Mar 28, 2020
The SUSY Experiments
An idea that would help answer the biggest questions of the universe
SUSY first appeared as a mathematical idea emerging from string theory. It was beautiful.
It appealed to us not only because it could resolve many questions about the nature of the universe but because it was balanced and elegant. It promised to bring symmetry to our scientific models, like the discovery of antimatter and its catastrophic but fulfilling place next to regular matter. SUSY could begin to unravel gravity, particle mysteries, dark matter, and even be a stepping stone to a grand unified theory. In short — it was going to lead us to a rich understanding of our universe as a whole.
It was half a century ago that SUSY was first applied to our everyday, four dimensional world. It elevated string theory to superstring theory since the acronym SUSY is short for supersymmetry. But supersymmetry itself is not a theory, it is a principle that can be used in many different theories and can result in varying models of the universe. There are dozens of theories that incorporate supersymmetry but experimentation has eliminated some of these over the years.
SUSY was born when fermions were incorporated into the strings of string theory. Particles of the Standard Model can fall into one of two categories. Fermions are particles of matter like the electron, the top quark, and the up quark. They have a half-integer spin. For the electron this is a spin of 1/2. The other category is force carrying particles called bosons. They have a whole integer spin, like the photon which has spin 1 and carries the electromagnetic force. SUSY is about relating these two particle categories with one another. Matter and forces are treated the same in supersymmetry. Fermions and bosons — all the known particles of the universe — would become interchangeable in its context. Previously, this interchange had been considered impossible.
This means that for every particle we’ve observed, there would be a sibling particle with the same mass but a different spin. The electron, then, would have a super sibling particle with equal mass and charge but, instead of having spin 1/2, the super sibling would have spin 1. What we’ve seen in our world is an electron acting as a fermion, what we’re searching for is an electron acting as a boson.
It’s a seemingly simple reevaluation of nature. Our observations reveal profound symmetries in other regions of science and the natural world, and while these symmetries can be — and often are — broken, they are essential to building a bigger picture. They are the patterns we hope will lead somewhere remarkable. For SUSY, this even promised to give us a serious candidate for the mystery of dark matter.
The super sibling of the photon is the neutralino. It’s a stable particle with neutral charge, affected only by gravity so that it would remain largely invisible to our probing. The idea is that there’s a swarm of neutralinos all around us. They formed in the moments when the universe was cooling down after the big bang. Calculations show they would have outnumbered the atoms of ordinary matter, with the amount of neutralinos we expect to be inhabiting our universe being in approximate correspondence to the amount of dark matter present today.
The problem with finding these SUSY particles — like the neutralino — is that our estimates for their masses were wrong. If they do exist, they are now expected to be much more massive than even the Large Hadron Collider could create. Hopes of finding elusive SUSY particles lies in adding drama. We need the higher energies of a supernova explosion or a gamma ray burst searing through the backdrop of space like the rupture of lightning on a charged, stormy night. These events contain billions of times more energy than anything we can achieve at the LHC. They are powerful, but rare. For now they represent one of our only hopes for proving supersymmetry.
Though the LHC has tried time and again to produce SUSY particles, it lies mostly spent and empty-handed. We have never seen evidence that these super siblings exist. Mexico, May of last year was one of the most recent announcements made by the LHC that they had run through fresh searches for supersymmetric particles. The ATLAS experiment pored through data from the years 2015–2018 but every search, no matter how detailed and challenging, resulted in a lack of new particles.
So why continue to hold onto this idea? What more is there to SUSY than just the gorgeous math?
In many ways, testing supersymmetry is the same as testing string theory. While proving supersymmetry wouldn’t prove that string theory is correct as a whole, it is an essential characteristic that gets us just that much closer to a theory of everything. Many in the world of physics believe that string theory is the only serious contender we have. To give up SUSY leaves us in an ocean of unknowns, without much of anything to hold onto.
And there is something which tells us supersymmetry might be correct.
The three quantum forces — electromagnetism, strong nuclear, and weak nuclear — have very different strengths. Yet at the big bang, this couldn’t have been the case. They were unified and equal, converging at the beginning of the universe. Using the Standard Model we arrive at calculations that these strengths are similar but not the same. It’s only when adding SUSY that they are all perfectly equal strengths. This is just one way in which the Standard Model is incomplete.
To get an idea of just how little we know, take the hierarchy problem with the Standard Model. It’s a problem that can be worded in different ways but always comes back to the same jarring numbers. The energy at which the three quantum forces unify is 15 orders of magnitude larger than the energy at which the weak and electromagnetic forces unify. And gravity is extremely weak compared to these three other forces. 10²⁴ times weaker than the weak nuclear force. The heaviest known quark — the top quark — is 40 times heavier than the bottom quark and 175 times heavier than the proton. These numerical discrepancies are abundant. The Higgs boson should have a Planck mass but is billions and billions of times lighter.
But to resolve all these problems we should already have seen SUSY particles showering from experiments like those at the LHC. While we continue to search for evidence of SUSY, it seems less and less able to provide natural and effortless answers for any of these questions. Its experimental absence means SUSY is unlikely to make dark matter particles, or to explain the hierarchy problem within the Standard Model. It isn’t yet a dead idea — but it’s one which increasingly divides scientists as time goes on. There are the scientists which have given up on SUSY and believe we must traverse new routes to a grand unified theory, and there are those who still believe it to be a discovery waiting to happen.
The continued supporters say that the symmetry of our universe must be broken. Broken symmetries are, after all, what allows us to be here in the first place. If there had been a perfect amount of matter to antimatter particles at the inception of the universe then we would never have formed. So maybe we exist in a region of spacetime that’s atypical — there is no supersymmetry here not because it’s unreal but because it has cracked apart and does not show itself at the low energies of our everyday life.
Without SUSY — and string theory — it’s unclear what stepping stone we’ll use to explain the universe. But we may have to admit that we’re much more lost than we’d hoped before we’re able to discover a better path.
