The standard model of particle physics, with its quarks, leptons, and bosons,
has served scientists incredibly well since it was first put forward in 1967. For the most part, it
has correctly predicted the existence of particles with such precision that it’s often hailed as the
most successful scientific theory of all time.
And yet scientists are not done with it, and they’re
constantly probing around its edges hunting for new particles. In fact several teams of
scientists are racing to discover what’s known as a Majorana fermion, which could be a major key to
settling some of the universe’s biggest mysteries.
Fermions are matter particles like the quarks
that make up protons and neutrons, as well as electrons and neutrinos. Fermions also include
corresponding antiparticles with very similar properties except they have opposite charge, so
the antiparticle of a negatively charged electron has a positive charge and is known as a positron.
Should a particle and its antiparticle meet, the two will annihilate each other, leaving behind
only energy. But a Majorana fermion would play by its own rules that could totally upend
our understanding of the Standard Model. In theory a Majorana particle doesn’t have a corresponding
antiparticle; it is its own antiparticle! That means when two of the same particles meet,
they could wipe each other out. So where would we even begin to look
for a Majorana particle? As it happens scientists have already identified a candidate
from the Standard Model; the neutrino. Neutrinos are bizarre little things for more reasons than
just their famous ability to pass right through whole planets. Unlike electrons and positrons
which both can have right or left-handed spins, neutrinos all have left-handed spins
while antineutrinos are all right-handed. To explain this, one idea is that maybe
antineutrinos aren’t antimatter after all, they’re just all the missing right-handed neutrinos.
Speaking of missing matter, if neutrinos are Majorana particles they could account
for that too. One of the great mysteries of the universe is why there’s… well, anything.
There’s no reason we can solidly point to that explains why there’s more matter than antimatter
today. There’s nothing inherently special about matter, and it probably formed in equal
amounts with antimatter after the Big Bang. That means by now everything should have been
annihilated, and yet here we are, made up of and surrounded by regular matter, not
getting spontaneously annihilated all the time. It’s possible the imbalance is the result of a
particular way some atoms decay. Beta minus decay is when a neutron in an unstable nucleus decays
into a proton and emits an electron and antineutrino. An extremely rare event
known as double beta decay occurs when certain nuclei have two neutrons decay simultaneously.
You see where I’m going with this right? If a neutrino and an antineutrino are actually
the same particle capable of annihilating itself, then sometimes double beta
decays will emit only electrons. This net gain of particles could help
account for the imbalance between matter and antimatter. Of course theorizing about
Majorana particles is one thing, actually finding evidence of them is quite another.
While neutrinos are notoriously hard to spot, neutrinoless double beta decay
should be detectable just by adding up the energy of the resulting two electrons and isotope.
Really the problem lies with luck and timing. Remember I said double beta decay is rare? Well
a double beta decay where the neutrinos annihilate each other should be at least 100 times rarer.
That doesn’t mean scientists aren’t still trying to spot it.
The preferred approach involves getting a huge amount of an isotope capable of double beta decay
and just… waiting. There are multiple experiments active and planned using elements like germanium
and xenon. They need to keep background radiation and the energetic movement of atoms
from ruining the data so many of them are shielded and kept cold, like the CUORE experiment in Italy
which is just 0.01 kelvin above absolute zero. What’s cooler than that? Maybe the fact that
it’s protected by 4 metric tonnes of lead recovered from a 2,000-year-old Roman shipwreck.
Seriously, the scientists borrowed it from a museum. If these experiments don’t see signs
of neutrinoless double beta decay, then maybe it’s even rarer than predicted, and
even bigger tanks of decaying isotopes will be necessary. Maybe it’s not possible at all and
the Majorana particle is a dead-end. Or, if luck is on our side, maybe we’ll see the telltale sign
of two neutrinos erasing each other, and the standard model and our understanding of the
universe will get a little bit more complete.
Fun fact: Majorana Fermions are named for Ettore
Majorana, a physicist who mysteriously disappeared without a trace in 1938. So about that
whole Standard Model being “The Most Successful Scientific Theory of All Time”. Turns out a recent
discovery has thrown a wrench in that. Amanda has muon that here.
So, what major mysteries about our universe do you want to see us cover next?
Let us know down in the comments. Be sure to subscribe, and I'll see you
next time on Seeker.