Question about majorana fermions:

@sonhouse said
https://phys.org/news/2020-04-sighting-mysterious-majorana-fermion-common.html

My question is if a particle is its own antiparticle how can it be stable, why doesn't it just disappear and emit gamma rays or some such?

It takes two.

@deepthought said
It takes two.

In a little more detail, when an electron annihilates with a positron a couple of gamma rays are emitted, but it doesn't have to be gamma rays, if there's enough centre of momentum energy it could produce pions or other particles. So first you have to have two neutrinos for an annihilation, since the only interact via the weak force the cross-section for an event is tiny. So yes, two majorana neutrinos could collide and produce two photons, there's nothing stopping that, except the cross-section would be small because the Z° mass is of the order of 80 GeV.

As a lone particle it can't self-annihilate, there really isn't any such thing. Also there aren't any different states for it to decay into. You'd need it to decay into a couple of spin 1 photons and a chargeless spin 1/2 particle - since there aren't any lighter spin 1/2 particles for it to decay into there isn't anything that can happen.

@sonhouse said
@DeepThought
What is it about these particles making it so hard to detect?

Neutrinos have been first detected almost 70 years ago. Confirming that they are majoranas is another problem.

@sonhouse said
@DeepThought
What is it about these particles making it so hard to detect?

If you mean neutrinos in general, it is because they only interact via gravity, which isn't a big thing in particle physics, and via the weak force. This means that the cross-section for detection is tiny. To give you an idea when SN1987a went off, the closest Supernova in several centuries, they saw an extra twenty five anti-neutrino detection events. It had a peak apparent magnitude of 3, which meant it was visible to the naked eye.