Intensity Frontier
Particle physics experiments at the Intensity Frontier explore fundamental particles and forces of nature using intense particle beams and highly sensitive detectors. One of the ways that researchers search for signals of new physics is to observe rarely interacting particles, such as neutrinos, and their corresponding antimatter particles. Some of these experiments search for evidence of the process theorists hypothesize allowed our universe full of matter to bloom rather than being annihilated by an equal amount of antimatter created in the big bang. Other experiments seek to observe rare processes that can give researchers a glimpse of unknown particles and unobserved interactions.
Neutrino Physics
Neutrinos are some of the most fascinating of the known particles. They abound in the universe but interact so little with other particles that trillions of them pass through our bodies each second without leaving a trace.
Neutrinos come in three types, called flavors: muon, electron and tau. They have no electric charge. Their mass is so small that the heaviest neutrino is at least a million times lighter than the lightest charged particle.
At Fermilab, physicists use a beam of protons from the Main Injector accelerator to create the most intense high-energy neutrino beam in the world. Magnets direct the protons onto a graphite target. When the protons strike this target, they take the form of new particles called pions. A magnetic lens called a horn focuses and collects the positively charged pions and discards the negatively charged ones. The positively charged pions travel through a long, empty space and ultimately decay into antimuons and muon neutrinos. Experimentalists filter the resulting mix of debris, antimuons, undecayed pions and muon neutrinos through a steel and concrete absorber, which stops all but the weakly interacting neutrinos. To make a beam of antineutrinos, they reverse the magnetic field of the horn to collect negatively charged pions that decay to negatively charged muons and muon antineutrinos.
The facility that creates Fermilab's neutrino beam is called NuMI, for Neutrinos at the Main Injector. The neutrinos travel between two detectors for an experiment called MINOS, or Main Injector Neutrino Oscillation Search. One sits at Fermilab; the other is located 450 miles away in the Soudan Underground Laboratory in Minnesota. The NuMI Beamline is aimed downward at a 3.3 degree angle toward the underground laboratory. Neutrinos interact so rarely with other particles that they can pass untouched through the entire Earth.
Although the beam starts out at 150 feet below ground at Fermilab, it passes as much as 6 miles beneath the surface as it travels through the earth toward Soudan. Neutrinos travel at the speed of light and make the trip from Illinois to Minnesota in just two and a half thousandths of a second. Researchers at Fermilab use the NuMI beamline as a source of neutrinos for other intensity-frontier experiments as well.
Muon Physics
Virtual Particles
Physicists also search intense beams of particles for signs of virtual particles. According to the Heisenberg Uncertainty Principle, even massive particles can pop briefly in and out of existence as virtual particles. The more massive the particle, the less frequently this happens. When virtual particles drop in, they are much less massive than usual, but physicists can detect them in intensity-frontier experiments by the effect they have on interactions between other particles like kaons and muons.
By colliding two concentrated beams of one of these types of particles into one another or by firing them into a target, physicists bring them into proximity in the hopes that they will interact. They then study those interactions with ultra-precise detectors, looking for unusual outcomes. They can identify the presence of virtual particles involved in an interaction by the effects they have.
Even if the particle colliders like the Large Hadron Collider grants physicists the chance to observe the particles physicists seek directly, they will need experiments like those at the intensity frontier to make precise measurements of their parameters.