arXiv subjects

Samuel D. McDermott

Publications and source records attributed to Samuel D. McDermott.

At least 19 recordsLinked to original sources

arXiv2023

A Phantom Menace: On the Morphology of the Galactic Center Excess

The characteristics of the Galactic Center Excess (GCE) emission observed in gamma-ray energies -- especially the morphology of the GCE -- remain a hotly debated subject. The manner in which the dominant diffuse gamma-ray background is modeled has been claimed to have a determining effect on the preferred morphology. In this work, we compare two distinct approaches to the galactic diffuse gamma-ray emission background: the first approach models this emission through templates calculated from a sequence of well-defined astrophysical assumptions, while the second approach divides surrogates for the background gamma-ray emission into cylindrical galactocentric rings with free independent normalizations. At the latitudes that we focus on, we find that the former approach works better, and that the overall best fit is obtained for an astrophysically motivated fit when the GCE follows the morphology expected of dark matter annihilation. Quantitatively, the improvement compared to the best ring-based fits is roughly 6500 in the chi^2 and roughly 4000 in the log of the Bayesian evidence.

astro-ph.HE↗

arXiv2023

WavPool: A New Block for Deep Neural Networks

Modern deep neural networks comprise many operational layers, such as dense or convolutional layers, which are often collected into blocks. In this work, we introduce a new, wavelet-transform-based network architecture that we call the multi-resolution perceptron: by adding a pooling layer, we create a new network block, the WavPool. The first step of the multi-resolution perceptron is transforming the data into its multi-resolution decomposition form by convolving the input data with filters of fixed coefficients but increasing size. Following image processing techniques, we are able to make scale and spatial information simultaneously accessible to the network without increasing the size of the data vector. WavPool outperforms a similar multilayer perceptron while using fewer parameters, and outperforms a comparable convolutional neural network by ~ 10% on relative accuracy on CIFAR-10.

cs.LG↗

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

Neutron Star Quantum Death by Small Black Holes

Neutron stars can be destroyed by black holes at their center accreting material and eventually swallowing the entire star. Here we note that the accretion model adopted in the literature, based on Bondi accretion or variations thereof, is inadequate for small black holes -- black holes whose Schwarzschild radius is comparable to, or smaller than, the neutron's de Broglie wavelength. In this case, quantum mechanical aspects of the accretion process cannot be neglected, and give rise to a completely different accretion rate. We show that for the case of black holes seeded by the collapse of bosonic dark matter, this is the case for electroweak-scale dark matter particles. In the case of fermionic dark matter, typically the black holes that would form at the center of a neutron star are more massive, unless the dark matter particle mass is very large, larger than about 10$^{10}$ GeV. We calculate the lifetime of neutron stars harboring a ``small'' black hole, and find that black holes lighter than $\sim 10^{11}$ kg quickly evaporate, leaving no trace. More massive black holes destroy neutron stars via quantum accretion on time-scales much shorter than the age of observed neutron stars. We find that the range where seed black holes inside neutron stars are massive enough that they do not quickly evaporate away, but not so massive that a fluid accretion picture is warranted is limited to between $\sim10^{11}$ and $10^{12}$ kg, but our results are key to accurately determine the actual critical black hole mass corresponding to the onset of neutron star destruction

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

The Return of the Templates: Revisiting the Galactic Center Excess with Multi-Messenger Observations

The Galactic center excess (GCE) remains one of the most intriguing discoveries from the Fermi Large Area Telescope (LAT) observations. We revisit the characteristics of the GCE by first producing a new set of high-resolution galactic diffuse gamma-ray emission templates, which are ultimately due to cosmic-ray interactions with the interstellar medium. Using multi-messenger observations we constrain the properties of the galactic diffuse emission. The broad properties of the GCE that we find in this work are qualitatively unchanged despite the introduction of this new set of templates, though its quantitative features appear mildly different than those obtained in previous analyses. In particular, we find a high-energy tail at higher significance than previously reported. This tail is very prominent in the northern hemisphere, and less so in the southern hemisphere. This strongly affects one prominent interpretation of the excess: known millisecond pulsars are incapable of producing this high-energy emission, even in the relatively softer southern hemisphere, and are therefore disfavored as the sole explanation of the GCE. The annihilation of dark matter particles of mass $40^{+10}_{-7}$ GeV (95$\%$ CL) to $b$ quarks with a cross-section of $σv = 1.4^{+0.6}_{-0.3} \times 10^{-26}$ cm$^{3}$s$^{-1}$ provides a good fit to the excess especially in the relatively cleaner southern sky. Dark matter of the same mass range annihilating to $b$ quarks or heavier dark matter particles annihilating to heavier Standard Model bosons can combine with millisecond pulsars to provide a good fit to the southern hemisphere emission. As part of this paper, we make publicly available all of our templates and the data covariance matrix we have generated to account for systematic uncertainties.[abridged]

astro-ph.HE↗

arXiv2022

Astrophysical and Cosmological Probes of Dark Matter

While astrophysical and cosmological probes provide a remarkably precise and consistent picture of the quantity and general properties of dark matter, its fundamental nature remains one of the most significant open questions in physics. Obtaining a more comprehensive understanding of dark matter within the next decade will require overcoming a number of theoretical challenges: the groundwork for these strides is being laid now, yet much remains to be done. Chief among the upcoming challenges is establishing the theoretical foundation needed to harness the full potential of new observables in the astrophysical and cosmological domains, spanning the early Universe to the inner portions of galaxies and the stars therein. Identifying the nature of dark matter will also entail repurposing and implementing a wide range of theoretical techniques from outside the typical toolkit of astrophysics, ranging from effective field theory to the dramatically evolving world of machine learning and artificial-intelligence-based statistical inference. Through this work, the theory frontier will be at the heart of dark matter discoveries in the upcoming decade.

hep-ph↗

arXiv2021

The Milky Way, Coming into Focus: Precision Astrometry Probes its Evolution, and its Dark Matter

The growing trove of precision astrometric observations from the Gaia space telescope and other surveys is revealing the structure and dynamics of the Milky Way in ever more exquisite detail. We summarize the current status of our understanding of the structure and the characteristics of the Milky Way, and we review the emerging picture: the Milky Way is evolving through interactions with the massive satellite galaxies that stud its volume, with evidence pointing to a cataclysmic past. It is also woven with stellar streams, and observations of streams, satellites, and field stars offer new constraints on its dark matter, both on its spatial distribution and its fundamental nature. The recent years have brought much focus to the study of dwarf galaxies found within our Galaxy's halo and their internal matter distributions. In this review, we focus on the predictions of the cold dark matter paradigm at small mass scales through precision astrometric measurements, and we summarize the modern consensus on the extent to which small-scale probes are consistent with this paradigm. We note the discovery prospects of these studies, and also how they intertwine with probes of the dynamics and evolution of the Milky Way in various and distinct ways.

astro-ph.GA↗

arXiv2021

Dark Matter Daily Modulation With Anisotropic Organic Crystals

Aromatic organic compounds, because of their small excitation energies ~ O(few eV) and scintillating properties, are promising targets for detecting dark matter of mass ~ O(few MeV). Additionally, their planar molecular structures lead to large anisotropies in the electronic wavefunctions, yielding a significant daily modulation in the event rate expected to be observed in crystals of these molecules. We characterize the daily modulation rate of dark matter interacting with an anisotropic scintillating organic crystal such as trans-stilbene, and show that daily modulation is an ~ O(1) fraction of the total rate for small DM masses and comparable to, or larger than, the ~ 10% annual modulation fraction at large DM masses. As we discuss in detail, this modulation provides significant leverage for detecting or excluding dark matter scattering, even in the presence of a non-negligible background rate. Assuming a non-modulating background rate of 1/min/kg that scales with total exposure, we find that a 100 kg yr experiment is sensitive to the cross section corresponding to the correct relic density for dark matter masses between 1.3-14 MeV (1.5-1000 MeV) if dark matter interacts via a heavy (light) mediator. This modulation can be understood using an effective velocity scale v* = Delta E/q*, where Delta E is the electronic transition energy and q* is a characteristic momentum scale of the electronic orbitals. We also characterize promising future directions for development of scintillating organic crystals as dark matter detectors.

hep-ph↗

arXiv2021

Cosmologically Degenerate Fermions

Dark matter (DM) with a mass below a few keV must have a phase space distribution that differs substantially from the Standard Model particle thermal phase space: otherwise, it will free stream out of cosmic structures as they form. We observe that fermionic DM psi in this mass range will have a non-negligible momentum in the early Universe, even in the total absence of thermal kinetic energy. This is because the fermions were inevitably more dense at higher redshifts, and thus experienced Pauli degeneracy pressure. They fill up the lowest-momentum states, such that a typical fermion gains a momentum ~ O(p_F) that can exceed its mass m_psi. We find a simple relation between m_psi, the current fraction f_psi of the cold DM energy density in light fermions, and the redshift at which they were relativistic. Considering the impacts of the transition between nonrelativistic and relativistic behavior as revealed by measurements of DNeff and the matter power spectrum, we derive qualitatively new bounds in the f_psi-m_psi plane. We also improve existing bounds for f_psi = 1 by an order of magnitude to m_psi=2 keV. We remark on implications for direct detection and suggest models of dark sectors that may give rise to cosmologically degenerate fermions.

hep-ph↗

arXiv2021

Find the Gap: Black Hole Population Analysis with an Astrophysically Motivated Mass Function

We introduce a novel black hole mass function which realistically models the physics of star formation and pair instability supernova with a minimal number of parameters. Applying this to all events in the LIGO-Virgo GWTC-2 catalog, we detect a peak at M_BHMG = 74.8^{+4.3}_{-8.0} MS, followed by a break in the mass function. Repeating the analysis without the black holes from the event GW190521, we find this feature at M_BHMG = 55.4^{+3.0}_{-6.1} MS. The latter result establishes the edge of the anticipated "black hole mass gap" at a value compatible with the expectation from standard stellar structure theory, while the former result is ~ 20MS higher, which would have far-reaching implications if confirmed. Using Bayesian techniques, we establish that our mass function fits a new catalog of black hole masses approximately as well as the pre-existing phenomenological mass functions. We also remark on the implications of these results for constraining or discovering new phenomena in nuclear and particle physics.

astro-ph.CO↗

arXiv2021

Late-Time Dark Matter Oscillations and the Core-Cusp Problem

The core-cusp problem persists as an unresolved tension between the predictions of $Λ$CDM cosmology and observations of dark matter (DM) profiles in dwarf spheroidal and other galaxies. We present a novel scenario for converting cusps into cores through reactivation of DM annihilation in galaxies at late times. This can happen in asymmetric DM models when there is a very small DM-number violating mass term that causes oscillations between DM and its antiparticle. Using analytic methods as well as gravitational N-body simulations, we show that this mechanism can robustly eliminate cusps from galactic DM profiles for light fermionic DM of mass $m_χ\sim (0.1-1)$ GeV and a lighter mediator into which the DM can annihilate. We identify regions of parameter space where annihilation of DM particles is more efficient than elastic scattering at reducing the inner density of the DM profile. Dark matter annihilation is therefore a qualitatively distinct alternative to the mechanism of elastic self-interacting dark matter for addressing the cusp-core problem.

hep-ph↗

arXiv2020

Beyond the Standard Model Explanations of GW190521

The LIGO/Virgo collaboration has recently announced the detection of a heavy binary black hole merger, with component masses that cannot be explained by standard stellar structure theory. In this letter we propose several explanations based on models of new physics, including new light particle losses, modified gravity, large extra dimensions, and a small magnetic moment of the neutrino. Each of these affect the physics of the pair-instability differently, leading to novel mechanisms for forming black holes inside the black hole mass gap.

gr-qc↗

arXiv2020

Missing in Action: New Physics and the Black Hole Mass Gap

We demonstrate the power of the black hole mass gap as a novel probe of fundamental physics. New light particles that couple to the Standard Model can act as an additional source of energy loss in the cores of population-III stars, dramatically altering their evolution. We investigate the effects of two paradigmatic weakly coupled, low-mass particles, axions and hidden photons, and find that the pulsational pair instability, which causes a substantial amount of mass loss, is suppressed. As a result, it is possible to form black holes of $72\msun$ or heavier, deep inside the black hole mass gap predicted by the Standard Model. The upper edge of the mass gap is raised to $>130{\rm M}_\odot$, implying that heavier black holes, anticipated to be observed after LIGO's sensitivity is upgraded, would also be impacted. In contrast, thermally produced heavy particles would remain in the core, leading to the tantalizing possibility that they drive a new instability akin to the electron-positron pair instability. We investigate this effect analytically and find that stars that avoid the electron-positron pair instability could experience this new instability. We discuss our results in light of current and upcoming gravitational wave interferometer detections of binary black hole mergers.

gr-qc↗

arXiv2020

Missing in Axion: where are XENON1T's big black holes?

We pioneer the black hole mass gap as a powerful new tool for constraining new particles. A new particle that couples to the Standard Model---such as an axion---acts as an additional source of loss in the cores of population-III stars, suppressing mass lost due to winds and quenching the pair-instability. This results in heavier astrophysical black holes. As an example, using stellar simulations we show that the solar axion explanation of the recent XENON1T excess implies astrophysical black holes of ~ 56 MS, squarely within the black hole mass gap predicted by the Standard Model.

hep-ph↗

arXiv2020

Supernova Muons: New Constraints on Z' Bosons, Axions, and ALPs

New light particles produced in supernovae can lead to additional energy loss and a consequent deficit in neutrino production in conflict with the neutrinos observed from Supernova 1987A (SN1987A). Contrary to the majority of previous SN1987A studies, we examine the impact of $Z'$ bosons, axions, and axion-like particles (ALPs) interacting with the muons produced in SN1987A. For the first time, we find constraints on generic $Z'$ bosons coupled to muons, and apply our results to particle models including gauged $L_μ-L_τ$ number, $U(1)_{L_μ-L_τ}$, and gauged $B-L$ number, $U(1)_{B-L}$. We constrain $Z'$ bosons with masses up to about 250-500 MeV, and down to about $10^{-9}$ in $Z'$-muon coupling. We also extend previous work on axion-muon couplings by examining the importance of loop-level interactions, as well as performing calculations over a wider range of axion masses. We constrain muon-coupled axions from arbitrarily low masses up to about 200-500 MeV, with bounds extending down to axion-muon couplings of approximately $10^{-8}$ GeV$^{-1}$. We conclude that supernovae broadly provide a sensitive probe of new lightly-coupled particles interacting with muons.

hep-ph↗

arXiv2020

Dark Photon Dark Matter in the Presence of Inhomogeneous Structure

Dark photon dark matter will resonantly convert into visible photons when the dark photon mass is equal to the plasma frequency of the ambient medium. In cosmological contexts, this transition leads to an extremely efficient, albeit short-lived, heating of the surrounding gas. Existing work in this field has been predominantly focused on understanding the implications of these resonant transitions in the limit that the plasma frequency of the Universe can be treated as being perfectly homogeneous, \ie neglecting inhomogeneities in the electron number density. In this work we focus on the implications of heating from dark photon dark matter in the presence of inhomogeneous structure (which is particularly relevant for dark photons with masses in the range $10^{-15} \, {\rm eV} \, \lesssim m_{A^\prime} \lesssim 10^{-12}$ eV), emphasizing both the importance of inhomogeneous energy injection, as well as the sensitivity of cosmological observations to the inhomogeneities themselves. More specifically, we derive modified constraints on dark photon dark matter from the Ly-$α$ forest, and show that the presence of inhomogeneities allows one to extend constraints to masses outside of the range that would be obtainable in the homogeneous limit, while only slightly relaxing their strength. We then project sensitivity for near-future cosmological surveys that are hoping to measure the 21cm transition in neutral hydrogen prior to reionization, and demonstrate that these experiments will be extremely useful in improving sensitivity to masses near $\sim 10^{-14}$ eV, potentially by several orders of magnitude. Finally, we discuss implications for reionization, early star formation, and late-time $y$-type spectral distortions, and show that probes which are inherently sensitive to the inhomogeneous state of the Universe could resolve signatures unique to the light dark photon...

astro-ph.CO↗

arXiv2020

Constraints on Primordial Black Holes From Big Bang Nucleosynthesis Revisited

As space expands, the energy density in black holes increases relative to that of radiation, providing us with motivation to consider scenarios in which the early universe contained a significant abundance of such objects. In this study, we revisit the constraints on primordial black holes derived from measurements of the light element abundances. Black holes and their Hawking evaporation products can impact the era of Big Bang Nucleosynthesis (BBN) by altering the rate of expansion at the time of neutron-proton freeze-out, as well as by radiating mesons which can convert protons into neutrons and vice versa. Such black holes can thus enhance the primordial neutron-to-proton ratio, and increase the amount of helium that is ultimately produced. Additionally, the products of Hawking evaporation can break up helium nuclei, which both reduces the helium abundance and increases the abundance of primordial deuterium. Building upon previous work, we make use of modern deuterium and helium measurements to derive stringent constraints on black holes which evaporate in $t_{\rm evap} \sim 10^{-1}$ s to $\sim 10^{13}$ s (corresponding to $M \sim 6\times 10^8$ g to $\sim 2 \times 10^{13}$ g, assuming Standard Model particle content). We also consider how physics beyond the Standard Model could impact these constraints. Due to the gravitational nature of Hawking evaporation, the rate at which a black hole evaporates, and the types of particles that are produced through this process, depend on the complete particle spectrum. Within this context, we discuss scenarios which feature a large number of decoupled degrees-of-freedom (\ie~large hidden sectors), as well as models of TeV-scale supersymmetry.

astro-ph.CO↗