Mysterious Cosmic Rays Shooting from the Ground in Antarctica Could Break Physics
NASA went searching for micro black holes in Antarctica. Instead, it detected cosmic rays shooting from the ground and some physicists think it could be evidence of a supersymmetric particle.
ANITA-IV after returning from the edge of space.
There’s something strange happening beneath the surface of Antarctica and it’s got nothing to do with Nazi UFOs. Rather, researchers are arguing that a decade-old experiment may have furnished the first evidence of a new type of particle that has evaded detection by some of the most sophisticated particle accelerators for years. If they turn out to be correct, it would change physics as we know it.
In 2006, NASA-affiliated researchers launched Antarctic Impulsive Transient Antenna (ANITA), a balloon experiment meant to observe high energy particles that shower the Earth from space, also known as cosmic rays. During ANITA’s flight, however, its instruments observed something that physicists couldn’t explain. In addition to detecting cosmic rays from space, ANITA also detected cosmic rays shooting from the ground as the high altitude balloon drifted over the Antarctic ice sheet.
Physicists have long known that high energy particles can penetrate deep into Earth, but none of the particles predicted by the Standard Model—the most accurate model of physics that has ever existed—should be able to pass all the way through the planet.
On Tuesday, a group of researchers led by the Pennsylvania State University physicist Derek Fox posted a new theory of these upward-shooting cosmic rays to arXiv that suggests they could be evidence of a particle that lies beyond the Standard Model. If Fox and his colleagues are correct, it would be the first evidence of a particle beyond the Standard Model of physics, the most accurate description of the universe humans have ever known.
What Did ANITA See In Antarctica?
The first ANITA mission launched from the McMurdo base in Antarctica in December 2006. The experiment flew to an altitude of about 120,000 feet where it spent a month drifting over Antarctica. It was equipped with sensors designed to detect pulses of radiation produced when ultra high energy neutrinos—a nearly massless particle with no electric charge—interact with the Antarctic ice sheet.
In the early 60s, the Soviet physicist Gurgen Askaryan theorized that when a high energy particle interacted with a dense dielectric medium—a type of insulating material that doesn’t conduct electricity—it would produce a shower of secondary charged particles whose radiation can be detected by standard radio antennas. This interaction, now known as the Askaryan effect, allows physicists to detect particles that hardly interact with normal matter (like neutrinos) by observing their secondary effects.
The goal of the ANITA mission was to use an array of antennas to detect the Askaryan radiation produced from high energy neutrinos interacting with the Antarctic ice sheet. Unlike photons, neutrinos don’t lose their energy as they propagate through the universe. This means that they can carry information from beyond the photon horizon (the limit that photon sources are still detectable from Earth) and provide a window onto the farthest reaches of the universe.
Furthermore, some models of physics that are “Beyond the Standard Model” predict the existence of incredibly small extra dimensions. Some of these theories predict that when cosmic rays interact with ice this produces micro black holes that open into these dimensions, which could be detected via the Askaryan effect.
Although the first ANITA mission didn’t detect any evidence of micro black holes, it did detect the Askaryan effect, the first time this had ever been observed from electron interactions with ice. Yet the researchers working on ANITA also got more than they bargained for when they also detected cosmic rays that appeared to be shooting out of the Antarctic ice sheet.
The first ANITA mission detected two “upward-pointing cosmic ray-like events” during its month-long sojourn above Antarctica. Unlike the cosmic rays that come from space and are reflected off the Antarctic ice sheet, which produce vertically polarized pulses of radiation, the two anomalous cosmic rays had nearly horizontal planes of polarization. This suggested that they either didn’t originate in space—or if they did, the radiation was produced by particles that had traveled all the way through Earth. In either case, this type of cosmic ray had never been observed before.
A second mission ANITA mission in 2009 as well as a third mission in 2014 detected another strange upward-pointing cosmic ray. The source of these cosmic rays remain a mystery, but a number of theories have been proposed. Some physicists think these upward-pointing cosmic rays are evidence of the decay of dark matter that exists in the Earth’s interior. In February, John Cherry and Ian Shoemaker suggested that these cosmic rays might be explained by sterile neutrinos, a type of high energy particle that hardly ever interacts with ordinary matter.
At first, physicists attempted to explain these strange events as the result of a type of particle called a tau-neutrino decaying as it passed through Earth. This would produce an elementary charged particle called a tau-lepton, which would produce the type of signature observed by the ANITA balloons.
There was just one problem. ANITA observed the particles coming in at extreme angles—27 degrees and 35 degrees—that aren’t permitted within the Standard Model of physics. This suggested that either the Standard Model would have to undergo “significant” revisions to account for the observation—or, as Derek Fox and his colleagues recently suggested, ANITA may have observed the first evidence of a supersymmetric particle.
What is Beyond the Standard Model of Physics?
The Standard Model of physics was cobbled together over the course of the past century and currently serves as the most accurate model of the physical universe ever created. It describes most of the fundamental forces and classifies elementary particles.
Although the Standard Model has proven remarkably successful for making experimental predictions over the last few decades, it’s not able to explain everything. Some phenomena, such as gravity, the accelerating expansion of the universe, and neutrino oscillations are not incorporated in the model.
These deficiencies in the model have led some physicist to begin thinking about physics beyond the Standard Model (BSM). You may have heard of some of these exotic theories, such as string theory or M-theory, but so far there isn’t much evidence to support one theoretical version of BSM physics over another.
In February, physicists According to the paper posted to arXiv this week, however, there are strong reasons to believe that the anomalous cosmic rays seen by ANITA could be evidence of a BSM particle.
This theory relies on a version of BSM physics called supersymmetry. Unlike string theory, which is a so-called “theory of everything” that overhauls the Standard Model, supersymmetry merely extends the Standard Model by adding a new class of massive particles into the mix.
“We argue that if the ANITA events are correctly interpreted then they require some beyond the Standard Model particle,” Fox told me on the phone. “The likely properties of the particle seem consistent in at least some ways with the predicted properties of the stau in some supersymmetric models.”
In supersymmetry, each of the elementary particles in the Standard Model has a heavier “superpartner.” Thus, leptons are matched with sleptons, electrons with selectrons, quarks with squarks, and so on. None of these theoretical supersymmetric particles, or sparticles, have been produced in a lab so far, which may be because the particles require too much energy to be made by contemporary particle accelerators such as the Large Hadron Collider. Thus some physicists hope to detect them by looking to astrophysical sources, which can produce the requisite amounts of energy to produce these more massive particles.
In the case of the upward-pointing cosmic rays, Fox and his colleagues argue that they are consistent with some of the predicted characteristics of the “stau,” the supersymmetric partner of the tau, which cannot be explained using the Standard Model of physics. The supersymmetric models predict that as a stau passes through Earth from space, it decays into a tau lepton and an as yet undetected lowest-mass supersymmetric particle before emerging on the other side of Earth—where the tau lepton could be registered by instruments like ANITA.
To arrive at this conclusion, Fox and his colleagues first demonstrated that the events observed by ANITA are not interpretable within the Standard Model. In the first place, the trajectories of the particles are “highly improbable” under the Standard Model. As Fox and his colleagues argue, to produce these trajectories using the Standard Model would require neutrino fluxes, or the number of neutrinos hitting a certain area in a certain amount of time, “well in excess” of those that have been cataloged by various cosmic ray observatories. Furthermore, they argued that the steepness of the angles of the cosmic rays are also highly improbable within the Standard Model.
Earlier this year, a team of researchers from Ohio State University’s Center for Cosmology and Astrophysics posted a paper to arxiv in which they detailed simulations of staus passing through the Earth to see if they would produce the same sort of signatures observed by ANITA when emerging on the other side. In particular, the researchers were interested in seeing whether simulations of high energy neutrinos interacting with nuclei in the Earth to create staus would reproduce the steep-angled particle trajectories detected by ANITA.
“Any proposed model for new physics would need to explain why such steep events are observed in the absence of a larger number of events near the horizon, and this turns out to be quite difficult to do,” Amy Connolly, the physicist at Ohio State University who led the research, told me in an email. “Our simulations found that even ultra-high energy neutrinos capable of propagating 10,000 km through the earth still do not give a preference for steep events. You still would expect even more such events from near the horizon.”
Nevertheless, Connolly and her colleagues argued that searching for staus with ANITA is a promising new research direction, but she cautioned against jumping to conclusions about the significance of the anomalous events seen by ANITA.
“Anytime an experiment has observed only two events of interest, there is a possibility that as more data is taken, the anomalous events may be found to be a background not previously anticipated,” Connolly told me. “Although it is exciting that ANITA can be sensitive to beyond the Standard Model physics, we must exercise caution and carry on assuming that the most likely possibility is that these events are an as yet unexplained background.”
This was a conclusion echoed by Fox, who said that it is difficult to make strong claims about anomalous events based on data from only one location. If what ANITA detected was actually evidence of a supersymmetric particle, researchers would expect to see similar signatures at other neutrino laboratories. Fox said this is why most physicists were hesitant to make any claims about the anomalous events when they were first published in 2016.
“If it was confirmed that this was evidence of a beyond the Standard Model particle then there would’ve been a lot of physicists working through the consequences right away,” Fox said. “That’s not what happened. It doesn’t feel right for a lot of people and some of them prefer to be more conservative and wait for confirmation from other facilities.”
Fox and his colleagues took the first step in this direction by examining observations from the IceCube Neutrino Observatory in the Arctic to see if similar phenomena was hidden among the available observational data. After adjusting for differences in the IceCube detection system and ANITA, Fox and his colleagues identified three distinct events among the data that were analogous to the upward-pointing cosmic rays observed in Antarctica.
For now, Fox and his colleagues’ theory is just one interpretation of the ANITA data among many, and more data analysis needs to be done. The researchers are hopeful that the data from the fourth and latest ANITA mission, which launched in 2016 and is now in its data analysis phase, might reveal more examples of these upward-pointing cosmic rays.
Even more tantalizing is the data from a decade of continuous observations at IceCube, which has registered far more neutrino events than all the ANITA observations combined and thus may have several upward-pointing cosmic ray events hidden among the data.
“The stau is a particle that physicists have been searching for since they first turned on the Large Hadron Collider,” Fox said. “ They’ve been looking for it, but they just haven’t seen it. What makes this so exciting is that it potentially forges a direct connection between cosmic rays and the LHC.”
Correction: A previous version of this article claimed that the Askaryan effect had been observed from the interaction with neutrinos and ice. In fact, this effect has only been observed from the interaction of electron beams and ice.