Leptons, the tiny particles that might explain huge swaths of the universe

Joseph Piergrossi

by Joseph W. Piergrossi

Last summer, scientists at the Large Hadron Collider announced the discovery of the Higgs boson, the particle that is supposed to give mass to all of matter. With recent results confirming the presence of Higgs bosons in the LHC data, an observer outside of physics might think this is a titanic leap forward for physics.

While it was a significant finding, for some physicists, it was also disappointing. Many were hoping the Higgs boson would be different than predicted. However, it turned out to be almost exactly as had been calculated over the past three decades. While scientists could pat themselves on the backs for accuracy, many hoped for more. The results meant there would be little new information that could explain heretofore unexplained phenomena. Why is there matter in the universe? Are there other particles we don’t know about? One family of particles could help answer these questions: the leptons.

Leptons are particles that come in two types: charged and neutrinos. One charged lepton most people know well: the electron, the tiny thing that orbits an atom’s central nucleus. We use plenty of electrons in our technology: it’s the motion of electrons in that gives us electricity, for instance. The other charged leptons – muons and taus – are not as well-known, and their uncharged cousins, the neutrinos, are downright mysterious. Together, leptons might be a major pathway to new physics.

The Standard Model of Elementary Particles, which shows all of the basic particles known by physicists. Leptons are in green in the lower left hand corner. (Image: PBS NOVA/Fermilab/Office of Science/US Dept of Energy)
The Standard Model of Elementary Particles, which shows all of the basic particles known by physicists. Leptons are in green in the lower left hand corner. Click image to enlarge. (Image: PBS NOVA/Fermilab/Office of Science/US Dept of Energy)

“The theory behind lepton experiments is that a rare particle is an indication of something else,” says University of Chicago physicist Dr David Schmitz. He works on the MINERvA neutrino experiment at Fermi National Accelerator Laboratory in Batavia, Illinois.

The lepton experiments are looking at how the universe began and how that affects its current nature. Physicists believe that when the universe first began, there were equal amounts of matter and its opposite-charged version, antimatter. If this were the case, though, the matter and antimatter would have destroyed each other as soon as they emerged. Yet there is matter in the universe, and no antimatter. That means there has to be some other difference between matter and antimatter that we don’t know about.

Scientists have previously found evidence of this difference in quarks, the particles that make up hadrons, but it wasn’t enough to account for a major difference between matter and antimatter. But when they look at neutrinos, the uncharged leptons, they show signs a possible difference.

Schmitz says he thinks of this problem as though the universe were a console with knobs. “If I added just a little bit of matter-antimatter difference, I could run my model and see if it’s enough to produce the overwhelming asymmetry that we know the universe to be today,” he says.

Another important question Schmitz and other physicists think leptons can answer is whether there are other symmetries. One proposed idea is called supersymmetry, in which particles have other versions of themselves outside of our field of view. Several charged lepton experiments are looking at the possibility that muons, a heavier cousin of the electron, can interact with these hidden particles and give away their existence. If so, it would be the strongest evidence yet for supersymmetry.

Many new experiments will be looking into lepton physics in the coming decades, which will help augment work at facilities like the LHC.

“At the end of the day, the approaches – whether you’re talking about the LHC or neutrino experiments or one of the muon experiments – are essentially the same,” he says. “We’re testing our models. Anywhere we see a deviation, it’s new physics that requires an explanation.”

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Joseph W. Piergrossi (@JWPiergrossi) is a science writer living in Boston who specializes in physics, natural history and human evolution

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