The rumored upcoming announcement of the discovery of the Higgs boson on July 4 would put in place the last major thread of the Standard Model of physics. This might sound like the case is closed on how the universe works, but though the Standard Model answers many questions and has been very effective at predicting the existence of particles that were subsequently discovered, it also spawns a whole new set of questions that could prove very tough to conquer.

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How Discovery of the Higgs Boson Could Break Physics

The Higgs Boson: Whose Discovery Is It?

Higgs Hunt Heats Up With Final Tevatron DataKnowing the mass of the Higgs would be a spectacular achievement, said theoretical physicist Lawrence Krauss of Arizona State University, whose book about the intricacies of particle physics,* Universe From Nothing: Why There Is Something Rather Than Nothing*, came out in January. "But if that's all they discover it could be bad for everyone because it doesn't tell you how to solve the problems of the Standard Model."

That's where supersymmetry comes in.

The Standard Model is the framework that particle physicists use to describe the behavior of all known subatomic particles and fundamental forces. Though it has been a triumph of late-20th century physics, it contains several looming unanswered issues. For years, many physicists have been looking for theories beyond the Standard Model and among the most popular is supersymmetry.

One of the questions keeping physicists up at night is why the four fundamental forces of the universe – gravity, electromagnetism, the weak force, and the strong force – have such differing values. More specifically, why the weak force is approximately 10 quadrillion times as powerful as gravity.

This is strange because down at the subatomic level, virtual quantum particles are constantly jiggling. This wavering, if left unimpeded, should push the energy scale of the weak force away from its observed value. Supersymmetry's popularity derives from its ability to prevent this from happening.

Like nearly everything having to do with subatomic particles, supersymmetry is weird. Essentially, it says that for every particle we know about – things like electrons, quarks, and neutrinos – there is a corresponding superpartner of higher mass. So the electron would be paired with a particle called a selectron, quarks would have corresponding squarks (much of supersymmetric nomenclature simply adds an "s" to the known particles.)

If these superpartners exist, they have the property of naturally canceling out the tiny quantum jiggles that would drive the weak force away from its observed range. "That's one of the things that makes supersymmetry so attractive: It can keep the scales separate," said Krauss.

Some supersymmetry theories have the added advantage of providing ideal candidates for dark matter. Lurking somewhere in all the strange new superpartners might be one that is massive but doesn't interact with light, which is exactly what a dark-matter particle should be.

The problem is that neither the LHC nor its recently decommissioned American counterpart, the Tevatron, have seen any strong evidence for new, heavy particles during their experiments. Though they keep searching at higher energy ranges, the particle accelerators don't turn up any new superpartners.

"As we exclude more and more energy ranges, the supersymmetry models that most easily keep the scales separate get more and more contrived," said Krauss.

Already, experiments have excluded the simplest supersymmetric theories. Physicists can keep tweaking their theories but after a while these fine-tunings begin to seem arbitrary.

Ideally, along with the Higgs boson, the LHC would also find the Higgs' superpartner in the coming years. But if the LHC doesn't ever see evidence for supersymmetry, where does that leave particle physics?

"We don't know," said Krauss. "The likelihood that we will build another large particle accelerator in our lifetime is not good. So, if this is it, then we're in a real quandary."

Image: A simulated supersymmetry event from the Large Hadron Collider's ATLAS experiment. ATLAS/CERN