Keith A. Olive

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.

We present a new determination of the primordial helium abundance based on new, high-quality Large Binocular Telescope (LBT) observations of 54 metal-poor H ii regions. These regions have been observed and analyzed uniformly. We also describe a number of updates to our methodology, including updated helium emissivities. Enabled by the large, high-quality dataset, we examine our sample targets for potential systematic errors, which could bias their results. We perform a standard 95% confidence level (CL) χ ² cut and find that a significantly larger fraction (47/54 = 87%) of our sample qualifies than for previous datasets. We also screen for quality and reliability, flagging targets that may introduce significant systematic errors, producing a dataset of 41 targets. In a significant breakthrough for the field, that dataset includes 15 high signal-to-noise ratio (SNR) targets with low metallicity (O/H ≤ 4 × 10 ⁻⁵ ). Due to this low-metallicity dataset, for the first time, a weighted average for determining the primordial helium abundance ( y _p ) is well justified and produces a robust result. By weighted average of our 15 low-metallicity targets, we determine y _p = 0.08146 ± 0.00058, with the corresponding mass fraction Y _p = 0.2458 ± 0.0013. This result achieves an unprecedented precision of 0.5%, and it is in good agreement with the Big Bang nucleosynthesis (BBN) result, Y _p = 0.2467 ± 0.0002, based on the Planck determination of the baryon density.

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%.

Particles that decouple relativistically from the Standard Model bath during reheating represent a versatile class of well-motivated cold dark matter candidates. In fact, ultrarelativistic decoupling ( T _FO ≫ m _χ ) is quite generic for beyond the Standard Model (BSM) heavy portal interactions with strong couplings and relatively low reheating temperatures. In this work, we study the direct detection prospects for ultrarelativistically frozen-out (UFO) candidates, using Z′ -portal dark matter as a case study. Although typical UFO cross sections are suppressed by a heavy mediator mass scale, we find that experiments such as LZ, XENONnT, PandaX, and DarkSide-50 have already excluded a large portion of the UFO parameter space and there remains viable space above the neutrino fog for 0.4 GeV ≲ m _DM ≲ 1 TeV. Moreover, SuperCDMS SNOLAB, which is expected to begin collecting data in 2026, should access a large region of UFO parameter space in the 0.5–10 GeV mass range. For heavy BSM portal interactions ( M ≳ 1 TeV), UFOs are typically more accessible to detection than freeze-in candidates due to the comparatively larger cross sections. We also carefully delineate regions of parameter space with degeneracy between UFO and non-relativistic freeze-out. In sum, UFOs are attractive candidates for ongoing and next-generation dark matter detection experiments in a looming post-WIMP era.

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.

A bstract: Supergravity provides the natural supersymmetric framework for early universe cosmology. A broad class of inflationary models in no-scale supergravity yields tree-level predictions for cosmic microwave background (CMB) observables that closely resemble those of the Starobinsky R + R ² model. Using results from global supersymmetry and supergravity, we analyze radiative corrections in models with canonical and non-canonical kinetic terms, focusing particularly on Starobinsky-like no-scale supergravity models. We derive conditions on the superpotential that keep the gravitino mass finite during inflation and ensure that loop-induced corrections to the Kähler potential remain either finite or subdominant relative to the tree-level potential. We show that in some models, most notably the original no-scale supergravity model with a Wess-Zumino superpotential, radiative corrections grow at large inflaton field values and can dominate the inflationary dynamics, rendering unreliable the model predictions for CMB data. However, we identify a class of no-scale Starobinsky-like models, including the Cecotti model, in which radiative corrections remain very small for inflaton field values ≲ 8 (in Planck units), preserving the agreement of the tree-level predictions with Planck CMB data.

We investigate the production of dark matter (DM) via a heavy Z' mediator during the post-inflationary reheating epoch. In particular, we study production from three mechanisms which are smoothly connected to one another: WIMP-like freeze-out, FIMP-like freeze-in, and ultra-relativistic freeze-out (UFO). This is the first systematic study of Z' portal DM which includes UFO. We find that much of the available parameter space for keV to TeV DM lies in the UFO regime for 1 TeV ≲ M _Z' ≲ 1 PeV. When the mediator mass M _Z' is greater than both the DM mass and the reheating temperature, UFO is a robust mechanism for producing cold DM. Although UFO DM is initially “hot” after freeze-out, it can easily become cold before structure formation if freeze-out occurs during post-inflationary reheating. Compared to standard freeze-in, UFO can accommodate significantly stronger interaction strengths (stronger couplings and/or smaller mediator masses).

We analyze the latest cosmic microwave background (CMB) constraints on the scalar spectral index n s and tensor-to-scalar ratio r from 2018, BICEP/Keck 2018, the Atacama Cosmology Telescope Data Release 6 (ACT DR6), and the South Pole Telescope (SPT-3G) data, focusing on their implications for attractor models of inflation. We compare systematically observational bounds with theoretical predictions for both E-model ( α -Starobinsky) and T-model potentials. The observational constraints accommodate E-models with α ≲ 25 , with the canonical Starobinsky model ( α = 1 ) predicting n s = 0.958 – 0.963 for reheating temperatures between 100 − 10 10 GeV , in good agreement with 2018 data and within the 95% CL region determined by the -ACT-SPT combination, but below the 95% confidence region of the -ACT-DESI combination. Higher reheating temperatures from near-instantaneous reheating improve the compatibility. T-models predict slightly lower n s values (0.956–0.961), in some tension with 2018 data, and we find an upper limit of α ≲ 11 in these models. We extend our analysis to generalized α -attractors with monomial potentials V ( φ ) ∝ φ k near the minimum, demonstrating that models with k ≥ 6 naturally predict n s ≃ 0.965 – 0.968 for typical number of e -folds, in better agreement with the ACT DR6 data. We also consider deformed E- and T-models, which allow significantly higher values of n s for low values of α ≃ 1 .