One Mysterious Particle Could Break String Theory
Physics today rests on two giant theories that… don’t get along.
On one side, we have the Standard Model, which describes all known particles and three of the fundamental forces (electromagnetism, weak and strong nuclear forces).
On the other side, there’s Einstein’s general relativity, which describes gravity and the geometry of spacetime.
The problem? These two theories are fundamentally incompatible. Gravity doesn’t fit neatly into the Standard Model.
String Theory: A Grand Idea With a Catch
For decades, physicists have hoped string theory could bridge the gap — a bold idea that says all particles are really tiny vibrating strings. Elegant, yes. Testable? Not so much.
The downsides:
- String theory only works with 10 or 11 dimensions — most of them curled up and invisible.
- It allows a vast “landscape” of possible universes, making predictions tricky.
- And crucially, it becomes obvious only at ridiculously high energies — way beyond what today’s experiments can reach.
But What If We Flip the Question?
Instead of asking what string theory predicts, what if we ask:
“What does string theory absolutely not allow?”
That’s exactly what physicists Jonathan Heckman and Rebecca Hicks at the University of Pennsylvania did. And they found something string theory seems incapable of producing: a 5-plet, or five-member particle family.
Heckman puts it like this:
“It’s like trying to order a Whopper at McDonald’s. No matter how creative you get, it’s just not on the menu.”
So What Is a 5-plet?
You may have heard of particle “families” like the electron and its neutrino sibling — these come in pairs, called doublets.
A 5-plet is an exotic version of that: five related particles tied together by the weak nuclear force, forming a unified package that’s mathematically consistent — but mysteriously absent from all known versions of string theory.
One of these five is a Majorana fermion — a particle that is its own antiparticle. Imagine a coin with two heads: strange, but theoretically possible.
And here’s the kicker: if the Large Hadron Collider (LHC) finds a 5-plet, it would directly contradict string theory. It could be the cleanest, most definitive blow ever landed on that theory.
How Would We Even Find It?
Finding a 5-plet isn’t easy. These particles are massive — potentially 10,000 times heavier than a proton — so it takes incredible energy to create them. And even if we do make one in a collider, it won’t stick around.
“The heavier members decay very quickly,” Hicks explains. “You end up with a soft pion and a neutral particle (X0) that passes straight through the detector.”
The result? A vanishing track — a particle that seems to appear and then suddenly disappears mid-flight. Physicists call these “disappearing tracks,” and the ATLAS and CMS detectors at CERN are trained to spot them.
Could This Be Dark Matter?
Here’s the really wild part: the neutral member of the 5-plet might also explain dark matter — the mysterious substance that makes up 85% of all matter in the universe.
If the 5-plet weighs around 10 TeV, it fits beautifully into current theories of how dark matter formed after the Big Bang.
“Detecting a 5-plet would be a double win,” Hicks says.
“We’d poke a hole in string theory and simultaneously uncover a new candidate for dark matter.”
What the LHC Has Already Shown
Using existing ATLAS data, the Penn team searched for signs of 5-plets by reinterpreting previous searches for other particles (like charginos). So far, they haven’t found anything.
But that’s still a result:
“We can now rule out 5-plets lighter than around 650–700 GeV,” Heckman says.
“That’s already heavier than the Higgs boson — and there’s still room for heavier ones.”
As the LHC gets upgraded, future experiments will dig even deeper.
Why This Matters
The goal isn’t to disprove string theory out of spite — it’s to test it like any real scientific idea.
“If string theory survives, amazing,” says Heckman.
“If it breaks, we’ve learned something fundamental about the universe.”
Either way, physics wins.
Paper: How to falsify string theory at a collider
Authors: M. Baumgart, J. Heckman, R. Hicks, et al. Journal: Physical Review Research, 2025
DOI: 10.1103/PhysRevResearch.7.023184