The two mesons whose discovery arguably did the most to advance our understanding of what a meson really is are the J/ψ (pronounced “J sigh”) and the Υ (Upsilon). The discovery of the J/ψ in 1974 at Brookhaven National Laboratory and SLAC National Accelerator Laboratory was the discovery that convinced physicists that the quark model was actually right. The discovery of the Υ here at Fermilab in 1977 was the discovery that made it clear that there should be (at least) six different flavors of quark.
With enough energy, such as one had in the Tevatron, it is possible to create both a J/ψ and an Υ simultaneously. What we learn from these events has a lot to do with the way quarks are stuck together inside the proton.
The gluon is the particle that carries the force that holds quarks together. When a proton collides with another proton, a quark or gluon from one proton can collide directly with a quark or gluon from the other proton. Occasionally, there will be two such direct collisions at the same time. Perhaps there will be two quark-quark collisions, or two gluon-quark collisions, or a gluon-gluon collision with a quark-gluon collision; all the possible combinations occur. This “double parton” process does not involve new forms of energy or matter, but it can look like that, and so they are important to understand.
The study of double parton collisions has a long history; see the Feb. 6, 2014, and Sept. 10, 2015, editions of Fermilab Today. Double-parton collisions that produce a J/ψ and an Υ are almost always a result of two gluon-gluon collisions. By comparing these collisions with other double-parton collisions involving quarks, we can find out if the distribution of gluons in a proton differs from the distribution of quarks in a proton. We use a number called σeff, the effective cross section; if it is small, the two collisions tend to happen close to each other.
Recently, DZero made the first measurement of the effective cross section in J/ψ-plus-Υ events, getting the result shown in blue in the above figure. A previous DZero result in which two J/ψ mesons are produced predominantly in gluon collisions also gave a low value of σeff, unlike the higher value of σeff for double collisions involving quarks.
It is fairly inescapable: The gluons in the proton tend to be clumped together more compactly than the quarks!