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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.
[MUSIC PLAYING]
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.
Thank you to CuriosityStream for
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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.
