Daniel Z. Freedman

Daniel Z. Freedman (born 1939) is an American theoretical physicist. The prediction of coherent elastic neutrino-nucleus scattering came from him — the effect that today makes neutrino detectors the size of a suitcase possible.

He is also known as a co-founder of supergravity, an entirely different branch of theoretical physics.

The prediction of 1974

In 1974 Freedman published a paper that drew an immediate consequence from the theory of the weak interaction, which had been confirmed shortly before.

Gargamelle had detected the neutral currents in 1973: processes in which a neutrino interacts via the Z boson without changing its identity.

Freedman asked what happens when a neutrino of low energy strikes a whole atomic nucleus. His answer: at sufficiently low energy the neutrino can no longer distinguish the individual nucleons. It then scatters not off a proton or a neutron, but off the nucleus as a whole — and the contributions of the individual neutrons do not simply add up, but reinforce one another coherently.

The quadratic gain

The consequence is remarkable: the cross section grows with the square of the neutron number.

For a heavy nucleus such as caesium with 78 neutrons that means a factor of around six thousand compared with scattering off a single nucleon.

For a particle whose main problem is that it almost never interacts, a factor of six thousand is a revolution. It is the reason why a detector of a few kilograms is sufficient where thousands of tonnes would otherwise be needed.

Why it took 43 years

Freedman himself considered the detection difficult, and he was proved right.

The catch lies in the recoil. The nucleus does take up energy, but very little — it moves by fractions of a kiloelectronvolt. Distinguishing such a signal from disturbances far exceeded the measurement technology of its time.

Only in 2017 did the COHERENT collaboration achieve the first detection, at a spallation source in Oak Ridge. Between prediction and confirmation lay 43 years — comparable with the 26 years between Pauli's proposal and the first detection of the neutrino.

What has come of it

The effect is no longer a curiosity today, but a tool.

It permits portable detectors of the kind discussed for monitoring nuclear reactors. It is the channel through which neutrino communication with smaller receivers could be conceived. And at the same time it is a limit for the search for dark matter: above a certain sensitivity such detectors inevitably see solar neutrinos.

In 2025 the CONUS+ experiment detected the effect for the first time with reactor antineutrinos as well — at markedly lower energies than at COHERENT.

The second career

Freedman's best-known work lies in an entirely different area. In 1976, together with Sergio Ferrara and Peter van Nieuwenhuizen, he developed supergravity — a theory that combines supersymmetry with Einstein's theory of gravitation.

For this work the three received the Special Breakthrough Prize in Fundamental Physics in 2019.

That the same person contributed a prediction of neutrino physics and a cornerstone of quantum gravity is unusual even in theoretical physics.

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