neutrino‑physics

Paper · 1957

Mesonium and Anti-Mesonium

Bruno Pontecorvo · Zh. Eksp. Teor. Fiz. 33, 549–551 / Soviet Physics JETP 6, 429

Abstract

Pontecorvo proposes that neutrinos, in analogy with the neutral kaon system K⁰–K̄⁰, may undergo particle-antiparticle oscillations during propagation. The proposal anticipates — though does not yet formulate in its modern form — the mechanism of flavor oscillation through a unitary mixing matrix between mass and interaction eigenstates.

Significance in the evidence base

The first suggestion of neutrino oscillation as a physical phenomenon. The idea was extended to flavor oscillation in Pontecorvo's 1967 follow-up paper and in the independent Maki-Nakagawa-Sakata formulation of 1962, producing the framework now known as PMNS mixing.

The analogy with K mesons

The neutral kaon system was, in 1957, the prototype example of a quantum-mechanical system whose mass eigenstates (K_S and K_L) differed from its flavor eigenstates (K⁰ and K̄⁰). Pontecorvo, working at Dubna, observed that if neutrinos were massive, an analogous phenomenon could occur in the lepton sector.

His 1957 paper is written in the two-neutrino language of the day — and as distinct particle-antiparticle states — and proposes oscillations of the type . The flavor-oscillation language () had not yet been introduced; the muon neutrino itself would be directly confirmed only in 1962.

The 1962 Maki-Nakagawa-Sakata formulation

Independently of Pontecorvo, Ziro Maki, Masami Nakagawa, and Shoichi Sakata in 1962 wrote down a framework in which the mass eigenstates of neutrinos differed from the weak-interaction (flavor) eigenstates by a unitary mixing matrix. Their formulation took essentially the modern form, though in two rather than three flavors.

Pontecorvo’s 1967 extension

In 1967, Pontecorvo returned to the problem in a second paper (“Neutrino experiments and the problem of conservation of leptonic charge”), extending the oscillation picture to flavor space. By that time the two-neutrino picture ( and ) had been experimentally established, and Pontecorvo’s formalism gave explicit oscillation probabilities that foreshadowed the modern expressions.

Together, the 1957 and 1967 papers — read alongside the independent Maki-Nakagawa-Sakata work — constitute the theoretical foundation of the mixing matrix now called PMNS (Pontecorvo–Maki–Nakagawa–Sakata).

Reception

Both Pontecorvo’s and the MNS work lingered at the periphery of the field through the late 1960s and 1970s. The solar neutrino problem, emerging from Davis’s Homestake experiment, gave oscillation its first experimental foothold. Even so, convincing confirmation had to wait for Super-Kamiokande (atmospheric, 1998) and SNO (solar, 2001) — forty-one and forty-four years, respectively, after Pontecorvo’s original proposal.

Significance

Pontecorvo’s paper is the earliest statement of an idea that is now central to particle physics:

  • that neutrinos have mass
  • that flavor eigenstates mix with mass eigenstates
  • that oscillation can be observed through the time-dependent survival and appearance probabilities at detectors separated from the source

The 2015 Nobel Prize to Kajita and McDonald — “for the discovery of neutrino oscillations, which shows that neutrinos have mass” — rewarded the experimental confirmation of what Pontecorvo had first suggested in the 1957 paper. He died in 1993, before the confirmation was achieved.