The Field Guide to Particle Physics

Are there antineutrini out there? Yes, surely. But, a better question is what are antineutrini?

Show Notes

The Field Guide to Particle Physics : Season 3
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The Antineutrino

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.

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 Decay

Neutrons 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, 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.

The math also works in reverse. If a nucleus absorbs an electron - which sometimes happens in certain isotopes of Vanadium, Nickel and Aluminum - it will convert a proton to a neutron, and spit out a regular neutrino. Conserving the number of leptons.

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.

Flavors of Antineutrini

Each electrically charged lepton: the electron, the muon and the tau, has it’s own flavor of neutrino. There’s an electron neutrino. A muon neutrino and a tau neutrino. Each electrically charged antilepton also has its antineutrino partner: antielectron neutrino. anti muon neutrino. Anti tau neutrino.

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.

And of course you might remember the famous experimental result that neutrinos can change their flavor as they move. So neutrinos flavors can get all mixed up, just like antineutrino flavors can get all mixed up. But do neutrini get mixed up with antineutrini?

They would if they were the same particle, wouldn’t they? Let’s think about it another way. In terms of annihilation. 

Do Neutrini and Antineutrini annihilate each other?

When an electron and positron collide, a pair of photons usually comes out. The antiparticle partners annihilate into pure electromagnetic energy. What do you suppose happens when a neutrino collides with an antineutrino?

A neutrino and an antineutrino - assuming it exists - would not annihilate to form photons. They have no electromagnetic charge and therefore no chance. They could potentially exchange a Z-boson, or even a Higgs Boson! Although the likelihood of the latter is proportional to the mass of the neutrini involved - and so very, very small.

If a neutrino-antineutrino pair of the same flavor smashed against each other violently enough, a pair of Z-bosons could come out. 

And.. if the neutrino were its own antiparticle partner, well, then any two neutrini of the same flavor could do this!

Such an annihilation of two regular electron neutrini would be strong evidence that the neutrino is its own antiparticle. But what a challenging experiment that would be! Where would you get dedicated, high energy neutrino beams?

Instead, physicists are looking for a slightly easier measurement with a clear signature: neutrinoless double beta decay.

Rarely, nuclei emit two electrons at time, converting their atomic number by changing two neutrons into two protons simultaneously. Germanium-76 and Xenon-136 are just a few of the many nuclei that undergo 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.

Neutrino Masses and the Seesaw

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.

In fact, if they do have both, then there is a very natural explanation for why the neutrino mass is so small compared with all the other fundamental particles.

If the Majorana mass is really, really big, say associated to some complicated physics we don’t yet understand, and the Dirac mass is “normal” by comparison to other particles, like a few thousand electron volts, the combination of those two masses we experience actually appears as a ratio of the two, rather than the sum. 

This is the famous seesaw mechanism

Neutrini are the only electrically neutral, elementary fermions known to science. Quarks all have electric charges. Electrons, muons and taus all do too. 

It is perhaps no surprise that neutrino physics is uniquely complicated. And if there’s one thing particle physics enjoys, it’s being complicated.

©2022 The Pasayten Institute cc by-sa-4.0

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.