It will surprise nobody that some particle physics theories are very, very, complicated
and take some serious work to understand.
But sometimes it’s possible to give a bird's-eye view of a theory.
A person hearing such an explanation won’t have a detailed understanding, but they’ll
have a rough idea as to what is going on.
So, in this video, I’m going to take a stab at explaining one of the most complex ideas
popular in modern particle physics.
This idea is called leptogenesis.
Leptogenesis is the name of a theory which may- and I emphasize the word may- answer
one of the deepest- and almost philosophical- questions of physics.
“Why is there something, instead of nothing?”
The question arises because Einstein’s theory of relativity and the theory of the Big Bang
predicts that when the universe began, equal amounts of matter and antimatter were formed.
For every proton we see, we should also see an antiproton.
For every electron, we should see an antimatter electron.
Yet, when we look around at the universe in which we live, we see only matter.
So, this is clearly a terrible prediction.
Nobody knows why we live in a matter-dominated universe, but there have been many theories
proposed that could explain the conundrum.
One such theory is called leptogenesis.
It’s important to remember that we don’t know that leptogenesis is correct, but it
could be correct, and- and this is important- it has testable consequences.
In fact, at Fermilab, we are preparing to investigate this interesting idea.
We are reconfiguring our accelerator complex and building a huge neutrino detector called
DUNE, short for Deep Underground Neutrino Experiment.
The word lepton is used to describe subatomic particles like the charged electron and another
particle called a neutrino, which interacts so weakly that it can pass through the entire
Earth.
Leptogenesis predicts that physical processes that govern leptons are the answer to the
mystery of the matter-dominated universe.
So, how does leptogenesis work?
Well, for starters, the theory unfolds during a time in the universe very shortly after
the Big Bang- far less than a trillionth of a second after the cosmos began.
At that early time, the universe was extremely hot- much, much, hotter than we can achieve
in any conceivable particle accelerator.
Now, that super high energy is a very big deal and it’s a big deal because the laws
that governed the universe were very different from the ones we experience today.
If you’ve watched a few of my videos, you probably have some understanding of the quarks
and leptons of subatomic physics.
You know that quarks are found inside protons and neutrons, and you know of the most common
lepton, which is the electron.
Hopefully you also know that there is a ghostly field in the universe called the Higgs field
that gives mass to subatomic particles.
But you might not know that the Higgs field depends on the temperature, which is to say
the energy, of the universe.
Above that temperature, the Higgs field is zero, meaning that all of the familiar particles
have no mass.
Below that temperature, the Higgs field isn’t zero.
And the theory of leptogenesis applies so early in the universe that the temperature
is above that threshold.
This is kind of hard to imagine, as the universe was way different than we currently understand,
with properties like mass and even electrical charge not existing yet.
But there’s an analogy that might make it a little easier to understand.
Suppose you were a theoretical physicist who happened to be a fish.
And you’re a very hardy fish, who is immune to being in hot water.
Now, such a fish physicist would study the world around them, learning about currents
and turbulence and viscosity and all of the properties of water.
They might raise and lower the temperature of the water and learn about how that works
too.
But we all know that there is a temperature above which water turns to steam.
And, of course, steam acts very differently from water.
A fish physicist who was a water expert would have to account for the differences between
water and steam.
I mean, the laws of physics that govern liquid water and steam are the same, but those two
forms of water sure have very, very different properties.
For leptogenesis, it’s kind of the same thing.
We’re talking about the equivalent of the steam world, that is to say a world in which
the temperature is so high that the Higgs field is zero, which means that no particle
mass exists.
And, in the same way that water and steam are both governed by atomic physics, the high
and low energy universe are both governed by the Standard Model of particle physics.
However, in the steam world, so to speak, the way the laws of physics manifest themselves
is pretty different.
Now remember, I’m just giving you a bird's-eye view, but the four key components of leptogenesis
are the following.
One, for every known neutrino, all of which have very low masses, there is a cousin neutrino
which has very high mass.
You should know that these heavy cousins have not yet been observed, but they haven’t
been ruled out either.
For technical reasons, we call these cousins “right-handed neutrinos.”
Notice that I said “high mass.”
That’s because these right-handed neutrinos are thought to get their mass from some unknown
physics that is similar to, but different from the Higgs field.
They already have mass in the steam world when all of the other particles are massless.
The second component is that matter neutrinos and antimatter neutrinos are the same particle.
The technical term is Majorana neutrinos, named after Ettore Majorana, who wrote about
particles that were their own antimatter particles.
This idea sounds kind of weird, but it might be true.
We still don’t know if the familiar neutrinos and antineutrinos are the same particle or
not.
The third component is that there is some unknown physical phenomenon that makes the
right-handed neutrino decay more often into an antimatter regular neutrino than it does
a matter neutrino.
The fourth key component is the hardest to explain, but is what is called the sphaleron
process.
And the reason that it’s hardest to explain is because it only exists in what we’re
calling the steam world, which is to say the world in which mass for the particles that
we know about doesn’t yet exist.
In the water world in which we live, we have protons, neutrons and electrons.
Protons and neutrons are of a class of particles called baryons.
Electrons are leptons.
Each baryon has a baryon number of plus one.
Six protons have a baryon number of plus six, as do three protons and three neutrons.
Antimatter protons and neutrons each have a baryon number of minus one.
So, four protons, one antimatter proton and one antimatter neutron have a combined baryon
number of plus two.
Lepton number is the same, with electrons and neutrinos having a lepton number of plus
one and antimatter electrons and antineutrinos having a lepton number of minus one.
In our water world, baryon and lepton number are conserved, which is a fancy way to say
that they don’t change.
Whatever complicated particle interaction that occurs, both the baryon and the lepton
number after the interaction is the same as before the interaction.
But in the steam world, neither baryon nor lepton number is conserved.
What is conserved is the surprising quantity of baryon number minus lepton number, or B
minus L for short.
So, this leads to an unexpected consequence.
The sphaleron process can convert antimatter leptons into matter baryons.
Let’s look at this a little more.
Suppose you had three antimatter leptons and no baryons.
The three antimatter leptons have a combined lepton number of minus three.
You have no baryons, so the baryon number is zero.
Therefore, the quantity B minus L is plus three.
Easy peasy.
Now, let’s compare that to there being three baryons and no leptons.
That also has a value of B minus L equal to three.
That means you can have some process- and we don’t have to look inside the box to
see what’s going on- and that process can convert antimatter leptons into matter baryons.
We now have the four key components of leptogenesis.
Let’s stitch them together.
In our theoretical steam world at energies where mass for most particles doesn’t exist,
we have these heavy right-handed neutrinos that are both matter and antimatter at the
same time.
For some reason that we don’t yet understand, they prefer to be antimatter.
They’re not antimatter all of the time- just more often than they’re matter.
Those right-handed neutrinos can decay into ordinary leptons and Higgs bosons.
Now, remember that we’re in the steam world, so these are massless leptons and massless
Higgs bosons and they’re different from our familiar ones, but they’re kind of the
same and I’m not going to worry about the differences.
This process makes more antimatter leptons than matter ones, which is good.
After all, we’re needing a matter/antimatter imbalance to explain our universe.
These antimatter leptons then undergo the sphaleron process to make an excess of baryons.
This is kind of like antimatter electrons turning into protons.
Now we have to be careful because, of course, at these energies in the steam world, mass
doesn’t exist.
Protons surely don’t exist.
So what I said is wrong in detail, but it eventually has that consequence, so I hope
you’ll forgive me the shortcut that I made.
We now have a steam world in which there are more matter than antimatter particles, albeit
without mass.
And the universe continues to expand and cool down.
Eventually, it becomes cold enough for the Higgs field to go from having zero strength
to a number different from zero.
This gives the familiar particles of the Standard Model their mass.
In what we’re calling the water world, quarks and leptons with the masses that we know about
come into existence at that point.
Eventually, this excess of matter quarks makes the excess of protons and neutrons we see
in the universe today.
So that’s the big picture.
At energies so high that mass doesn’t exist, a heavy cousin of the neutrino is simultaneously
matter and antimatter and it prefers to decay into antimatter.
Those decays result in antimatter leptons that are then converted into matter baryons
and then the universe cools down with more regular baryons than antimatter baryons.
No problem.
Now if you’re a skeptical consumer of science, at this point there’s only one reaction
you can have.
Cool story, bro… tell it again.
And there’s absolutely no reason you should believe any of that.
Researchers certainly don’t.
But what if there were a signature that we can see in our water world that at least gives
us some evidence that this crazy-sounding chain of assumptions is really true?
And that’s where the Fermilab research program comes in; because, if this idea is true, then
some of the properties of the steam world are imprinted on the behavior of particles
of the water world.
And the one that we can test is the preference of right-handed neutrinos to be antimatter
neutrinos instead of matter ones.
That tendency can be observed using the familiar neutrinos and antimatter neutrinos of the
Standard Model.
It’s like a vestigial or a fossil artifact of the high energy world that we can see in
the universe today.
So what Fermilab is doing is developing a fantastic facility that can shoot both neutrinos
and antimatter neutrinos from our campus near Chicago to a location 800 miles away in Lead,
South Dakota, called the Sanford Underground Research Facility or SURF.
SURF is a laboratory located deep underground and it will house the DUNE detector, which
can detect neutrinos.
Neutrinos change their identity as they travel, turning from one type of neutrino into another.
What DUNE will do is study this morphing behavior of neutrinos and compare it to antimatter
neutrinos.
If they’re different, this whole leptogenesis thing could be true.
Now, some caution is in order.
We have to be careful about the logic.
If leptogenesis is true, we should see differences in the morphing behavior of neutrinos and
antimatter neutrinos, but the converse is not necessarily true.
Seeing those differences in the morphing of neutrinos and antineutrinos doesn’t mean
that leptogenesis is true.
After all, we still need to establish that right-handed neutrinos exist and that neutrinos
are their own antimatter particle.
o, there will still be some work to do.
But seeing that matter and antimatter neutrinos change their identity in different ways will
be a huge clue and maybe, just maybe, we’ll have found a key clue to unravel one of the
most pressing mysteries of modern physics.
DUNE won’t start operations until the second half of the 2020s, so we’ll have to wait
a while to know the answer, but that’s research for you.
Yeah, yeah- I know.
Waiting is hard.
But good things come to those who wait.
Okay, so this topic was super interesting as it is a proposed answer to one of the most
puzzling mysteries of modern physics.
I’d like to offer a big shout out to Dr. Jessica Turner, who helped me decide the best
way to explain the subject.
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