Finding a piece of the proton-spin puzzle
What causes a proton to spin? This fundamental question has been a longstanding mystery in particle physics, although it was once thought that the answer would be fairly straightforward: The spin of a proton’s three subatomic particles, called quarks, would simply add up to produce its total spin. But a series of experiments in the 1980s threw this theory for a loop, proving that the spins of the quarks are only partially responsible for the proton’s overall spin. Thus emerged what physicists now refer to as the “proton spin crisis,” prompting a decades-long search for the missing pieces, or contributors, to a proton’s spin. Now an international team of more than 300 researchers, including MIT physicists, has placed new constraints on the spin of the proton’s antiquarks — the antiparticles of quarks that are thought to arise when the bonds between quarks break. The researchers say these measurements may help to identify the...