How big is a neutrino? We’re finally starting to get an answer

Our estimates of the size of a neutrino span from smaller than an atomic nucleus to as large as a few metres, but now we are starting to narrow down its true value

By Alex Wilkins

12 February 2025

Pinning down the size of the neutrino is a tricky task

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The first direct measurement of the size of the neutrino, a fundamental particle, suggests they are at least larger than an atomic nucleus – but they could potentially be trillions of times larger.

Part of the problem in answering this question is that, rather than being spherical, quantum mechanics tells us that particles are inherently fuzzy waves, moving and vibrating as they travel through space. Physicists mark the boundaries of a particle, and thus its size, by looking for its wave packet, an area inside which the wave vibrates strongly, and beyond which it sharply trails off.

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For neutrinos, measuring the wave packet is particularly challenging because these particles rarely interact with normal matter. Until now, we have only calculated the wave packet’s size indirectly, with estimates spanning a range of 13 orders of magnitude – from smaller than an atomic nucleus to as large as a couple of metres, or 10 trillion times bigger.

Now, Joseph Smolsky at the Colorado School of Mines and his colleagues have made the first direct measurement of the wave packet, finding that neutrinos must be at least hundreds of times larger than the previous smallest estimate, making them larger than typical atomic nuclei.

To do this, Smolsky and his team measured radioactive beryllium as it decayed into lithium, a process called electron capture. When this happens, an electron in the beryllium atom combines with a proton in its nucleus, producing a neutron. This transforms the beryllium atom into lithium, producing a kick of energy that fires the atom in a certain direction and generating a neutrino that fires in the opposite direction to balance the momentum. By putting the beryllium inside very sensitive superconducting detectors and studying it using a particle accelerator, they could then measure the lithium atoms extremely precisely and infer the neutrino’s properties.