Coming Closer to Solving the Proton Spin Crisis
Normally one would expect that an object can be described as the sum of its parts, depending on what specific property you are describing. For protons, many believed that their spin was the combination of the spins of the particles that make it up, but in the 1980s experiments showed this was not the case. Since then researchers have been working to solve the proton spin crisis, and those at MIT have some new evidence.
According to the standard model, protons are made of two up quarks and one down quark, which do add up to the positive charge of the larger particle. Adding the spins of the quarks up does not result in the spin of the proton though, so researchers have been searching for where the additional spin must come from. One theory is that the bonds between the quarks occasionally break, and this allows pairs of quarks and antiquarks to briefly appear and annihilate, contributing to the proton's spin while they exist. To test this, the researchers collided a number of protons, which produced some W bosons. These bosons would have the spin of any antiquarks that were present when the proton collided, thereby allowing the researchers to determine how great an influence the quark-antiquark pairs have on proton spin.
As it turns out, the spin of the antiquarks is only marginal and not enough to solve the crisis. All is not lost though, in a larger sense, as the information collected gives a much better understanding of how up-flavored antiquarks behave and come to exist. This will aid future studies into the crisis as well.