Brian D. Fields

The primordial abundance of ⁴ He plays a central role in Big Bang nucleosynthesis (BBN) and in the cosmic microwave background (CMB). The LBT Y _p Project’s new measurement of the primordial ⁴ He mass fraction, Y _p = 0.2458 ± 0.0013, is the most precise determination to date. In this paper, we combine our new Y _p value with the latest primordial deuterium measurement and assess the consequences for cosmology. For standard BBN, where the number of light neutrino species is fixed at N _ν = 3, the single free parameter is the cosmic baryon density; the CMB measures this independently, with results consistent with each other. Combining Y _p , D/H, BBN, and the CMB gives the cosmic baryon-to-photon ratio η = (6.120 ± 0.038) × 10 ⁻¹⁰ , corresponding to a baryon density parameter Ω _B h ² = 0.02236 ± 0.00014. We then allow N _ν to vary and thus measure relativistic species present during nucleosynthesis. We find η = (6.101 ± 0.044) × 10 ⁻¹⁰ or Ω _B h ² = 0.02229 ± 0.00016, and N _ν = 2.925 ± 0.082, and for N _ν ≥ 3, Δ N _ν = N _ν − 3 ≤ 0.125 (95% confidence limit) during BBN and the CMB. Our results demonstrate consistency with the Standard Model of particle physics and the standard cosmology that links BBN at ∼ 1 s and the CMB at ∼400,000 yr.

Extremely low-metallicity H ii regions have been observed with the goal of determining the primordial helium abundance ( Y _p ). Y _p , combined with standard Big Bang nucleosynthesis and the half-life of the neutron, provides a direct measurement of the number of neutrino families, but Y _p must be measured very precisely to provide meaningful constraints on physics beyond the Standard Model. Here we describe a program to combine new Large Binocular Telescope (LBT) observations with a new analysis methodology to significantly improve the determination of Y _p . The LBT, with its MODS and LUCI instruments, produces spectra, which, when combined with our new analysis methodology, are capable of delivering He abundances in individual H ii regions with uncertainties of approximately 2% or less. Archival LBT/MODS spectra of standard stars over a 4 yr period enable the determination of a wavelength-dependent uncertainty in the MODS spectral response, resulting in improved relative emission line uncertainties. An optimized sample of low-metallicity galaxies has been selected with the goal of producing a determination of Y _p with a precision of ∼0.5%, sufficient to provide an independent constraint on the effective number of neutrino families of ∼3%.

The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 3,200 new measurements from 903 papers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as supersymmetric particles, heavy bosons, axions, dark photons, etc. Particle properties and search limits are listed in Summary Tables. We give numerous tables, figures, formulae, and reviews of topics such as Higgs Boson Physics, Supersymmetry, Grand Unified Theories, Neutrino Mixing, Dark Energy, Dark Matter, Cosmology, Particle Detectors, Colliders, Probability and Statistics. Most of the 118 reviews are updated, including many that are heavily revised. The Review is divided into two volumes. Volume 1 includes the Summary Tables and 96 review articles. Volume 2 consists of the Particle Listings and contains also 22 reviews that address specific aspects of the data presented in the Listings. The complete Review (both volumes) is published online on the website of the Particle Data Group ( pdg.lbl.gov ) and in a journal. Volume 1 is available in print as the PDG Book. A Particle Physics Booklet with the Summary Tables and essential tables, figures, and equations from selected review articles is available in print, as a web version optimized for use on phones, and as an Android app. The PDG API (Application Programming Interface) provides access to the data published in the Review in machine-readable format.

The vertical redistribution of materials in the lunar regolith—ranging from continuously produced space-weathering products to sporadic pulses of supernova- or kilonova-derived isotopes—remains a fundamental problem in planetary science. We present a unified stochastic model of regolith gardening induced by the impact flux. Treating gardening as a competition between impact-driven advection and diffusion predicts the maturity profiles of cores over more than 2 orders of magnitude in time ( 1.4 × 10 7 to 4.5 × 10 8 yr ). This model describes well the depth profiles of live Fe 60 in regolith samples, suggesting that supernova dust capture is independent of native iron abundance, and is consistent with a uniform influx at the latitudes of the landing sites. We extend our model to predict lunar signals for live r -process species that might originate from supernovae or kilonovae: Pu 244 tied to terrestrial detections, and I 129 , Hf 182 , and Cm 247 based on r -process calculations. The Pu 244 / Fe 60 depth profile can probe the origin of Pu 244 , motivating searches in regolith samples down to depths O ( 100 ) cm .

The LIGO–Virgo–KAGRA gravitational-wave network, which enabled the discovery of the kilonova (KN) counterpart to GW170817 during observing run 2 (O2), has just reached the end of O4 with no new confirmed neutron star mergers, suggesting the intrinsic rate of these events must be even lower than previously estimated. As a result, building a sample of KNe will remain challenging even with continued gravitational wave observations, motivating complementary discovery strategies that do not rely on gravitational-wave triggers. In this work, we consider how leveraging bright short gamma-ray burst afterglows can aid in the discovery of KNe with the Vera C. Rubin Observatory’s upcoming Legacy Survey of Space and Time (LSST), whose unprecedented depth will make such detections feasible. We find that nearly on-axis ( θ _view ≤30°) afterglows can enhance KN detection rates in the LSST g band from 2 9 − 21 + 51 yr − 1 to 9 1 − 65 + 160 yr − 1 . We further show how the colors of the observed events can be used to distinguish between neutron star merger counterparts with and without KN emission. This study demonstrates how critical multiwavelength and multisurvey observations are for these rare events, especially without context from gravitational waves. Fortunately, detectable events will likely be discovered near peak with LSST, allowing for rapid follow-up and confirmation. We discuss key uncertainties in our study, particularly the volume rate of merger events, and the degeneracy between the empirically determined explosion energy and ambient medium density.

Supernova (SN) blasts envelop many surrounding stellar systems, transferring kinetic energy to small bodies in the systems. Geologic evidence from ⁶⁰Fe points to recent nearby SN activity within the past several Myr. Here, we model the transfer of energy and resulting orbital changes from these SN blasts to the Oort Cloud, the Kuiper Belt, and Saturn’s Phoebe ring. For the Oort Cloud, an impulse approximation shows that a 50 pc SN can eject approximately half of all objects less than 1 cm while altering the trajectories of larger ones, depending on their orbital parameters. For stars closest to SNe, objects up to ∼100 m can be ejected. Turning to the explored solar system, we find that SNe closer than 50 pc may affect Saturn’s Phoebe ring and can sweep away Kuiper Belt dust. It is also possible that the passage of the solar system through a dense interstellar cloud could have a similar effect; a numerical trajectory simulation shows that the location of the dust grains and the direction of the wind (from an SN or interstellar cloud) has a significant impact on whether or not the grains will become unbound from their orbit in the Kuiper Belt. Overall, nearby SNe sweep micron-sized dust from the solar system, though whether the grains are ultimately cast toward the Sun or altogether ejected depends on various factors. Evidence of SN-modified dust grain trajectories may be observed by New Horizons, though further modeling efforts are required.