You probably know that atoms contain neutrons. But there’s a strange, long-standing discrepancy plaguing one of the neutron’s most basic measurements – something that a pair of scientists think might have to do with dark matter, the mysterious substance that supposedly accounts more than five times the amount of mass in the universe as regular matter does. The trouble is, we have no way of detecting dark matter directly, though we can see its gravitational effects on distant stars.
Neutrons don’t last forever, but decay into other particles, usually a proton, electron and “electron antineutrino”, after a little less than 15 minutes. But there are two different kinds of experiments frequently used to measure this particle’s lifetime, and their results differ by around 10 seconds.
This discrepancy has been around for at least 20 years, and physicists don’t know why, the authors from the University of California, San Diego write in the paper published recently on the arXiv preprint server. Perhaps the difference is due simply to errors in how one of the experiments does its measurements. Or maybe, they write, “the theory itself provides inaccurate predictions”.
The two neutron lifetime measurement techniques are called “bottle” and “beam” experiments. In the first, scientists store neutrons in a container for around 15 minutes, then count how many remain. This method puts the neutron’s lifetime at around 879 seconds, give or take a half a second. The “beam” requires the scientists to measure both the neutrons and the protons they decay into in a beam. Using those numbers leads to a lifetime around 888 seconds, give or take two seconds.
The difference between the two measurements has a four-sigma confidence level, meaning the discrepancy is highly unlikely to be caused by chance, but it’s not quite the ironclad proof required to demonstrate that something new is occurring. More data could either remove the discrepancy, or bring it to the five-sigma level that physicists associate with a discovery.
Bartosz Fornal and Benjamín Grinstein propose a solution to the problem – perhaps the difference is due to something called “dark decay”. Essentially, in those beam experiments, the assumption is that a neutron will pretty much always decay into a proton (plus some junk). But perhaps only 99 per cent of the time it decays into a proton, and the other one per cent it decays into some dark matter particle or other combination of particles that the beam experiments aren’t detecting.
Here’s the thing – if these rare neutron dark decays really occurred, researchers might be able to detect non-dark particles that carry a signature of dark matter – such as photons with a certain amount of energy, for example.
This is just a hypothesis, though theoretical physicist Susan Gardner from the University of Kentucky told science writer Edwin Cartlidge at PhysicsWorld that the idea is “feasible”. Cartlidge also points out that UCNA and UCNtau collaborations at Los Alamos are looking for the decay signature.
Just know that, yes, there are questions about even the most common particles you learned about in high school that make for interesting theoretical physics. And perhaps these particles are hiding the secret of the elusive dark matter.