Hacker News Reader

Comment by dmix | Hacker News Reader

> Examining the number of electron–positron pairs at different angles of separation, the researchers found an unexpected surplus of pairs at a separation angle of about 140º.

I keep reading about this "angle" in regards to beryllium-8 and X17. Can someone ELI5 for a non-physics person what it means for particles to come off at a certain angle?

View on HN

Imagine a slow moving blob heading towards a target on a wall, a small charge causes it to explode at some point along its path to the wall. The larger the charge the wider the splatter. In this case, the mass is the analogy to that detonation charge. If that blob was photons alone (which are the standard model assumption) you'd expect a smooth decreasing amount of splatter as you go away from the center of the target, what they seem to be seeing is an unexpected increase around 115 - 140 degrees.

Its pretty much what it sounds like, when there are particle collisions or decays, the particles come off at certain characteristic angles relative to the beam that are signatures of certain types decay and energies.

https://en.wikipedia.org/wiki/Rutherford_scattering

In this video, look for the forks that happen in the vapor trails being generated, that is a particle collision, and the corresponding sort of angle being talked about here.

https://www.youtube.com/watch?v=-B2cvZgLToY

This technique works because the air is essentially super saturated with moisture, so even very slight atomic level disturbances will cause the water to condense to form mini "clouds" (hence the term cloud chamber).

Its difficult to find videos about, but antimatter pair production will cause patterns that look like this for instance

https://i.stack.imgur.com/9CKRA.jpg

muons, electrons, and alpha particles of different energies will will cause patterns that look like this

https://i.stack.imgur.com/NwNcS.jpg

and so on.

Check out http://avtanski.net/projects/cc/virtual_cloud_chamber.html and https://www.nucleonica.com/wiki/index.php?title=Help%3AVirtu... and see if you can identify some of the particles :)

Thanks very helpful, the angles being signatures to help determine what forced/objects are involved makes a lot more sense to me now. That's always been something I've struggled with in biology/chemistry and physics is the higher level abstractions used to infer smaller phenomenons that are not as easily observable.

I'm too used to compsci where it's more explicit and every "but why?" doesn't lead you down a Wikipedia rabbit hole.

Edit: that cloud chamber video is very cool!

It's a literal angle, like you learned in geometry class. After the decay, two particles fly off in different directions. They mean the angle between those directions.

Apologies I meant what the significance of the particles bouncing off at certain angles, particularly down to the degree as others have mentioned the different ranges reported. My short google search earlier didn't help me much. Others have provided some links I will dig into.

You can use the angle (in conjunction with trig applied to various conservation principles such as conservation of momentum) in an attempt to derive quantities like mass.

I might be mistaken but from my college chemistry I believe this is referring to VSEPR theory. Where electron pairs around a nucleus will push against each other to maximize the distance between other pairs in spherical space. So two electron pairs would be separated by 180° (line) and 3 by 120° (3 lines on same plane), 4 by 120° (tetrahedron) and so on.

So if it's 140° then something else is pushing against them as it should never be 140°.

Someone more knowledgeable correct me if I'm mistaken.

Reply on news.ycombinator.com