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For something called the Standard Model, the foundational theory of particle physics is confronting a lengthy list of non-standard data. Neutrino oscillations, dark matter and energy, the imbalance between matter and anti-matter, quantum gravity—there’s a growing list of natural phenomena that don’t seem to fit.
Now there might be a new entry. Two years ago, a balloon-born experiment floating high above the Antarctic ice looking for neutrinos saw something unusual: particle signatures traveling up out of the Earth at angles suggesting that they’d passed through 5000 kilometers of rock. The particles had energies that were high enough for the Standard Model to prohibit that kind of careless disregard for matter.
The data gave rise to a series of speculations. Some, like decaying dark matter inside the Earth, or a new form of neutrino, represented new physics beyond the Standard Model. Others were more mundane: The signals could have resulted, for example, from some unconsidered physics that made a regular, downward-going neutrino look as if it were traveling up.
Then, two weeks ago, a team of physicists from Penn State University pointed out that the two ANITA detections, plus three unusual neutrino detections at a completely different experiment—called IceCube, also in Antarctica—could all be explained by a chain of particle collisions and decays that involved the stau.
The stau is a hypothesized, never-detected particle that is proposed by supersymmetry. According to supersymmetry, every fermion particle—like electrons, or tau particles—has a partner with a different spin (spin is a quantum mechanical property of subatomic particles). These superpartners, for convenience, carry the names of their regular (non-super) twins, but preceded with an “s.” The superpartner of the electron is the selectron. The superpartner of the tau is the stau.
If supersymmetry were proven, it would have the potential to solve many outstanding problems in physics, potentially including dark matter. The team from Penn calculated the probability that the Standard Model could explain the data—it is tiny—and argued that the three IceCube events, which are currently interpreted as standard muon particle tracks, are really the result of tau and its superpartner.
I caught up with Derek Fox, the first author of a preprint published by the team, last week.
How much of a problem are these events for the Standard Model?
The two ANITA events clearly contradict the Standard Model. The standard model is dead for these events. You can’t do it. It’s excluded at a few parts in a trillion. The three IceCube events are subject to more ambiguity because a Standard Model explanation exists. Our bottom line number there is 91 percent confidence that the IceCube events depart from the Standard Model. That is not a high level of confidence. In some situations, you would not even publish it. We just include it in our paper because we think it’s very interesting in context.
How do you propose the anomalous events got produced?
It’s quite a crazy chain of events. An ultra-high energy neutrino from an astrophysical source hits the Earth, and it passes through the atmosphere without noticing it. Inside the Earth, it only gets a couple hundred kilometers, then it interacts and makes a pair of beyond Standard Model particles. One particle of the pair, called the stau, will then travel a few thousand additional kilometers through the Earth before decaying.
How can the stau travel through so much of the Earth before decaying?
The stau is the supersymmetric partner of the tau. It decays into the tau lepton (which we also just call the tau), which is a known Standard Model Particle, plus the least-mass supersymmetric particle, which is a candidate for dark matter. But the dark matter candidate is very weakly interacting. Therefore, the decay rate of the stau to this particle, plus the tau, is relatively low. It lives a lot longer than any standard-model particle at this mass. It’s like a big Mack truck cruising through the Earth.
The dinosaurs were here and there were these crazy cosmic ray showers erupting from the surface of the planet.
What happens after the stau decays?
The dark matter candidate particle it produces is for all intents and purposes undetectable, so it escapes. The tau it produces can maybe get 100 kilometers or so before decaying into a tau neutrino, which can go another 200 kilometers. If the tau neutrino decays, then it makes the tau again in a sort of regeneration process. Eventually, if you’re lucky, the tau neutrino makes a tau close enough to the surface of the Earth for the tau itself to escape before decaying. If it escapes and decays in the atmosphere, it will make an air shower like the ones ANITA saw.
Why does the dark matter particle not decay?
In supersymmetric models, the least-mass supersymmetric particle is stable for billions of years. That’s because it comes with a new symmetry and a new conservation principle. If the dark matter particle decays just on its own, it violates that conservation principle. It’s sort of like the decay of an electron. The electron is the least-massive charged particle and therefore it doesn’t decay, thank goodness. Imagine how likely it would be for us to be here if electrons decayed.
The sequence you’re proposing seems like such a coincidence. The numbers have to exactly line up for us to be able to see any of this.
I do agree completely with you that we have some crazy cosmic coincidence going on. The properties of this particle plus the properties of the diffuse flux of high-energy neutrinos hitting the Earth have to be right to make it happen. And then, the Earth has to have a certain size, and an atmosphere of a certain density that is transparent in a certain way. It’s a little crazy. I was thinking about this when I finally came to appreciate this scenario, and I was like, oh my gosh, this has been happening our whole lives. It’s happening right now on Earth. These crazy high-energy neutrinos are interacting in the crust and making these crazy supersymmetric particles, which scream through the Earth and then through a chain of decays, create upgoing cosmic ray showers, popping out like little Roman candles. And it’s been happening for billions of years, for the whole history of humanity. The dinosaurs were here and there were these crazy cosmic ray showers erupting from the surface of the planet.
Besides the upgoing showers that ANITA sees, you argue that taus are also detected by IceCube. How?
If the tau is close to the surface of the Earth and it streaks through the ice of the IceCube detector, then it will leave a track type signature in the IceCube detector. We assert that the easiest explanation for some of the very high-energy tracks in the IceCube data is ultra-high energy tau particles.
How did you do your search through IceCube events?
The IceCube collaboration determined that they were going to publish a catalog of their events above a cutoff energy of 200 TeV. This catalog includes 36 events. Luckily, these are the events we’re really interested in for identifying possible hidden SEECRs, as we call them (Sub-EeV Earth-emergent Cosmic Rays). That’s the data we used, and I think we have three pretty good tau candidates. It might be possible to make a more detailed analysis of the data that IceCube has. The tau is 17 times more massive than the muon [Ed: to which the three candidate tracks are currently attributed], and I suspect that just has to have some sort of impact on the pattern of light deposition in the detectors. But I can’t prove it. I don’t have an analysis ready to go. I do have some ideas how to do that.
Why did IceCube not recognize these tracks as coming from tau particles?
IceCube is a set of photo detectors in ice. When a highly relativistic charged particle passes through the ice, it spews out this sort of electromagnetic sonic boom of blue Cherenkov light. That’s what IceCube detects. Using all those photon detections, its scientists reconstruct the angle and the trajectory of the particle. But the light measurement is not telling you the mass of the particle, or its energy. It’s telling you the Lorentz factor, which is energy divided by mass. So, if you know that it’s a muon, then you know the mass and then you know the energy. But, if you’re wrong and it’s actually a tau, since the tau is 17 times more massive than the muon your energy estimate is 17 times too low. If one or two taus slipped through, how would you know? The answer is you wouldn’t, unless you go looking for them specifically, which is what we did. There’s no sore thumb sticking out there.
One of the big attractions of supersymmetry is that it offers us an actual candidate for dark matter.
Were you the first to suspect camouflaged tau tracks at IceCube?
We were not the first. The paper I credit for this more than any other is by Kistler and Laha. The publication year is 2018, but it was actually out on the arXiv last year, so it’s been around for about a year and a half at his point. They identified this particular event, IceCube 140611, that we spend a whole section in our paper talking about. They said that the properties of the event are really weird if you think it’s a muon, and maybe they make more sense if you treat it as a tau, even though when you treat it as a tau, it’s at much higher energy and therefore is a much less likely sort of neutrino for IceCube to encounter. Really what we’ve done is sort of put some hard numbers down. They never planted any stake in the ground about the statistical unlikelihood of that event and we’ve done that now. We also identified two further candidate events.
If we are seeing new evidence for supersymmetry, what does that mean for physics?
If you think of the Standard Model as a Lagrangian that reflects all known particles and interactions of those particles, then the prospect of doubling the terms in the Lagrangian, as you do in supersymmetric theories, is a pretty dramatic change in our picture of the universe. One of the big attractions of supersymmetry is that it offers us an actual candidate for dark matter, in the lowest mass super symmetric partner particle.
What if all of this is a hardware error?
It’s just impossible. You can go ahead and talk to the ANITA people if you want, but these events are detected across multiple flights with multiple radio antennae each. The characteristic signature across the antennae and across frequencies is consistent with the air shower interpretation. We’re way beyond hardware glitch at this point.
What alternative explanations is the ANITA team considering?
I’m sort of reading tea leaves here, but I think they may be worried that it’s possible for a down-going cosmic ray shower to execute a double bounce on its way, so that it hits the ice and comes back to the facility, which would mimic a direct observation. But, frankly I think this extremely unlikely to happen.
If one or two taus slipped through, how would you know? The answer is you wouldn’t, unless you go looking for them specifically.
Why hasn’t the Large Hadron Collider (LHC) seen this particle?
The Atlas and CMS experiments at the LHC have been searching for slepton particles, including the stau, since the LHC turned on. They haven’t found it yet, but they’ve been pushing the lower mass bound for the stau upward with time. They just don’t have enough data yet.
What is the range of possible masses for the stau?
Supersymmetry basically breaks if the scale for the supersymmetric partner particle masses goes above 1,000 GeV. And we need a mass greater than, let’s say 400 GeV to avoid producing events that would have shown up in LHC searches so far.
Why did you use Monte Carlo simulations in your paper?
We were very grateful to the Alvarez-Muñiz team that put together the Monte Carlo software we used. The Earth is not a single homogenous sphere of uniform density—as physicists say, it’s not a spherical cow—and so, if you really want to address this question of how often a tau neutrino can propagate through the Earth and generate an outgoing tau on the far side, a Monte Carlo realization is the way to go. The software goes neutrino by neutrino and it tracks it through the Earth. It’s sort of beyond the capabilities of pencil and paper, mainly because of the regeneration process.
How does it feel to be helping interpret such potentially revolutionary data?
I would say nobody is more astounded than me. But it sort of fell into my lap this way. Since realizing how nicely all of the pieces of the puzzle were coming together, which was about a month ago, I’ve just been trying to rise to the occasion. Working really hard and trying to consider all angles: Possibly negative or refutory aspects, ways in which the thing doesn’t work, most promising avenues of future research, highest priorities, that sort of stuff. It was a real sprint—29 days from realization to submission on the arXiv.
That is a breakneck speed for science.
It is breakneck. That actually is a little too apropos. I was getting a little ragged.
Michael Segal is Nautilus ’ editor in chief.
Lead image from Drummermean / Wikimedia.
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