Why does matter exist

Here is the deepest question in physics, and it is not rhetorical: the Big Bang should have made equal amounts of matter and antimatter — which should have annihilated each other into pure light. Yet here we are. Every atom in every star, planet, and person is the leftover of a tiny early imbalance — for every billion antimatter particles, the universe made roughly a billion and one of matter; the billion pairs annihilated, and the "+1" became everything. Explaining that surplus — the problem of baryogenesis — is one of the great unsolved problems of physics, and the Zeptoverse program of rare-decay hunting is one of the laboratories built to find it.^[1]

What we'd need — Sakharov's three conditions

In 1967, Soviet physicist Andrei Sakharov — dissident, Nobel laureate, father of the hydrogen bomb — wrote down what any successful matter-making process requires. The list is short and brutal:

1. Baryon-number violation — some process must create or destroy matter particles; if baryon number is perfectly conserved, the matter count can never change
2. C and CP violation — the physics must treat matter and antimatter differently: charge symmetry (C, swapping matter↔antimatter) and charge-parity symmetry (CP) must both be broken, or every matter-making reaction has an antimatter twin that cancels it
3. Departure from thermal equilibrium — the process must run in a non-equilibrium moment (like the early universe's expansion); in equilibrium, rates balance and nothing net can be created

All three have been observed — CP violation exists (discovered in 1964 in neutral kaons, earning Cronin and Fitch the 1980 Nobel, and since found in beauty particles — the same quark the Zeptoverse team hunts). The problem is amount: the Standard Model's built-in CP violation is orders of magnitude too small to explain the observed surplus. Something else — outside our current theory — did the job.^[2]

The suspects

• More CP violation in the quark sector — beauty physics is exactly where tiny rare asymmetries surface; it's why the One in ten billion decay matters beyond the headline. LHCb already found CP violation in baryons in 2025 — evidence the asymmetry runs deeper than the Standard Model accounts for
• Leptogenesis — the leading elegant idea: if neutrinos are their own antiparticles (a "Majorana" property) and violate CP in their own sector, a matter-asymmetry in leptons could have been converted into the baryon asymmetry — a testable idea being hunted by the neutrino experiments chasing neutrino mass
• New particles entirely — any heavy particle that decayed preferentially to matter in the early universe would leave the same fingerprint: a rate measurably off from the Standard Model in today's rare-decay hunts

How this connects to the smallest signals

The chain is unusually direct. Rare decays — the B⁰ → μμ class the zeptoverse opened — are precision tests of the symmetries Sakharov's conditions require to be broken. A measured rate that deviates from the Standard Model would mean new particles interfering in the decay — particles that could be the missing agents of baryogenesis. The reason physicists spent 42 years hunting a one-in-ten-billion decay is that the answer to "why does anything exist" is encoded in processes that happen almost never — the smallest signals answering the biggest question.^[3]

What we know for sure

• The asymmetry is real — measured precisely in the cosmic microwave background: about 6.1 matter particles per 10⁹ photons, the "+1 in a billion" that became everything^[4]
• CP violation is real — found in kaons (1964), beauty mesons (2001), and beauty baryons (2025)
• The Standard Model's amount is too small — by many orders of magnitude
• Therefore: physics we haven't found yet made the universe's matter — the honest standing of the field

See also

• Zeptoverse — the frontier hunting the missing physics
• One in ten billion decay — the golden channel
• Neutrino — the leptogenesis suspect
• Proton decay — if baryon number can be violated at all

References

• Sakharov — "Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe" (1967)
• Christenson, Cronin, Fitch & Turlay — CP violation in K⁰ decay (1964); Nobel 1980
• Planck collaboration — baryon-to-photon ratio
• LHCb collaboration — CP violation in baryon decays (2025)