The hunt is on…. https://t.co/jNJQcsWw4a
— DUNE Science (@DUNEScience) December 26, 2023
"The neutrino is perhaps the most fascinating inhabitant of the subatomic world. Nearly massless, this fundamental particle experiences only the weak nuclear force and the much fainter force of gravity. With no more than these feeble connections to other forms of matter, a neutrino can pass through the entire Earth with just a tiny chance of hitting an atom. Ghosts, who are said to be able to pass through walls, have nothing on neutrinos.
The neutrinos’ phantom properties are not the only thing that sets them apart from other fundamental particles. They are unique in that they don’t have a fixed identity. The three known forms of neutrinos are able to transform into one another through a cyclical process called neutrino oscillation. In addition to being subatomic specters, they are also quantum chameleons.
Although the phenomenon of neutrino oscillation has been studied in many experiments, the data don’t tell a unified story. Based on the evidence of some experiments, some scientists have begun to suspect that there may be more than three types of neutrinos. These hypothetical additional neutrino types, unlike their familiar counterparts, would not even interact via the weak nuclear force and thus would be called sterile neutrinos.
Sterile neutrinos are not part of the Standard Model, the accepted theory of matter and energy in the subatomic world. If these extra neutrinos exist, they will force physicists to revisit the theory and possibly substantially revise it. A new experiment set to begin measurements soon may be able to settle the question of whether previous investigations have seen sterile neutrinos or not.
CONFUSING SIGNALS
The three known types of neutrinos are the electron neutrino, muon neutrino and tau neutrino, each named for the charged particle that is produced simultaneously with it. Early in our understanding of neutrino physics, each of these types seemed to be different from the other two. However, the situation became murkier in the 1960s and 1970s, when experiments began to show puzzling results.
Electron neutrinos are produced in nuclear reactions, and the biggest nuclear reactor around is the sun. Researchers used the energy output of our home star to calculate how many electron neutrinos they expected to arrive here on Earth. However, measurements yielded a third as many electron neutrinos as predicted. In addition, the cascade of particle interactions that result when high-energy cosmic protons hit our planet’s atmosphere was expected to produce twice as many muon neutrinos as electron ones. Yet experiments measured roughly equal quantities.
In 1957 physicist Bruno Pontecorvo made the daring proposal that neutrinos could oscillate, thereby changing their identity. Between 1998 and 2001, detectors studying the flux of neutrinos from both the sun and Earth’s atmosphere proved that neutrinos were changing into other flavors on their way to us.
Even prior to these observations, researchers used particle beams to investigate the possibility of neutrino oscillation. One experiment using the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory produced a sample of nearly pure positive muons. As the muons decayed, they created muon antimatter neutrinos. Taking into account the setup of the experiment, physicists expected to detect electron antimatter neutrinos at a rate of about 0.06 percent of the amount of muon antimatter neutrinos. Instead they measured that electron antimatter neutrinos were about 0.31 percent of interactions, well above predictions.
Scientists can determine which neutrino they’ve detected by studying the particles that are created when neutrinos collide with atoms. When neutrinos do happen to impact an atom of matter, electron neutrinos will create an electron, and muon neutrinos will create a muon. Tau neutrinos react similarly, but it is challenging to identify tau particles.
Using their measurements (and others performed elsewhere), the LSND scientists concluded in 2001 that three neutrino variants could not simultaneously explain both their data and the array of solar and atmospheric neutrino measurements that existed at the time. However, if there were a fourth, sterile neutrino, then the experiments were consistent. The only problem was that other accelerator-based neutrino measurements didn’t support the idea of a fourth neutrino. Another measurement was necessary.
To help resolve this quandary, researchers at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., built MiniBooNE (Mini Booster Neutrino Experiment). The idea was to construct a detector using a similar technology as LSND but with a different source of particles and enhanced detector capabilities to see if scientists could clarify the situation.
MiniBooNE collected data from 2002 to 2018. The 2007 publication of its early results ruled out the simplest explanation of the LSND finding, though it did observe a different excess. When scientists collected more data and performed a more sophisticated analysis, they concluded in 2018 that there was a persistent mystery.
Yet other experiments tell a different story. A separate Fermilab project called MINOS (Main Injector Neutrino Oscillation Search) saw no evidence for sterile neutrinos. Nor was such evidence found by the IceCube experiment in Antarctica, which uses a cubic kilometer of ice to study neutrinos from space.
Nuclear reactors provide another source of electron neutrinos, and researchers have also used them to look for sterile neutrinos. In 2011 scientists reported a 6 percent deficit of electron neutrinos at a reactor in China, compared with what they expected to see. More recently, other researchers have claimed that the earlier calculations were in error and that no deficit exists...."
Full article by Don Lincoln, senior physicist at Fermilab, here.
Don't miss this video about neutrino oscillations:
MFBB.
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