Supersymmetric Particle Found?

Supersymmetry remains unconfirmed, but two puzzling ultra-high-energy radio bursts detected by ANITA have revived interest in a specific SUSY candidate: the stau. The events appeared to originate from directly below Antarctica—meaning the responsible particles would have had to traverse thousands of kilometers through Earth’s rock, magma, and iron—an outcome that clashes with expectations for standard-model neutrinos at those energies. With conventional explanations struggling to account for the timing and geometry, a proposed supersymmetric mechanism offers a way to make the signals fit.

The core problem starts with the hierarchy problem: the Standard Model cannot naturally explain why gravity is so vastly weaker than the other fundamental forces. Supersymmetry (SUSY) was designed to address this by introducing a symmetry between fermions (matter) and bosons (force carriers). In SUSY, every Standard Model particle gains a partner of the opposite type, and—crucially for collider searches—those partners are expected to be much heavier than their known counterparts. Many versions of SUSY would place the relevant masses near the electroweak energy scale, where the electromagnetic and weak forces unify. The Large Hadron Collider (LHC) has thoroughly tested the Standard Model and found the Higgs boson in 2013, yet it has not produced the expected supersymmetric particles. That absence doesn’t rule SUSY out; it may simply mean the new particles are heavier than anticipated, demanding higher-energy probes than the LHC can deliver.

Instead of building a larger accelerator, the search can use the universe as a particle accelerator. Ultra-high-energy cosmic rays interact with the cosmic microwave background (CMB), producing extremely energetic neutrinos. Neutrinos are hard to detect because they rarely interact, but they can pass through Earth if they’re not too energetic. ANITA (the Antarctic Impulsive Transient Antenna) is designed to catch the highest-energy neutrinos by scanning about 15 million square kilometers of Antarctic ice from a balloon platform roughly 37 kilometers above. When such a neutrino interacts in the ice, it can trigger a radio-frequency Cherenkov signal. ANITA’s key advantage is directional: it can distinguish neutrino-induced signals from other cosmic-ray radio flashes by focusing on neutrinos arriving from below.

ANITA reported two extremely high-energy radio bursts consistent with particles that traveled through the planet. Under standard-model expectations, the probability of seeing two tau-neutrino events of this type during ANITA’s observation window is estimated at about 1 in 3 trillion. To bridge that gap, astrophysicists Derek Fox, Steinn Sigurdsson, and collaborators point to a SUSY scenario involving the stau, the supersymmetric partner of the tau lepton. In their picture, an ultra-high-energy neutrino produces a stau on the far side of Earth; the stau then traverses Earth before decaying into a tau lepton near the detector, generating the characteristic double-burst radio signature.

Still, SUSY isn’t the only candidate. Other explanations include sterile neutrinos, unusually large bursts of ordinary neutrinos from astrophysical sources like supernovae or gamma-ray bursts, or simply gaps in understanding of neutrino propagation through Earth. One of the two ANITA events may line up with a distant supernova, but the chance association is only about 3%, and the supernova’s brightness seems insufficient to plausibly generate the required ultra-high-energy neutrinos. IceCube, the other major neutrino observatory, has limited sensitivity to these exact geometries, though a retrospective look found ambiguous possible tau-like events. The bottom line: the stau hypothesis is a tantalizing fit, not a discovery—confirmation will depend on more data and cross-checks across observatories.