How many quarks in a proton?

Although we usually say that a proton contains three quarks (up, up and down), there are many more quark-antiquark pairs at fine scales.

Matter is made of molecules, which are made of atoms, which are primarily made of protons and neutrons, which are made of quarks. In each case, however, “made of” takes a subtly different meaning. Protons are not made of quarks the way that a wall is made of bricks but rather like the way that a fire is made of flames. They are seething balls of spontaneously forming and annihilating quarks.

Yet this tempest has structure. For instance, quarks and antiquarks can only be created or destroyed in pairs, so when we say that a proton contains three quarks, it is because the total number of quarks minus the total number of antiquarks is always three (two more up quarks than anti-up and one more down quark than anti-down). Adding a few more quark-antiquark pairs doesn’t change the difference.

The number of quarks plus the number of antiquarks depends on how closely you look. Just as a coastline seems to get longer as you zoom in (because the true coastline winds around every grain of sand on the beach), the number of quarks and antiquarks increases at finer scales. High energies are sensitive to small scales, so high-energy protons appear to be denser and are more likely to collide.

This affects the rate of production of every kind of particle made by the LHC. But because these high energies had never been explored before first collisions in 2010, no one knew for sure what the rates of particle production would be. In addition to searching for the Higgs boson and other new particles, physicists have been measuring familiar processes to get a clearer picture of how the proton’s density scales with energy.

One very precise way to do that is to count the ratio of W+ bosons to W− bosons. A W+ boson is formed when an up quark and an anti-down quark combine, and a W− is formed from down and anti-up. Since protons contain more up quarks than down, W+ is somewhat more likely than W−. The exact ratio depends on the density of antiquarks, so CMS scientists carefully measured tens of millions of W+ and W– bosons with impressively small uncertainties (0.2 to 0.4 percent). These measurements are already helping to nail down the structure of the proton at the smallest scales.

Jim Pivarski

The U.S. physicists pictured above made major contributions to this study of W charge asymmetry.
After years of effort, the above physicists are stepping down from senior leadership roles in CMS. As a group, they held managerial roles in computational resources, physics analyses and the detector upgrade. CMS thanks them for their effort and wishes them well in future endeavors.