X marks the what?

The peak on the left indicates an excess of events consistent with the X(4140) state, which could be a new type of particle composed of two quarks and two antiquarks. A second possible excess can also be seen, although this is more consistent with a background fluctuation.

Physicists at particle colliders often perform searches for specific particles that are predicted to exist in our models. The Higgs boson is an excellent example of such a focused search effort, and indeed we found it in the end! However, this isn’t always the way that science works. Sometimes there are surprises, discoveries of new particles that aren’t expected and that don’t fit within our existing theories. Such findings can challenge our understanding of the universe.

In recent years, there have been several observations and hints of this kind: a class of particle that appears to stretch our comprehension of the way that quarks interact. We currently know of only two ways that quarks combine into composite particles, either in quark-antiquark pairs ("mesons") or in threesomes ("baryons") like the proton. The “new” particles behave like mesons, but their properties show that there must be some other quarks inside as well: We could be seeing four-quark states for the first time! These unexpected particles are so mysterious that they are called simply X, Y or Z states in the official listings, and scientists are working hard to understand and identify them more fully.

Inspired by one of these hints from the CDF collaboration, a new search at the DZero experiment looks for the snappily named X(4140) state decaying into a charm-anticharm meson and a strange-antistrange meson. The analyzers indeed see an excess of possible signal events, sharply peaked in mass, consistent with this fledgling state, with less than a 0.3 percent chance that it is a natural fluctuation of background events.

Interestingly, while the CDF and DZero results agree, additional searches from other experiments in Europe and Japan give inconclusive results, and so the X(4140) enigma goes on: Further work is needed before the state can be confirmed. This shows the value of building and operating independent experiments, allowing measurements like this to be cross-checked and fully investigated. By combining information from all the different particle accelerators, we hope to compile a list of these "exotic" end-of-alphabet states and maybe even add a third sibling to the meson and baryon family.

Mark Williams

These physicists all made major contributions to this analysis.
The chairperson (left) and deputy chairperson (right) of the DZero Institutional Board, which represents the interests of 72 institutions from 18 nations that make up the DZero collaboration. The IB sets the rules that govern this collaboration, liaising with the spokespeople on important matters to guide the good ship DZero on its scientific exploration.