Determining the precise details of how the strong nuclear force assembles quarks into hadrons is not always easy.

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Method A:
3 quarks
chromodynamic glue

Step 1. Glue the quarks together.
Step 2. Voila!

Method A will produce a baryon; most of your more massive hadrons are baryons. The type of baryon will be determined in part by the flavor of the quarks used and how the quarks are aligned. For example, if you have glued together one up quark, one down quark and one strange quark, you have created a Lambda baryon or maybe a Sigma baryon. If you glued together two up quarks and a down quark, you have created a proton. (Of course, the work going on now out at the Fermilab Proton Assembly Building has nothing to do with any of this.)

Method B:
1 quark
1 antiquark
chromodynamic glue

Step 1. Glue the quark and antiquark together.
Step 2. Voila!

Method B will produce a meson, typically a lighter kind of hadron. Again, the type of meson will be determined by the flavors of the specific quark and antiquark. If for example you glued together an up quark to a strange antiquark, you have created a K+ meson. (Of course, the work going on now out at the Fermilab Meson Assembly Building has nothing to do with any of this.)

Method C:
There is no Method C
Or is there?

Chromodynamics is the theory of the forces — the glue — that binds quarks and antiquarks to make hadrons. The theory has a great deal of experimental support, but there are times when it is hard to make exact computations using it. In particular, theory doesn’t clearly say that baryons and mesons are the only kinds of hadrons that can be constructed. Starting a little over a decade ago, increasingly strong experimental evidence emerged in favor of the existence of other hadrons — neither baryons nor mesons. But, as is so often the case in science, different experiments do not always get the same result, and those results need to be confirmed.

An example is the X(4140). This particle, sometimes called the Y(4140), was first reported by the CDF experiment in 2009. It does not seem to be the result of following either Method A or Method B. It seems to be some other sort of thing — perhaps it is two quarks and two antiquarks, or perhaps some other method was followed in assembling this hadron.

Shortly afterwards, the Belle experiment in Japan, using a different method, reported that they did not see the X(4140). LHCb at CERN also did not see it. But the CMS experiment reported that they did see it. Last year, DZero published the results of our first search for the X(4140): We saw it.

The three experiments with positive results all looked for X(4140) created as a result of the decay of a B+ meson. If the X(4140) really exists, it should have been possible to create it at the Tevatron directly, without making first a B+ meson.

DZero has recently searched for the X(4140) without requiring it to be the result of a B+ meson decay, and we discovered that there were 1,964 ± 248 such events in the Tevatron data. The number of X(4140) from meson decays is only 809 ± 175, so this is evidence that X(4140) can indeed be created directly.

Further study is needed before we can be completely certain of all of the properties of the X(4140). But we now have already strong evidence that there is a third way to assemble a hadron.

Leo Bellantoni

 Avdhesh Chandra (Rice University) and Daria Zieminska (Indiana University) are the primary analysts for this measurement.
 The DZero collaboration relies upon many of its collaborators to carefully review analyses for scientific quality before they are released. This analysis was guided by Editorial Board Chairs Brendan Casey and Gene Fisk (both of Fermilab).