Thursday, January 11, 2018

Is Dark Matter Hiding Right Under our Noses?

Consider the lowly neutron. Neutrons make up about half the mass in your body, but they contribute nothing to chemistry. They just lounge quietly inside the atomic nuclei and get carried along for the ride. But this past week, two physicists at the University of California, San Diego, suggested that the neutron might be the key to unlocking the mystery of dark matter in the universe.

Their argument is based on a weird anomaly in the properties of the neutron. Neutrons are stable when they live inside of nuclei, but a lonely free neutron is unstable and will decay into a proton, with a lifetime of about 10 minutes. And that lifetime is surprisingly hard to measure. That's because it's very hard to keep neutrons in captivity. Imagine trying to hold a bunch of neutrons in a bottle -- since they have no charge, there's no force to hold them inside the bottle, and they'll leak right out. So scientists have resorted to two different approaches to measure how fast neutrons decay. One is to cool the neutrons down until they are so cold that they just bounce off of the bottle walls.  Then scientists can count how fast the neutrons are disappearing. And the other approach is to make a beam of neutrons and then count the protons that these neutrons decay into.

Surprisingly, these two methods give two different answers for the neutron decay rate. This anomaly has been chalked up to "systematic effects" (the experimentalist's catchphrase for defects in the experiment that throw off the results). But the California physicists have suggested something far more radical: maybe the reason these experiments give different results is that not all of the neutrons are decaying into protons. This could account for the difference in the two experiments because the bottle experiments measure the total decay rate of the neutrons, while the beam experiments measure the decay rate only into protons. But if a small fraction of the neutrons are decaying into something else besides protons, what is the "something else"? Maybe they are decaying into dark matter!

As usual, the reader is advised to treat all speculations of this sort with skepticism. But it would be deeply ironic if the dark matter particle were produced in plain sight from the decay of the most ordinary particle of all, the neutron.

And now let me close with a classic very bad physics joke.  A neutron walks into a bar and orders a drink. The neutron asks, "How much for the drink?" and the bartender replies, "For you, no charge."


Dave Caudel said...

Glad to see you back at it =D

Fun to read as always.

Rob Knop said...

Hmm... presumably one could envision an experiment to look for this the same way the neutrino was originally "discovered". (Of course, the neutrino was proposed as a way to keep conservation of energy and momentum sound.) The neutron decays into the proton and the positron, both of which are easy to detect. If you can make a neutron beam (that sounds hard), look for an admixture of decays that aren't proton + positron, and that is missing energy and momentum.

Robert Scherrer said...

The paper suggests a few possible experimental signatures. The mass of the dark matter particle in this scenario ends up being very close to the neutron mass.

Kathy said...

I rather hope this is the case. I kept wondering why dark matter should be absent from Earth when it makes up the major part of the universe, and it is affected by gravity.

A police officer stops a photon on the interstate and asks: "Do you know how fast you were going?"

"No," answers the photon,"but I can tell you where I've been."