Neutrinos are both not so small - and also even smaller - than we thought.....
- tomtierney
- May 15
- 2 min read
There is much that I don't know or understand about neutrinos but I have always clung to two simple facts: that they are neutral, and that they are very, very small. But now it turns out I may be wrong about the latter point!
I came across this story in New Scientist recently that suggest neutrinos are actually quite big - at least bigger than an atomic nucleus, and possibly much, much bigger - and its been at the back of my mind ever since, so I just sat down to try make sense of it.
Basically the research looked at the decay of beryllium, during which an electron combines with a proton to produce a neutron, thus converting the beryllium into lithium. The energy released then manifests itself in the movement of the new lithium atom in one direction, and a neutrino in the opposite direction, following the principle of conservation of momentum.
By surrounding the beryllium with detectors, a team of researchers including Joseph Smolsky in Colorado were able to measure the momentum of the lithium atoms, and use that to calculate the size of the neutrino - and found it to be at least 6.2 pm in diameter. Which is considerably larger than most nuclei!
But it turns out, thankfully, that this isn't one of those moments where I have to accept that everything I thought I knew is wrong. Because this measurement is essentially of the dimensions of the quantum wave packet, which I am simplifying in my own thoughts as its volume. And the mass of the particles is still very, very small.
And it turns out in another story from last month - their may be even smaller than previously thought.
A different research group, in Germany, studied the decay of tritium (H3) - during which an electron and and an electron anti-neutrino are created. By carrying out an impressive 36,000,000 measurements of the energy of the resulting electrons, and noting that energy must be conserved, the mass of the neutrino could be inferred. And apparently it is at most 0.45 eV. Which is about one-millionth the mass of an electron.
And this figure is half of previous estimates.
So it turns out that neutrinos possibly occupy a bit more space than we previously thought. But that they do so while having even less mass than we previously thought. So they're simultaneously not as small as we previously thought.....and also considerably smaller.
But they're still neutral, so at least that bit is easy enough to understand!