Here’s what you’ll learn when you read this story:
String theory—the idea that particles are not point-like, but instead one-dimensional strings—is a popular theoretical framework that attempts to combine general relativity and quantum field theory into an all-encompassing Theory of Everything.
However, scientists have yet to provide any experimental evidence of its validity.
Now, a new study suggests how detecting a certain family of particles (known as a “5-plet”) using the Large Hadron Collider at CERN could, at the very least, prove that string theory is incomplete.
There’s a mystery lying at the heart of physics. Two long-held theories—general relativity and the standard model of particle physics—do an incredible job at explaining the universal and the subatomic, respectively. But they don’t exactly play well with each other—quantum mechanics doesn’t accommodate Albert Einstein’s description of gravity (at least not yet). This discrepancy has inspired decades of theoretical investigations into how these two theories could be united using various theoretical frameworks, including string theory.
At its most basic, string theory is the idea that the foundations of matter and energy are not particles, but one-dimensional strings that vibrate in varying patterns. However, string theory is famously mathematically dense, and relies on nearly a dozen spacetime dimensions. The other problem is that the predicted behaviors of string theory only present themselves at immense energies—even more immense that what’s currently possible at the Large Hadron Collider at CERN.
Instead of trying to prove string theory, scientists from the University of Pennsylvania and Arizona State University have identified an exotic particle family—known as a “5-plet”—that would (at the very least) confirm that some of string theory’s assumptions are wrong if it was discovered at the LHC. This is known as “falsifying,” and researchers highlighted the experimental process in the journal Physical Review Research.
At the center of this experiment is the analysis of how string theory deals with particle “families,” one example being how electrons and neutrinos can form a two-member family known as a doublet. But researchers in this study were interested in a rarer form of particle families known as the “5-plet,” which includes the Majorana fermion—a particle that’s its own antiparticle. As its name suggests, this family contains a five-member particle package that is noticeably absent from string theory calculations, according to the authors.
“We scoured every toolbox we have, and this five-member package just never shows up,” Rebecca Hicks, a co-author of the study from Penn, said in a press statement. “The LHC has to slam protons together hard enough to conjure these hefty particles out of pure energy. As the masses of these particles climb toward a trillion electron volts, the chance of creating them drops dramatically.”
Producing a 5-plet isn’t the only challenging aspect—detecting them is arguably even harder, as the charged particles that make up the 5-plet decay into near invisible products, including a low-energy pion. “The result is a track that vanishes mid-detector, like footprints in snow suddenly stopping,” Hicks said.
The authors looked at data from CERN’s ATLAS experiment and reinterpreted searches originally designed to detect “charginos”—charged particles predicted by supersymmetry—and instead searched for 5-plet signatures. Unfortunately, they didn’t come across any, but they were able to establish a new lower limit for a hypothetical 5-plet weight: about 650–700 Gigaelectronvolts (GeV), which is roughly five times heavier than the Higgs boson.
The authors hope that future tests at the LHC will provide even better data, which should either strengthen the long-standing theory or finally snap its strings.
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