Fixedtarget

Fixed-target experiments are a class of particle and nuclear physics experiments in which a high-energy particle beam is directed onto a stationary target, as opposed to collider experiments where two beams are made to collide. The interaction products are analyzed to study hadron production, nuclear structure, and fundamental processes.

The center-of-mass energy in fixed-target collisions is set by the beam energy and the target mass. For a high-energy proton beam on a proton target, sqrt(s) is approximately sqrt(2 m_p E_beam) when E_beam >> m_p. This yields lower s than a modern collider with the same beam energy, but fixed-target setups can achieve higher luminosities because dense targets and long data-taking periods are practical.

Fixed-target methods have a long history in high-energy and nuclear physics. They enabled discoveries in hadron spectroscopy, measurements of deep inelastic scattering, and the determination of parton distribution functions. They are also central to neutrino production experiments, which generate intense neutrino beams by striking thick targets.

Experiments typically consist of a beamline delivering protons, pions, electrons, or other particles, a solid or liquid target, and a detection system downstream—often a magnetic spectrometer complemented by calorimeters and muon detectors—to reconstruct trajectories, momenta, energies, and identities of outgoing particles.

Prominent programs include Fermilab's fixed-target Drell-Yan measurements (E866/NuSea), CERN's NA61/SHINE hadron-production studies, and Jefferson Lab's high-luminosity electron-fixed-target scattering program. Neutrino programs at Fermilab use fixed targets to generate neutrino beams for oscillation experiments. Fixed-target science remains active at facilities worldwide and underpins future projects such as neutrino-beam experiments and high-intensity hadron physics.