
Neutrino Experiment Intensifies Effort to Explain Matter-Antimatter Asymmetry - jonbaer
https://www.simonsfoundation.org/quanta/20131010-neutrino-experiment-intensifies-effort-to-explain-matter-antimatter-asymmetry/
======
SaberTail
It's a stretch to call this an attempt to explain matter-antimatter asymmetry.

First, let's start with how fermions (spin 1/2 particles like electrons and
quarks) get mass. They get it through a coupling with the Higgs boson, and the
strength of that coupling determines the particle's mass. Neutrinos are
fermions, so they could get their mass in the same way. However, that would
require a coupling to the Higgs much, much, much smaller than any other
fermion has, and so it's a bit of an unsatisfying solution.

However, neutrinos could be their own particles (Majorana particles), as the
article mentions. If that's the case, they can acquire mass in a different way
than all other fermions. Some of the mechanisms (called see-saw mechanisms)
that lead to small neutrino masses predict the existence of heavy neutrinos
(so heavy that we haven't observed them).

Now, a heavy neutrino would decay to the lighter particles we see today. So
there aren't any around anymore. But in the early universe, shortly after the
big bang, there was enough energy to create them.

Particles can preferentially decay to matter instead of antimatter. It's
called CP violation, and we observe it in things like heavy mesons. So if the
heavy neutrino violates CP when it decays, then that could lead to our
universe being made of matter.

But I've strung together a lot of hypotheticals: 1\. Neutrinos must be
Majorana particles. 2\. Majorana neutrinos acquire mass through a see-saw
mechanism with heavy neutrinos. 3\. Heavy neutrinos violate CP when they
decay.

This experiment might answer #1.

Number 2 would probably require studying cosmology. The mass of the heavy
neutrinos is probably greater than we could create in a collider on earth
(like grand unified theory scale).

For number 3, it can be argued that if we observe CP violation in light
neutrinos (in this case, CP violation would mean neutrinos oscillate
differently than antineutrinos), then heavy neutrinos should exhibit it, too.
But we're still trying to look for CP violation in light neutrinos. And it's
not clear that CP violation in light neutrinos must imply that heavy neutrinos
do it, too.

I agree that finding out the nature of the neutrino is important and really
cool, but I've always thought trying to argue that double-beta decay
experiments are going to solve the matter-antimatter asymmetry problem is
hyping things a bit too much. Isn't potentially measuring the mass of
neutrinos cool enough?

------
ars
How can a neutrino annihilate with another one? Where would the energy go? It
can't make a photon, and there is no lighter particle for it.

Or can this only happen with the assistance of another particle?

~~~
yk
If the neutrinos have enough energy, then they can just produce a lepton-anti-
lepton pair. But for low energies the simplest process is creation of a
virtual W pair, that annihilates to two photons. Trying to explain a bit,
because of the uncertainty principle you can borrow energy from the vacuum, as
long as the resulting particles are destroyed quickly enough. So for
intermediate stage in a process you do not need to exactly satisfy energy
conservation, but instead the resulting cross section is suppressed and the
reaction becomes more unlikely.

But the neutrino less double beta decay works somewhat differently. In a
normal beta decay, a nucleus transmutes to a daughter nucleus, a electron and
a anti-neutrino. ( X, X' are the nuclei, nubar the anti neutrino, e- the
electron.)

    
    
        X -> X' + nubar + e-
    

In a neutrino less double beta decay, this is what the article is talking
about, the anti-neutrino then reacts with the daughter nucleus to produce yet
another nucleus and an additional electron. Here the energy to create the two
electrons comes from the mass difference between the initial nucleus and the
final nucleus.

    
    
        X' + nubar + e- -> X'' + 2 e-
    

But it is only possible if the anti-neutrino is actually a normal neutrino,
since otherwise the second reaction would need to create a positron, which
would mean that the initial and final nucleus are identical, which means that
there is no mass difference to create the electron positron pair. So detection
of a neutrino less double beta decay is immediate proof that the neutrino is
its own anti particle.

~~~
ars
Thank you!

But I find it odd that the neutrino which doesn't participate in
electromagnetic force can make a charged particle (the lepton or the W).

~~~
yk
Well, there is charge conservation as well as lepton number conservation. So
at the vertex a (electron) neutrino can couple to a W+ and a e-, since the
lepton number of the neutrino is 1 and the reaction products do not have a
total charge. ( But energy-impulse conservation dictates that at least one of
the two needs to be virtual.)

Actually there is something called effective field theories, which is
essentially a technique to derive simple rules to calculate specific
processes. ( For example, one can derive a effective theory containing only up
and down quarks from the standard model. Such a theory would be sufficient for
nuclear physics, since only up and down quarks are present in protons and
neutrons.) In such a theory, which only describes the interaction between
neutrinos and photons, the neutrino would acquire a small but non zero charge.
( Or equivalently the photon would be charged under the weak force.)

------
linusekenstam
This is really insane, intense and mind-blowing...

------
gnator
It's mind-boggling how complex the universe is yet somehow we are capable of
understanding it.

