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arxiv: 2605.26824 · v1 · pith:KQWYQEQ2new · submitted 2026-05-26 · ✦ hep-ph · astro-ph.CO

Improved Big Bang Nucleosynthesis constraints on decaying massive relics

Sara Bianco , Jonas Frerick , Marco Hufnagel , Kai Schmidt-Hoberg This is my paper

Pith reviewed 2026-06-29 17:11 UTC · model grok-4.3

classification ✦ hep-ph astro-ph.CO
keywords Big Bang Nucleosynthesisdecaying relicsbeyond Standard Modelprimordial abundanceshadronic decaysphotodisintegrationexclusion contoursfreeze-in
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The pith

Updated BBN modeling produces revised exclusion contours on lifetime, mass and abundance of decaying heavy relics for each two-body channel.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper recomputes Big Bang Nucleosynthesis limits on long-lived beyond-Standard-Model particles that decay into pairs of ordinary particles. It folds in newer measurements of primordial element abundances, current nuclear reaction rates, injection spectra generated by PYTHIA 8, an improved account of neutron-proton interconversions that includes kaon channels and dynamical equilibrium, and a hadrodisintegration treatment that remains valid while nucleosynthesis is still occurring. These changes together shift the regions of lifetime, mass and abundance that are ruled out for every decay mode examined, while also noting the unavoidable freeze-in population from inverse decays. Readers care because the resulting contours supply tighter, more reliable boundaries on any early-universe particle that could alter light-element yields.

Core claim

Adopting up-to-date primordial abundance measurements, the latest nuclear reaction rates, PYTHIA 8 spectra that properly include final-state radiation and hadronisation, an improved p↔n interconversion treatment that accounts for dynamical equilibrium and kaon-induced processes, and a refined hadrodisintegration formalism applicable while BBN is active yields updated exclusion contours on the lifetime, mass and abundance of the relic φ for each decay channel considered, together with discussion of the irreducible freeze-in contribution from inverse decays.

What carries the argument

The refined modeling of hadronic and electromagnetic injections that modifies the neutron-to-proton ratio and drives hadro- and photodisintegration of light nuclei, implemented with PYTHIA 8 spectra and updated rates throughout the BBN epoch.

If this is right

  • Exclusion regions shift for every two-body decay channel once the new data and methods are applied.
  • The irreducible freeze-in population from inverse decays must be included when setting limits.
  • Constraints now cover the full parameter space of lifetime, mass and abundance with the refined treatment active during BBN.
  • Updated nuclear rates and abundance inputs directly alter the allowed relic parameter space.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same injection machinery could be applied to three-body or multi-particle decay modes not treated in the present work.
  • The revised contours supply a ready input for joint analyses that combine BBN with CMB or large-scale structure data.
  • Models predicting relic particles with lifetimes near the BBN window can be checked directly against the new channel-by-channel limits.

Load-bearing premise

The leading effects of the decays are captured by the listed modifications to the neutron-to-proton ratio and to hadro- and photodisintegration, and that the PYTHIA 8 spectra plus the chosen nuclear rates accurately represent the relevant physics throughout the BBN epoch.

What would settle it

A new high-precision measurement of the primordial helium-4 or deuterium abundance that lies outside the range predicted when the updated injection spectra and interconversion rates are used would falsify the revised exclusion contours.

read the original abstract

We present updated and improved Big Bang Nucleosynthesis (BBN) constraints on heavy, long-lived beyond the Standard Model (BSM) relics $\phi$ decaying into pairs of Standard Model particles, covering a comprehensive set of two-body decay channels. We treat the leading effects of these injections in detail, discussing the modification of the neutron-to-proton ratio from hadronic interconversions, as well as hadro- and photodisintegration of the light elements. Our analysis incorporates several important refinements with respect to earlier work. We adopt up-to-date primordial abundance measurements, including the new $^4$He determination and the latest nuclear reaction rates. The hadronic and electromagnetic injection spectra are computed using PYTHIA 8, providing a proper treatment of final-state radiation and hadronisation. We further implement an improved treatment of $p\leftrightarrow n$ interconversions, accounting for dynamical equilibrium, kaon-induced processes, and updated rates. Additionally, we make use of a refined hadrodisintegration formalism which allows us to also consider disintegration processes while BBN is still active. Together, these improvements yield updated exclusion contours on lifetime, mass, and abundance of the relic for each decay channel considered. Furthermore, we discuss the irreducible freeze-in contribution from inverse decays.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

0 major / 3 minor

Summary. The paper updates BBN constraints on long-lived heavy BSM relics φ decaying to SM pairs across a comprehensive set of two-body channels. It refines the treatment of neutron-to-proton ratio modifications via hadronic interconversions, hadro- and photodisintegration, using updated primordial abundances (including new ⁴He), latest nuclear rates, PYTHIA 8 spectra with FSR and hadronization, dynamical-equilibrium p↔n interconversions including kaons, and a refined hadrodisintegration formalism active during BBN. These yield new exclusion contours in lifetime-mass-abundance space per channel, plus discussion of irreducible freeze-in from inverse decays.

Significance. If the numerical implementation and modeling choices hold, the work supplies tighter, more comprehensive BBN limits on BSM relics by incorporating modern inputs and previously omitted effects during the BBN epoch. The systematic coverage of channels and explicit use of PYTHIA 8 plus updated rates constitute a clear incremental advance for constraining new physics in the early universe.

minor comments (3)
  1. [Introduction] The abstract and introduction would benefit from a brief explicit statement of the energy range over which PYTHIA 8 spectra are applied and any extrapolation or matching procedure used below the PYTHIA validity threshold.
  2. [Section 3] Notation for the relic abundance parameter (e.g., Y_φ or Ω_φ h²) should be defined once at first use and used consistently in all contour plots and tables.
  3. [Figures 4-8] Figure captions for the exclusion contours should state the precise 95% CL criterion and the treatment of theoretical uncertainties from nuclear rates.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of our work and for recommending minor revision. The report provides a clear summary of our contributions but does not raise any specific major comments requiring response.

Circularity Check

0 steps flagged

No significant circularity; derivation uses external inputs

full rationale

The paper's central derivation computes updated exclusion contours on relic parameters by incorporating external primordial abundance measurements, updated nuclear rates, PYTHIA 8 injection spectra, and refined modeling of p↔n interconversions and hadrodisintegration. These inputs are independent of the output contours and are not fitted or redefined within the paper itself. No load-bearing step reduces by construction to a self-citation, fitted parameter renamed as prediction, or self-definitional relation. The analysis is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the accuracy of external nuclear reaction rates, the new 4He measurement, and the fidelity of PYTHIA 8 hadronisation and final-state radiation modeling; no new free parameters or invented entities are introduced in the abstract.

axioms (1)
  • standard math Standard BBN framework with known nuclear reaction network and initial conditions from the Standard Model.
    Invoked implicitly when the paper states it updates constraints within BBN.

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