Could a Mysterious Particle Break String Theory? Physicists Say Yes

In modern physics, two towering theoretical frameworks attempt to explain the universe, yet they remain fundamentally incompatible. On one hand, we have the Standard Model of particle physics, which successfully describes all known fundamental particles and three of the four fundamental forces—electromagnetism, the strong nuclear force, and the weak nuclear force. On the other hand, Einstein’s general theory of relativity describes gravity and the fabric of spacetime itself.

“The problem,” says Jonathan Heckman, a theoretical physicist at the University of Pennsylvania, “is that these two models speak very different languages.” The Standard Model treats forces as interactions among quantum fields and particles, while general relativity sees gravity as the smooth warping of spacetime. Gravity, therefore, “doesn’t fit into physics’s Standard Model,” Heckman explains.

But what if we could find a place where one of these grand theories—notably string theory, which aims to unite them—breaks down?

Turning the Question Around

In a recent paper published in Physical Review Research, Heckman and Rebecca Hicks, a Ph.D. student at Penn, along with their collaborators, approached string theory from a new angle. Rather than asking what string theory predicts, they asked a sharper question: What is it fundamentally incapable of producing?

Their surprising answer: a specific kind of exotic particle—a five-member particle family known as a “5-plet.” If such a particle were to appear in experiments, like those conducted at CERN’s Large Hadron Collider (LHC), it could spell serious trouble for the foundations of string theory.

String Theory: Elegant but Elusive

For decades, string theory has been a leading candidate for unifying gravity with quantum mechanics. It proposes that all particles, even the graviton (the hypothetical particle of gravity), are tiny vibrating strings. String theory promises a beautifully unified framework, but with a serious catch: it’s nearly impossible to test.

“The mathematics of string theory demands more than the four dimensions we experience,” says Heckman. “Most models require 10 or 11 dimensions, with the extra ones curled up at subatomic scales.”

Hicks explains: “At low energies, these strings behave like the point-like particles we’re used to. But crank the energy high enough, and their true stringy nature is revealed.” Unfortunately, the energies required to see those behaviors—far beyond what current colliders like the LHC can produce—make it extremely difficult to confirm or falsify the theory.

The 5-plet: A Particle That Shouldn’t Exist?

Instead of chasing down elusive predictions, the Penn team flipped the logic. If you can find a particle that string theory can’t produce, then observing it would be like catching a ghost in a theory that says ghosts don’t exist.

Enter the 5-plet—a hypothetical family of five related particles bound by the weak nuclear force. These families, or “multiplets,” are common in particle physics; for instance, the electron and its neutrino form a two-member “doublet.” But a five-member version? That’s something string theory simply doesn’t accommodate.

“We’ve scoured every tool in the string theorist’s toolkit,” says Heckman. “The 5-plet just doesn’t show up.”

“It’s like trying to order a Whopper from McDonald’s,” he jokes. “No matter how creative you get with the menu, it’s just not there.”

What Would the 5-plet Look Like?

The 5-plet, explains Hicks, is a supersized version of a doublet, mathematically described in the particle physics formula known as the Lagrangian. The particle family includes a Majorana fermion—a particle that is its own antiparticle, like a coin with heads on both sides.

Detecting such a particle at the LHC would challenge string theory at its core.

Disappearing Tracks: The Signature Clue

So why hasn’t the 5-plet been found yet?

Hicks points to two major hurdles: production and subtlety.

According to Einstein’s famous equation E = mc², enough energy can be converted into mass. But as particle mass increases—especially if it reaches into the trillions of electron volts (TeV)—it becomes extremely rare to produce in collisions.

And even if it’s created, the 5-plet is tricky to spot. “The heavier members decay into a soft pion and an invisible neutral particle,” says Hicks. “The pion is too low-energy to detect, and the neutral particle passes straight through the detector.”

The result? A track that suddenly disappears mid-flight, like footprints vanishing in snow.

These strange signals can be caught by ATLAS and CMS, two massive detectors at the LHC. Hicks and her team are part of the ATLAS Collaboration, sifting through data to find these vanishing-track signatures.

A Link to Dark Matter?

The implications of finding a 5-plet extend far beyond testing string theory.

“The neutral member of the 5-plet might explain dark matter,” says Hicks, referencing the invisible substance that makes up about 85% of the universe’s mass. “If it weighs around 10 TeV, it fits nicely into theories of how dark matter formed after the Big Bang.”

Even lighter 5-plets could still contribute to a broader dark matter framework, she adds.

“If we detect a 5-plet,” Hicks concludes, “it’s a double discovery—we’d falsify key aspects of string theory and possibly unravel one of the deepest mysteries in astrophysics.”

What We Know So Far

Using existing LHC data, the team has already searched for 5-plet signals—repurposing searches originally designed to find “charginos”, hypothetical particles predicted by supersymmetry.

So far, no 5-plets have been found, which rules out any lighter versions of the particle (those under 650–700 GeV, about five times heavier than the Higgs boson).

“But that still leaves the door wide open for heavier 5-plets,” says Heckman. And future upgrades to the LHC could make the search even more precise.

What’s at Stake?

“We’re not rooting for string theory to fail,” says Hicks. “We’re stress-testing it—pushing its limits to see if it holds.”

“If it survives,” adds Heckman, “great. If not, we’ve just learned something profound about nature—and that’s the ultimate goal of science.”