Particle physics has a lot of non-intuitive
and hard to imagine concepts.
Particles too small to see are governed by
forces too alien to experience and combine
to make up the universe we see around us.
It’s all pretty mind-boggling.
It’s hard to pick one single phenomenon
that is the weirdest, but one might involve
the spin of neutrinos.
So spin in the quantum world is a complicated
thing.
Rather than telling you all of the nuances,
I’m going to tell you the core points.
If you want to know the entire story, then
you’ve got a whole lot of studying ahead
of you.
So let’s start with the quarks and leptons,
which are of a class of particles called fermions.
Point-like fermions are particles that can
have a spin of plus a half or minus a half.
I made a video that talks about fermions in
more detail.
But for this video, we can dispense with the
half and just focus on the plus and minus.
What do they mean?
Subatomic particles aren’t little spinning
balls, but that mental image can be useful
in some instances.
Luckily, this is one of those times.
So if you imagine, say, an electron as a spinning
ball, the axis of rotation is parallel or
antiparallel to the electron’s direction
of motion.
If you take your two hands and point your
thumb in the direction the electron is moving,
the electron has two ways in which it can
spin.
It can spin in the direction that the fingers
of your right hand wrap or it can spin in
the direction your left hand wraps.
If the sense of the spin is like your right
hand, the electron has a positive spin, and,
if the sense of the spin is like the fingers
of your left hand, then the electron has a
negative spin.
It turns out that both the strong nuclear
force and electromagnetism doesn’t care
a bit about whether a particle has plus or
minus spin, but that’s not true for the
weak force.
The weak force only interacts with fermions
with negative spin…what we call left handed
particles…and with antimatter fermions with
positive spin…or what we call right handed
antiparticles.
Never in any experiment have we seen a right
handed particle interact with the weak force.
So, since we can only study neutrinos with
the weak force, we’ve only seen left-handed
or negative spin neutrinos.
Does that mean that there are no right-handed
neutrinos?
Well…strictly speaking…no.
We could imagine that there are right handed
neutrinos that just don’t feel the weak
force.
That’s possible.
We have a name for these hypothetical neutrinos
that don’t interact with the weak force.
They are called sterile neutrinos.
But you have to be careful with the nomenclature.
In principle, any particle that doesn’t
interact with the strong, electromagnetic
or weak forces can be considered to be sterile-
they could be right or left handed.
But since right handed neutrinos fit the bill,
they are examples of sterile neutrinos- well,
of course, if they exist.
So it’s important to remember that nobody
knows if sterile neutrinos exist.
There have been some measurements investigating
how ordinary neutrinos can oscillate, which
is a fancy way to say change their identity,
and, in those experiments, the data leans
towards the possibility of there being sterile
neutrinos.
On the OTHER hand, other experiments lean
the other way.
The bottom line is that the jury is still
out on the subject of sterile neutrinos.
So there are experiments underway that will
hopefully figure this all out.
If you want to know my opinion on whether
they exist or not, I don’t really have one,
except to say that most new ideas are wrong.
But time will tell.
There is an interesting idea that is part
of the right handed neutrino story.
It also may be wrong, but it’s worth knowing.
It starts with the Higgs boson, which you’ve
probably heard gives subatomic particles their
mass.
And that’s true, as far as we understand.
Now there appear to be a couple of different
classes of particles.
There are the massless ones, like the photons
and gluons, with their zero mass.
And then there are the heavy ones like the
quarks and non-neutrino leptons.
Using a unit of mass called the electron volt,
these particles have masses in the range of
about a million to a couple hundred billion
electron volts.
So there are these two classes – particles
with zero electron volts of mass and those
with a mass of about a million electron volts
and more.
Now the neutrinos seem to be yet another class.
We don’t know their mass, but they seem
to be very low…very close to zero, but not
exactly zero.
Given that the neutrino masses are so low,
that seems odd and hard to understand using
Higgs theory, which seems to prefer a mass
of million or billions of electron volts.
Why is the mass of neutrinos so light, but
not exactly zero?
One possible answer is that perhaps the mass
of neutrinos doesn’t come from the Higgs
mechanism.
Maybe there is another cause.
So this is where right handed neutrinos come
in.
Physicists have hypothesized a mechanism for
giving mass to neutrinos which connects the
masses of ordinary left handed neutrinos and
hypothetical right handed ones.
They are tied together so that if you multiply
their masses together, the product is a constant.
And, if you are mathematically adept, you
see that this means that if one mass gets
high, the other must get low.
We scientists call this the seesaw mechanism
for perhaps obvious reasons.
I was going to film me and a little kid on
a seesaw, but it turns out it’s hard to
find seesaws nowadays.
Apparently, they’re dangerous.
It makes you wonder how the kids of my generation
ever survived.
But this graphic shows the key point.
You shouldn’t believe this seesaw idea,
because it is purely supposition at this point.
But it is credible supposition and therefore
you should keep it in mind.
Reputable scientists are at this very minute
testing this hypothesis.
So this whole idea of sterile or massive or
right handed neutrinos is all very speculative.
In fact, the data supporting any of these
ideas are very, very thin.
I mean, there are a few measurements here
and there that one can explain by invoking
sterile neutrinos, but there are often other
experiments that rule out whatever idea the
first experiment hypothesized.
Clearly one of them are wrong.
I think the bottom line is that you need to
just wait and see on this topic.
The Fermilab research program for the next
few decades will focus on studying neutrinos,
so hopefully they will eventually sort this
all out.
Until then, let’s take another look at those
kids who are taking their lives into their
own hands.
