Particle erahow quarks, antimatter and neutrinos appeared

The Particle era runs from 10⁻¹² s after the Big Bang to 3 minutes. In this time quarks first roamed free and were then confined into protons and neutrons, electrons and positrons dominated the universe, and at about 1 second neutrinos began to travel freely. The most intriguing item is a ledger: for every billion pairs of particles and antiparticles, there was one extra matter particle. Today's stars, mountains and people all come from that tiny surplus.

The earlier the state, the harder it is to observe directly. Researchers recreate extreme conditions in colliders or work them out in theory. As you read, notice whether each item rests on experimental evidence or on a theoretical prediction.

Six events

• 10⁻¹² s · Quark epoch: Quarks roam free in a hot soup, like ingredients swimming in a boiling pot.
• 10⁻¹¹ s · Baryon asymmetry: One extra matter particle per billion particle–antiparticle pairs. This is why matter survived.
• 10⁻⁶ s · Hadron epoch: Quarks are confined into protons and neutrons, and a lone quark is never seen again.
• 1 s · Lepton epoch: Electrons and positrons dominate the universe.
• 1 s · Neutrino decoupling: Neutrinos now travel freely and almost never interact with other matter.
• 10 s · Electron–positron annihilation: A small number of electrons remain, and photons increase greatly.

Quarks: from model to liquid

First came an idea that brought order to the particle family: protons and neutrons are made of smaller "quarks". It tidied the chaotic particle zoo of the time, in a paper titled "A schematic model of baryons and mesons" (Gell-Mann, Physics Letters, 1964). The next question was why quarks are never seen alone. The answer is called asymptotic freedom: the closer quarks get, the weaker their interaction, and the farther apart, the more tightly they are pulled together, so they stay locked up. The paper is titled "Ultraviolet behavior of non-abelian gauge theories" (Gross and Wilczek, Physical Review Letters, 1973).

The free-roaming quark state has since been recreated in the lab. Heavy-ion collisions reproduced the quark–gluon plasma of the first few microseconds, which behaves like a liquid with almost no viscosity (STAR Collaboration, Nuclear Physics A, 2005).

Antimatter: first on paper, then in cosmic rays

Paul Dirac combined quantum mechanics and relativity to describe the electron, and a solution of his equation unexpectedly predicted antimatter. His paper is titled "The quantum theory of the electron" (Dirac, Proceedings of the Royal Society A, 1928). A few years later the prediction was confirmed: a positively charged electron was photographed in cosmic rays, the first time humans saw antimatter. The paper has a plain title, "The positive electron" (Anderson, Physical Review, 1933).

Why matter won

Particles and antiparticles appear in pairs, so how did matter end up in surplus? Andrei Sakharov listed three conditions needed for the universe to produce more matter than antimatter, which are still the starting point for research. His paper's title names the topic directly: "Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe" (Sakharov, JETP Letters, 1967). Another clue came from experiment: the decay of the K meson showed that the laws of physics are not perfectly symmetric between matter and antimatter. The paper is titled "Evidence for the 2π decay of the K₂⁰ meson" (Christenson, Cronin, Fitch and Turlay, Physical Review Letters, 1964).

Neutrinos: ghost particles

Neutrinos hardly interact with anything, so catching one is hard. Researchers waited for years beside a nuclear reactor and finally caught this "ghost particle", in a paper titled "Detection of the free neutrino: a confirmation" (Cowan, Reines et al., Science, 1956). Later it was found that neutrinos change between three types, which means they have mass. The observation came from the Super-Kamiokande detector, in a paper titled "Evidence for oscillation of atmospheric neutrinos" (Fukuda et al., Physical Review Letters, 1998).

How the game shows it

In the game, the time range of the Particle era is written "10⁻¹² s – 3 min" and its resource is particles. The generators are Quark–gluon plasma, Matter–antimatter annihilation, Hadronization and Neutrino decoupling, corresponding to the real processes above. The generators are only a way to feel the order of these stages. No such machines exist in reality.