Temperature-Dependent CPT Violation: Constraints from Big Bang Nucleosynthesis

TL;DR

This paper investigates temperature-dependent CPT violation during Big Bang Nucleosynthesis, constraining electron-positron mass asymmetries with a modified BBN code and presenting models for the temperature dependence.

Contribution

It introduces a novel temperature-dependent CPT violation model with $b_0(T) \\propto T^2$ and derives the most stringent early-universe constraints using observational data.

Findings

Electron-positron mass differences are constrained to -6 GeV^{-1} for keV-scale masses.

No 7 overlap in all three observables, but pairwise overlaps constrain parameter space.

Toy models show how 7 temperature dependence can arise from field-theoretic mechanisms.

Abstract

In this study, we explore temperature-dependent CPT violation during Big Bang Nucleosynthesis (BBN) through electron-positron mass asymmetries parametrized by $b_0(T) = \alpha T^2$. The $T^2$ scaling naturally evades stringent laboratory bounds at zero temperature while allowing for significant CPT violation at MeV scales in the early universe \cite{ParticleDataGroup:2024cfk}. Using a modified version of the BBN code \faGithub \href{https://github.com/vallima/PRyMordial}{\,\texttt{PRyMordial}} with dynamically-solved chemical potentials and appropriate finite-mass corrections, we constrain electron-positron mass differences from observed abundances of Helium-4, Deuterium, and $N_{\rm eff}$. We find that $\alpha$ must be greater than or approximately equal to $10^{-6}$ GeV$^{-1}$ for keV-scale mass differences at BBN. All three observables show no simultaneous $1\sigma$ overlap, though pairwise combinations allow for constrained regions of parameter space. We present three toy models demonstrating how $b_0(T) \propto T^2$ arises from field-theoretic mechanisms, including temperature-driven phase transitions. These results provide the most stringent constraints on early-universe CPT violation in this regime, probing parameter space inaccessible to laboratory experiments.