Could the Higgs Boson Lead Us to Dark Matter?

We would like to thank NordVPN for supporting PBS. The discovery of the Higgs

boson

ten years ago at the Large Hadron Collider was the culmination of decades of work and the collaboration of thousands of brilliant and passionate people. It was the last piece needed to confirm the Standard Model of particle physics as it currently stands. But there are still many outstanding questions, for example, it seems that nothing in the Standard Model can explain what

dark

matter

is. So the discovery of the Higgs was not 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 from studying the Higgs. In fact, we already have the first tantalizing evidence. The matter we perceive in the universe is a small fraction of the matter that exists. We see and feel atoms (electrons and quarks) through protons and neutrons. These particles dominate our experience of the universe because they interact strongly. They pull and push each other through strong electromagnetic and nuclear forces. But there are other particles of matter that interact only weakly, so we don't see them even though they are incredibly abundant.

Approximately one hundred trillion neutrinos emitted by the sun pass through your body every second, but you don't realize it because they rarely interact with the electrons and quarks that make up the atoms that make you up. And then there is dark matter. We know that there is some source of gravity in the universe that is NOT caused by Standard Model particles. We see its effect in the way galaxies move and how the universe evolves on the largest scales. This “dark matter”

could

be a new type of particle or, in fact, there

could

be a whole family of different particles that interact with each other but not with us.

We talked about this so-called dark sector in a previous episode. So how can you detect a particle whose defining quality is being almost undetectable? Let's start by looking at how we detect new particles in general. We can group the different methods into three broad categories, each represented by a different Feynman diagram. A Feynman diagram is simply a way of representing particle interactions, plotting time versus space, so we have two particles that come together, undergo some interaction that involves the exchange of force-carrying particles, and then we have particles that they leave the interaction, maybe the Same thing that entered, maybe not.

This particular diagram shows a dark matter particle scattering in some way from a standard model particle. The standard model particle could be a quark, an electron, or whatever makes up normal matter. We would call this a direct detection experiment, because a dark matter particle actually interacts with one of the particles in our detector. Of course, this type of interaction is incredibly rare; Otherwise, we would have already detected dark matter. But with enough particles and enough time, we should eventually see an interaction between a dark matter particle and a matter particle. Therefore, dark matter detectors consist of huge containers of liquid or huge pieces of glass, placed at great depths to avoid cosmic rays.

But either our detectors are not big enough or we have not waited long enough, because we have not yet detected 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. The nice thing about these diagrams is that if one orientation is possible, all other orientations are also possible. In this case, the time and space axes are reversed, so we are now looking at the annihilation of a pair of antiparticle particles from the dark sector, resulting in the creation of a pair of antiparticle particles of a known type.

For example, two dark matter particles somewhere in space could annihilate to produce gamma-ray photons, which could be captured by telescopes like the Alpha Magnetic Spectrometer. If we were to find excess gamma radiation in high-density regions of our galaxy, then this could come from dark matter annihilations. But it is very difficult to separate this source of gamma rays from other astrophysical sources such as pulsars, supernovae and objects devoured by black holes. So far we have no clear evidence of this method. Okay, let's try rotating our Feynman diagram one more time. Now our annihilating dark matter. The particles become dark matter which is created from the annihilation of some standard model particles.

Now, some theorists believe we could see this by observing high-energy collisions of standard model particles in experiments with colliders like the LHC. This is where our Higgs

boson

will come into the picture again. At the LHC we crush particles of regular matter, such as protons or heavier nuclei. In these collisions all kinds of exotic particles are created. Sometimes those particles are detected directly when they hit one of the many detectors surrounding the collision point. Or they may decompose because they are hopelessly unstable, in which case we detect their decomposition products. But of all the particles produced in these events, we think the elusive Higgs boson has the best chance of producing a dark matter particle.

Let's talk about why. First of all, particles with electric charge OR color charge cannot decay into Higgs bosons, because the Higgs itself does not have that charge; If it had it, it would interact through electromagnetism and the strong force and we would have already detected it. That excludes electrically charged leptons: electrons, muons, and tau particles; excludes quarks and everything made of quarks; excludes the W boson from the weak nuclear force. It excludes colored charged gluons from the strong nuclear force. We also exclude photons, because not interacting with light is the first defining characteristic of dark matter.

We don't have much left in the standard model. Neutrinos could potentially decay into dark matter particles, but if they do it will be almost impossible to detect the event, so we'll leave it aside for now. There are ways to test whether dark matter and neutrinos are connected on cosmic scales by looking at the cosmic microwave background, but that's another story. We are left with two neutral bosons: the Z boson of the weak force and the Higgs boson. Z was studied thoroughly at the Large Electron Position Collider, but no evidence was found to support interactions with dark matter, so Z is probably a dead end.

So

higgs

seems to be the only game left in town. But this is not a last, desperate Hail Mary. There are good reasons to think that the Higgs could interact with dark matter. We know that the Higgs field is what gives mass to most Standard Model particles. Well, dark matter definitely has mass; That's how we know it exists. Therefore, it wouldn't be too surprising if it turns out that dark matter also gets its mass from the Higgs. There is a whole family of potential theories for how the Higgs could interact with dark matter, all under the umbrella of Higgs portal models.

Physicists jokingly called it a portal, since the Higgs could be the door connecting our standard particle sector to the dark universe. So how exactly are we going to find dark matter through 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 improvements, producing Higgs bosons better than ever. But once we create them, how would we know if a Higgs decays into dark matter? After all, those particles will fly through all our detectors.

Well, if you've seen this show before, you probably know that physicists are very stubborn and very good and find ways to do things that they shouldn't be able to do. And there is a trick to detect undetectable particles. In this case, a 350-year-old law of physics known as conservation of momentum is used. Conservation of momentum tells us that the product of the velocity and the mass of all the particles that enter the collision must be the same product for all the particles that leave. We know the momentum of the particles entering our collision quite well, and we can measure and sum the momentum of all the final state particles that we actually see leaving.

If the total impulse appears to have decreased, this implies that something invisible has managed to pass that missing impulse through the detectors. Now, if we only looked at the total momentum, there would be a lot of uncertainty due to the fact that there is variation in the speed of the colliding particles. But there is another part of this trick that makes it very precise. We can perform our boost audit in a way that ensures the incoming boost is known with complete accuracy. In fact, it is precisely zero. The total momentum 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 transverse momentum and is zero by definition. And it has to remain at zero. The various products of the collision can disperse in any direction: forward, backward or to the side. But any amount of that lateral or transverse momentum has to add up to zero. For every particle that scatters to the left, you need something that scatters to the right to balance it. Here's a real example from the LHC's ATLAS detector: This event has caused a jet of several visible particles to shoot to the side.

Conservation of momentum tells us that there should have been more stuff shooting in the opposite direction of the jet, but no visible particles appeared on that side. The only explanation is that the particles were projected in that direction, they are simply invisible. You might be wondering: couldn't those invisible particles just be neutrinos? If you can. But each neutrino has to be created with an electron, a muon or a tau particle. And that lepton will be detected, which means we will be able to explain the momentum lost by the neutrinos. There are certain Higgs generating reactions that are especially promising for our search for dark matter.

This is, for example, the Higgs production vector boson fusion channel, in which a pair of quarks in colliding protons fire a W or Z boson at each other. Those bosons then annihilate each other to produce a Higgs. And the Higgs lives only a fraction of a second before decaying. The hope is that it will sometimes decay and become a dark matter particle. Most of the visible disorder products from this particular type of event are thrown in the same direction as the ray, making the calculation of the transverse momentum very simple. Physicists at the Large Hadron Collider's ATLAS experiment have been adding up the outgoing transverse moments of many, many events like this one.

The results are summarized in a number: the so-called branching fraction. This number indicates the fraction of times a Higgs decayed into particles that cannot 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 data from all known Higgs production channels, the true branching fraction could be as high as 26%. If this number holds, then the Higgs could be decaying into new invisible particles! The error bars in this measurement are still large, so we need to observe more Higgs boson decays.

The LHC and ATLAS were recently turned back on after a 3 year upgrade, so we're at it again. And there are multiple plans to build colliders specialized in producing Higgs bosons, although this is still a few 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 will reveal: hopefully a dark matter particle, maybe an entire dark sector, maybe much more. But it is certainly a portal beyond the familiar physics of our luminous space-time.

We would like to thank Nord (creators of NordVPN) for supporting PBS. NordVPN is a cybersecurity tool that offers you a VPN solution for computers and mobile devices. Imagine a VPN as an encrypted tunnel for online traffic to flow. No one can see through the tunnel and access your Internet data. And with Nord Threat Protection, users get a service that protects them from ads, trackers, and malware. NordVPN can help users analyze their data to detect suspicious websites and an accountNordVPN allows you to connect up to 6 devices. For more information about NordVPN, see https://nordvpn.com/spacetime.