Deep inelastic scattering

Deep inelastic scattering (DIS) is the high-energy scattering of leptons (typically electrons or muons) off hadrons, in which the momentum transfer is large enough to resolve the internal structure of the target hadron. The experiments conducted at the Stanford Linear Accelerator Center (SLAC) in the late 1960s provided the first direct evidence that protons and neutrons contain point-like constituents — later identified as quarks — and that these constituents interact weakly when probed at short distances, a phenomenon now understood as asymptotic freedom in quantum chromodynamics.

In DIS, the lepton emits a virtual photon or Z boson that interacts with a single quark inside the hadron. The cross-section can be factorized into a leptonic part (calculable from QED) and a hadronic part encoded in parton distribution functions (PDFs), which describe the momentum fraction carried by each type of quark and gluon. The QCD evolution of these PDFs with energy scale is governed by the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) equations — a renormalization-group equation that describes how the parton content of the proton changes as the resolution scale increases.

DIS remains the primary experimental method for determining parton distribution functions, which are essential inputs for predictions at the Large Hadron Collider and for understanding the spin structure of the nucleon.

Deep inelastic scattering is often taught as a historical experiment — the one that discovered quarks. But its deeper significance is methodological: it demonstrated that the renormalization group is not just a theoretical tool but an experimental observable. The scaling violations in DIS — the subtle deviations from exact Bjorken scaling — are direct measurements of the QCD beta function. The experiment that discovered quarks also discovered the running of the coupling.