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Since the discovery of the Higgs boson,
physicists have searched and searched for any hint
of new particles.
That search has been fruitless until, perhaps, now.
Today on "Space Time" Journal Club,
we'll look at a paper that reports
a compelling hint of a new particle
outside the standard model, the sterile neutrino.
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Regular neutrinos are a bit aloof.
They don't interact by the electromagnetic or strong
nuclear forces, only by the weak nuclear force and gravity.
They are so weakly interacting that they pass through matter
like it is isn't there.
To have a 50/50 chance of stopping any given neutrino,
you need a wall of lead 1 light year thick.
If regular neutrinos are aloof, then sterile neutrinos
are complete loners.
They don't even interact via the weak interaction.
Even so, detection of a sterile neutrino
would be incredibly important.
Besides being the first expansion of the standard model
family since the Higgs boson, sterile neutrinos
are a candidate for dark matter, and their existence
would have had a huge influence on the expansion
of the early universe.
We can detect regular neutrinos by watching
for the rare interaction between a neutrino
and an atomic nucleus in some huge volume of matter,
an entire glacier in the IceCube experiment or a huge vat of oil
in the experiment we're about to discuss.
Those regular neutrinos are spotted
when they interact with matter via the weak nuclear force.
So, how on earth do you spot a sterile neutrino
that doesn't even undergo that interaction-- well,
by being extremely clever, obviously.
This may have been achieved as reported in the 2018
paper, "Observation of a Significant
Excess of Electron-like Events in the MiniBooNE Short Baseline
Neutrino Experiment."
Catchy name.
Before we get to that, we're going
to need to go a few inception layers deep to set up
the knowledge.
We're going to drop through the standard model of particle
physics, electric charge and antimatter, the bizarreness
of quantum chirality and the Higgs mechanism,
and finally, why all of this points to sterile neutrinos.
Hold on to your butts.
First, the standard model of particle physics--
as we'll see in upcoming episodes,
these particles are divided into the bosons which
carry the fundamental forces and the fermions which
comprise matter.
The latter include the quarks, up/down,
which comprise protons and neutrons,
and the more exotic top, bottom, strange, and charm
as well as the antimatter versions of these.
And then there are the leptons, the ubiquitous electron
and its heavier cousins, the muon and tauon, and again,
each with its antimatter counterpart.
Neutrinos are also leptons, and each of the heavier lepton
flavors has a neutrino version.
These have far lower mass, and unlike quarks and leptons,
they have no electric charge, hence neutrino
or little neutral one.
Each flavor of neutrino also has an antimatter counterpart.
So let's drop down to antimatter.
An antimatter version of a particle
has the same mass and the opposite electric charge.
So an electron has a charge of negative 1
and an antielectron has a charge of plus 1.
Neutrinos don't have charge, so what's
the difference between a neutrino and an antineutrino?
Well, there's actually another property that
gets reversed in antiparticles.
And that brings us to the next level, chirality.
The physical interpretation of chirality is pretty abstract.
To explain, we need to start with helicity.
Helicity is just the direction of a particle's spin
relative to its direction of motion.
Helicity can be right-handed, which
means clockwise rotation, or left-handed or anticlockwise.
Like helicity, chirality can be left- or right-handed.
However, the physical interpretation
is much more abstract.
It's related to the direction in which the particle's phase
shifts under rotations.
Helicity depends on your own motion
relative to the particle in question.
It flips direction if you start moving faster
than the particle.
However, chirality is fundamental to the particle
and doesn't depend on your own velocity.
This is where we need to expand our picture of the particles
of the standard model a little and open up
the possibility of the sterile neutrino's existence.
There are actually two versions of each fermion,
one with right-handed chirality and one with left.
And that's on top of the matter-antimatter split.
As we saw in our episode on the Higgs mechanism,
real quarks and electrons are actually
a combination of left and right chiral particles
that oscillate back and forth between those particles
through interactions with the Higgs field.
That oscillation is what gives these particles their mass.
Still with me?
Good.
Like electric charge, chirality is also reversed in antimatter.
For example, both left and right chiral negatively
charged electrons have their own positively
charged antimatter particles, which are right
and left chiral, respectively.
These different chiralities are thought
of as completely separate particles,
and there's a good reason for this.
Chirality determines whether a particle can interact
with the weak nuclear force.
The left chiral electron feels this force
and the right chiral electron does not.
This interaction is opt for antimatter.
The right chiral antielectron feels the weak force,
while the left chiral antielectron does not.
OK, got all of that?
We're finally ready to bring it back to neutrinos.
Every neutrino we've ever observed
was spotted using the weak interaction.
That means we've only ever seen left-handed neutrinos
or right-handed antineutrinos.
The opposite chirality, right-handed neutrinos
or left-handed antineutrinos, should only
interact gravitationally so would
be near impossible to detect.
These are the sterile neutrinos.
They aren't part of the standard model because, until now,
we had no concrete evidence that they exist.
But there's good reason to suspect their existence.
If neutrinos gained their mass by the same mechanism
as quarks and electrons, that means
their chirality oscillates.
That would require regular left-handed neutrinos
to spend at least a bit of their time
as sterile right-handed neutrinos.
Now, we know that neutrinos have mass due to a completely
different type of oscillation.
We've observed a neutrino's flavor can change.
Electron neutrinos can become muon neutrinos
can become tau neutrinos.
In order to evolve that way, neutrinos
must experience the flow of time,
which means they can't be moving at the speed of light, which
means they must have mass.
That mass may indicate the existence
of the sterile neutrino, but it could also
come from some more exotic mechanism--
for example, the Majorana mechanism
that would require the neutrino to be its own antiparticle
and would break the standard model even more
than the existence of the sterile neutrino.
OK, let's get to the experiment.
MiniBooNE is an experiment at Fermilab in Illinois.
Neutrinos are created by colliding protons together
to produce a beam of mostly muon neutrinos.
These then travel to an 800-ton vat of mineral oil.
Rare interactions with nuclei in the oil
reveal the nature of the neutrinos.
Now the evidence for sterile neutrinos is subtle,
and they certainly weren't directly detected.
Instead, MiniBooNE detected way more electron neutrinos than
expected.
So I told you that neutrinos oscillate between type--
electron, muon, tau.
So the MiniBooNE experiment starts with muon neutrinos,
and some of these transform into electron neutrinos
by the time they hit the vat.
According to the standard model, that oscillation
should be extremely rare over the very tiny distance
of the neutrino beam.
A lot more muon neutrinos made the transition
to electron neutrino than was expected according
to the basic standard model.
But one way to speed up that transition
is to introduce sterile neutrinos
as an intermediate step in the oscillation.
If muon neutrinos can flip their chirality
and become sterile neutrinos, then it's
an easier transition from sterile neutrino
to electron neutrino.
And that's a proposed explanation
of the MiniBooNE team.
The team finds an overabundance in electron neutrinos
at the 4.8 sigma level.
Now that's actually slightly below the critical 5-sigma
level required for claiming a high-confidence detection.
However, MiniBooNE then combined their results
with that of an older experiment that has also
detected a hint of this excess.
That was the Liquid Scintillator Neutrino Detector, LSND,
experiment at Los Alamos, which in 2001 published a 3.8-sigma
excess in electron neutrinos.
Combined with the 4.8-sigma MiniBooNE result,
the author's claim a 6.1-sigma signal,
which would be considered extremely significant.
If this is right, then it's the first particle
outside the standard model since the Higgs boson.
And if we really have seen the influence
of the sterile neutrino, we now know something about it.
It would have a relatively low mass at around 1 electronvolt.
Forgive the particle-physics energy units for mass.
That's heavier than regular neutrinos but way too light
to be a candidate for dark matter.
OK, I know you're excited, but don't crack the champagne
bottle yet.
This result is in conflict with some other measurements.
The IceCube neutrino detector in Antarctica
has found no evidence of the existence of sterile neutrinos
based on the transition of muon to electron neutrinos
as they travel through the body of the Earth.
An analysis of the cosmic microwave background radiation
by the Planck satellite shows that the early rates
of expansion of the universe is consistent with only
three neutrino types.
Add more neutrino types like the sterile neutrino
and the early universe would have expanded faster.
There's an interesting conflict here.
The MiniBooNE result looks compelling.
Hopefully it isn't some sort of experimental error,
which it might be.
Remember those faster-than-light neutrinos?
Yeah, we don't talk about that anymore.
If this one is real, then something
is missing in our understanding of physics,
and glitches between experiment and theory
are exactly how new physics gets discovered.
The sterile neutrino may have been discovered
or we may have just spotted something even more
interesting.
Either way, we'll have peered just
a little deeper into the fundamental building
blocks of space time.
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In the last two episodes we covered
the black-hole-information paradox and asteroid mining.
Today, we're going to cover your questions from both.
oiSnowy asks about a statement I made
in the information-paradox episode.
With perfect knowledge of the current universe,
it should be possible to perfectly trace the universe
backwards and forwards in time.
So, how does that work with the cosmic event horizon?
Shouldn't regions outside the event horizon
be lost to our access and therefore
impossible to trace backwards?
Well sure, absolutely.
The statement about the retraceability of the universe
doesn't actually care about event horizons, whether cosmic
or black hole.
The idea is that if the information is still
existent somewhere, then the universe could be put in rewind
and it would end up back where it started.
Even a black hole should unravel in that case.
But if information is actually destroyed,
then the rewind would get stuck at the point of information
destruction because the universe wouldn't
know which of multiple possible histories
that led to that point.
But you're absolutely right in thinking
that we can't have perfect knowledge of the universe.
We also can't rewind the universe,
so this is just a thought experiment.
A few of you wonder whether adding extra mass to Earth
from asteroid mining could lead to problems like with our orbit
or Earth's gravitational pull.
So Earth is around 2,000 times more
massive than the entire asteroid belt.
Even if the entire belt were brought to Earth,
you wouldn't notice the difference in gravity.
And no one is proposing we do that.
Only the very tiny fraction of precious and rare-earth
elements are likely to be profitable to bring back
to Earth.
Some of you also wondered whether mining
the Moon could be more efficient than mining the asteroid belt.
Well, the moon is definitely an option for mining,
and it's going to have some useful heavy elements
from crashed asteroids.
It's not clear that it's better than near-earth asteroids.
To get stuff off the Moon, you have
to contend with its admittedly low gravitational field
compared to essentially no such field in the case of asteroids.
Also, mining the moon is going to have
a lot of political complications compared to asteroids.
For one thing, our moon is protected from exploitation
by the Outer Space Treaty, which prohibits nations from claiming
any sovereignty there.
Asteroids, however, are fair game.
Lisa Vaughn likes this channel because when she repeats
this stuff to her friends they're like, damn dude,
how did you get so smart?
Weird thing, that's actually how I got started.
One minute I'm learning the constellations or Einstein
quotes to impress my friends, and next, well, it's
a long, sad down spiral to where I am today.
I heard this one guy, he started out
memorizing pi to impress chicks, ended up
inventing the atomic bomb.
People, this stuff may be fun at parties, but please, everyone,
nerd responsibly.