The discovery of the Higgs boson ten years ago in the Large Hadron Collider was the culmination of decades of work and the collaboration of thousands of brilliant and passionate people.

It was the final piece needed to confirm the standard model of particle physics as that model now stands.

But there are still many outstanding questions for example, it seems like nothing in the standard model can explain what dark matter is.

So the discovery of the Higgs wasn’t the end of particle physics - but it may be the way forward.

Many physicists think that the secret to finding the elusive dark matter particle will come by studying the Higgs.

In fact, the first tantalizing evidence is already in.

The matter that we perceive out there in the universe is a small fraction of the matter that exists.

We see and we feel the atoms - the electrons and the quarks - via the protons and neutrons.

These particles dominate our experience of the universe because they are strongly interacting.

They tug and push at each other via the electromagnetic and strong nuclear forces.

But there are other matter particles that interact only weakly, and so we don’t see them even though they’re insanely abundant.

Roughly a hundred trillion neutrinos emitted by the sun pass through your body every second, but you don’t notice because they extremely rarely interact with the electrons and quarks that make up the atoms that make up you.

And then there’s dark matter.

We know that there’s some source of gravity out there in the universe NOT caused by the particles of the standard model.

We see its effect in the way galaxies move and in how the universe on the largest scales evolves.

This “dark matter” might be a new kind of particle, or, in fact, there could be an entire family of different particles that interact with each other but not with us.

We’ve talked about this so-called dark sector in a previous episode.

So how do you go about detecting a particle whose defining quality is being almost undetectable?

Let’s start by looking at how we detect new particles in general.

We can bundle the different methods into three broad categories, each represented by a different Feynman diagram.

A Feynman diagram is just a way to represent the interactions of particles, plotting time versus space so we have two particles coming together, undergoing some interaction that involves the exchange of force-carrying particles, and then we have particles leaving the interaction - perhaps the same that went in, perhaps not.

This particular diagram shows a dark matter particle scattering off a standard model particle in some way.

The standard model particle could be a quark, an electron, or anything that makes up normal matter.

We would call this a direct detection experiment - because a dark matter particle is actually interacted with one of the particles in our say detector.

Of course this sort of interaction is incredibly rare - otherwise we’d have detected dark matter already.

But with enough particles and enough time, we should eventually see an interaction between a dark matter particle and a matter particle.

So dark matter detectors consist of huge tubs of liquid or massive chunks of crystal, placed deep underground to avoid cosmic rays.

But either our detectors aren’t big enough or we haven’t waited long enough, because we have yet to spot even a single collision compatible with the dark matter particle hypothesis.

The second family of experiments, indirect detection, can be represented by rotating our Feynman diagram.

A cool thing about these diagrams is that if one orientation is possible, then all other orientations are possible.

In this case the time and space axes are flipped, so now we’re looking at the annihilation of a particle anti-particle pair from the dark sector, resulting in the creation of a particle anti-particle pair of a known type.

For example, two dark matter particles somewhere in space could annihilate to produce gamma ray photons, which could be picked up by telescopes like the Alpha Magnetic Spectrometer.

If we were to find excess gamma radiation from high-density regions of our galaxy, then this might come from dark matter annihilations.

But it’s very difficult to disentangle this source of gamma rays from other astrophysical sources like pulsars, supernovae, and things being eaten by black holes.

So far we have no clear evidence from this method.

OK, let’s try rotating our Feynman diagram one more time.

Now our annihilating dark matter.

particles become dark matter that’s created from the annihilation of some standard model particles.

Now some theorists believe we could see this, by looking at the high-energy collisions of standard model particles in collider experiments like the LHC.

This is where our Higgs boson is going to come back into the picture.

In the LHC we smash together particles of regular matter, like protons or heavier nuclei.

All sorts of exotic particles get created in those collisions.

Those particles are sometimes detected directly when they smash into one of the many detectors surrounding the collision point.

Or they may decay due to being hopelessly unstable, in which case we detect their decay products.

But of all the particles produced in these events, we think that the elusive Higgs boson has the best shot at producing a dark matter particle.

Let’s talk about why.

Firstly, particles with electrical charge OR color charge can’t decay into Higgs bosons, because the Higgs itself doesn’t have such charge - if it did it would interact via electromagnetism and the strong force and we would have spotted it already.

So that excludes the electrically charged leptons: electrons, muons and tau particles; it excludes the quarks and whatever is made of quarks; it excludes the W boson of the weak nuclear force.

It excludes the color-charged gluons of the strong nuclear force.

We also exclude photons, because not interacting with light is the first defining characteristic of dark matter.We aren’t left with much in the standard model.

Neutrinos could potentially decay into dark matter particles, but if they do it’s going to be near impossible to spot the event, so we’ll leave that aside for now.

There are ways to check whether dark matter and neutrinos are connected on cosmic scales by looking at the cosmic microwave background, but that’s another story.

We’re left with two neutral bosons: the Z boson of the weak force and the Higgs.

The Z was thoroughly studied at the Large Electron-Position Collider, but no evidence was found supporting interactions with dark matter, and so the Z is probably a dead end.

So the higgs is looking like the only game left in town.

But this isn’t quite a last desperate hail mary.

There’s good reason to think the Higgs might interact with dark matter.

We know that the Higgs field is what gives most of the standard model particles their masses.

Well, dark matter definitely has mass - that’s how we know it exists.

So it wouldn’t be too surprising if it turns out dark matter also gets its mass from the Higgs.

There is a whole family of potential theories of how the Higgs could interact with dark matter, these all fall under the umbrella of Higgs portal models.

Physicists playfully called it a portal since the Higgs could be the doorway that connects our standard sector of particles to the dark universe.

So, how exactly are we going to find dark matter via the Higgs?

Well first, we need a place where we can reliably create Higgs bosons.

The best place is still the place where the Higgs was discovered - the LHC.

Since that discovery, the LHC has undergone major upgrades, and so it’s better at producing Higgs bosons than ever.

But once we create them, how could we possibly tell if a Higgs decays into dark matter?

After all, those particles are going to fly right through all of our detectors.

Well if you’ve watched this show before you probably know that physicists are very stubborn and very good and finding ways to do things they should not be able to.

And there is a trick for detecting undetectable particles.

In this case, it’s by using a 350 year old law of physics known as conservation of momentum.

Conservation of momentum tells us that the product of velocity times mass of all particles going into a collision has to be the same as the same product for all particles going out.

We know pretty well the momentum of the particles going into our collision, and we can measure and add up the momentum of all the final state particles that we actually see going out.

If total momentum seems to have decreased, this implies something invisible has sneaked that missing momentum past the detectors.

Now if we just looked at the total momentum, there’d be a lot of uncertainty due to the fact that there’s variation in the speed of the colliding particles.

But there's another part to this trick that makes it very precise.

We can do our momentum audit in a way that ensures that in-going momentum is known completely precisely.

In fact, it’s precisely zero.

Total moment in a collision is conserved, but also the momentum in each separate direction is conserved independently of the other directions.

The momentum perpendicular to the direction of the particle beams is called the transverse momentum, and it’s zero by definition.

And has to stay at zero.

The various collision products can scatter in any direction, forwards, back or sideways.

But any of that sideways or transverse momentum has to add up to zero.

For every particle scattering to the left, you need something scattering the right to balance it out.

Here’s a real-life example from the ATLAS detector at the LHC: this event has caused a jet of various visible particles to shoot off to the side.

Conservation of momentum tells us that there should be more stuff firing out in the opposite direction of the jet, but no visible particles appeared on that side.

The only explanation is that particles were projected in that direction, they're just invisible.

You might ask - can’t those invisible particles just be neutrinos?

Yes they can.

But every neutrino has to be created with an electron, muon or tau particle partner.

And that lepton will be detected, meaning we can account for the momentum lost to neutrinos.

There are certain Higgs-generating reactions that are especially promising for our dark matter hunt.

For example, this is the vector boson fusion channel of Higgs production, in which a pair of quarks in the colliding protons shoot a W or Z boson at each other.

Those bosons then annihilate each other to produce a Higgs.

And the Higgs lives for only a fraction of a second before decaying.

The hope is that sometimes it decays into a dark matter particle.

Most of the visible messy products of this particular type of event are launched in the same direction as the beam, making the calculation of the transverse momentum very straightforward.

Physicists at the Large Hadron Collider’s ATLAS experiment have been adding up the outgoing transverse momenta for many, many events like this.

The results are summed up in one number: the so-called branching fraction.

This number tells you the fraction of times a Higgs decayed into particles that can’t be detected.

The standard model predicts that up to 17% of Higgs bosons should decay into invisible neutrinos, so the null hypothesis would be for a branching fraction of 0.17.

So what did the ATLAS people find?

Adding up data from all known channels of Higgs production, the true branching fraction could be anything up to 26%.

If this number holds up, then ,the Higgs could be decaying into new invisible particles!

The error bars on this measurement are still large, and so we need to watch more Higgs bosons decay.

The LHC and ATLAS recently switch on again after a 3 year upgrade, so we’re back to it.

And there are multiple plans to build colliders specialized in making Higgs bosons, although those are some years away.

The discovery of the Higgs boson was the end of one era of particle physics but very much the beginning of another.

We are entering the era of Higgs physics, and we don’t know what it’ll reveal —- hopefully a dark matter particle, perhaps an entire dark sector, perhaps much more.

But certainly a portal beyond the familiar physics of our luminous space time.