenomenology of axion-axion and axion-photon interactions, recasting present and future single-axion bounds onto the companion-axion parameter space. Most remarkably, we predict that future axion searches with haloscopes and helioscopes may well discover two QCD axions, perhaps even within the same experiment.

hep-ph↗

arXiv2021

Probing Quadratic Gravity with Binary Inspirals

In this paper, we study gravitational waves generated by binary systems within an extension of General Relativity which is described by the addition of quadratic in curvature tensor terms to the Einstein-Hilbert action. Treating quadratic gravity as an effective theory valid in the low energy/curvature regime, we argue that reliable calculations can be performed in the early inspiral phase, and furthermore, no flux of additional massive waves can be detected. We then compute the massive dipole (-1PN) leading corrections to the post-Newtonian (PN) expansion of the standard waveform. By confronting these theoretical calculations with available experimental data, we constrain both unknown parameters of quadratic gravity to be $0 \leq γ\, \lesssim 5.7\cdot 10^{76}$, and $-\fracγ{4} \leq β\, \lesssim - 4.2\cdot 10^{75}$.

gr-qc↗

arXiv2021

Comment on: Cosmological black holes are not described by the Thakurta metric

Recently, Hütsi et al.[arXiv:2105.09328] critiqued our work that reconsidered the mathematical description of cosmological black holes. In this short comment, we highlight some of the conceptual issues with this criticism in relation to the interpretation of the quasi-local Misner-Sharp mass, and the fact that our description of cosmological black holes does not impose any assumptions about matter accretion.

astro-ph.CO↗

arXiv2021

Navigating the asteroid field: New evaporation constraints for primordial black holes as dark matter

Primordial black holes (PBHs) in the asteroid-mass range $(10^{-17}$--$10^{-12})~M_{\odot}$ almost entirely escape constraints as dark matter candidates. In the early universe, however, the Schwarzschild metric is no longer applicable and we must carefully consider PBH metrics with a proper cosmological embedding. In particular, the Thakurta solution stands out as perhaps the most adequate such solution. Notably, the Thakurta metric contains a time-dependent apparent horizon, proportional to the cosmological scale-factor. We show that this implies the PBHs are hotter than in the standard Schwarzschild case, and so evaporate significantly faster via Hawking radiation. By matter-radiation equality, PBHs up to masses of $\sim 10^{-12}~M_{\odot}$ have totally evaporated, and so cannot be a viable dark matter candidate.

astro-ph.CO↗

arXiv2021

Eliminating the LIGO bounds on primordial black hole dark matter

Primordial black holes (PBHs) in the mass range $(30$--$100)~M_{\odot}$ are interesting candidates for dark matter, as they sit in a narrow window between microlensing and cosmic microwave background constraints. There are however tight constraints from the binary merger rate observed by the LIGO and Virgo experiments. In deriving these constraints, PBHs were treated as point Schwarzschild masses, while the more careful analysis in an expanding universe we present here, leads to a time-dependent mass. This implies a stricter set of conditions for a black hole binary to form and means that black holes coalesce much more quickly than was previously calculated, namely well before the LIGO/Virgo's observed mergers. The observed binaries are those coalescing within galactic halos, with a merger rate consistent with data. This reopens the possibility for dark matter in the form of LIGO-mass PBHs.

astro-ph.CO↗