Students of particle physics learn early that the proton is made up of three quarks: two up and one down. But that simple picture doesn’t tell the whole story. The quark masses, at a few MeV each, contribute just 1% of the proton’s total mass of 938 MeV. The rest comes from the quarks’ kinetic energy, the gluon field that holds them together, and a fluctuating sea of fleeting pairs of quarks and antiquarks.
At low energies, protons behave like self-contained particles, and their internal structure is of little consequence. But at the high energies of particle accelerators, the reaction between two colliding protons is really a reaction between their constituent quarks and gluons. Among the possible reaction channels is the Drell–Yan process, in which a quark in one proton and an antiquark in another annihilate to produce a virtual photon. The photon promptly decays into a muon and antimuon, whose measurable properties open a window into the sea.
Some 20 years ago, the NuSea experiment at Fermilab yielded a result that left researchers scratching their heads: For the most part, down antiquarks outnumbered up antiquarks in the sea. But among those rare antiquarks that carried more than 30% of their host protons’ momentum, the situation reversed, and up outnumbered down.
That there should be a difference in number at all was initially a surprise. Gluons don’t couple to quark flavor, so they should produce up–antiup and down–antidown pairs with equal probability. Perturbative quantum chromodynamics, the simplest framework for quantifying strong-interaction processes, also predicts a flavor-symmetric sea. To explain the antidown excess, theorists have incorporated additional phenomena—for example, Fermi statistics may play an outsized role, and the presence of two up quarks in a proton significantly forestall the formation of a third. But none of the theories could also reproduce the flavor reversal at higher fractional momentum.
Now, at long last, NuSea’s intellectual successor—the SeaQuest experiment, also based at Fermilab—may have come to the rescue. Whereas NuSea’s data at high fractional momentum are tenuous and noisy, SeaQuest was specifically designed to study collisions of those high-momentum antiquarks. The new results show no flavor reversal after all: Down antiquarks outnumber up antiquarks by a nearly constant 50% over a range of high momenta.
That result makes more theoretical sense, and it suggests a new way of thinking about the sea—not as a perturbation on top of the original three-quark state but as an integral part of the proton. In search of more clues, SeaQuest is transforming into SpinQuest. That new experiment will study collisions of spin-polarized protons, with the hope of eventually answering a lingering question: Why, with such ever-changing internal structure, do protons all look the same? (J. Dove et al., Nature 590, 561, 2021.)
