arXiv subjects

Djuna Croon

Publications and source records attributed to Djuna Croon.

At least 37 records · Page 2Linked to original sources

arXiv2024

Cosmology and Astrophysics of CP-Violating Axions

We study the cosmology and astrophysics of axion-like particles (ALPs) with CP-violating Yukawa couplings to nucleons. At finite nucleon density, the ALP's dynamics is governed by an effective potential which is the sum of the bare periodic potential and a linear potential whose strength depends on the nucleon density. We identify a critical nucleon density $ρ_c$ controlling the dynamics. At densities smaller than $ρ_c$ the effective potential is a tilted sinusoidal curve and the field is displaced from its zero-density minimum. At densities larger than $ρ_c$ the minima (and maxima) are absent, and the ALP is destabilized. Astrophysically, this implies that neutron stars can source a radial ALP field, providing a complementary probe to equivalence principle tests. Cosmologically, the ALP may have been destabilized in the early Universe and could have made large field excursions. We discuss model-building applications of our results for such early universe scenarios.

hep-ph↗

arXiv2024

Dark Matter Annihilation and Pair-Instability Supernovae

We study the evolution of heavy stars ($M\ge40{\rm M}_\odot$) undergoing pair-instability in the presence of annihilating dark matter. Focusing on the scenario where the dark matter is in capture-annihilation equilibrium, we model the profile of energy injections in the local thermal equilibrium approximation. We find that significant changes to masses of astrophysical black holes formed by (pulsational) pair-instability supernovae can occur when the ambient dark matter density $ ρ_{\rm DM} \gtrsim10^9 \rm \, GeV \, cm^{-3}$. There are two distinct outcomes, depending on the dark matter mass. For masses $m_{\rm DM}\gtrsim1$ GeV the DM is primarily confined to the core. The annihilation increases the lifetime of core helium burning, resulting in more oxygen being formed, fueling a more violent explosion during the pair-instability-induced contraction. This drives stronger pulsations, leading to lighter black holes being formed than predicted by the standard model. For masses $m_{\rm DM}\lesssim0.5$ GeV there is significant dark matter in the envelope, leading to a phase where the star is supported by the energy from the annihilation. This reduces the core temperature and density, allowing the star to evade the pair-instability allowing heavier black holes to be formed. We find a mass gap for all models studied.

astro-ph.HE↗

arXiv2024

Multi-Generational Black Hole Population Analysis with an Astrophysically Informed Mass Function

We analyze the population statistics of black holes in the LIGO/Virgo/KAGRA GWTC-3 catalog using a parametric mass function derived from simulations of massive stars experiencing pulsational pair-instability supernovae (PPISN). Our formalism enables us to separate the black hole mass function into sub-populations corresponding to mergers between objects formed via different astrophysical pathways, allowing us to infer the properties of black holes formed from stellar collapse and black holes formed via prior mergers separately. Applying this formalism, we find that this model fits the data better than the powerlaw+peak model with Bayes factor $ 9.7\pm0.1$. We measure the location of the lower edge of the upper black hole mass gap to be $M_{\rm BHMG}=84.05_{-12.88}^{+17.19}{\rm M}_{\odot}$, providing evidence that the $35{\rm M}_{\odot}$ Gaussian peak detected in the data using other models is not associated with the PPISN pile-up predicted to precede this gap. Incorporating spin, we find that the normalized spins of stellar remnant black holes are close to zero while those of higher generation black holes tend to larger values. All of these results are in accordance with the predictions of stellar structure theory and black hole merger scenarios. Finally, we combine our mass function with the spectral siren method for measuring the Hubble constant to find $H_0=36.19_{-10.91}^{17.50}$ km/s/Mpc and discuss potential explanations of this low value. Our results demonstrate how astrophysically-informed mass functions can facilitate the interpretation of gravitational wave catalog data to provide information about black hole formation and cosmology. Future data releases will improve the precision of our measurements.

astro-ph.HE↗

arXiv2023

Prediction of Multiple Features in the Black Hole Mass Function due to Pulsational Pair-Instability Supernovae

Using high-resolution simulations of black hole formation from the direct collapse of massive stars undergoing pulsational pair-instability supernovae (PPISN), we find a new phenomenon which significantly affects the explosion and leads to two peaks in the resulting black hole mass function (BHMF). Lighter stars experiencing the pair-instability can form a narrow shell in which alpha ladder reactions take place, exacerbating the effect of the PPISN. The shell temperature in higher mass stars ($>62 {\rm M}_\odot $ at the onset of helium burning for population-III stars with metallicity $Z=10^{-5}$) is too low for this to occur. As a result, the spectrum of black holes $M_{\rm BH} (M_i)$ exhibits a shoulder feature whereby a large range of initial masses result in near-identical black hole masses. PPISN therefore predict two peaks in the mass function of astrophysical black holes -- one corresponding to the location of the upper black hole mass gap and a second corresponding to the location of the shoulder. This shoulder effect may explain the peak at $35_{-2.9}^{+1.7}{\rm M}_\odot$ in the LIGO/Virgo/KAGRA GWTC-3 catalog of merging binary black holes.

astro-ph.HE↗

arXiv2023

TASI lectures on Phase Transitions, Baryogenesis, and Gravitational Waves

These lectures, presented at the 2022 TASI summer school, give an introductory overview of first-order phase transitions in the early Universe, baryogenesis, and the resulting gravitational wave phenomenology. We introduce thermal field theory via the imaginary time formalism, and comment on the pitfalls of 1-loop calculations and alternative approaches. Then, we discuss how to calculate the false vacuum decay rate in first order phase transitions, of which we give various examples in theories beyond the Standard Model. Baryogenesis is presented via the Sakharov conditions, and how they are met in important classes of examples. Finally, we explore gravitational waves from the early Universe, first reviewing the basics of gravitational wave generation and then focusing on the specific example of first order phase transitions.

hep-ph↗

arXiv2023

Light Axion Emission and the Formation of Merging Binary Black Holes

We study the impact of stellar cooling due to light axion emission on the formation and evolution of black hole binaries, via stable mass transfer and the common envelope scenario. We find that in the presence of light axion emission, no binary black hole mergers are formed with black holes in the lower mass gap ($\rm M_{\rm BH} < 4 M_\odot $) via the common envelope formation channel. In some systems, this happens because axions prevent Roche lobe overflow. In others, they prevent the common envelope from being ejected. Our results apply to axions with couplings $ g_{a γ} \gtrsim 10^{-10}\, \rm GeV^{-1}$ (to photons) or $α_{ae} \gtrsim 10^{-26} $ (to electrons) and masses $ m_a \ll 10 \, \rm keV$. Light, weakly coupled particles may therefore apparently produce a mass gap $\rm 2 M_\odot < M_{\rm BH} < 4 M_\odot $ in the LIGO/Virgo/KAGRA data, when no mass gap is present in the stellar remnant population.

astro-ph.HE↗

arXiv2023

Non-perturbative methods for false vacuum decay

We propose a simple non-perturbative formalism for false vacuum decay using functional methods. We introduce the quasi-stationary effective action, a bounce action that non-perturbatively incorporates radiative corrections and is robust to strong couplings. The quasi-stationary effective action obeys an exact flow equation in a modified functional renormalization group with a motivated regulator functional. We demonstrate the use of this formalism in a simple toy model and compare our result with that obtained in perturbation theory.

hep-th↗

arXiv2023

The Postdoc Accord in Theoretical High Energy Physics

We present the results of a survey meant to assess the opinion of the high-energy physics theory (HET) community on the January 7th postdoc acceptance deadline - specifically, whether there is a preference to shift the deadline to later in January or February. This survey, which served for information-gathering purpose only, is part of a community conversation on the optimal timing of an acceptance deadline and whether the community would be better served by a later date. In addition, we present an analysis of data from the postdoc Rumor Mill, which gives a picture of the current hiring landscape in the field. We emphasize the importance of preserving a universal deadline, and the current results of our survey show broad support for a shift to a later date. A link to the survey, frequently asked questions, a running list of supporters, and next steps can be found on our companion web page.

hep-ph↗

arXiv2023

Gravitational wave constraints on extended dark matter structures

We generalise existing constraints on primordial black holes to dark objects with extended sizes using the aLIGO design sensitivity. We show that LIGO is sensitive to dark objects with radius $O(10-10^3~{\rm km})$ if they make up more than $\sim O(10^{-2}-10^{-3})$ of dark matter.

astro-ph.CO↗

arXiv2022

Tidal Love Numbers of Novel and Admixed Celestial Objects

A sub-fraction of dark matter or new particles trapped inside celestial objects can significantly alter their macroscopic properties. We investigate the new physics imprint on celestial objects by using a generic framework to solve the Tolman-Oppenheimer-Volkoff (TOV) equations for up to two fluids. We test the impact of populations of new particles on celestial objects, including the sensitivity to self-interaction sizes, new particle mass, and net population mass. Applying our setup to neutron stars and boson stars, we find rich phenomenology for a range of these parameters, including the creation of extended atmospheres. These atmospheres are detectable by their impact on the tidal love number, which can be measured at upcoming gravitational wave experiments such as Advanced LIGO, the Einstein Telescope, and LISA. We release our calculation framework as a publicly available code, allowing the TOV equations to be generically solved for arbitrary new physics models in novel and admixed celestial objects.

gr-qc↗

arXiv2022

Report of the Topical Group on Cosmic Probes of Dark Matter for Snowmass 2021

Cosmological and astrophysical observations currently provide the only robust, positive evidence for dark matter. Cosmic probes of dark matter, which seek to determine the fundamental properties of dark matter through observations of the cosmos, have emerged as a promising means to reveal the nature of dark matter. This report summarizes the current status and future potential of cosmic probes to inform our understanding of the fundamental nature of dark matter in the coming decade.

hep-ph↗

arXiv2022

Dark Matter In Extreme Astrophysical Environments

Exploring dark matter via observations of extreme astrophysical environments -- defined here as heavy compact objects such as white dwarfs, neutron stars, and black holes, as well as supernovae and compact object merger events -- has been a major field of growth since the last Snowmass process. Theoretical work has highlighted the utility of current and near-future observatories to constrain novel dark matter parameter space across the full mass range. This includes gravitational wave instruments and observatories spanning the electromagnetic spectrum, from radio to gamma-rays. While recent searches already provide leading sensitivity to various dark matter models, this work also highlights the need for theoretical astrophysics research to better constrain the properties of these extreme astrophysical systems. The unique potential of these search signatures to probe dark matter adds motivation to proposed next-generation astronomical and gravitational wave instruments.

hep-ph↗

arXiv2022

Snowmass Theory Frontier: Astrophysics and Cosmology

We summarize progress made in theoretical astrophysics and cosmology over the past decade and areas of interest for the coming decade. This Report is prepared as the TF09 "Astrophysics and Cosmology" topical group summary for the Theory Frontier as part of the Snowmass 2021 process.

hep-ph↗

arXiv2022

Early-Universe Model Building

Theoretical investigations into the evolution of the early universe are an essential part of particle physics that allow us to identify viable extensions to the Standard Model as well as motivated parameter space that can be probed by various experiments and observations. In this white paper, we review particle physics models of the early universe. First, we outline various models that explain two essential ingredients of the early universe (dark matter and baryon asymmetry) and those that seek to address current observational anomalies. We then discuss dynamics of the early universe in models of neutrino masses, axions, and several solutions to the electroweak hierarchy problem. Finally, we review solutions to naturalness problems of the Standard Model that employ cosmological dynamics.

hep-ph↗

arXiv2022

Archimedean Lever Leptogenesis

We propose that weak scale leptogenesis via $\sim 10$ TeV scale right-handed neutrinos could be possible if their couplings had transitory larger values in the early Universe. The requisite lifted parameters can be attained if a light scalar $ϕ$ is displaced a long distance from its origin by the thermal population of fermions $X$ that become massive before electroweak symmetry breaking. The fermion $X$ can be a viable dark matter candidate; for suitable choice of parameters, the light scalar itself can be dark matter through a misalignment mechanism. We find that a two-component DM population made up of both $X$ and $ϕ$ is a typical outcome in our framework.

hep-ph↗

arXiv2022

Machine learning a manifold

We propose a simple method to identify a continuous Lie algebra symmetry in a dataset through regression by an artificial neural network. Our proposal takes advantage of the $ \mathcal{O}(ε^2)$ scaling of the output variable under infinitesimal symmetry transformations on the input variables. As symmetry transformations are generated post-training, the methodology does not rely on sampling of the full representation space or binning of the dataset, and the possibility of false identification is minimised. We demonstrate our method in the SU(3)-symmetric (non-) linear $Σ$ model.

hep-ph↗

arXiv2022

Axion Instability Supernovae

New particles coupled to the Standard Model can equilibrate in stellar cores if they are sufficiently heavy and strongly coupled. In this work, we investigate the astrophysical consequences of such a scenario for massive stars by incorporating new contributions to the equation of state into a state of the art stellar structure code. We focus on axions in the "cosmological triangle", a region of parameter space with $300{\rm\,keV} \lesssim m_a \lesssim 2$ MeV, $g_{aγγ}\sim 10^{-5}$ GeV$^{-1}$ that is not presently excluded by other considerations. We find that for axion masses $m_a \sim m_e $, axion production in the core drives a new stellar instability that results in explosive nuclear burning that either drives a series of mass-shedding pulsations or completely disrupts the star resulting in a new type of optical transient -- an \textit{Axion Instability Supernova}. We predict that the upper black hole mass gap would be located at $37{\rm M}_\odot \le M\le 107{\rm M}_\odot$ in these theories, a large shift down from the standard prediction, which is disfavored by the detection of the mass gap in the LIGO/Virgo/KAGRA GWTC-2 gravitational wave catalog beginning at $46_{-6}^{+17}{\rm M}_\odot$. Furthermore, axion-instability supernovae are more common than pair-instability supernovae, making them excellent candidate targets for JWST. The methods presented in this work can be used to investigate the astrophysical consequences of any theory of new physics that contains heavy bosonic particles of arbitrary spin. We provide the tools to facilitate such studies.

hep-ph↗

arXiv2022

Cosmology with the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational wave observations by LISA to probe the universe.

astro-ph.CO↗