Are massless particles always their own anti-particle? Thanks in advance.
So, the Standard Model allows for the hypothetical existence of a particle that has zero mass and is not equal to it's own anti-particle?
Neutrinos are not massless.The standard model is a particular model with a well defined particle content. It does contain a massless particle which is not its own anti-particle in the massless neutrino. It also contains gluons of different sorts which have different colour charges and differently colour charged particles may or may not be considered as different. In addition, all standard model particles are massless before spontaneous symmetry breaking.
Neutrinos are massless in the standard model as it does not contain a right-handed neutrino and a neutrino mass cannot be added with any renormalisable and gauge invariant operator. That neutrinos are massive is one of the few indications we have that the standard model does not give the whole picture. This is not very strange as the only gauge invariant dimension five operator you can add gives rise to neutrino masses after electroweak symmetry breaking.Neutrinos are not massless.
There is absolutely no reason to expect this. The introduction of the right-handed neutrino field allows it a Majorana mass term, which immediately leads to neutrinos being Majorana, or at the very least pseudo Dirac. This was well known long before the discovery of neutrino oscillations (the seesaw mechanism was introduced in the 70s).Before it, most people probably thought that the SM meant that neutrinos were Dirac particles like every other fermion, and their right handed components were sterile, just as you would expect.
While I will agree that the SM doesn't include undiscovered fields, it certainly permits some, in the sense that some are excluded (a massive vector that doesn't couple to the Higgs) and some are not excluded.
Interesting discussion. Learning Particle Physics and the Standard Model is on my bucket list.
Question: Is the original Dirac Equation still relevant to my question? Because, during the derivation of that equation, the matter-antimatter duality arises only after the mass term is introduced - when the 2X2 Pauli Matrices are not sufficient and the 4X4 Dirac Matrices become necessary. This seems to imply that anti-matter is strictly mass related.