One of the most successful ways for scientists to examine neutrinos is to use particle accelerators to create intense neutrino beams. Only a few facilities in the world, like the Japan Proton Accelerator Research Center (J-PARC), the European Organization for Nuclear Research (CERN), and the US Department of Energy’s Fermi National Accelerator Laboratory, can produce such powerful neutrino beams.
Muon neutrinos are the most common neutrinos produced in accelerators, and the machine can be set to produce either neutrinos or antineutrinos. Scientists start with protons to create a beam (usually from a bottle of hydrogen gas). Before being slammed into a target constructed of beryllium or graphite, these bunches of protons are accelerated close to the speed of light. The collision produces new particles, including the pion, which is a short-lived particle.
Pions have charge, which allows scientists to steer and focus them using magnets. Operators can regulate whether the beam consists predominantly of neutrinos or antineutrinos by using magnets to pick positively or negatively charged pions. The pions begin to decay as they approach a detector downstream, creating neutrinos (or antineutrinos). Other particles are absorbed by large slabs of metal, steel, or concrete, while slippery neutrinos flow through with ease. The birth of a neutrino beam!
Experiments in both the near and long future
Neutrino beams can be used for two types of experiments, depending on how far the detectors are from the source of the neutrinos.
A short-baseline neutrino experiment occurs when neutrinos travel a short distance to the detectors—that is, when the detectors are close to the beam’s source.
It’s a long-baseline neutrino experiment when they travel a long distance.
When it comes to learning about neutrinos, each type of experiment has its own set of advantages.
A neutrino beam widens out like a flashlight beam as it travels. Short-baseline experiments are located close to the neutrino source, resulting in a very concentrated beam when it reaches the detector. With an extremely pure neutrino beam, this indicates a substantially higher number of neutrino interactions. It’s useful for characterizing the beam (determining how many neutrinos are present and their energy) as well as learning about the neutrinos before they have a chance to oscillate or change flavors. It’s also a good area to look for sterile neutrinos and observe their interactions with other particles. Short-baseline experiments include ND280 in Japan, as well as MicroBooNE, MINERvA, ICARUS, and Fermilab’s Short-Baseline Near Detector.
Long-baseline experiments, on the other hand, concentrate on the oscillations that propagate long distances across the Earth. Sending a beam through a few hundred miles of rock gives neutrinos plenty of opportunities to interact with stuff while also providing enough space for flavors to change (even though the trip takes only fractions of a second, since neutrinos move close to the speed of light). Researchers can use oscillation studies to determine which of the three neutrinos is the heaviest or lightest, as well as what kind of symmetry neutrinos and antineutrinos have. T2K and Super-K in Japan, as well as NOvA and DUNE at Fermilab, are examples of long-baseline experiments. A near detector close to the neutrino source is used in most long-baseline investigations to measure the particles before they oscillate.
Do you want to know what detectors see when they’re in the neutrino beam? View real-time data from NOvA’s near- and far-field detectors.