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The distinction between fundamental particles and composite particles is more a statement about the extent of human knowledge rather than anything physical abou

The distinction between fundamental particles and composite particles is more a statement about the extent of human knowledge rather than anything physical about those particles. That is, saying "this is a fundamental particle" tells you that we haven't discovered any constituent parts to it, not that there aren't any. Protons used to be fundamental particles until we discovered they were made of quarks, and quarks may yet turn out to be composite particles built from preons. So yes, if you were attracted to this article because you thought they found a new fundamental particle, then it was clickbait. But the real point is that it's a Majorana fermion (with well-defined momentum eigenstates, a scattering matrix, etc.). It's possible in the future that we discover neutrinos are Majorana fermions https://en.wikipedia.org/wiki/Double_beta_decay#Neutrinoless_double_beta_decay https://en.wikipedia.org/wiki/Double_beta_decay#Neutrinoless... at which time they will be "fundamental Majorana fermions"... and then discover after that the neutrinos are just composite particles. https://arxiv.org/abs/1510.07988 https://arxiv.org/abs/1510.07988 (There are some subtleties here. Some quasi-particles don't have all the properties we normally associate with fundamental particles. And there are weak theoretical arguments that some fundamental particles can't be composite unless their constituents have properties that are subjectively inelegant.)

gus_massa 9y ago
The gyromagnetic ration of a truly elementary particle is g=2. With the current theories, a electron has a cloud of virtual particles, but no internal structure. With this assumptions the theoretical value is approximately g=2+2.alfa/2pi+...~=2+1/(137*pi). The experimental value of the g of the electron agree with the theoretical value up to 14 digits, so it's a very good hint that the electron is really an elementary particle. Obviously there can be future theory were the electron has internal structure, but it will be a breakthrough, not a small fix. The theoretical and experimental value of g for a muon almost agree, but only 10 digits. So there is a small chance of something interesting there. But IIRC most people think that it may be a new particle in the cloud of virtual particles, not an internal structure of muons. Protons and neutrons have internal structure, a lot of internal structure, so the value of g is a difficult to calculate number that is not 2. In particular, g=+5.6 and g=-3.8.
- jessriedel 9y ago
  This is just a way of saying that we have probed the electron for internal structure to a significantly more precise level than the level necessary to reveal the known internal structure of nucleons. But it's just an upper bound. For all we know, the next accelerator (if there is one) could reveal composite structure just below the bound.

gus_massa 9y ago

The gyromagnetic ration of a truly elementary particle is g=2. With the current theories, a electron has a cloud of virtual particles, but no internal structure. With this assumptions the theoretical value is approximately g=2+2.alfa/2pi+...~=2+1/(137*pi). The experimental value of the g of the electron agree with the theoretical value up to 14 digits, so it's a very good hint that the electron is really an elementary particle. Obviously there can be future theory were the electron has internal structure, but it will be a breakthrough, not a small fix. The theoretical and experimental value of g for a muon almost agree, but only 10 digits. So there is a small chance of something interesting there. But IIRC most people think that it may be a new particle in the cloud of virtual particles, not an internal structure of muons. Protons and neutrons have internal structure, a lot of internal structure, so the value of g is a difficult to calculate number that is not 2. In particular, g=+5.6 and g=-3.8.

jessriedel 9y ago
This is just a way of saying that we have probed the electron for internal structure to a significantly more precise level than the level necessary to reveal the known internal structure of nucleons. But it's just an upper bound. For all we know, the next accelerator (if there is one) could reveal composite structure just below the bound.