Muon surplus leaves physicists searching for answers

Muons, electrons’ heftier cousins, rain down through the Earth’s atmosphere in numbers higher than physicists expect. The discrepancy could simply point to a gap in physicists’ understanding of the nitty-gritty physics of particle interactions, or perhaps something unexpected is going on, such as the creation of a new state of matter.

When cosmic rays — spacefaring protons or atomic nuclei — smash into the atmosphere at ultrahigh energies, they launch a cascade of many other types of particles, including muons. New observations made at the Pierre Auger Observatory detect about 30 percent more muons than simulations predict, scientists report October 31 in Physical Review Letters.

The Auger observatory, located in Argentina, uses telescopes to observe faint light from particle showers in the atmosphere, and detects particles that reach the ground using tanks of water. By comparing simulated particle showers to real data, and allowing for possible miscalibration of their detectors, the scientists concluded that the predicted numbers of muons don’t match up with reality. Hints of the muon excess have been popping up since the ’90s, says physicist Thomas Gaisser of the University of Delaware. But the new measurement is “a better job, which confirms the excess compared to what’s predicted by the models.”

The ultrahigh energy cosmic rays that the researchers analyzed probe physics at energies 10 times those reached at the world’s most powerful particle accelerator, the Large Hadron Collider, potentially allowing scientists to detect new phenomena. But, says Spencer Klein of the Lawrence Berkeley National Laboratory in California, “it’s premature to say that this is something really interesting.” He suggests that the discrepancy could simply be due to an incomplete grasp of the physics of how protons and neutrons inside a nucleus behave when nuclei collide. The complexities of that behavior could result in particles that eventually decay into more muons than scientists naïvely expected, thus explaining the glut.

But, says Auger physicist Glennys Farrar of New York University, scientists have unsuccessfully tried to explain the muon surplus using standard physics for many years. “That’s in a way the most convincing reason to think that there may be new physics.” An explanation Farrar favors is a phenomenon in which a new state of matter appears at high energies. In such a state, large numbers of gluons — particles that transmit the strong nuclear force — may behave collectively, like photons in sync in a laser. If enough energy is pumped in by the cosmic rays, the gluons could “start to develop a life of their own,” Farrar says. The gluons might then gang up into hypothetical particles called glueballs, which could decay into particles that produce more muons.