The 2012 discovery of the Higgs boson at CERN's Large Hadron Collider (LHC) near Geneva was a spectacular vindication of the Standard Model—a framework that describes all known particles and forces in physics. The Higgs, whose existence was first predicted in the 1960s, was the final missing piece of the puzzle. Since then, however, physicists have been stuck. The so-called superpartner particles scientists hoped to find at the LHC—particles whose detection would help solve long-standing problems with the Standard Model—never appeared.

Physicists have been talking for decades about a collider that could find those missing particles. Three years ago an international team of physicists and engineers finished its design. Called the International Linear Collider (ILC), this 31-kilometer-long accelerator would smash electrons and positrons together underneath the mountains of the Kitakami region in northern Japan, producing matter-antimatter annihilations that would release 250 billion electron volts of energy. (A later upgrade would double the ILC's energy output.) Any day now Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT) is expected to decide whether the ILC should go forward. We believe it should.

The Standard Model has a hole where a 125-billion-electron-volt Higgs boson would fit perfectly. And that is what scientists found at the LHC. The twist is that physicists cannot explain why the Higgs has that mass. (Physicists generally measure the mass of particles in electron volts, which works because energy and mass are equivalent.) In fact, they have known since the early 1980s that virtual quantum effects should make the Higgs millions or billions of times more massive.

The theory of supersymmetry, or SUSY, offers a solution. It posits an underlying link between matter particles, such as quarks and leptons, and force-carrying particles, such as photons, gluons, and W and Z particles. It also predicts a host of new partner particles with such whimsical names as squarks (partners of quarks) and Higgsinos (partners of the Higgs boson). These partner particles interact with Standard Model particles in a way that cancels out the virtual quantum effects, producing the masses predicted by the Standard Model and observed at the LHC.

Physicists thought they might find these superpartners when the LHC's predecessor, CERN's Large Electron-Positron collider, came online a quarter of a century ago. They did not. When superpartners also failed to appear in the much bigger and more powerful LHC, some physicists panicked.

But there is hope. Recent theoretical research suggests that Higgsinos might actually be showing up at the LHC—scientists just cannot find them in the mess of particles generated by the LHC's proton-antiproton collisions.

This is where the International Linear Collider would shine. The ILC's collisions involve significantly lower energies than the LHC, but the ILC's great advantage is that, unlike its European cousin, it would collide electrons and positrons. Unlike protons and antiprotons, which are made up of quarks and antiquarks, electrons and positrons are truly elementary. Their collisions are much tidier, making any Higgsinos that emerge much more straightforward to detect.

The ILC would take no less than $10 billion to build—about twice the cost of building the LHC. Indeed, the cost of the ILC is probably too much for any single country to bear, so that international participation is vital. But it would be worth it.

Theory predicts that the ILC should create abundant Higgsinos, sleptons (partners of leptons) and other superpartners. If it does, the ILC would confirm supersymmetry, vindicating a model of the subatomic universe physicists have long suspected must be true. Because the Higgsino could make up at least some of the still undetected dark matter that pervades the cosmos, it could also help solve one of the outstanding mysteries of astrophysics. If the superpartners still do not show, science advances nonetheless, as high-energy theorists focus their energies on other theories. Either way, the insights gained would deepen our understanding of the laws of nature—and their implications for the origin and evolution of the universe itself.