A Wide-Angle Lens for the Subatomic World ✛
At Jefferson Lab, electrons are accelerated to 12 GeV and smash into protons to pry out their secrets. But existing detectors are like cameras with a narrow field of view: you either see a lot but dimly, or brightly but only a tiny patch. Electron scattering is the main tool, but statistics are insufficient for subtle effects.
The new SoLID spectrometer (Solenoidal Large Intensity Device) changes the game. It's a wide-angle lens for the subatomic world: large acceptance, meaning the fraction of particles the detector catches, combined with luminosity up to $10^{39}$ cm^-2 $s^{-1}$, a measure of collision intensity. For comparison, the Large Hadron Collider's luminosity at startup was five orders of magnitude lower. SoLID will collect scattered electrons and produced particles over a huge range of angles without losing sharpness.
SoLID's three main tasks are like three different lenses in one frame. The first is three-dimensional tomography of the nucleon via semi-inclusive deep inelastic scattering: by measuring not only the scattered electron but also the outgoing hadron, physicists can reconstruct the momentum distribution of quarks and gluons. The second is near-threshold J/ψ production: a rare process that provides access to gluon gravitational form factors, which characterize the distribution of mass and pressure inside the proton. The third is parity violation in deep inelastic scattering: the difference in scattering of left- and right-handed electrons, sensitive to weak interactions and possible new physics.
Before SoLID, experimentalists faced a choice: either large acceptance or high luminosity. CLAS12 and GlueX catch many particles but operate at moderate luminosity around $10^{35}$ cm^-2 $s^{-1}$. The magnetic spectrometers HRS, SBS, HMS, and SHMS provide high resolution, but their acceptance is small. SoLID is the first to combine both qualities, opening the door to measurements that were previously impossible due to lack of statistics. This will sharply reduce uncertainties in quark and gluon distribution functions and also test the Standard Model at low energies, where traces of new interactions might hide.
SoLID's wide-angle lens is aimed at the very heart of matter. In a few years, it will start collecting the light of subatomic events, and then the proton will no longer be a blurry spot; we will see its inner mechanics with unprecedented clarity.
SoLID's luminosity in the PVDIS experiment will reach $10^{39}$ cm^-2 $s^{-1}$, which is roughly equivalent to $10^{39}$ particles passing through a square centimeter of target every second. For comparison, the Large Hadron Collider's luminosity at startup was about $10^{34}$ cm^-2 $s^{-1}$, that is, five orders of magnitude lower.