6 Freaky Particles That Could Fix Physics—If They Exist

For all practical purposes, the Standard Model of particle physics is almost perfect. The tried-and-true framework predicts the behavior of particles with chilling accuracy—but most physicists also agree that it’s far from complete.

For instance, the mathematical framework of the model doesn’t play nice with general relativity—another “nearly perfect” theory—particularly with the Big Bang or black holes. The model also fails to address several unsolved mysteries in physics concerning widely accepted ideas like neutrino oscillations, matter-antimatter symmetry, or dark matter and dark energy.

An entire field dedicated to this very disparity, unsurprisingly called “physics beyond the Standard Model,” focuses on the theory’s gaps while offering alternatives. Often, this means physicists will come up with new particles to supplement those known to exist. These hypothetical particles are, well, hypothetical, but that doesn’t mean they’ll forever be out of reach. After all, it was only in 1897 that humanity discovered the electron and the Higgs boson as late as 2012.

Read on for some popular hypothetical particles that could fix physics—if they exist.

1. Squarks and sleptons

“Squarks” and “sleptons” refer to broader categories for “superpartners” of more familiar fundamental particles, like quarks, electrons, or neutrinos. So the superpartner of an up quark would be a “sup squark,” whereas an electron would be paired with a “selectron.”

These sound like jokes, but I promise they’re not. They’re based on complex predictions having to do with supersymmetry, which hypothesizes that all force-carrying particles (like photons) and matter particles (like electrons) come in pairs. If valid, the particle partners should have spins that differ by half a unit, theoretically pushing them to equal, very high energies that mathematically unite different forces—a coveted dream for physicists, including Albert Einstein.

2. Gravitons for quantum gravity

Another major, unsolved issue in physics is reconciling general relativity and quantum mechanics, two successful, groundbreaking theories that famously don’t get along. This is such a serious issue that, arguably, a good chunk of string theory is dedicated to resolving this conflict.

But before string theory, scientists like British theoretical physicist Paul Dirac had other ideas. He entertained the possibility of quantized gravity, possibly in the form of a particle. Physicists later dubbed this hypothetical particle the “graviton.”

If the graviton is real, it would mediate the forces of gravitational interaction, demonstrating that “gravity is actually a quantum force, giving us a direct clue for how to build a theory of quantum gravity,” Daniel Whiteson, a particle physicist at CERN and the University of California, Irvine, told Gizmodo.

3. Particles for taming neutrino “misfits”

Neutrinos are near-massless, fundamental particles that permeate nearly every corner of the universe. According to Symmetry Magazine, these neutrinos are the “Standard Model misfits.” To start, the Standard Model initially predicted that neutrinos were massless. They were not. In fact, experimental results demonstrated that neutrinos (1) have mass, (2) come in three different “flavors” (electron, muon, and tau) that mix between mass states, and (3) oscillate between those types, presumably due to quantum mechanical effects.

All this raises some serious red flags for the Standard Model, and it’s a discrepancy physicists have yet to solve. As a remedy, researchers have introduced hypothetical particles like the sterile neutrino (much heavier relatives of neutrinos), majorons, and triplets of zero-spin particles, to name a few, that would give neutrinos mass as the Higgs boson gives regular matter its mass.

4. Most dark matter candidates

Dark matter is the invisible stuff making up around 85% of the universe’s matter. It would be reasonable to assume, therefore, that something that makes up so much of the universe might be a particle. And indeed, this has been the prevailing assumption for scientists seeking to detect signs of dark matter.

If dark matter is indeed a particle, it would be an odd one at that, not absorbing, emitting, or reflecting any light and barely interacting with other particles. None of the known Standard Model particles fit that bill, so physicists have come up with some alternatives, a popular one being a class of particles called weakly interacting massive particles, or WIMPs. But that represents one end of a broad spectrum. Another well-received hypothesis argues that primordial black holes could be dark matter, for example.

Accordingly, different dark matter particle candidates can be “anywhere within several tens of orders of magnitude, and it may or may not couple to Standard Model particles,” Andrea Thamm, a theoretical physicist at the University of Massachusetts at Amherst, explained to Gizmodo.

5. Axions for our evil twin

Many unsolved mysteries in physics intersect, meaning the same hypothetical particle can wear many different hats. One multipurpose, prominent hypothetical particle is the axion, first proposed to explain the strong CP problem. The strong CP problem, very simply speaking, arises from the issue of charge-parity (CP) violation, in which the behavior of particles and their antimatter counterparts do not align. But that doesn’t seem to be the case in systems involving only the strong nuclear force, or the force holding protons and neutrons together. The strong CP problem considers how to make sense of this outlier.

The late Italian physicist Roberto Peccei and Australian physicist Helen Quinn proposed a well-regarded solution to this problem that involves axions. According to Frank Wilczek’s column on the discovery, the presumably lightweight particle “cleaned up” the strong CP problem through a new symmetry associated with a system’s directional properties. (Wilczek, an American Nobel laureate, officially named the axion after a brand of laundry detergent). Incidentally, the axion’s properties make it a strong candidate for dark matter as well.

6. Particle X

Here’s the craziest idea of them all: the hypothetical particle of humanity’s wildest dreams—the thing that solves all our problems in the most unexpected ways—could be something we’ve never even bothered to think of.

Then again, the chances of that are probably slim. After all, physicists seem to have no shortage of ideas, devising all sorts of possible scenarios and theories (check out the Wikipedia article on hypothetical particles to see what I mean). My list reflects a sliver of that overarching exercise, centered around more mainstream issues. That is to say, there’s a lot this list didn’t cover. And with major upgrades coming to humanity’s most powerful colliders, who knows what we’ll find (or won’t find)?

“The best thing about particle colliders is that you don’t need to anticipate new particles to discover them,” Whiteson said. “And the best discoveries, the ones that teach you the most about the universe, are the surprises!”