arXiv subjects

Glennys R. Farrar

Publications and source records attributed to Glennys R. Farrar.

At least 19 recordsLinked to original sources

arXiv2026

The Random Magnetic Field of the Milky Way

We constrain the large-scale isotropic random component of the Galactic magnetic field using the Planck reconstruction of the 408 MHz synchrotron sky. Our analysis simultaneously fits the random-field structure, the synchrotron contributions of the coherent field and local foregrounds, and isotropic and dipolar offsets. A new element of the analysis is the use of excess polarized emission at 30 GHz to model local foregrounds, allowing us to retain 97% of the sky without absorbing these structures into Galaxy-wide features of the magnetic field. Across variations in the coherent-field model, cosmic-ray electron distribution, sky mask, and field profile, we consistently find that the dominant random-field component is a vertically compact disk with a local rms strength of about 4 $μ$G and a 1/e scale height of approximately 1 kpc, substantially thinner than in most previous models. An annular enhancement in the inner Galaxy improves the fit to the data, whereas we find no evidence for either a large-scale spiral pattern or a thick random-field disk. The fits also yield an intensity monopole of 4-7 K, whose possible origin we discuss. We quantify the implications of the inferred random field for the angular smearing of ultrahigh-energy cosmic rays. Compared with previous models, the sky-median smearing angle is smaller by up to a factor of 1.7, and by up to 2.4 in individual directions.

astro-ph.GA↗

arXiv2026

A New Method to Simulate Dark Matter-Baryon Interactions and Application to an Isolated Disk Galaxy

We report on a new method for incorporating interactions between dark matter (DM) and baryons in cosmological simulations, capable of handling the challenging regime in which the dark matter particle mass is comparable to or lighter than the baryon mass. The method hybridizes two distinct approaches: gas particles receive momentum and energy transfer according to a mean-field calculation while DM particles undergo Monte Carlo scatterings. These approaches are derived from the Boltzmann equation and shown to be statistically equivalent. We present an open-source implementation of this method in the simulation code GIZMO. As a first application, we investigate the effects of DM-baryon interactions on an isolated Milky Way-like disk galaxy for dark matter having twice the proton mass, which roughly maximizes the average energy transfer per collision. For cross sections of order 1 barn ($10^{-24}$ cm$^2$), these interactions cause strong changes to the mass distribution in the center of the galaxy in less than 1 Gyr, even when bar formation is suppressed by hand. For cross sections typical of hadronic interactions ($\lesssim 30$ mb), high-fidelity galaxy formation simulations will be needed to assess the effects on observable features of galaxies.

astro-ph.CO↗

arXiv2026

How well is the local Large Scale Structure of the Universe known? CosmicFlows vs. Biteau's Galaxy Catalog with Cloning

Knowledge of the actual density distribution of matter in the local universe is needed for a variety of purposes, for instance as a baseline model for ultrahigh energy cosmic ray sources in the continuum limit and for predicting the diffuse dark matter annihilation signal. Determining the local mass density and velocity distribution is the aim of the CosmicFlows project. An alternate approach is based on catalogs of galaxies, supplemented with some scheme for filling in unseen galaxies. Here we compare the density field proposed by J. Biteau (2021) with the quasi-linear density field of CosmicFlows2 (Y. Hoffman et al. 2018) and the mean posterior field of CosmicFlows4 (A. Valade 2026). We find factor-of-two level differences in some regions and even larger discrepancies toward the Galactic center zone of avoidance (ZoA), defined by absolute longitude less than 30 degrees and absolute latitude less than 20 degrees, as filled by Biteau using cloning. Within 11 Mpc the density field is well determined by the Local Volume catalog (I. D. Karachentsev et al. 2018), which is directly incorporated by Biteau, and is preferred over the CosmicFlows modeling. At larger distances Biteau (2021) is also preferred for the direction and integrated mass of non-obscured structures, but the radial mass distribution is less robust due to line-of-sight peculiar velocities. Within the ZoA the galaxies of Biteau (2021) are entirely fictitious and their use is ill advised except where they do not contribute to the observable of interest. Moreover, as shown by the South Pole Wall, discovered by CosmicFlows but absent from galaxy catalogs, obscuration can be significant even outside the ZoA. Unexpectedly, we find that the angular positions of structures in CosmicFlows are sometimes misaligned with their true positions as seen in galaxy catalogs outside the ZoA.

astro-ph.CO↗

arXiv2025

Ultrahigh Energy Cosmic Ray Production in Binary Neutron Star Mergers

Having previously argued that binary neutron star mergers are the principle source of ultrahigh energy cosmic rays~\citep{fBNS-prl25}, we exploit here the highly constrained initial conditions to make quantitative predictions for the cutoff energy of various nuclei. UHECRs heavier than helium are accelerated in the magnetized turbulent outflow outside the jets to a rigidity $\mathcal{R}_{\rm cut} \equiv E_{\rm cut}/eZ \approx 6-9$ EV, consistent with the measured value $\mathcal{R}_{\rm cut} = 6.3^{+6.3}_{-2.3}\,$EV from fitting data. This agreement strengthens the case that BNS mergers are the main site of UHECR production. The jets may accelerate protons and/or helium to cutoff energies $\approx 11.5$ and $\approx 35$ EeV, respectively. Such a jet component and its spallation products could explain the indication of a secondary light population at higher energy found in the analysis of~\citet{muf19}. The relative abundances of different elements and the total energy in UHECRs per merger event will become calculable, pending advances in our understanding of the mechanism of ion uptake into the acceleration process and input from nuclear physics experiments. This scenario implies that each neutrino above 1 PeV is co-directional with a gravitational wave arriving $\approx 1$ day earlier, and that the highest energy UHECRs have masses heavier than iron.

astro-ph.HE↗

arXiv2025

The Galactic Magnetic Field and UHECR Deflections

Ultrahigh-energy cosmic rays (UHECRs) experience deflections as they traverse the Galactic magnetic field (GMF), which must be accounted for when tracing them back to their sources. After briefly summarizing our results on uncertainties in cosmic-ray deflections from the UF23 ensemble of GMF models (Unger & Farrar, 2024), we report a new preliminary fit of the GMF including foreground emission from the Local Bubble. This fit uses the analytic model of Pelgrims et al. (2024) for the magnetic field in the thick shell of Galactic bubbles. We also discuss how variations in toroidal halo field modeling account for the key differences between the Jansson & Farrar (2012) GMF model and the UF23 ensemble. Furthermore, we extend our previous analysis of the origin of the highest-energy "Amaterasu" event observed by the Telescope Array to include the four highest-energy events detected by the Pierre Auger Observatory. Amaterasu and PAO070114 are the UHECR events with the smallest localization uncertainties of 4.7% and 2.4%, respectively. Neither of their back-tracked directions aligns with any compelling candidate for a continuous UHECR accelerator. This strengthens the evidence that at least a fraction of the highest energy events originate from transient sources.

astro-ph.HE↗

arXiv2025

Binary neutron star mergers as the source of the highest energy cosmic rays

We propose that ultrahigh energy cosmic rays are produced in binary neutron star mergers. This scenario can account for the heretofore inexplicable narrow rigidity range of UHECRs, because the jets of BNS mergers are generated by a gravitationally-driven dynamo and thus are nearly identical due to the narrow range of BNS masses. Observed UHECRs with energies well beyond 100 EeV can be explained as $r$-process nuclei, without invoking an exotic source class. Evidence for this mechanism, and its prediction of coincidences between neutrinos above 10 PeV and gravitational waves, are discussed.

astro-ph.HE↗

arXiv2025

Ideas and Requirements for the Global Cosmic-Ray Observatory (GCOS)

After a successful kick-off meeting in 2021. two workshops in 2022 and 2023 on the future Global Cosmic-Ray Observatory (GCOS) focused mainly on a straw man design of the detector and science possibilities for astro- and particle physics. About 100 participants gathered for in-person and hybrid panel discussions. In this report, we summarize these discussions, present a preliminary straw-man design for GCOS and collect short write-ups of the flash talks given during the focus sessions.

astro-ph.IM↗

arXiv2024

The large-scale anisotropy and flux (de-)magnification of ultra-high-energy cosmic rays in the Galactic magnetic field

We calculate the arrival direction distribution of ultra-high-energy cosmic rays (UHECRs) with a new suite of models of the Galactic magnetic field (GMF), assuming sources follow the large-scale structure of the Universe. Compared to previous GMF models, the amplitude of the dipole component of the UHECR arrival flux is significantly reduced. We find that the reduction is due to the accidentally coinciding position of the peak of the extragalactic UHECR flux and the boundary of strong flux demagnification due to the GMF toward the central region of the Galaxy. This serendipitous sensitivity of UHECR anisotropies to the GMF model will be a powerful probe of the source distribution as well as Galactic and extragalactic magnetic fields. Demagnification by the GMF also impacts visibility of some popular source candidates.

astro-ph.HE↗

arXiv2024

Ultra-High-Energy Cosmic Rays Accelerated by Magnetically Dominated Turbulence

Ultra-High-Energy Cosmic Rays (UHECRs), particles characterized by energies exceeding $10^{18}$ eV, are generally believed to be accelerated electromagnetically in high-energy astrophysical sources. One promising mechanism of UHECR acceleration is magnetized turbulence. We demonstrate from first principles, using fully kinetic particle-in-cell simulations, that magnetically dominated turbulence accelerates particles on a short timescale, producing a power-law energy distribution with a rigidity-dependent, sharply defined cutoff well approximated by the form $f_{\rm cut}\left({E, E_{\rm cut}}\right) = {\text{sech}}\left[ ( {{E}/{E_{\rm cut}}} )^2 \right]$. Particle escape from the turbulent accelerating region is energy-dependent, with $t_{\rm esc} \propto E^{-δ}$ and $δ\sim 1/3$. The resulting particle flux from the accelerator follows $dN/dEdt \propto E^{-s} {\text{sech}}\left[ ( {{E}/{E_{\rm cut}}} )^2 \right]$, with $s \sim 2.1$. We fit the Pierre Auger Observatory's spectrum and composition measurements, taking into account particle interactions between acceleration and detection, and show that the turbulence-associated energy cutoff is well supported by the data, with the best-fitting spectral index being $s = 2.1^{+0.06}_{-0.13}$. Our first-principles results indicate that particle acceleration by magnetically dominated turbulence may constitute the physical mechanism responsible for UHECR acceleration.

astro-ph.HE↗

arXiv2024

The Coherent Magnetic Field of the Milky Way

We present a suite of models of the coherent magnetic field of the Galaxy (GMF) based on new divergence-free parametric functions describing the global structure of the field. The model parameters are fit to the latest full-sky Faraday rotation measures of extragalactic sources (RMs) and polarized synchrotron intensity (PI) maps from WMAP and Planck. We employ multiple models for the density of thermal and cosmic-ray electrons in the Galaxy, needed to predict the skymaps of RMs and PI for a given GMF model. The robustness of the inferred properties of the GMF is gauged by studying many combinations of parametric field models and electron density models. We determine the pitch angle of the local magnetic field (11+/-1 deg.), explore the evidence for a grand-design spiral coherent magnetic field (inconclusive), determine the strength of the toroidal and poloidal magnetic halo fields below and above the disk (magnitudes the same for both hemispheres within 10%), set constraints on the half-height of the cosmic-ray diffusion volume (>2.9 kpc), investigate the compatibility of RM- and PI-derived magnetic field strengths (compatible under certain assumptions) and check if the toroidal halo field could be created by the shear of the poloidal halo field due to the differential rotation of the Galaxy (possibly). A set of eight models is identified to help quantify the present uncertainties in the coherent GMF -- spanning different functional forms, data products and auxiliary input, and maximizing the differences in their predictions. We present the corresponding skymaps of rates for axion-photon conversion in the Galaxy, and deflections of ultra-high energy cosmic rays.

astro-ph.GA↗

arXiv2024

Constraints on UHECR sources and extragalactic magnetic fields from directional anisotropies

A dipole anisotropy in ultra-high-energy cosmic ray (UHECR) arrival directions, of extragalactic origin, is now firmly established at energies E > 8 EeV. Furthermore, the UHECR angular power spectrum shows no power at smaller angular scales than the dipole, apart from hints of possible individual hot or warm spots for energy thresholds $\gtrsim$40 EeV. Here, we exploit the magnitude of the dipole and the limits on smaller-scale anisotropies to place constraints on two quantities: the extragalactic magnetic field (EGMF) and the number density of UHECR sources or the volumetric event rate if UHECR sources are transient. We also vary the bias between the extragalactic matter and the UHECR source densities, reflecting whether UHECR sources are preferentially found in over- or under-dense regions, and find that little or no bias is favored. We follow Ding et al. (2021) in using the Cosmic Flows 2 density distribution of the local universe as our baseline distribution of UHECR sources, but we improve and extend that work by employing an accurate and self-consistent treatment of interactions and energy losses during propagation. Deflections in the Galactic magnetic field are treated using both the full JF12 magnetic field model, with random as well as coherent components, or just the coherent part, to bracket the impact of the GMF on the dipole anisotropy. This Large Scale Structure (LSS) model gives good agreement with both the direction and magnitude of the measured dipole anisotropy and forms the basis for simulations of discrete sources and the inclusion of EGMF effects.

astro-ph.HE↗

arXiv2024

Where Did the Amaterasu Particle Come From?

The Telescope Array Collaboration recently reported the detection of a cosmic-ray particle, "Amaterasu", with an extremely high energy of $2.4\times10^{20}$ eV. Here we investigate its probable charge and the locus of its production. Interpreted as a primary iron nucleus or slightly stripped fragment, the event fits well within the existing paradigm for UHECR composition and spectrum. Using the most up-to-date modeling of the Galactic magnetic field strength and structure, and taking into account uncertainties, we identify the likely volume from which it originated. We estimate a localization uncertainty on the source direction of 6.6\% of $4π$ or 2726 deg$^2$. The uncertainty of magnetic deflections and the experimental energy uncertainties contribute about equally to the localization uncertainty. The maximum source distance is 8-50 Mpc, with the range reflecting the uncertainty on the energy assignment. We provide sky maps showing the localization region of the event and superimpose the location of galaxies of different types. There are no candidate sources among powerful radio galaxies. An origin in AGNs or star-forming galaxies is unlikely but cannot be completely ruled out without a more precise energy determination. The most straightforward option is that Amaterasu was created in a transient event in an otherwise undistinguished galaxy.

astro-ph.HE↗

arXiv2023

Report of the 2021 U.S. Community Study on the Future of Particle Physics (Snowmass 2021) Summary Chapter

The 2021-22 High-Energy Physics Community Planning Exercise (a.k.a. ``Snowmass 2021'') was organized by the Division of Particles and Fields of the American Physical Society. Snowmass 2021 was a scientific study that provided an opportunity for the entire U.S. particle physics community, along with its international partners, to identify the most important scientific questions in High Energy Physics for the following decade, with an eye to the decade after that, and the experiments, facilities, infrastructure, and R&D needed to pursue them. This Snowmass summary report synthesizes the lessons learned and the main conclusions of the Community Planning Exercise as a whole and presents a community-informed synopsis of U.S. particle physics at the beginning of 2023. This document, along with the Snowmass reports from the various subfields, will provide input to the 2023 Particle Physics Project Prioritization Panel (P5) subpanel of the U.S. High-Energy Physics Advisory Panel (HEPAP), and will help to guide and inform the activity of the U.S. particle physics community during the next decade and beyond.

hep-ex↗

arXiv2023

New approach to finding invisible states in $e^+e^-$ annihilation and application to BESIII data

We compare precision $e^+e^-$ to $μ^+μ^-$ cross section measurements by BESIII in the E_cm =3.8-4.6 GeV range, to predictions based on measured R_had data. The consistency is poor (p-value <0.012). Allowing for resonance contributions not seen in R_had gives an excellent fit, with the state at 4421 MeV ( 4.6 sigma) giving insight into the psi(4415) and the 3.1 sigma structure at 4211 MeV, if confirmed, being a new, very narrow resonance. This analysis shows the power of precision $e^+e^-$ to $μ^+μ^-$ measurements to uncover or probe otherwise difficult to access states.

hep-ex↗

arXiv2023

Relic Neutrino Helicity Evolution in Galactic Magnetic Field and Its Implications

We simulate the evolution of the helicity of relic neutrinos as they propagate to Earth through a realistic model of the Galactic magnetic field, improving upon the rough estimates in the literature. For magnetic moments consistent with experimental bounds and several orders of magnitude smaller, we confirm that the helicity of relic neutrinos and anti-neutrinos rotates so much that the spin projection changes by $\mathcal{O}$(1). However, as we show, the total event rate in an inverse tritium beta decay (ITBD) experiment changes by less than a few percent, unless the lightest neutrino has mass of order 0.001 eV or less. Such a tiny reduction in the absolute rate relative to the standard model value would be very difficult to establish, even if detecting relic neutrinos were routine. However as we show, the \emph{directional anisotropy} of the rate in a \emph{polarized} ITBD detector is $\gtrsim \mathcal{O}$(10\%) as long as the lightest neutrino mass is $\gtrsim \mathcal{O}$(0.01 eV). Thus with percent-level error bars on the absolute neutrino flux and its directional anisotropy, both the mass and magnetic moment of the relic neutrinos can in principle be probed if they are within a few orders of magnitude of current bounds.

hep-ph↗

arXiv2023

Wave function of the sexaquark or compact H-dibaryon

We derive and explicitly display the internal wave function of a color-, flavor- and spin- singlet dibaryon composed of uuddss quarks in a spatially symmetric state, in the approximation of exact SU(3) flavor symmetry. This wavefunction shows that the often-used superposition of Lambda Lambda, N Xi and Sigma Sigma baryons, relevant for di-baryon molecules, accounts for only 1/5 of a spatially symmetric six-quark color-flavor-spin-singlet state, with the remaining 4/5 consisting of products of color-octet baryons. Using the correct wavefunction has important implications for calculations of the mass of the lowest-lying flavor-singlet dibaryon, as we illustrate using the Cornell potential. We also provide a compact representation of the state in terms of creation operators, and comment on the impact of not using the optimal operator in lattice studies of this system.

hep-ph↗

arXiv2023

Constraints on long-lived di-baryons and di-baryonic dark matter

A color-flavor-spin singlet state of six quarks $uuddss$ ($S$, or sexaquark) has been argued to be a potentially undiscovered deeply bound, long-lived hadron. Theoretical calculations of the $S$ mass have been made in the literature, widely varying from deeply-bound $\sim 1.2$ GeV to weakly-bound $\sim2.2$ GeV. Given the spread of the mass predictions, it is vital to derive observational constraints on the state as a function of the mass. The transition rates between $S$ and two baryons are governed by $\tilde{g}$, the effective Yukawa coupling between $S$ and two baryons with the same quantum numbers as $S$. In this paper, we place strong observational constraints on $\tilde{g}$, improving on various previous limits; additional limits assuming $S$ is dark matter are also presented.

hep-ph↗

arXiv2022

Constraints on the hosts of UHECR accelerators

Interactions of ultrahigh energy cosmic rays in the surroundings of their accelerators can naturally explain the observed spectrum and composition of UHECRs, including the abundance of protons below the ankle. We show that astrophysical properties of the UHECR source environment such as the temperature, size, and magnetic field can be constrained by UHECR and neutrino data. Applying this to candidate sources with a simple structure shows that starburst galaxies are consistent with these constraints, but galaxy clusters may be in tension with them. For multi-component systems like AGNs and GRBs the results are indicative but customized analysis is needed for definitive conclusions.

astro-ph.HE↗