A little over a year ago, physicists put the finishing touches to the most successful scientific theory of all time: the Standard Model of particle physics. When the Higgs boson was found at the Large Hadron Collider in July 2012, it was the final piece in our picture of the universe at the smallest, subatomic scales.
Champagne corks flew in physics labs around the world at this vindication of quantum field theory, which had been more than 80 years and dozens of Nobel prizes in the making.
Inevitably, a hangover followed. The leading idea for how to push physics beyond the Standard Model — and explain the many remaining mysteries of the universe — is looking shaky. Thousands of physicists have spent their career carefully constructing the theory, called supersymmetry. It has taken almost four decades. But, so far, the most powerful particle accelerator ever built — the Large Hadron Collider (LHC) at Cern, near Geneva — has not found any hard evidence to back up the theory.
This conspicuous lack of proof has led a growing number of physicists, particularly those who are less invested in supersymmetry, to publicly call time on the idea. Perhaps, despite all the work, the theory is just plain wrong.
Beyond the standard model
The Standard Model describes all the fundamental particles that make up the matter and forces in the universe — including electrons, quarks and photons — but it has some worrying omissions. It fails, for example, to include a description of the familiar force of gravity, which not only keeps us rooted to the ground but also shapes the universe at the scale of stars and galaxies. Neither is it able to explain the presence of so much matter, as opposed to anti-matter, in the universe.
Supersymmetry is physicists’ best shot so far at explaining what happens in the subatomic universe beyond the Standard Model. A theory that has been worked on and refined for almost 40 years, it proposes the existence of a set of new particles, each one a supersymmetric partner of the particles that already exist in the Standard Model.
Such particles would be more massive than any we have seen until now, and would therefore require high-energy experiments to isolate them and confirm their existence. At least one of the hypothetical particles, the neutralino, might be a candidate for the elusive dark matter that makes up some 30 percent of the universe.
Theoretical work in physics — however beautiful or compelling it may be — is never accepted as fact until it makes predictions that are confirmed through experiment. Supersymmetry’s predictions are its hypothetical particles, and, even after the Large Hadron Collider at CERN has been in operation for almost five years, none of these has been found.
Worse, the particles and forces in the Standard Model can account for only around 4 percent of the mass of the universe. The remaining 96 percent is dark matter and dark energy, and scientists have no idea what either of these things might be. The Standard Model has been a great success, but it can take us only so far in understanding the fabric of reality.
Enter supersymmetry. First formulated in the early 1970s, and with more than 10,000 scientific papers written about it, the theory has fought off rival ideas to emerge as the leading candidate to explain physics beyond the Standard Model.
Its central proposition is that every particle in the Standard Model has a heavier, as-yet-unseen “superpartner.” The superpartners of quarks and electrons, for example, are called squarks and selectrons; the superpartners of the Higgs, and of force carriers such as the photon, are the higgsino and photino.
“This theory is founded on such a lovely idea: that you have this additional symmetry in nature that unites force and matter and gives a deep, intimate connection between them, that tells them they’re not distinct entities by themselves, that the universe is really rather simpler than you might have thought,” says Professor Tara Shears, a particle physicist at Liverpool University.
Supersymmetry is attractive for many reasons, not least because its lightest predicted supersymmetric particle, the neutralino, could be a candidate for the universe’s dark matter.
The theory also solves a fundamental problem with the Higgs boson. The natural mass of the boson should be subject to huge fluctuations as it interacts with other fundamental particles. Left unchecked, this could mean that its mass could grow bigger than any value we have observed. To get around this, supersymmetry proposes that the superpartners of every fundamental particle also interact with the Higgs, but in such a way that each one almost exactly cancels out the fluctuations of their normal partners.