The Antineutrino

The Field Guide to Particle Physics : Season 3https://pasayten.org/the-field-guide-to-particle-physics©2022 The Pasayten Institute cc by-sa-4.0The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov.Also check out the links embedded this description. Or also check out those same links at:https://pasayten.org/the-field-guide-to-particle-physics/antineutrinoThe Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch.The AntineutrinoThe neutrino is a curious particle. As fundamental as the electron or the muon, but rarely interact with other particles. This makes the study of these neutrini quite challenging. But also quite interesting.Are there antineutrini? Yes, surely. But, a better question is what are antineutrini?Antiparticles with an electric charge are easier to identify. Positrons and electrons have opposite charges and behave oppositely in most respects. Photons and neutral pions do not have any electric charge. They are their own antiparticle partners! But this isn’t always the case with neutral particles.  As we have antineutrons and two distinct kinds of neutral kaons: the K0 and K0bar which are antiparticles of each other.Neutrini - those smallest of massive matter particles in the Standard Model - are electrically neutral. So it is natural to ask: are they their own antiparticle? Or are there distinct antineutrini? And importantly, how can we tell the difference?The short answer is, we don’t know yet. End of story. But the short answer is boring.Neutrini are famously shy and interact only via the weak nuclear force - and gravity - so detecting them so detecting them is no small task.So without further ado, let’s go ahead with the long answer.Beta DecayNeutrons decay to protons by emitting an electron. This is usually called beta decay, and is mediated by the W- boson. Other nuclei experience it as well.  Detailed studies of beta decay suggest that the neutron should decay into two particles rather than one. That second particle was need to make sure that energy, momentum and spin angular momentum was conserved. As it should be.The neutrino - the small neutral one - was discovered nearly 26 years after their proposal.Now, electric charge is conserved in beta decay. The uncharged neutron decays to a positively charged proton and a negatively charged electron and a neutrino. The neutrino also has no electric charge, but carries away some of the energy and some of the momentum.So far as we can tell, energy, momentum and spin like electric charge, is always conserved. Such conservation laws are useful organizing principles for understanding the laws of particle physics. Some might argue they are foundational.Another thing that seems to be conserved in nature - usually anyway - is the number of leptons in the universe. There are actually quantum effects that can change the number of leptons, but in ordinary decays - like beta decay -  they seem to conserve the number of leptons.Neutrini - like electrons, <a h