Sciency Words: The X17 Particle

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:


If you want to know everything there is to know about particle physics, there are only four things you need to understand:

  • Gravity
  • Electromagnetism
  • The weak nuclear force
  • The strong nuclear force

See?  Particle physics is easy!

Okay, maybe particle physics is not so easy.  Even the professionals will tell you they don’t understand those four fundamental forces nearly well enough.  And now, to make matters worse, we may have to add a fifth fundamental force to the list, thanks to the newly discovered X17 particle.

The story of the X17 particle begins with this 2015 paper published in Physical Review Letters.  A team of researchers in Hungary were studying a radioactive isotope of beryllium when they noticed something odd.  Click here if you’re interested in more details about what the Hungarians noticed and what was so odd about it; but for our purposes here on Sciency Words, I think it’s enough to say that this odd thing implied the existence of a previously unknown subatomic particle.

After doing some calculations, the Hungarians determined that this unknown particle (or X particle) must have a mass just shy of 17 megaelectronvolts (hence the name X17 particle).  Follow up research at the University of California, Irvine, strengthened the case that this new particle is real, and furthermore the Irvine team argued that X17 might even be a force carrying particle—meaning we might have discovered a fifth fundamental force of nature!

The latest update is that the same team of Hungarian researchers have noticed something odd happening with another radioactive isotope, this time an isotope of helium.  And according to this prerelease paper, we are once again dealing with an unknown particle with a mass just shy of 17 megaelectronvolts.

This could be a huge breakthrough in the field of particle physics, and according to the Irvine team the X17 particle might even shed some light on the mystery of dark matter.  However, as reported in this article from Quanta Magazine, this particular team of researchers in Hungary have a history of discovering “new” particles that turn out to be errors in their data.  Furthermore, there’s some suspicion that the Hungarians are withholding some of their experimental data regarding X17.  As I’ve written before, withholding data is a huge red flag.

That being said, both professional physicists and the popular press seem to be abuzz with rumors about X17, and I can’t tell you how many people have been asking me about this whole “fifth force” thing.  So I definitely think it is worth knowing about the X17 particle (and now you can impress your friends at parties by explaining what the name means!).

However, do not be surprised if, in another few years, the X17 particle gets thoroughly discredited and debunked.

6 thoughts on “Sciency Words: The X17 Particle

    1. I don’t know enough about particle physics to know how easy a 17 MeV particle should be to find. But if CERN has been looking for this thing and can’t find it, that makes me think there’s nothing to find.

      Liked by 1 person

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