Supersymmetric 'Sleptons' Might Exist. But They'd Have to Be Huge.

The world's largest atom smasher might be losing its dark matter. But physicists are getting a clearer picture of what that lost dark matter might look like — if it even exists.

ATLAS, the detector of very large particles at the Geneva-based Large Hadron Collider (LHC), is best-known for discovering the Higgs boson back in 2012. Now it has moved on to hunt for even more exotic particles — including theoretical "supersymmetric" particles, or partner particles to all the known particles in the universe.

If supersymmetry is real, some of those particles could explain the unseen dark matter spread across our universe. Now, a pair of results presented at an ATLAS-focused conference in March has offered the most precise description yet of what those hypothetical particles would have to look like.

Unseen matter

Let's back up. [Beyond Higgs: 5 Elusive Particles That May Lurk in the Universe]

Dark matter is the unseen stuff that may make up most of the universe. There are a number of reasons to suspect it exists, even though no one can see it. But here's the most obvious one: Galaxies exist.

Looking around our universe, researchers can see that galaxies don't seem massive enough to bind themselves together with the gravity of their visible stars and other ordinary matter. If the stuff we could see was all there is, those galaxies would drift apart. That suggests some unseen dark matter is clustered in galaxies and holding them together with its gravity.

But none of the known particles can explain the cosmic web of galaxies. So most physicists assume there's something else out there, some sort of particle (or particles) that we've never seen, that's making up all that dark matter.

Experimental physicists have built many detectors to hunt them. [The 18 Biggest Unsolved Mysteries in Physics]

These experiments work in different ways, but in essence, many amount to putting a big chunk of stuff in a very dark room and watching it very carefully. Eventually, the theory goes, some particle of dark matter will bang into the big chunk of stuff and cause it to glitter. And depending on the nature of the stuff and the glittering, physicists will learn what the dark matter particle looked like.

ATLAS is taking the opposite approach, looking for dark-matter particles in one of the brightest places on Earth. The LHC is a very big machine that smashes particles together at unbelievably high speeds. Inside its miles of tubes is a sort of ongoing blast of new particles formed in those collisions. When ATLAS discovered the Higgs boson, what it saw was a bunch of Higgs bosons that were actually created by the LHC.

Some theorists think that the LHC might also be creating specific kinds of dark matter particles: supersymmetric partners of known particles. The word "supersymmetry" refers to a theory that many of the known particles in physics have undiscovered "partners" that are much harder to detect. This theory hasn't been proved, but if it were true it would simplify a lot of the messy equations that currently govern particle physics. [Photos: The World's Largest Atom Smasher (LHC)]

It's also possible that supersymmetric particles with the right properties could account for some or all of the missing dark matter in the universe. And if they're being made at the LHC, ATLAS should be able to prove it.

The hunt for supersymmetric particles

But there's a problem. Physicists are increasingly convinced that if those supersymmetric particles are being made at the LHC, they're flying out of the detector before decaying. That's a problem, as Live Science has previously reported, because ATLAS doesn''t directly detect exotic supersymmetric particles, but instead sees the more common particles that supersymmetric particles transform to after they decay.. If supersymmetric particles are shooting out of the LHC before decaying, though, then ATLAS can't see that signature. So its researchers came up with a creative alternative: Hunting, using statistics from millions of particle collisions in the LHC, for evidence that something else is missing.

"Their presence can only be inferred through the magnitude of the collision's missing transverse momentum," the researchers said in a statement.

Accurately measuring the missing momentum is adifficult task though.

"In the dense environment of numerous overlapping collisions generated by the LHC, it can be difficult to separate genuine from fake" momentum, the researchers said..

So far, that hunt hasn't turned up anything. But that's useful information. Whenever a particular dark matter experiment fails, it provides researchers with information about what dark matter doesn't look like. Physicists call this narrowing-down process "constraining" dark matter. [8 Ways You Can See Einstein's Theory of Relativity in Real Life]

Those two March results, based on that statistical hunt for missing momentum, show that if certain supersymmetric dark matter candidates (called charginos, sleptons and supersymmetric bottom quarks) do exist, they have to have particular characteristics that ATLAS hasn't yet ruled out.

If current models of supersymmetry are correct a pair of charginos must be at least 447 times the mass of a proton, and a pair of sleptons must be at least 746 times the mass of a proton.

Similarly, based on current models, the supersymmetric bottom quark would have to be at least 1,545 times the mass of a proton.

ATLAS has already finished hunting for more lightweight charginos, sleptons, and bottom quarks. And the researchers said they are 95% confident that they don't exist.

In some respects, the hunt for dark matter seems to constantly produce null findings, which can be disappointing. But these physicists remain optimistic.

These results, they said in a statement, "place strong constraints on important supersymmetric scenarios, which will guide future ATLAS searches."

As a result, ATLAS now has a new method for hunting dark matter and supersymmetry. It just hasn't happened to find any dark matter or supersymmetry yet.

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Originally published on Live Science.