What is the concept of supersymmetry?

Faster than a speeding bullet, more powerful than a locomotive, and capable of leaping tall buildings in a single bound – supersymmetry (or SUSY, in its more casual form) is the unsung superhero of the universe. It might not battle villains or save the world, but it holds the key to understanding the tiniest building blocks of the cosmos. Unlocking the universe could reveal answers to challenges we haven't even considered.

So, who is our elusive hero? Think more Peter Parker than suave Spider-Man. Supersymmetry isn't a flashy superhero but rather a principle designed to address gaps in other theoretical frameworks. While physicists once held it in high regard, some now worry that supersymmetry may not be as formidable as once thought. Its fate may very well be decided at the Large Hadron Collider, where it might face its ultimate test.

To understand the universe at its core, physicists rely on the Standard Model, which describes the fundamental particles and the four forces interacting with them. These particles include quarks and leptons, like protons and neutrons (from the quark family), and electrons and neutrinos (from the lepton family). The four forces are the strong, weak, electromagnetic, and gravitational forces, which keep everything in balance.

The Standard Model also posits that each of these forces is mediated by a corresponding particle, called a boson. These bosons enable energy exchange between particles, allowing matter to transfer energy to one another [source: CERN]. The most significant find in recent years has been the discovery of the Higgs boson, which forms a larger Higgs field that imparts mass to particles.

Here's an oddity: If the Standard Model holds true, the Higgs field gives subatomic particles their mass. But it doesn't specify the amount of mass, nor does it explain why the Higgs boson is so light—based on predictions, it should be incredibly heavy if the Standard Model interactions are correct.

This is where supersymmetry enters the picture. As noted by Fermilab, supersymmetry is not a theory but a principle, meaning there are multiple supersymmetric theories, each with its own variations. However, all these theories share supersymmetric equations that treat both matter and forces as interchangeable [source: Fermilab]. In other words, matter and forces can be swapped.

How does this balance work? Supersymmetry proposes that every particle in the Standard Model has a superpartner with a different mass. In this framework, every matter particle (fermion) pairs with a force particle (boson), and vice versa. For instance, an electron is a fermion, and a photon is a boson. One of the most remarkable properties of these superpartners is their ability to cancel out the large mass the Standard Model predicts for the Higgs. This is exciting because, as we know, the Higgs we found wasn't that massive. Could supersymmetry be the answer? Long live supersymmetry!

Hold up, though—there's a major issue with supersymmetry and its predicted superpartners: we haven't actually observed them. It's great that we've found the Higgs boson at the mass that supersymmetry predicted, but we should also be seeing these superpartners. Despite running the Large Hadron Collider for years, we haven't found any.

Yeah, it's tough to keep supporting supersymmetry at this point. We're just assuming that all these superpartners exist because the Standard Model would be far more coherent if they did. Doesn't this seem like bad science?

But don't jump to conclusions just yet. Supersymmetry could resolve more than just the Higgs mass issue, and scientists love a solution that tackles multiple puzzles at once [source: Fermilab]. For instance, physicists once struggled to explain why galaxies spin so quickly despite their immense mass. This led to the concept of dark matter, but the real question remained: what exactly is dark matter made of? We can't see it, so we can't identify its composition. Supersymmetry offers a solution, as the lightest supersymmetric particle could very well be dark matter.

Another advantage of supersymmetry? It could unite the three fundamental forces we currently understand on a subatomic level (strong, weak, and electromagnetic) into a single force. While the Standard Model suggests that these forces start to resemble each other at extremely high energies, supersymmetry predicts that they unify at a single energy level [source: Fermilab]. This might not be strictly necessary for understanding the universe, but physicists love natural, elegant solutions, and supersymmetry could deliver just that when it comes to unifying the forces.

However, it's essential to remember that none of this matters if we don't find those elusive superpartners. Without them, we won't be able to explain the Higgs mass, dark matter, or the unification of forces. But it feels premature to declare supersymmetry dead before we've really given it a fair shot.

Hope may be on the horizon in the form of a massive proton explosion. Yes, our expectations still rest with the Large Hadron Collider, the particle accelerator that made the pivotal discovery of the Higgs boson in 2012. While finding the Higgs was undoubtedly a monumental achievement for supporters of supersymmetry — and for the scientific community at large — their real aim was to uncover a series of particles. More precisely, they sought the elusive superpartners, which would offer the proof needed to establish that supersymmetry is a valid concept.

It’s not an exaggeration to say that the LHC’s discovery of only the Higgs — without any other superpartners — has caused a bit of turmoil in the physics world. For the Higgs mass to be logically consistent, the superpartners should have been found in roughly the same region. As the LHC is set to restart in 2015, it will smash protons at even greater energies in hopes of finding superpartners at higher masses. However, this does not fully resolve the issue: Even if heavy superpartners are detected, the useful effects of supersymmetry — like canceling out the Higgs’ enormous mass — may not work as effectively, leaving us once again in a supersymmetry deadlock.

As many have pointed out, supersymmetry is a principle rather than a theory. In certain supersymmetric models, the Large Hadron Collider may not have been able to detect the superpartners due to the limitations of its experiments, which are not designed to spot less stable particles. So, while supersymmetry likely needs to make a quick entrance with a convincing reason for its delay, it’s still too early to rule it out completely.