Temperature-dependent C P T violation: Constraints from big bang nucleosynthesis

In this study, we explore temperature-dependent C P T violation during big bang nucleosynthesis (BBN) through electron-positron mass asymmetries parametrized by b 0 ( T ) = α T 2 . The T 2 scaling naturally evades stringent laboratory bounds at zero temperature while allowing for significant C P T violation at MeV scales in the early Universe [S. Navas (Particle Data Group), ]. Using a modified version of the BBN code 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 eff . We find that α 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 σ overlap, though pairwise combinations allow for constrained regions of parameter space. We present two toy models demonstrating how b 0 ( T ) ∝ T 2 arises from field-theoretic mechanisms, including temperature-driven phase transitions. These results provide the most stringent constraints on early-Universe C P T violation in this regime, probing parameter space inaccessible to laboratory experiments.