A subterranean neutrino shot
The Japan Proton Accelerator Research Complex, or J-PARC, is a sprawling high-intensity proton accelerator facility in Tokai, on Japan's east coast. Its primary mission involves one of the most precisely aimed experiments on the planet: the T2K (Tokai to Kamioka) experiment. Here, scientists create the world's most intense beam of neutrinos and fire it 295 kilometers straight through the Earth's crust to a massive detector called Super-Kamiokande. The neutrinos, which are nearly massless and interact very weakly with other matter, complete this journey without a tunnel.
The process begins with a trio of accelerators. First, a 400 MeV linear accelerator (LINAC) gives protons their initial push. From there, they enter the 3 GeV Rapid Cycling Synchrotron (RCS), a ring with a circumference of about 350 meters. The final stage is the 30 GeV Main Ring, a much larger synchrotron measuring 1,567.5 meters in circumference, which accelerates the protons to 99.95% of the speed of light. This high-energy proton beam is then slammed into a graphite target. The collision produces a shower of short-lived particles called pions, which decay into muons and muon neutrinos as they travel down a 250-meter-long tunnel. A system of three powerful magnetic horns focuses these pions before they decay, ensuring a tightly-collimated beam of neutrinos is aimed with extreme precision toward the distant detector. The main goal is to study neutrino oscillation, observing how many of the muon neutrinos transform into electron neutrinos during their flight.
Beyond the ghost particle
J-PARC's capabilities extend beyond the T2K experiment. The complex houses two other major experimental areas: the Materials and Life Science Experimental Facility (MLF) and the Hadron Experimental Facility. Both utilize the powerful proton beams for different kinds of science.
The MLF uses the 1-megawatt proton beam from the 3 GeV RCS. This beam strikes a mercury target, producing the world's most powerful pulsed spallation neutron beams. Scientists use these neutrons and secondary muon beams to investigate the atomic structure and properties of a vast range of substances. The research has applications in developing materials for new batteries, high-performance plastics, and pharmaceuticals. The Hadron Experimental Facility uses the 30 GeV beam from the Main Ring. Protons collide with a target to create secondary particles like kaons and pions. Physicists use these particles to study the fundamental forces that hold atomic nuclei together and to probe the structure of exotic matter, showing the conditions of the early universe.