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?

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next time on Seeker.