
Is the neutrino its own antiparticle? - jonbaer
http://www.symmetrymagazine.org/article/is-the-neutrino-its-own-antiparticle
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snewman
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.

~~~
dukwon
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

~~~
andrewflnr
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.

~~~
dukwon
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.

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halosghost
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?

~~~
evanb
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)?

~~~
rotorblade
> 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.

~~~
cygx
And before electroweak symmetry breaking?

~~~
dukwon
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

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techforx
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.

~~~
saboot
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.

~~~
77pt77
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.

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

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peter303
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.

~~~
marcosdumay
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.

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

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marvel_boy
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."

~~~
JadeNB
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.

