- When a theory makes a prediction that disagrees
with an experimental test.
Sometimes it means we should throw that theory away.
But what if that theory has otherwise produced
the most successful predictions in all of physics.
Then that little glitch maybe pointing the way to layers
of physics deeper than we've yet imagined.
Well Fermilab's Muon g-2 experiment has been chasing
what might be the most promising glitch of all,
and they've just announced their results.
(bright music)
The Standard Model of particle physics
describes the elementary building blocks of nature
with incredible success.
At some level it's right in a very fundamental way,
but it is not the whole picture.
For one thing, it doesn't explain gravity.
In fact, it doesn't play nice
with Einstein's entire theory of gravity,
which itself is clearly right in its own way.
Our search for a theory of everything
which will bring these theories together
is perhaps the next great question physics.
To find the path forward,
we need to find glitches in these theories.
Loose threads,
that might lead us to deeper layers of physics.
We don't have many leads.
These theories are woven pretty tight
but there is one glitch,
one stray thread that is just begging to be tugged.
And that's the anomalous magnetic dipole moment
of the muon.
And the scientists at Fermilab have just tugged it hard
with the Muon g-2 experiment.
Today, we're gonna see what's been unraveled
and what might lie beneath.
But first, what is all this technobabble?
What's an anomalous magnetic dipole moment?
What is a muon?
What is g-2?
Let's take the one at a time.
We've actually talked at about
the anomalous magnetic dipole moment before
in terms of the electron.
That's an episode well worth checking out
for a deeper dive, but briefly.
Of all the incredible successes of the Standard Model,
this seems the most miraculous.
It comes from the part of the Standard Model
that describes how particles with electric charge
interact via the electromagnetic force,
quantum electrodynamics.
One of the interactions that QED describes
is how a charge particle will tend to rotate
to align with a magnetic field.
The strength of that interaction
is defined by something called,
the g-factor for the particle.
That's the g in our g-2,
and we'll come back to that later.
QED predicts the value for the electrons g-factor
that matches experimental measurements
to one part in a billion.
By far the most accurate prediction in all of physics.
If this works so well for electrons
surely it works for other particles too.
Well, actually not so much.
The muon, is a close cousin to the electron
identical in all properties besides its heavier mass.
Starting in 20 years ago,
experimental measurements of the Muon g-factor did not agree
with the QED calculation.
And that's not because QED is just wrong,
rather it tells us that the calculations
have missed something.
And that something maybe physics beyond the Standard Model.
Let's start by talking about quantum spin.
We're actually gonna do a deep dive into this topic soon.
So today we'll just cover what we need to get by.
Every particle with electric charge also has quantum spin.
This isn't the same thing as simple rotation.
The particles with quantum spin
do generate a magnetic field,
same as if you send an electric charge around a looped wire,
or have electrical currency in Earth's spinning core.
The result is a dipole magnetic field
with a North and a South pole.
Place an object
with such a field inside a second magnetic field,
and the object will tend to rotate to align with that field.
The strength of that rotational pool or the talk,
is defined by the objects dipole moment.
For a rotating charge,
that depends on the objects rate of rotation,
or angular momentum,
it's charge and it's mass.
Here's the equation for the classical dipole moment
for a nonquantum rotating charge.
Yes this will be on the test,
and it's also useful for the next thing I'm about to say.
An electron also has a dipole field and a dipole moment
which depends on the electron spin charge and mass.
But the electron dipole moment
is different from the classical one
by this factor g.
For the electron, g is allowed too.
So the electron responds to a magnetic field
twice as strongly compared to what you'd expect
for an equivalent classical rotating charge.
Quantum electrodynamics,
tells us exactly what this difference is.
To understand this,
let's look at the QED picture of the world.
In this theory,
electromagnetic interactions
result from charge particles communicating
by exchanging virtual photons.
In QED,
you figure out the strength of an interaction
by counting up all the ways
that this interaction could occur.
For example, a pair of electrons could repel each other
by exchanging one virtual photon,
or two virtual photons,
or three et cetera.
All those virtual photons could do something weird
like momentarily becoming an electron-positron pair.
We depict these interactions in Feynman diagrams.
Each Feynman diagram represents one family of ways
that the interaction could proceed.
And the sum of all possible Feynman diagrams,
gives you the interaction strength.
For a deeper dive in Feynman diagrams,
virtual particles,
and quantum electrodynamics,
we have you covered,
episode list in the description.
We can represent an electron interacting
with a magnetic field,
with the simplest possible Feynman diagram.
Really not even a full Feynman diagram.
We have an electron being deflected
by a single photon from that field.
If you calculate the g-factor from just the simplest case,
you get a value of exactly two.
Paul Dirac first calculated g equals two
from his eponymous equation.
But there are other ways this interaction can happen.
The next simplest is for the electron
to emit a virtual photon just prior to absorbing
the magnetic field photon,
and then reabsorbing that virtual photon.
Same particles in and same particles out,
but a slightly more complicated sequence of events.
Adding this interaction allowed Julian Schwinger
to calculate a slightly higher value of g equals 2.0011614.
Over time, more and more complicated interactions
were added.
Each layer of complication involve
many more Feynman diagrams,
but also added less and less to the interaction.
The latest calculations rely on powerful computers
to add many thousands of Feynman diagrams,
and get us our g-factor to 12 significant figures.
And a latest calculated electron g-factor
of 2.001159652181643.
By the way,
the anomalous in the anomalous magnetic dipole moment,
refers to that little bit extra after the two.
So the g-2 in Fermilab's experiment name
refers to that leftover bit.
Measure that leftover bit
and you are testing the sudless interactions
of the particle.
For the electron,
the current measurement is precise to one part in a billion.
And to that degree it perfectly agrees
with the theoretical number.
An obvious next step is to do the same for other particles.
The electron is the lightest and most common
of the lepton family.
It has two heavier cousins,
the muon and the tau particle.
Muons are nice,
because they're easily produced in radioactive decay.
They live only a few microseconds,
but that's long enough to work with them.
During their brief existence,
they're very similar to electrons.
They have the same exact charge,
interact with the same forces,
and have the same quantum spin.
They have a different g-factor
because there are slowly different ways
that the muon can interact with the quantum fields.
So you add up all of the Feynman diagram
to all possible electromagnetic interactions of the muon,
but you don't stop there.
The quantum vacuum is seething with an incredible variety
of possible virtual particles.
There can be very subtle interactions
that involve the other forces, weak, strong,
and even the Higgs field.
All of these tweak the muons g-factor by a tiny degree.
And when we include every possibility encompassed
by the Standard Model of particle physics,
we get a g-factor
that's ever so slightly off the experimental results.
So why would we get the wrong value for the muon
but not for the electron?
Well, I can tell you what the physicists hope is the reason.
The muon is 200 times more massive than the electron.
The probability of interaction between a particle
and some massive virtual particle
is proportional to mass squared.
So the muon,
is 40,000 times more likely to be perturbed in this way.
It's 40,000 times more likely
than the electron to encounter,
say a virtual Higgs boson,
or virtual proton or other hadrons.
And it's 40,000 times more likely
to encounter any completely unknown virtual particles
that might be hiding out there.
Accounting for all of the known particles,
still gives a Muon g-factor that's off.
So the rising hope,
is that an as yet unknown particle is at work here.
So, final we get to the Fermilab Muon g-2 experiment.
Prior to this experiment,
various labs over the past 20 years
have refined the Muon g-factor measurement.
Until now, it had been measured precisely enough
to claim a 3.7 sigma difference,
compared to theory.
That was at Brookhaven National Laboratories in 2001.
There's roughly a one in 10,000 chance
that random fluctuations could lead
to that degree of difference
just by chance.
Physicists however,
refer a five-sigma signal
before declaring a discovery.
For five-sigma,
there's only one in 3.5 million chance
of random noise resulting in the same signal.
The Muon g-2 experiment at Fermilab,
hopes to push closer to this level of confidence,
and is designed to achieve four times the sensitivity
of the Brookhaven experiment.
At the Fermilab experiment,
physicists send muons flying at nearly the speed of light
around a 50 foot diameter magnetic tube.
The muons interact with the magnetic field
and their own magnetic dipole axis,
rotate like a top just before it falls.
We call this Larmor precession,
and its frequency depends on the g-factor.
The frequency of the precession
also governs the energy of the particles
that these muons decay into.
So by measuring the energies of those particles,
positrons in particular,
the researchers can determine the precession rate
and so measure the g-factor.
So what did they find?
- [Narrator] Before I get to that,
you might remember around five years ago,
physicists at the Large Hadron Collider
thought they detected a new particle
based on a slight bump at a particular energy
of the decay products.
That was a three point something sigma detection.
More data was collected, and the bump went away.
It really was just a random statistical fluctuation.
So has the Muon g-factor deviation gone away?
No, no, it hasn't.
The Muon g-2 team literally just announced
their latest result.
And the confidence is now at 4.2 sigma.
So not yet a slam dunk detection
but definitely moving in the right direction,
and with all confidence than the LHC bump ever achieved.
The chance of randomly getting a 4.2 fluctuation
is just a little over one in a 100,000.
The hallowed five sigma confidence
will take time and many more muans.
Unless of course this was a systematic error,
some unknown factor influencing the measurement
that is not a new particle,
the g-2 team worked very hard
to rule anything like that out.
But the only way to be sure
is to repeat the measurement
with a new independent experiment.
But if all that goes well,
then this may be a fake glimmer of light
in the current theoretical wasteland.
You can bet that physicists will be chasing down
that glimmer with great enthusiasm.
In fact, I expect a flurry of theoretical papers
in the very near future.
Perhaps this time, some of them will be right.
- So there you have it.
That's how we peer beneath the hood of reality.
We scratch our heads and scroll on chalkboards
for about a 100 years.
Then we build a giant magnet and watch the muans dance.
And that dance,
may just have revealed to us the next step on our path
to a more complete understanding
of our quantum space time.
(heavy metal music)