People are made of cells, cells are made
of molecules, molecules are made of atoms, atoms are made of electrons and proton and neutrons, and those last two are made up of three quarks. Simple stuff, right?
All except for that last part. Protons and neutrons are actually made of many, many quarks that just happen to look like three quarks when you look at them in a particular way. And even then, sometimes they’re
made of 5 quarks - including the charm quark The humble proton may seem simple enough,
and they’re certainly common. Most of the visible matter in the universe comes from
protons, either as lonely hydrogen nuclei or bound with neutrons into the nuclei
of the various elements of the periodic table. You’d think by now we’d have a pretty
good idea of the properties of the proton. But actually, their interiors remain a profound
mystery. It wasn’t until the late 60s that we even confirmed out that protons were not themselves
elementary particles, but consistent of 3 quarks. Now, thanks to AI we have evidence that
suggests they may be made of five quarks at least sometime. Two of them heavier than the proton itself. To make sense of this we need
to understand how physicists probe the smallest scales of nature. And that means
understanding the physics of scattering. Every time you open your eyes you’re performing
a scattering experiment. Photons from some light source like the Sun bounce - or scatter -
off objects in the world around you into your particle detectors—aka your eyes. Your
analysis computer—aka your brain—then builds up a colour map of the world around you
based on the properties of those photons. Well, it’s also possible to
“see” the subatomic world if we do a different type of scattering experiment. In general, the higher the energy
the scattering particle has, the smaller the object you can see. To see subatomic
scales, it’s better not to use photons at all, but instead to use particles of matter. That's
how Earnest Rutherford first discovered the nucleus. He shot a beam of alpha particles
- which we now know to be helium nuclei - at a thin gold foil. He found that, while
most particles passed straight through, a small fraction scattered off something very small and very dense. That’s how he figured out that the atom is mostly empty space, with most of its
mass concentrated in a tiny central nucleus. Since Rutherford’s 1911 experiment we’ve
gotten much better at doing this. For example, we now have electron microscopes which
shoot electrons into a sample and measure how they’re deflected. Sophisticated software
then reconstructs the structure of the sample, just as our brain reconstructs
the structure of the world. Electron scattering can be used to study
the structure of anything larger than an electron. That sounds pretty useful, because
electrons have no size—they’re point-like. Actually not so fast—electrons are quantum
objects and so have a wavelength that gets shorter the more energy the electron has.
So really I should say that electrons can be used to study anything that’s larger than the
electron’s wavelength, which depends on energy. So if you want to study the interior of a
proton you need a pretty high energy electron. Below a certain energy, the electron will just
bounce off the proton as a whole. But with enough energy the electron will punch into a proton and
then scatter off the proton's internal parts. I should add that such an electron is energetic
enough to destroy the proton. Which sounds bad, but it’s actually okay - sometimes you need
to break something to see what it’s made of. And that’s what physicists at the Stanford Linear
Accelerator Center—SLAC—managed to do in the 50s and 60s. They accelerated electrons to higher energies than had previously been achieved and slammed them into protons, then watched what came
out the other side. In 1968 they published their results. From those scattering products and the
deflection of the electrons, the SLAC physicists deduced that protons must be made of three
point-like particles. These particles matched the properties of the so-called quarks—theoretical
components of nucleons proposed by Murry Gell-Mann and George Zweig earlier that decade. And that’s
how we get the proton model that you’re probably familiar with—two up and one down quark, with
three different charge colours bound to each other by gluons carrying the strong nuclear force,
as we’ve talked about in previous episodes. And same for the neutron except it's two down and one up. I'll be talking about protons from now on, but everything I say probably applies to neutrons too. While the SLAC electron beam was
the most energetic of its time, we quickly figured out how to make more and
more powerful accelerators. And as we did so, we started to see something strange.
Remember I said that higher energy scattering allows us to see smaller
things and finer resolution. Well, as we powered up our accelerators that’s exactly
what we saw—the detailed guts of the proton, and it turned out to be much more complicated
than just three quarks bound to each other. The interior of the proton was revealed to be
a complex cluster of energy—a dense network of gluons that are constantly transforming
into pairs of virtual quarks and antiquarks, which quickly annihilate each other, turning
back into gluons. We call this the quark sea, and this constantly shifting, flickering
mess is a stormy ocean indeed. But because the quantum properties of
charge, spin, and colour must be conserved, there is order within the chaos. If you took all
the contents of the quark sea in one instant, you could list all the quarks and all the
antiquarks, and see that they cancel each other out except for two up quarks and one down
quark. These “valence” quarks will constantly exchange energy and colour charge via the quark
sea and may even be annihilated by a virtual particle or antiparticle, but are instantly
replaced by that virtual particle’s partner. These valence quarks are what the first SLAC
experiments detected. But as we increased the electron beam’s energy, those electrons started
to interact with the transient entities of the quark sea. They scattered off gluons and virtual
quarks, revealing that complex inner structure. The higher the energy of the electrons, the
more fine detail we saw. We saw the virtual up and down quarks of the quark sea, the gluon
network. But also some weird stuff. For example, around 1% of the time there was evidence of
a charm quark inside the proton. Well that’s odd. The charm quark weighs more than
the entire proton—36% more in fact. It’s like opening a 1 kg box of I dunno,
apples, and finding a 1.3 kg melon inside. Fortunately there’s a perfectly plausible
explanation in the case of the proton. It turns out that the particles we discover after smashing
an electron into a proton aren’t necessarily inside the proton to start with. That’s because
the energy carried by the electron can create brand new particles. Remember, E=mc^2, which
tells us that mass and energy can be converted into each other. As the ingoing electron transfers
its substantial kinetic energy into the quark sea, new particle-antiparticle pairs can be created
that were not part of the proton to start with. As the energy of the electron beam increases,
we get a scattering signal from finer and finer structure. But this signal is increasingly muddied
with new particles created in the collision. We call the particles that are inside the proton
to start with intrinsic particles, and they include the valence quarks and the virtual quarks
and gluons of the quark sea. On the other hand, we call particles created in the collision itself
extrinsic particles. After a particle collision we see both intrinsic and extrinsic particles,
as well as the decay products of those particles in cases where the outgoing particle is too
unstable to reach the detector before falling apart. So if the ingoing electron has enough
energy, perhaps it could produce charm quarks that are detected in some cases. If the charm
quarks are extrinsic then there’s no problem. But even early on, there was some evidence
of intrinsic charm quarks. There was the hint of the presence of charm quarks even in cases
where the electron didn’t carry enough energy to produce one. So if the charm quark is too
massive to be produced by the incoming electron, and is too massive to be a regular part
of the entire proton, how can it exist? Well it’s not quite as simple a conundrum as I
make it sound. As you reduce the energy of the collision, the probability of finding
an extrinsic charm quark also goes down, but only reaches zero at zero energy, which
means when no collision is happening. In these early experiments charm quarks were appearing only slightly more often at low energies than expected from calculations. and so the evidence for intrinsic charm quarks was weak. Let’s talk about how those calculations and how they're done to see why this uncertainty exists. The interior of the proton is the realm of
the strong nuclear force, which is described by quantum chromodynamics. With its multiple
different charge types and different charge carriers, QCD is far more complicated than say the single-charged electrodynamics. One weird thing about “QCD” is that the calculations are easier at
high energy than at low energy because it’s easier to implement a favorite hack of quantum mechanics
called perturbation theory. That means it’s easier to calculate the result of a high-energy
scattering event than a low one. It’s extra hard to properly calculate the state of the interior
of the proton in the absence of a collision. For this reason it's kinda straight foward to explain all the extrinsic quarks that we see at high energies, but it is very difficult to explain the behavior of the
three intrinsic quarks we see at low energies, and it's even harder to explain why we seem to find
these charmed quark at those same low energies Although it’s hard to model the interior of the
proton at low energy, our intuition still tells us that it shouldn’t contain things more massive
than the proton itself—like the charm quark. But there is actually a way to get quantum
chromodynamics to give us intrinsic charm quarks. We do that by taking advantage of the Heisenberg
uncertainty principle, which, among other things tells us that we can borrow energy from out of
nowhere to create massive particle-antiparticle, as long as those particles
vanish again very quickly. The more massive the particles, the
shorter they’re allowed to live. For this reason, it should be possible
to generate a charm-anticharm quark pair, even inside a proton, as long as it
exists only for a very short duration. Due to the large mass of the charm quark pair,
their momentary appearance is a bigger deal for the proton’s inner structure than is the
continuous flickering of virtual up and down quarks. In that instance of their appearance, the proton looks more like a 5-quark particle than a 3-quark particle. However I wouldn’t go so far as to say that
the proton briefly more massive. The proton’s measured mass comes from the average of
its internal energy over the time of measurement. So if the charm quarks only exist
for a tiny fraction of the time, then they contribute only a fraction of
their own hefty mass to the proton’s mass. I just gave a crude outline of the theory
of intrinsic charm, proposed by Stanley Brodsky in 1980 to explain the weak evidence of charm quarks in protons. But proving this was challenging due to the difficulty of
modeling low-energy particle collisions. One issue is that there are typically many
different models of the proton interior that can lead to the prediction of the same
scattering results. So, you might come up with a model that includes intrinsic charm
that gives a perfect match for your average particle output over many collisions. But you
may have gotten that model right just by chance, and if you analyzed even more collisions you'd realize
the correct model is actually totally different. Without testing all possible models, it’s
impossible to know whether a model of intrinsic charm was really better at predicting the outcome
of these collisions, versus some unknown model that doesn’t include intrinsic charm. So
now you have a better idea of why there’s so much uncertainty about the
existence of charm quarks in the proton —it just wasn’t possible to sift through
all of the possible models to be sure that intrinsic charm was needed to
explain the scattering experiments. That is, until Artificial Intelligence
came along. This new tool allowed scientists in the NNPDF collaboration to do
something that was previously impossible: Instead of testing one model, they could test
hundreds or thousands of models of the proton interior at the same time. They trained a neural network to analyze
nearly 30 years of proton collisions, not constrained by a single model, but in the
limit of all possible models. The network was then able to create new models that approach
the scattering data as closely as possible. This was incredible. Instead of a few scientists
trying a few models every couple of years, they could have this machine test thousands
of models in just a couple of days, and while a scientists may be biased to trying to
to find evidence in favor of their favorite model, the network just wants to find the best
answer, even if it contradicts intrinsic charm. And now we get to the punchline. This
Neural Network was able to find a model with intrinsic charm that predicts the
data much better than any previous model. Now currently this is still tentative. The team reports a 3-sigma result which means a 1 in 1000 chance they found the wrong model in favor of intrinsic charm just by random chance. Of course the gold standard for claiming a victory is 5-sigma, which means a 1 in a million chance the result came from an unlucky streak. That sounds like a high standard, but
with so many different experiments happening around the world, we do get a lot
of 3-sigma results that end up being wrong. So will this one pan out in the end? Will
we find the charm quark in the proton? The melon in the apple box? Well we’ll need to blast a lot more protons to find out, but we may also get closer to our answer by improving this shiny
new tool of machine learning. That technology is developing at a rapid pace, as it’s increasingly
used across all areas of experimental physics. Computers can come up with and test models
much faster than any physicist, which means the physicists can get on with the more interesting
work of interpreting the successful models. Wherever we land on the question
of intrinsic proton charm, there’s something charming about
this cooperation between artificial and natural intelligences working towards the common goal of deciphering the inner workings of space time Hey everyone, comment responses will return next week. But if you really want to watch me try to answer difficult questions with mixed success, head over to StarTalk, where my long time colleague Dr. Neil Tyson tries to stump me with some real doozies. Link in the description. And feel free to tell them we sent you.
