A proton is composed of three valence (core) quarks—two up quarks and a down quark — held together by gluons. Surrounding the valence quarks is an indistinct sea of quark-antiquark pairs, primarily consisting of up, anti-up, down, and anti-down quarks. The quark-antiquark pairs are believed to originate from processes such as gluon splitting or virtual-pion production (see figure to the right). Until the early 1990s, the scientific community had assumed that there were equal numbers of anti-up and anti-down quarks in this “nucleonic sea,” a condition known as flavor symmetry. Experimental evidence, however, began to suggest that there might be more anti-down quarks in the sea than anti-up quarks, and a need arose for a more accurate measurement.
Fermilab Experiment 866 was a fixed target experiment to measure the asymmetry of down and up anti-quarks in the nucleon sea, and ACU students were heavily involved in its construction and running. The experiment aimed an 800 GeV proton beam at either proton or deuterium targets. When a quark from a proton in the beam strikes a sea anti-quark in a target nucleus, the quark-antiquark pair annihilate and form a virtual photon, which can then decay into a pair of oppositely charged muons; production of a pair of muons in this way is called the Drell-Yan process (see figure to the right). By detecting this pair of muons, E-866 was able to reconstruct important information such as the ratio of anti-up and anti-down quarks in the nucleonic sea. E-866 was a groundbreaking experiment whose results have already been incorporated into theoretical models to help us better understand the complex forces and interactions within the proton.
The Fermilab E-906/SeaQuest Experiment is a follow-up experiment to E-866. Using a 120 GeV proton beam extracted from the Fermilab Main Injector, it extends the range of down-to-up antiquark measurements, and examines the modifications to the antiquark structure of the proton from nuclear binding. This experiment is an exciting opportunity for ACU undergraduates to make contributions to this important exploration of the proton.