The forgotten particle

Anyone who knows the fundamentals of cosmology knows that the existence of our universe, dominated by matter, is linked to a microscopic but fundamental primordial anomaly. If the Big Bang had produced matter and antimatter in perfectly symmetric amounts, they would have annihilated each other during expansion, leaving a cosmos filled only with radiation. The exact mechanism that originated this matter-antimatter asymmetry is still one of the great unsolved problems of particle physics. A study produced by the University of Southampton (United Kingdom) and published last month in Physical Review Letters shows a possible theory that resolves the question. One of the most solid theoretical frameworks is rooted in an extension of the standard model known as the Type I seesaw mechanism, originally formulated to explain the infinitesimal masses of neutrinos we observe today. This model predicts the existence of three right-handed neutrinos, extremely massive primordial particles. Of these three, physicists have historically focused their phenomenological analyses only on the two lightest, systematically neglecting the third and heaviest, deeming its interactions too weak to significantly influence cosmological evolution. This study shows, however, that ignoring this third neutrino represents a serious omission. Precisely because of its almost non-existent interactions with the primordial plasma, this massive particle results in an exceptionally long lifetime. Surviving as the universe cooled, its energy density ended up behaving like matter in a spacetime still dominated by radiation. The result is the triggering of a temporary but crucial epoch of matter dominance, which profoundly altered the rate of expansion of the primordial cosmos. Modifying the rate of space expansion means altering the thermodynamics of the particles within it. Specifically, the study reveals that this cosmic brake generates a substantial delay in the moment when the fundamental constituents (leptons) stabilize their mutual interactions. In practical terms, the alteration of the expansion rate changes the temperatures at which these particles separate and assume distinct identities (electron, muon, tauon), forcing physicists to recalculate the dynamics of thermal leptogenesis, that is, the asymmetric decay process that allowed matter to prevail. "This heavy neutrino, now decoupled, was long considered an irrelevant background detail in particle physics," explains Rishav Roshan, the first author of the study. "However, surviving long enough, it has come to constitute the main fraction of the energy density of the universe, altering its evolution." The theoretical model also offers precise and falsifiable predictions. To trigger this specific chain of events and justify the observed asymmetry, the equations require that the lightest active neutrino must have a mass exactly between 10-12 and 10-14 eV. This is a limit that can be explored and potentially confirmed by high-precision active experiments, such as KATRIN or the Project 8 collaboration. Furthermore, the slowing of expansion and the subsequent injection of entropy due to the decay of this neutrino would have left indelible imprints on spacetime. These anomalies would today be observable as perturbations in the spectrum of primordial gravitational waves, providing a direct channel to observe the first moments after the Big Bang. To learn more: - Read the article "Delayed Charged Lepton Yukawa Equilibration in Minimal Seesaw" in Physical Review Letters, by Rishav Roshan and Sudipta Show