Ultra-Cold Atoms and Neutrino Masses

Physics World‘s news aggregator had a story yesterday with the headline Chilly solution to neutrino mass problem, and the one-sentence teaser

Ultracold atoms could be used to measure the mass of the neutrino.

This creates a wonderful image of somehow turning a magneto-optical trap or a Bose-Einstein Condensate into a neutrino detector, which is a nice thought, but highly improbable. Even a BEC has a density a million times less than the density of air, and a volume that’s way too small to catch any neutrinos.

So what’s going on? The answer is cool in its own right, and the path from the proposal to the news story is a decent example for the endless discussion of bloggers vs. journalists.

The story is based on a proposal by Mark Raizen and Joshua Klein for a way to use cold tritium atoms to measure the mass of the neutrino. Tritium is hydrogen with two extra neutrons, and it tends to decay by turning into helium: one of those neutrons turns into a proton (through the weak nuclear interaction), spitting out an electron and a neutrino.

The proposed experiment is to trap a large amount of tritium at very low temperatures (meaning that the atoms are very nearly stationary), and look at the recoil of the helium that’s produced. When the tritium decays into helium, one of two things happens: either the helium captures the electron on the way out, becoming neutral helium, in which case the atom recoils in a direction opposite the direction of the neutrino; or the electron and neutrino both escape, in which case the helium ion recoils in a direction that depends on the exit direction of both the electron and the neutrino. In either case, the helium is moving, and if everything is done right, it’s moving considerably faster than the trapped tritium atoms.

To measure the neutrino mass, then, all you need to do is detect the helium and measure both the magnitude and direction of its velocity. If the electron was captured, that alone is enough to let you find the momentum (and thus mass) of the neutrino; if the electron escaped, you need to determine its velocity as well, but again, you can calculate the momentum of the neutrino.

It’s a nifty idea. How well does it work? Well, the paper in question is just a proposal, so all they can do is estimate what it might be able to do. From the news story:

Raizen and co-workers have worked out that by trapping some 1013 tritium atoms — the maximum that can feasibly be attained over the course of about a year – they would reach a somewhat disappointing upper limit on the neutrino mass of around 9 eV with the first approach, but could get down to roughly the limit possible with KATRIN — 0.2 eV — with the second.

These figures, however, were generated using a computer simulation. Whether the 0.2 eV limit could be reproduced in an actual experiment — which the researchers hope to carry out within the next decade — depends on overcoming several significant engineering challenges, not least of which is finding out how many atoms can in fact be trapped.

(KATRIN is a neutrino mass experiment being built by a large collaboration.)

This is where the bloggers vs. journalists thing comes in. While it’s a neat idea for an experiment, the headline and teaser sentence make it sound a lot more solid than it is. In reality, this is a long way from being a real neutrino mass experiment, and there are an awful lot of “if”‘s involved in those estimates. Cold atom physics has provided one of the few examples of Murphy violation, but that’s not something you can count on.

One of the persistent complaints of people who hold that science bloggers are more noble and virtuous than grubby old journalists is that news stories tend to exaggerate results to make them sound sexier than they really are. This might well count as an example of that, and I’m sure that there are some physics bloggers out there who would probably mutter angrily about how this story distorts science. And I do agree that the headline and teaser are a little exaggerated.

On the other hand, though, this is a pretty straightforward translation of what’s in the original paper. And it is a very cool idea, and having the idea out there as a potential neutrino detection method might inspire somebody else to think about this sort of thing from a slightly different angle, and come up with some refinement of the idea that will work even better.

So it’s a tough call. In the end, I tend to think that the idea is cool enough to be worth promoting, even if it involves a little bit of exaggeration. But then, I’m a sucker for table-top atomic physics experiments that can get at high-energy physics problems. Other people might feel differently.