Tempest in a pinpoint

A proton contains two up quarks, a down quark and a soup of quark-antiquark pairs, seething below the surface.

It is often said that a proton is made of three quarks: two of the same type, called up quarks, and one of a different type called a down quark. But that’s not the whole story. In the space between these three stable quarks there is a boiling soup of quark–antiquark pairs. That is, a quark and an antimatter quark spontaneously come into existence, drift a while, and then recombine, destroying one another. This happens all the time— in every proton in every atom of every cell of our bodies, and in all of the matter in the universe.

When two protons collide in the LHC, most of the individual quarks miss each other. Often only one quark or antiquark from each proton collides directly. When an up quark collides with an anti-down quark, the two can combine to form a W+ boson; similarly, a down quark and an anti-up quark can combine to form a W¯ boson. In both cases, an antiquark is involved. Thus, each of the millions of W bosons produced at the LHC must come from at least one of these transient particles, caught before it had a chance to sink back into the soup.

CMS scientists recently measured the ratio of W+ to W¯ production in proton collisions at the LHC. The number of W+ bosons exceeds the number of W¯ bosons by about 40 percent, partly because each proton has two stable up-quarks for every stable down-quark. However, the exact ratio also depends on the density of the quark-antiquark soup.

Counting W+ and W¯ bosons yields new insight into the dynamic structure of protons, which is too complicated to compute from first principles with current techniques. It also informs predictions of new physics: The rate at which hypothetical particles would be produced depends on the density of quark-antiquark pairs, for the same reason that W bosons do. It is important to know the thickness of this soup when imagining what else might spring from it.

— Jim Pivarski

The U.S. physicists pictured above contributed to this analysis, which required input from around the world.
The above physicists had important roles in developing and maintaining the Level 1 Endcap Muon Trigger, which recorded many of the collision events used in this analysis.