The name isn’t particularly catchy—X(3872)—but the discovery of a new subatomic particle adds to the evidence that physicists’ standard model of how matter behaves is seriously incomplete.
Stephen Olsen of the University of Hawaii and his colleagues, working at the KEK laboratory in Tsukuba, Japan, uncovered the new particle last summer while smashing together electrons and their antimatter twins, positrons. The experiment churns out mesons, intermediate-mass particles that quickly decay into more stable entities. Current models of particle physics make detailed predictions of the expected masses and behaviors of various types of mesons, but X(3872) does not seem to fit in. “When we measured its mass it was way off what the theory says it should be,” Olsen says. Physicists at the Fermilab accelerator in Illinois confirmed the odd properties of the particle and dubbed it the “mystery meson.”
One possibility is that the new particle is actually a hybrid of previously known ones. Olsen notes that the mass of X(3872) almost precisely matches the combined heft of two mesons known as D and D*. “This strange coincidence has people wondering if the thing might be a ‘molecule’ of a D meson and an anti-D* meson,” he says. Just as the electromagnetic force binding atoms together can cause them to stick to one another into a molecule, the strong force that holds mesons together might leak out, causing two to cling to each other. “People had speculated that this could happen, but this is the first time we’ve seen anything that fits the bill so well,” Olsen says. Conventional theory says such meson “molecules” should not exist, he notes: “The theories we derive from the standard model will absolutely have to be changed.”