Are there antineutrini out there? Yes, surely. But, a better question is what are antineutrini?
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The 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.
Antiparticles with an electric charge are easier to identify. Positrons and electrons have opposite charges and behave oppositely in most respects.
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.
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.
Neutrini - like electrons, muons and taus - are leptons. Naively you might think that beta decay creates two leptons: a neutrino and an electron. The thing is, the neutron actually emits an electron and an antielectron neutrino. Like electric charge, antineutrinos count as minus one lepton.
Now, before your eyes glaze over, I know. Talking about weird conservation rules like lepton number is tricky, because it seems like a bunch of silly rules the details quickly spiral out of control. Neutrino physics is nothing if not complicated.
So let’s talk more about some of the reactions.
When a muon decays into an electron, it actually emits three particles: the electron, the antielectron neutrino and a regular muon neutrino.
Given that there are so many cosmogenic muons around us, muon neutrinos - and anti electron neutrinos - are also fairly ubiquitous here on Earth.
Do Neutrini and Antineutrini annihilate each other?
And.. if the neutrino were its own antiparticle partner, well, then any two neutrini of the same flavor could do this!
Instead, physicists are looking for a slightly easier measurement with a clear signature: neutrinoless double beta decay.
If neutrini are their own antiparticle partners, it’s possible that those two electrons could come out, and the pair of neutrini would annihilate each other just as the decay happens.
If no neutrini are produced, conservation of momentum suggests that the electrons will be emitted in opposite directions, and conservation of energy suggests that their energy should sum exactly to difference in atomic mass of the parent and child nucleus.
To date, all double beta decays observed have been consistent with the emission of neutrini. Studies from experiments like EXO, NEMO, GERDA have shown that it takes nuclei over 10,000 times longer to decay without neutrini. But of course if it cannot happen - if the neutrino is NOT its own antiparticle in any capacity - then it never will.
But the search is one. The CUORE and KamLand-Zen experiments are still taking data and nEXO is still be planned.
Finally, we know that neutrini have tiny masses. Super tiny. A million times smaller than the electron, at least.
If neutrini are their own antiparticle partners, they have a special kind of mass called the “Majorana” mass. If the antineutrini are distinct particles, then their mass might well be a “Dirac” mass - which is the usual kind mass that leptons pick up in the standard model.
This distinction is of course reductive. There is no reason why they couldn’t have both a Majorana mass and a Dirac mass.
Neutrini are the only electrically neutral, elementary fermions known to science. Quarks all have electric charges. Electrons, muons and taus all do too.
What is The Field Guide to Particle Physics?
This is your informal guide to the subatomic ecosystem we’re all immersed in. In this series, we explore the taxa of particle species and how they interact with one another. Our aim is give us all a better foundation for understanding our place in the universe.
The guide starts with a host of different particle species. We’ll talk about their masses, charges and interactions with other particles. We’ll talk about how they are created, how they decay, and what other particles they might be made of.