Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space. He contributed this article to Space.com's Expert Voices: Opinions and Insights.
Supersymmetry is the idea that the fundamental particles of nature are connected through a deep relationship. This theory predicts the existence of brand-new particles in the world's largest collider experiments.
But according to a recent report, there have been no signs of supersymmetry, and the theory is looking a little shaky.
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The mysterious symmetry
The subatomic universe is composed of two fundamental kinds of particles, called the fermions (in honor of Enrico Fermi) and bosons (named for Satyendra Nath Bose). In essence, fermions are the building blocks of the natural world: the quarks, the electrons, the neutrinos. If you zoomed into your own cells and molecules and atoms, you would find a bunch of fermions buzzing around, doing their thing.
In contrast, the bosons are the carriers of the fundamental forces of nature. The electromagnetic force is carried by the photon, a type of boson. The weak nuclear force has a trio of bosons to carry it around, and eight different bosons conspire to make the strong nuclear force happen. Gravity has a hypothetical boson associated with it, called the graviton, but we don't have an understanding of that particle yet.
We also don't have an understanding as to why the universe is split into these two major camps. Why aren't there more "families" of particles? Why do the fermions have the properties they do? Why are the bosons connected with the forces? And are there any connections at all between those two worlds?
There just might be a connection between fermions and bosons, and the name for the theoretical connection is supersymmetry. Mathematical symmetry plays a central role in modern physics. It's through the discovery of deep mathematical relationships that physicists have been able to understand the forces of nature and other wondrous ideas like the conservation of energy.
By searching for symmetries, physicists can understand the world.
In supersymmetry, there's a new kind of mathematical relationship that connects the fermions and the bosons. In fact, it's more than a mere connection: supersymmetry states that fermions and bosons are really two sides of the same (supersymmetric) coin. Every single fermion has a mirror-like particle in the boson family, and every boson has a twin over the fermion world.
In the jargon of supersymmetry, the mirror-like twins of particles get rather fanciful names. Every supersymmetric partner of a fermion gets an "s" attached to the front, so the partner of a quark is a squark, the partner of an electron is a selectron, and so on. For the bosons, their partners get "ino" attached at the end, so photons are paired with photinos and gluons (the carriers of the strong force) are paired with gluinos. So to find evidence for supersymmetry, all you have to do is find a stray gluino or selectron floating around.
This sounds cool, but it's not that easy. In a perfectly supersymmetric world, we would see these twinned particles everywhere we look. For every fermion we could find an associated boson, and vice versa.
But we don't.
The reason we don't see the symmetry made manifest in our universe is that it's a broken symmetry. A long time ago, when the universe was much hotter and denser, this symmetry could survive. But as the universe expanded, it cooled and broke the symmetry, dividing the fermions and bosons. The breaking of the symmetry caused all the supersymmetry twins to drastically inflate in mass, and in the world of particle physics, the more massive you are, the more unstable you are.
The only way to access the realm of supersymmetry to recreate the conditions of the early universe. Like, for example, in a giant particle collider.
ATLAS holding up the world
The Large Hadron Collider (LHC) is, like the name suggests, a giant particle collider. It's capable of accelerating particles to nearly the speed of light and then smashing them together, achieving the highest energies possible — conditions not found in the universe since the first moments of the Big Bang. The Large Hadron Collider was explicitly designed to hunt for signs of supersymmetry by finding evidence for supersymmetric particle partners in the collision debris.
One of the detectors at the LHC is called ATLAS, for "A Toroidal LHC ApparatuS" (yes, it's a little clumsy as acronyms go, but it's an awesome name). The ATLAS collaboration, made up of hundreds of scientists from around the world, have released their latest findings in their search for supersymmetry in a paper appearing in the preprint journal arXiv.
And their results? Nothing. Nada. Zilch. Zero.
After years of searching and loads of accumulated data from countless collisions, there is no sign of any supersymmetric particle. In fact, many supersymmetry models are now completely ruled out, and very few theoretical ideas remain valid.
While supersymmetry has enjoyed widespread support from theorists for decades (who often portrayed it as the obvious next step in advancing our understanding of the universe), the theory has been on thin ice ever since the LHC turned on. But despite those initial doubtful results, theorists had hoped that some model of tuning of the theory would produce a positive result inside the collider experiment.
While not every possible model of supersymmetry has been ruled out, the future of the theory is in serious doubt. And since physicists have invested so much time and energy into supersymmetry for years, there aren't a lot of compelling alternatives.
Where will physics go from here, in a universe without supersymmetry? Only time (and a lot of math) will tell.
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