Supersymmetric Particle Found? - Script (Patreon)

With the large hadron collider running out of places to look for clues to a deeper theory of physics, we need a bigger particle accelerator. We have one - the galaxy. And the particles it flings at us may have finally revealed physics beyond the standard model.

Physics is currently in a weird place. Historically, no matter how crazy our theories got, there were always new ways to test them. Your theory predicts a new particle? Build a particle accelerator big enough to see it. But once your collider spans entire countries – like the Large Hadron Collider in Switzerland – there’s only so much larger you can go  - at least on the surface of the Earth. The LHC has thoroughly tested the standard model of particle physics. The last component of that model – the Higgs boson – was verified in 2013. But the standard model isn’t the end of the story – there MUST be a more fundamental theory that explains the origins of this rich family of particles. Proposals for such grand unified theories proliferate, unconstrained by even the tiniest hint of new physics from the LHC.

One potentially VERY important ingredient for grand unification is supersymmetry. This is one that physicists had really hoped to nail down with the Large Hadron Collider. SUSY is a proposed extension to the standard model, designed to fix certain issues with the theory. The most serious being that the standard model can’t explain the minuscule weakness of gravity compared to the other three forces. This is the hierarchy problem. SUSY provides a very natural explanation for the discrepancy by introducing a new symmetry between the fermions which comprise matter and the bosons which communicate the fundamental forces. As well as fixing the hierarchy problem, this connection between fermions and bosons is in general a step towards unifying the particles of the standard model. It’s a key feature of some grand unified theories as well as modern string theory – leveling it up to superstring theory.

SUSY predicts that every single standard model particle has a supersymmetric partner particle of the opposite type. The partners of fermions are bosons, and the partners of bosons are fermions. We’ll come back to all of this in detail another time, but the one property that’s relevant for today’s episode is that these supersymmetric particles are all expected to be way more massive than their known partners in the standard model. To solve the hierarchy problem perfectly, those particles would need to have masses at around what we call the electroweak energy – that’s the energy at which the electromagnetic and weak nuclear forces merge into the same force.

Physicists had hoped that, by smashing particles together hard enough in the Large Hadron Collider, there’d be enough energy in those collisions to produce a supersymmetric particle. In fact there should have been – at least for the versions of SUSY that most neatly solve the hierarchy problem. But the LHC has seen nothing. This doesn’t necessarily kill supersymmetry – it may just be that these new particles are more massive than expected. If so, they can still help with the hierarchy problem, though not in as neat a way as hoped.

To detect more massive supersymmetric particles you need higher energy particle collisions. So what? Build an accelerator the size of the planet? The solar system? Give up and let the theorists just tell their stories? Actually, there is a way to probe energies far higher than is possible with the Large Hadron Collider. The universe itself is a pretty good natural particle accelerator.

Supernova explosions, gamma-ray bursts, black hole magnetic fields are all expected to blast high-energy particles like electrons and atomic nuclei into the universe. These are cosmic rays. The highest energy cosmic rays can have energies around a billion times that of the LHC. Unfortunately for particle physics experiments, cosmic rays at these energies are extremely rare. So it’s not surprising that we haven’t seen supersymmetric particles in our cosmic ray observatories yet. Or have we?

Let’s talk about this thing. No, it’s not a downed Imperial probe droid. Your rebel base is safe. This is ANITA, the Antarctic Impulsive Transient Antenna. It’s a cosmic ray experiment of a very special sort. In fact it’s a cosmic ray detector disguised as a neutrino detector disguised as a radio antenna disguised as a hot air balloon. Probably I should explain that.

When ultra-high energy cosmic rays travel through space, they bump into the photons of the cosmic microwave background – that’s the left over heat-glow of the very early universe. Those cosmic rays lose energy to the CMB, which is partly why the most energetic cosmic rays are so rare here on Earth. But in those interactions, cosmic rays can create extremely high-energy neutrinos. Neutrinos are almost ghost-like particles that travel through the CMB unimpeded. So detecting the highest energy neutrinos allows us to learn about the cosmic rays that produced them.

These neutrinos don’t just ignore the CMB, they can pass through solid matter. Lower energy neutrinos can flow right through the Earth as though it isn’t there. We detect neutrinos because very VERY rarely one will interact with an atomic nucleus

and produce a shower of particles. For example, the IceCube observatory is a 1km cube of Antarctic glacier laces with photon detectors. It spots neutrinos when they decay into electrons, muons, or tau particles, which in turn produce visible light as they streak through the ice. This is Cherenkov radiation, and IceCube’s photo-detectors track these flashes. ANITA works in a similar way, but it’s focused on catching the very highest-energy neutrinos – the ones that are produced by cosmic ray interactions with the CMB.

In order to see those extremely rare neutrinos, ANITA scans not 1 cubic kilometer of ice – but 15 million square kilometers of Antarctic ice sheet. That’s where the balloon comes in. ANITA is a cluster of radio antennae that hovers 37 km above Antarctica. If an ultra-high-energy neutrino decays in the ice anywhere within 700km of ANITA, the resulting radio-frequency Cherenkov can be seen by ANITA’s antennae. ANITA is designed to detect neutrinos that are coming from below – passing through the Earth into the ice sheet. That allows it to sort out neutrino radio flashes from the flashes produced by other cosmic rays coming in from above.

In fact ANITA expects to see its most interesting, MOST energetic neutrinos coming in at an angle – skimming the arc of the Earth on a shallow trajectory. They should not come from directly below, which would require them to pass through the entire planet. That’s because the most energetic neutrinos actually do lose energy passing through the Earth. They aren’t expected to make it all the way through without slowing down significantly. So you can imagine that ANITA scientists were a little confused when they spotted two extremely high-energy radio bursts that could only have been produced by a high-energy particles passing all the way through the middle of the planet. That’s several thousand kilometers of rock, magma and iron.

These events looked like what you expect when a particular flavor of neutrino – the tau neutrino – interacts with the ice and transforms into a tau lepton – that’s the heavier cousin to the electron. The tau is cool because it’s so short lived – it produces a Cherenkov burst when it’s created and then a second burst when it decays into a shower of secondary particles. But seeing these very high energy tau events from directly below doesn’t make sense. But based on our understanding of the normal background rate of high-energy neutrinos, it’s estimated that there’s around a 1 in 3 trillion chance that 2 tau neutrinos could have been seen in the amount of time ANITA has been looking. Physicists are having trouble accounting for these events with any known standard model particle, which brings us back to supersymmetry.

Astrophysicists Derek Fox, Stein Sigurdson and team point out that there’s a version of supersymmetry that predicts exactly the right particle to do this job. It’s the supersymmetric partner of the tau lepton – the stau particle. Yeah, you put in s in front to get the SUSY particle – selectron, squark, stau. Supersymmetry is that easy. Here’s the scenario: a stau particle was produced on the opposite side of the planet by an incoming ultra-high energy neutrino plowing into the Earth. The stau is theoretically capable of zipping straight through the Earth before decaying into a regular tau lepton on the other side. This then causes a high-energy radio flash coming from directly below.

That’s quite a story, but it fits the observations pretty well. There are, of course, other possibilities. It could be a so-called sterile neutrino – that’s … , This particle is also not in the standard model, but has nothing to do with supersymmetry. Hints of its existence have been found in a Fermilab particle accelerator experiment – as we discussed previously. It may also be that there were some gigantic bursts of regular neutrinos at the time of the observed events. Hit the Earth with enough high neutrinos – for example from a supernova explosion - and at least some of the ultra-high energy could make it through. In fact one of the two events may have been associated with a distant supernova that was observed around the same time and location. The probability of a chance association with a supernova is around 3%, so it’s unlikely but does happen. On the other hand, the supernova in question wasn’t nearly bright enough to make even one Earth-penetrating ultra-high energy neutrino likely.  And remember there were two events at different times. The other event wasn’t associated with any supernova or gamma-ray burst.

The final possibility is just that we’re missing something. Perhaps our understanding of neutrino propagation through the Earth is flawed, or perhaps penguins use cell phones now. This is going to require more observation and confirmation. The first step would be to look to the other big neutrino observatories. IceCube is really the only one that could have potentially have spotted similar events. Actually, given the amount of time IceCube has been in operation it probably should have seen them. Fox and team looked back into the IceCube archive and actually did find some possible high-energy tau lepton events that may have come from directly below. The data is a little ambiguous – they may have been boring old muon neutrinos, which can easily pass through the planet. But you can bet people will be paying a lot more attention to these sorts of events from now on.

So have we gone beyond the standard model and proved supersymmetry? Hell no. What we have is a tantalizing hint. Given the painful absence of new particles from the Large Hadron Collider, ANY hint of something new is bound to get physicists excited. ANITA will keep flying, IceCube will start carefully scrutinizing its data, and if these mysterious events-from-below keep arriving, you can be sure that physicists will find an ingenious way to confirm their nature. Perhaps we’ll verify the existence of the stau, and with it confirm the supersymmetric nature of s-space-stime.