The neutrino is perhaps the most intriguing particle in the atomic world. This lightweight fundamental particle interacts only with the weak nuclear force and the much weaker gravitational force. Thanks to these weak links with other forms of matter, the neutrino can pass through the entire Earth with an incredibly low chance of colliding with an atom. Ghosts, said to be able to pass through walls, have nothing on the neutrino.
The Neutrino and Its Oscillation
The neutrino has no fixed identity. The three known flavors of neutrinos can transform into one another through a periodic process called neutrino oscillation. In addition to being tiny ghosts, they are also quantum superpositions.
Confusing Signals
The three known types of neutrinos are the electron neutrino, muon neutrino, and tau neutrino, named after the charged particle produced alongside them. Early in our understanding of neutrino physics, all these types seemed distinct from one another. However, the situation became murkier in the 1960s and 1970s when experiments began to show confusing results.
The MiniBooNE Experiment
To help solve this puzzle, researchers at the Fermi National Accelerator Laboratory in Batavia, Illinois, built an experiment called MiniBooNE (Mini Booster Neutrino Experiment). The idea was to construct a detector using technology similar to the LSND experiment but with a different source of particles and enhanced detector capabilities to see if scientists could clarify the situation.
Short-Baseline Neutrino Program
To determine whether sterile neutrinos actually exist, researchers at Fermilab built two new detectors hoping to resolve the situation once and for all. The public research project is called the Short-Baseline Neutrino Program (SBN). The name reflects the fact that the detectors will be separated by a shorter distance than traditional neutrino oscillation experiments.
The Final Answer
To determine whether sterile neutrinos exist, researchers at Fermilab constructed two new detectors aiming to resolve the situation once and for all. The public research project is called the Short-Baseline Neutrino Program (SBN). The detectors are located on the same neutrino beam that was used in the MiniBooNE and MicroBooNE experiments. The detectors use liquid argon to detect neutrino interactions. The composition of the neutrino beam at the detectors is measured, and the number of neutrinos that changed flavor while transitioning from one detector to another is determined.
What’s Next?
Particle physics experiments rarely lead to swift announcements of results, and this is especially true for neutrino experiments that have an incredibly low interaction rate. Researchers will need to record collisions for several years to gather enough data to determine whether they support the sterile neutrino hypothesis. In addition to searching for sterile neutrinos, scientists expect the SBND detector to record 20 to 30 times more interactions between neutrinos and argon atoms than what has been observed in the past. This important data will provide significant contributions to Fermilab’s other neutrino-related efforts, such as the Deep Underground Neutrino Experiment (DUNE), which will be much larger than anything that preceded it. DUNE will focus on exploring the properties of neutrino oscillation that differ from those studied by SBN and will address whether matter neutrinos and antineutrinos oscillate in the same way. Currently under construction, DUNE is expected to begin operations in the late 2020s or early 2030s. In addition to the crucial investigations of the SBN program into sterile neutrinos, the improved understanding of neutrino interactions with matter that can be achieved by this program will contribute to the analyses of DUNE, leading to faster conclusions.
The neutrino has a long history of exciting scientists’ curiosity, from the particle’s first proposal in 1930 to the discovery of multiple flavors of neutrinos in 1962, to the finding in the early 21st century that neutrinos can change their identity. If sterile neutrinos are found to exist, physicists will have to add yet another surprise to the list. Regardless of the outcome, it is clear that the humble neutrino still has stories to tell.
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