Astronomical observations, such as the rotation of galaxies and their relative velocities in galaxy clusters, have provided compelling evidence that the universe contains much more matter than is visible in stars. Although this so-called “dark matter” is thought to be as much as ten times larger than the visible kind, its nature is still a mystery. Numerous searches have failed to find dark matter and many others are in progress or planned. One of those, an experiment called “DarkLight”, is prepared by a collaboration headed by MIT. It will search for low-mass photon-like dark-matter particles created in electron-proton collisions.
Since the chances of creating these particles are necessarily extremely small, an extremely large number of collisions must be created and examined. It is planned to collide the world’s most intensive beam of 100-MeV electrons form the energy-recovering linear accelerator at the Jefferson Laboratory in Newport News, VA, with a very dense hydrogen gas target to be designed and built at the MIT Bates Laboratory. In contrast to several other experiments, it is planned to measure all particles emitted in the electron-proton collisions including very low-energy recoil protons which would not escape from a liquid hydrogen target (hence the gas target). This is vital in identifying a dark-matter particle amidst the enormous number of conventionally produced particles.
Since a solid beam entrance window to the gas target would produce unwanted beam scattering and would be melted by the powerful electron beam, the beam must enter the target through a small tube through which gas will escape into the beam vacuum where it is pumped away by powerful booster pumps. In a test at Jefferson Lab, an electron beam of 0.45 Mega-Watt power was transmitted through a 2-mm diameter tube with losses of less than 3 parts-per-million, proving the feasibility of the proposed gas target.
The target, placed inside a solenoid magnet, is to be surrounded by position-sensitive detectors which track and identify the particles emitted by the collisions and measure their energy.
Funding was approved by the National Science Foundation to build a prototype target and detectors and test them in a full-power beam at Jefferson Lab. The MIT-Bates Lab. will engineer and build the target for this Phase-I experiment and assemble and commission the magnet and detector systems supplied by the collaboration.