A superconducting solenoid magnet, the largest of its kind, is part of the Large Hadron Collider, which is searching for the Higgs boson. (Fabrice Coffrini/Agence France-Presse via Getty Images)

The Higgs boson is a scientific Loch Ness monster. Some people believe it exists, want it to exist and have seen small glimpses of evidence that suggests it exists. Some physicists predict that this is the year we find it. Still, after years of research and billions of dollars invested, we haven’t produced definitive evidence that the Higgs boson is real, leaving open the possibility that a decades-old, widely believed theory is completely wrong.

For those who haven’t yet caught Higgs fever, here’s a brief background on the particle.

It all begins with a collection of theories known as the “standard model.” The standard model answers two important and fundamental questions: What is matter made of? And why does it behave the way it does?

As to the first question, the standard model says that all matter is made of 12 particles with zany names such as the “charm quark” and the “muon-neutrino.”

The answer to the second question comes down to the four forces observed in the universe. Ordinary people are familiar with gravity and electromagnetism, but there are also the strong and weak forces, which operate on the subatomic level. Under the standard model, bosons transmit forces between particles of matter.

One of the most cherished goals of physics is a “unified field theory,” which would explain the relationships among these forces. For the most part, this theory has been elusive. However, in the 1960s, scientists came up with an idea that would unify at least two of the forces: electromagnetism and the weak force. (They called it the electroweak force, which is rather disappointing for a field that has such a knack for colorful monikers.) But there was a problem with the theory: In order for it to be true, the particles that carry the electroweak force would have to be without mass, and such particles have never been observed.

Enter English physicist Peter Higgs, who came up with a way to save the electroweak force. Higgs theorized that, in the moments after the big bang, mass didn’t exist. However, as things cooled down, a field settled over the universe. When certain particles interact with that field, they gain mass. Physicists don’t know how to observe the Higgs field, but the theory is that when the field acts on a particle, a boson particle is left behind.

This is not easy to conceptualize. Indeed, very little is easy to conceptualize in this field, but one physicist suggests thinking of it as a kind of condensation: The Higgs boson is to the Higgs field as a water droplet is to the water vapor from which it emerges. Higgs’s theory goes a long way toward explaining an enduring problem of physics: why mass exists at all.

Higgs’s theory set off a race to find the Higgs boson, which would help prove the theory. Theoretical physicists worked out a range for the particle’s mass, and experimental scientists got to work smashing things together and hunting for particles within that mass range. At the end of last year, scientists at the Large Hadron Collider at the European Organization for Nuclear Research (CERN) laboratory observed traces of a particle in the range, but the evidence wasn’t definitive. Now they’ve turned the machine, a particle accelerator, back on after a winter slumber, and this is a make-or-break year for the Higgs.

“We’re looking in a small mass window,” says Rob Roser, a physicist with Fermilab near Chicago who is involved with the CERN experiments. “So, if the machine performs the way it’s supposed to, this year’s results should settle the question of whether there is a particle” in the mass range of the theorized Higgs boson.

There has been a lot of confident talk among physicists over the past few months, but when contacted, several said they thought the odds of the Higgs boson’s existing were less than 50-50. Which opens up an interesting question: If the Higgs doesn’t exist, what then?

Obviously, our dominant theory for why mass exists would pretty much go out the window. There are also financial concerns.

“Unless we’re missing something in the existing data, a failure to find the Higgs boson would mean building an accelerator that can work at even higher energies,” says Lawrence Krauss, a physicist at Arizona State University and author of “A Universe From Nothing: Why There Is Something Rather Than Nothing.”

The Large Hadron Collider has cost nearly $10 billion to build and operate. It conducts a variety of research, but the Higgs search was its main selling point. If scientists find nothing in the range of energies that the collider can generate, they will need to start looking for other particles in higher energy ranges, which probably would require a new collider that’s even more expensive to build and operate.

Still, despite the costs, some physicists are almost rooting against the Higgs boson.

“If we don’t find the Higgs, and we can exclude it, it would be an interesting result. Almost more interesting than finding it,” says Markus Klute of MIT. For a theoretical physicist, you see, finding the Higgs would be a bittersweet event. While it would mostly settle one of the biggest mysteries of nature, scientists like questions as much as they like answers.

In addition, while finding the elusive particle would bring glory to Peter Higgs, it might spell the end for other theories of mass. Technicolor theory posits that mass is the result of two particles called techniquarks. The supersymmetry theory posits the existence of numerous undiscovered particles, rather than one in the Higgs theory or two for technicolor. Not everyone would be sad to see these theories go the way of the humoral theory of disease or Lamarck’s theory of evolution, though.

“These other theories are not as elegant,” says Roser. “Physicists like easy equations that you can write on T-shirts, like E=mc2 and F=ma. The Higgs boson has that kind of simplicity.”

Of course, it’s entirely possible that none of these theories are correct, which would be a major gift to theoretical physicists. The great challenge of explaining mass would be open once again.