The world’s most powerful particle accelerator, the Large Hadron Collider at the CERN laboratory near Geneva, has failed to find any of the hoped-for particles that would lead physicists beyond the Standard Model of particle physics. But it’s possible that the LHC has been producing such pivotal new particles all along, and that we’re just not seeing them.

“The core of the story,” said David Curtin, a physicist at the University of Maryland, “is that the LHC could be making particles which are totally invisible, which decay some distance away from the production point, whether it’s millimeters or many kilometers, and which are connected at the most fundamental level to some of the most important theoretical mysteries that we have.”

Such hypothetical particles are called “long-lived,” because their lifetimes would far exceed those that the LHC was designed to detect. If the LHC is indeed producing these particles, then it’s likely that some are fleeing the accelerator’s underground tunnel, shooting up through the earth, and potentially exploding like fireworks in the sky above the nearby farm fields as they decay back into ordinary matter.

To catch the flash of those fireworks, assuming they exist, Curtin and collaborators Henry Lubatti of the University of Washington and John Paul Chou of Rutgers University have proposed building an enormous new detector that would stand in those fields, looking rather like a really big barn. The three recently published their proposal in Physics Letters B, christening their detector Mathusla (which, in the grand tradition of tortured physics acronyms, stands for MAssive Timing Hodoscope for Ultra Stable neutraL pArticles). The name is a nod to the mythical figure who lived for over 900 years.

David Curtin, a physicist who will be moving to the University of Toronto in January, is one of the leaders of a proposed experiment to find long-lived particles at the LHC. Raul Cunha

“I think an experiment like that is really an expression of our times,” said Gian Giudice, who heads the theory department at CERN. For many years there has been “a sense of conviction about the right direction for exploration,” he said, with the field led by efforts to find short-lived particles predicted by a theory called supersymmetry. Yet those particles haven’t shown up as planned. “Now we are looking for new directions and new motivations, and that is what is really changing.”

Unfortunately, long-lived particles are difficult to detect. The good news is that our best hope for seeing them might stem from the one new particle the LHC has discovered and continues to produce: the Higgs boson.

Higgs Twins

The discovery of the Higgs in 2012 gave physicists two things at once: the triumph of finding the last missing piece of the Standard Model, but also compelling evidence that that same model is missing something essential.

The problem in the Standard Model lies in the fact that the measured mass of the Higgs is about 100 million billion times smaller than what quantum mechanics suggests it should be. From the standpoint of the Standard Model, this can be true only as a result of an extremely unlikely coincidence involving the values of some of the universe’s fundamental building blocks. (The coincidence is also exceptionally fortunate, because without it, atoms and everything they’re made of couldn’t exist.) Physicists call this situation the “hierarchy problem” and see it as evidence that the Standard Model is only an approximation of a more comprehensive theory that would explain the Higgs mass “naturally”—as the result of some mechanism other than an apparent miracle.

Supersymmetry has long been the leading candidate for the more comprehensive theory. It solves the hierarchy problem by means of new particles—one partner particle for each particle in the Standard Model. But with no sign of superpartners emerging from the LHC, some physicists are seriously considering the possibility that particles addressing the hierarchy problem belong to what’s called a hidden sector.

Raman Sundrum, a physicist at the University of Maryland, devised ways to solve the naturalness problem using hidden sectors. Faye Levine/University of Maryland

A hidden sector is a family of particles that may interact with each other but which don’t feel the effects of the Standard Model’s three forces—strong, electromagnetic and weak. They don’t directly interact with ordinary matter, which makes them very difficult to detect. But a hidden sector of particles could help solve the hierarchy problem, said Zackaria Chacko, a physicist at the University of Maryland who was one of the first to propose this idea in the early 2000s. “Naively, the way you can think about it is, the Standard Model particles want to pull their Higgs up and make the Higgs heavy,” he said. “And you have this hidden sector that is pulling it back down.”