In the world of physics, nothing gets the blood flowing like the thought that a new particle has been discovered. For decades now, physicists have been hunting for evidence that modern physics isn’t right. But, the smoking gun has remained elusive. In November, people started polishing a Nobel prize for a group of physicists who seemed to have found new boson. And, a new boson means a new force, which is even more exciting. Is that gun smoking yet?
Sometimes the gun just smolders
This result has been cooking for quite some time. The first experimental results date back to 2015, with publication in 2016. Essentially, the scientists took some lithium and shot protons at it. By choosing the energy of the protons correctly, Beryllium in a particular excited state is produced, which quickly decays back to its ground state by emitting an electron and a positron, and from there, back to lithium. Now, in these experiments, energy and momentum must be conserved. The lithium nucleus is quite a complicated beast and can rattle around in all sorts of ways, meaning that the electron and positron have a certain amount of freedom in the direction in which they are emitted.
By contrast, the researchers observed that some electrons and positrons seem to be correlated in their emission direction. Computer modeling confirmed that this was not due to their equipment and could not be explained by the nuclear physics of beryllium, lithium, or any known background process. The correlation could, however, be explained by a new boson that decayed by emitting a positron and an electron. As long as the production was reasonably inefficient, and the mass was about 17MeV (million electron volts), then the data was beautifully explained.
The paper really got the juices flowing. Theorists jumped on the result so fast they inadvertently broke special relativity. Experimental physicists went back through old data searching for confirmation. Operating experiments were tweaked to look for the new particle.
Experimentally, it all seemed a wash. Old data revealed nothing, but, at the same time, those experiments weren’t exactly set up to look for the right particle. New experiments were still too fresh to be conclusive (even if they had started taking data).
Look, if we just tweak the Universe…
The theory situation is even more of a mess. It is always possible to extend our models of the Universe to include new particles, including new bosons and new forces. But, it isn’t good enough to match a single experimental result. You have to match all of them. The end results are particles that look a bit like a backyard panel-beating job. Yeah, the paint matches, but you can still see the wavy patches where the filler hasn’t been sanded flat. The problems arise from the mass—17MeV is at the low end of well-explored territory.
So, why did this story flare back up again? A new paper, by the same scientists that published the beryllium results. This time, they measured electron-positron emissions from excited helium. Same experiment, different atom, but the same 17MeV boson was found.
The new result is pretty strong evidence. If the experiment has some kind of systematic error in it, then we would expect that the “new” particle would change mass between helium and beryllium. It doesn’t, though; the results are very consistent between experiments. That means that if it is an error, it is an unfortunately flukey one.
I think more scientists would be happier to accept the result if it fit their expectations. An axion with a mass as small as a few MeV? Sure thing. A giant WIMP with a mass of many GeV? Ok. But, a boson that is lighter than a proton and kind of middle of the road? Why haven’t we seen that before?
There may also be, I think, a certain amount of unconscious snobbery in the background. The experimental results haven’t come from any of the big labs. And now the big labs are going to be putting planned experiments on hold to see if a result that they won’t get credit for stands up. If they find the boson, then, great, they’ve won plaudits for someone else. But, if that gun doesn’t smoke, there will be a long and painful search for what makes the original experiment different from the rest.
ArXiv.org, 2019, ID: 1910.10459
https://arstechnica.com/?p=1635665