

Japanese neutrino experiment has first three candidate electron antineutrinos - dnetesn
http://www.symmetrymagazine.org/article/july-2015/a-new-first-for-t2k

======
gus_massa
A few years ago it was a popular (unproved) idea in particle physics that the
neutrinos were Majorana particles and so the neutrinos and the antineutrinos
were the same particle. Has that expectation changed? Does this experiment
disprove it?

~~~
drostie
This experiment should neither prove nor disprove it. It measures a parameter
of a phenomenon (neutrino oscillation) which is well-known and which is only
weakly affected by the premise that neutrinos are Majoranas.

So, just as a quick overview, there are two ways within the Standard Model to
give a particle mass, one is the Higgs mechanism where the particle couples
with a field generating an effective Dirac mass; another is to sum up certain
Feynman diagrams (loops and other particles and such) generating a Majorana
mass. These Majorana mechanisms effectively require that the particle be a
charge-neutral fermion, so bosons (force-carriers) are out, as are quarks,
electrons, muons, and taus. That leaves the neutrinos as the only known
particles which could get mass this way.

For a while, it wasn't even known if neutrinos had mass; we now know that they
do, but it's (m) at a scale much smaller than any of the known masses (M) of
the Standard Model, and just postulating, "hey, the diagrams work out just
right so m/M is a small number" feels like a cop-out. So one of the most
intriguing Majorana-mechanisms is the so-called "seesaw mechanism": this
postulates some symmetry breaking between the left-handed neutrinos of the
Standard Model and some new right-handed neutrinos; the left-handed get ratio
m/M and the right-handed I think get ratio M/m and it all sort of balances
out.

There are two clear "signatures" that neutrinos are Majoranas: one would be to
observe these huge right-handed neutrinos. The other is to observe two
neutrinos perfectly annihilate.

What I think you're thinking of: In the early 2000s there were claims that
experiments had observed the latter. This is a little tricky and the process
is called "neutrinoless double beta decay," since a beta-decay produces an
antineutrino and two antineutrinos can only annihilate if they are their own
antiparticle.

The general consensus now is that those early experiments weren't sufficiently
reproducible and we've never seen neutrinoless double-beta decay. However,
that doesn't shut the door completely since double-beta-decay is already
pretty rare: 76-Ge for example decays via double-beta-decay into 76-Se with a
half-life of 10^21 years, or a hundred billion times the age of the universe.
The GERDA experiment has been studying this stuff and can conclusively say
that the half-life for _neutrinoless_ double beta decay must be at least 10^25
years, and they're looking to increase their resolution to 10^26 years: so
it's less than 1/10,000 of their decays observed so far, but they can capture
it if it's more frequent than 1/100,000.

