For all the neatness of the Standard Model, particle physics can be a messy business. It’s difficult enough to keep track of just the major players — the huge family of quarks, neutrinos, electrons, photons, leptons, bosons and so forth — without worrying about a bewildering extended “family” of X and Y particles that don’t seem to fit the usual paradigm for how quarks and anti-quarks combine to make matter.
One of those was discovered in 2009 by physicists at Fermilab’s CDF experiment. Dubbed Y(4140), the particle first showed up in B meson decays and joins a handful of other exotic particles observed in accelerator experiments around the world over the past decade.
Now physicists at the Large Hadron Collider’s CMS collaboration think they have spotted this elusive object in their own data, confirming that Y(4140) is a real structure — even if they’re not quite sure what, exactly, it is. All they know for sure is that its mass is 4140 MeV and that there is less than a 1 in 3.5 million chance that the bump in the data is due to a random statistical fluctuation.
There are six different “flavors” of quarks: top, bottom, charm, strange, up and down. Most elementary particles are the result of quarks and antiquarks combining to make mesons (quark-antiquark pairs) or baryons (made up of three quarks).
Y(4140) has the same basic building blocks, but doesn’t fit into either the meson or baryon category — a characteristic it shares with similar strange particles like X(3872) — discovered in 2003 — and Y(2460), discovered in 2005.
“Apparently there are a lot more ways of putting things together than we thought,” Syracuse University’s Sheldon Stone told National Geographic back in 2009.
One possibility is that these unusual X and Y particles are hybrids, perhaps four quarks bound together. They might be more akin to molecules than standard subatomic particles. In much the same way that hydrogen and oxygen atoms combine to form water (H2O), two mesons, for example, might briefly combine to form Y(4140). Or they might be something entirely new.
Now that CMS has confirmed Fermilab’s 2009 result, physicists will try to make better measurements of Y(4140)’s basic properties, like mass and how it decays, in hopes of better understanding what they’re dealing with. “I don’t know which explanationn is right,” the University of Iowa’s Kai Yi told Symmetry Breaking. “We can’t point at something and say, yes, this is it, yet.”
Image credit: CERN