Is the neutrino its own antiparticle? | Hacker News Reader

Is the neutrino its own antiparticle?

symmetrymagazine.org

92 pointsjonbaer33 comments

The article says this could explain "why matter won out over antimatter in the early universe." Can someone explain that to me?

> If neutrinos are their own antiparticles, it’s possible that the antineutrinos emitted during double beta decay could annihilate one another and disappear, violating lepton number conservation. This is called neutrinoless double beta decay.

> Such a process would favor matter over antimatter, creating an imbalance.

If the initial universe contained equal amounts of matter and antimatter, wouldn't there also have been "anti double beta decay"? (Where an antimatter nucleus decays into a different anti-nucleus and emits two positrons and two neutrinos; and then the two neutrinos could annihilate one another and disappear.) They haven't explained where the asymmetry comes in.

CP violation.

The rates of some weak-force processes are measurably different when you replace particles with their antiparticles. This has been observed in processes involving quarks for over 50 years now. We haven't quite done it with leptons yet because of the difficulties involved in detecting neutrinos.

See also the Sakharov conditions for baryogenesis

All this time I thought it was a big mystery why there's more matter than anti-matter, but now you're telling me we've known this whole time that the rules were different between them? All we're waiting for is to find out what specific processes made it work out? I've always gotten this breathless "how could it be?" impression when people talk about that issue.

To be fair, that's a pretty common reaction to most of advanced physics.

CP violation is a necessary component, but it doesn't completely explain the whole matter-antimatter asymmetry.

We still haven't ever observed a process that doesn't conserve baryon or lepton numbers, which is another crucial component.

Disclaimer: I am not a particle physicist

In the theoretical physics courses I took in college, we spent a while on anti-matter, and one of the things that the professor mentioned was that it is widely accepted that photons are their own anti-particles as well. Researching this now (read: googling for sources that seem legitimate), that seems to still be widely accepted.

However, I have never read anything that suggests that photon collisions would lead to annihilation. Is there a reason such might be the case for {anti-,}neutrinos?

Physicist here: yes, photon collisions can lead to annihilation ... and production of a particle--anti-particle pair.

Then why don't things happen when you point two lasers at each other?

Scattering happens in this situation, but for ordinary lasers, it's an undetectably tiny effect. You need particle accelerator scale energies and very high densities to see anything.

Because photon density in your little laser is far below the levels required to make this phenomenon observable.

It can be done, but it requires very high power laser beams, focused very tightly, and even then the events are pretty rare.

Yes. One quick and not-exactly-honest answer is that photons are gauge bosons and thus are automatically their own antiparticle, while neutrinos are fermions and the symmetry is not guaranteed. Beyond that, though, I don't know how to explain it without delving into some of the structure of quantum field theory and the Standard Model.

Edit: and yes of course, as aroberge points out, two photons can collide and produce a particle/antiparticle pair. The real question is: can two neutrinos collide and pair produce? Or do you need a neutrino and an antineutrino (assuming the are Dirac in nature)?

> photons are gauge bosons and thus are automatically their own antiparticle

So is W^+, but it is charged, hence not its own antiparticle. W^- should in this case be considered the antiparticle.

And before electroweak symmetry breaking?

Before EWSB you have four massless bosons: W+, W−, W0 and B0

The W0 and B0 mix to give the photon and Z0

The W+, W− and Z0 gain mass and the photon is left massless

> I have never read anything that suggests that photon collisions would lead to annihilation.

You may at least have heard that a particle antiparticle collision can lead to the production of two photons. Viewed in reverse this would be two photons annihilating and producing a particle antiparticle pair.

Since the laws of physics work equally well in reverse two photons must therefore be able to annihilate.

Two photons "annihilating" to two photons is... well.. photon-photon scattering, but you can also produce other particles. High energy gamma rays pair produce into electron positron pairs with the cosmic microwave background, so at high enough energies, they don't go very far through the universe.

https://en.wikipedia.org/wiki/Two-photon_physics

Is anyone able to explain the comment by Friedland:

"Theoretically it would cause a profound revolution in our understanding of where particles get their mass”? [6 paragraphs from the end]

Presumably neutrinos and antineutrinos being the same entity would somehow indicate our current understanding of the Higg's field is wrong (since it's the interaction of particles with the Higg's field that we understand creates mass, right?) - but there isn't any indication of why this is the case.

I'm not 100% positive on this so please take my comment with a grain of salt.

In the same paragraph, Friedland says "It would also tell us there has to be some new physics at very, very high energy scales—that there is something new in addition to the Standard Model we know and love."

Since the Higgs was discovered by using more and more power to power the particle, I believe they're implying that there is something above that level. Since energy and mass are intertwined, if they can generate different particles at higher energy collisions, there could more to it then just the Higgs field.

Hopefully, someone can explain better then I.

I'm not sure how to explain it in non-technical terms, but the problems of neutrinos and their mass is basically:

One is not allowed to add mass-terms to the Standard model (that is a term in the expression that explains the particle physics we know of), because they break symmetries that we know hold (gauge symmetry). So in order to add a mass-term it must be generated by a dynamical field -- the Higgs field -- such that those symmetries are still respected. Hence, the Higgs is able to give the mass-terms to the fermions.

However, the neutrinos are not like the other fermions. Fermions can have different chirality, but the neutrino only has one chirality. Due to the neutrinos being different, the Higgs field needs to be different in order to also generate mass-terms to the neutrinos (or another mechanism than the Higgs-mechanism must be used). Whatever that is, it is probable that it involves something that is beyond the Standard model.

Might be a bit technical, hopefully someone else can make it more understandable.

If neutrinos are their own anti-particle, they do not get their mass from the higgs field. Most particles have a 'dirac mass' which arises from a coupling with the Higgs Field. An alternative is to have a 'majorana mass' due to the interactions of the two majorana particle fields. As a consequence a super-heavy neutrino with very large mass would be predicted.

This is the correct answer. the term people should search for is indeed majorana mass.

I would just like to add that most of the mass we observe doesn't come from the higgs but from QCD.

the higgs part is minuscule.

As a non-physicist, I very much appreciated the clarity and simplicity with which the article explains what is going on. No mean feat.

Does math always predict physics? Diracs equation predicted anti-matter and SU(3) symmetry additional baryons. Einsteins GR equation has oscillatory solutions. But no experiment has seen gravity waves yet.

* Maybe. http://www.skyandtelescope.com/astronomy-news/about-this-wee...

Well, it is a good feature of a theory. Imagine that you would have two competing theories but they give different predictions, then a measurement on those predictions would determine which is which. (If they give the same predictions, an interesting questions would be 'is there a mathematical proof that they are the same?'.)

For example with GR, it explained for example the precession of Mercury's elliptical orbit, and predicted time dilation (used now in GPS for example) and bending of light. It is true that gravity waves remains to be directly detected.

This is how physics have usually progressed in the past: Have an old theory, have some experiments not explained by that theory, invent new theory that can be reduced to the old theory and explains these new experiments and give new predictions. (The subject of string theory is somewhat different however.)

We keep the math that predicts physics, and throw everything else away.

So, it's too easy to assemble a small narrative of history that gives this impression, and forget every other model that people crated trying to get to those ones that hold.

Even though (LIGO's rumored result notwithstanding) there is no direct evidence for gravity waves, the Hulse-Taylor binary pulsar was good enough indirect evidence for the Nobel committee.

We haven't seen direct evidence of gravitational waves yet (unless the current LIGO rumors pan out!), but there's already compelling indirect evidence. The most specific example I know of comes from the Hulse–Taylor binary system (https://en.wikipedia.org/wiki/PSR_B1913%2B16), in which a pulsar and another neutron star orbit each other every 7.75 hours (separated by a variable distance of roughly the diameter of our Sun).

The period of their orbit is decaying over time, at a rate that is in precise agreement with the predicted energy loss due to gravitational radiation in such a system (the observed rate of energy loss is 99.7% +/- 0.2% of the gravitational wave prediction). So while we haven't seen them directly yet (they're really, really hard to detect!), there's very little doubt that they're out there more or less exactly as Einstein predicted.

You have some theory x. Based on that theory, you can use math and logic to predict what else should be true. As a consequence of x, let's say that following the math predicts that y should exist. We already know that y exists, so we have some reason for believing x to be true. x also predicts that z exists, but we haven't discovered that z exists -- based on that should we assume that z exists but we haven't discovered it, or that x is wrong? I think probably either conclusion is fine. But if we later discover that z exists, that gives us _really_ strong evidence that x is true.

It's not exactly that math always predicts science, but that math allows us to filter theories based on what we else we know to be true. If working out the math based on your theory gives us answers that don't match reality, then there's probably something wrong with your theory.

That's a pretty deep question that a lot of smart people have tried to address. For example: https://en.wikipedia.org/wiki/The_Unreasonable_Effectiveness...

I'm curious what the practical implications of this might be. Perhaps an easier way to store antimatter or modulate it's reaction.

This reminds me a Latin proverb "Homo homini lupus" meaning "A man is a wolf to another man," or more tersely "Man is wolf to man."

Is that just because it's a description of a thing standing in (any) relation to itself? It doesn't seem to have anything to do with the physics, which is the interesting part here.

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