A joint Fermilab/SLAC publication

A new way to measure neutrinos

06/13/08

The rate at which certain unstable atoms decay can be affected by neutrino mixing, according to a recent experiment at the Experimental Storage Ring (ESR) of the GSI nuclear physics laboratory in Darmstadt, Germany. What should be a relatively straightforward tale of radioactive decay is complicated by the role of quantum physics. The process could provide physicists with a new way of measuring neutrinos.

Radioactive decay is a fundamentally random process. However, when you look at a large number of decaying atoms, you see characteristic patterns that reveal the rules behind the randomness. The rule for radioactive decay is pretty simple: the probability of an atom decaying in a given period of time is a certain constant number for each species of atom. However, that simple rule can lead to behavior that seems complicated.

For example, imagine you have a radioactive atom that has a 10 percent chance of decaying in any one second period. If you start with 100 of those atoms, then after one second, you would expect 10 to have decayed and to have 90 left. For those remaining 90 atoms, they still each have a 10 percent chance of decay in the following second-the chance of decay in any particular second does not increase as time goes on. So after two seconds, we expect another nine decays, leaving 81 undecayed atoms. And so the process goes. This leads to what is called an exponential decay curve which, for a large number of atoms, is a nice smooth curve that tells you how many atoms are left in the sample.

In the experiments at GSI, praesodymium-140 and promethium-142 decay rates were measured but there wasn't a smooth exponential decay. Instead, there were periodic bumps in each of the graphs. The number of decays each second rose and fell over a cycle about 7 seconds long, but superimposed on the overall exponential decay. The experimenters went to great lengths to rule out any kinds of effects from the equipment they were using and were led to conclude that the variation is due to neutrinos.

The basic idea of the process is this: The radioactive atom starts with all but one of its electrons stripped away (partly for ease of measuring phenomena such as this). A process called electron capture occurs, in which the remaining electron is absorbed into the nucleus, turning a proton into a neutron. (It's the opposite of beta decay.) At the end of this process, an electron neutrino is also emitted, although there is no attempt to measure that neutrino directly.

This should be a straightforward process with no real opportunity for neutrino mixing to play a role, and no reason for the odd bumps in the decay pattern. However, quantum mechanics rears its head due to the particular configuration of this experiment. (Some readers will have noticed that quantum mechanics already comes into this to produce the randomness of the decays in the first place, but here we are talking about an additional role for quantum physics.)

Because the neutrino is never detected and the energy and momentum of the atoms are never measured precisely enough, quantum mechanics allows the decay to happen via a number of different paths involving different neutrino states but which cannot, even in principle, be differentiated. Precisely because the scientists can't look at all the internal workings of the decay, extra quantum stuff can go on.

The different paths to decay involve different energies of the neutrino states, and that means, because of Heisenberg's uncertainty principle, the timing of the decays can be affected. So as the different types of neutrino get mixed together in this hidden process (just as happens on larger scale neutrino oscillation experiments), that mixing is converted into oscillations of decay time. It is that oscillation which is observed in the GSI experiments. The period of oscillation is determined by how much the neutrinos mix together. Future theoretical calculations should be able to match up what is expected with what is measured.

Of course, this is a very complicated process and the experimenters don't know for sure that this is the actual process. However it seems reasonable based on a toy model created by Harry Lipkin, which indicates that experimenters should expect to see the strange bumps with approximately this period in the decay.

The most exciting aspect of this work for many people interested in neutrinos is that it provides physicists with an entirely new technique to measure properties of neutrinos that is complementary to the large long-baseline neutrino oscillations experiments and other large-scale experiments.

You can read more about the experiment in a more technical review of the work at Nature.

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