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Top Comments: Experiment in China May Determine Neutrino Mass Ordering

Feb 13, 2024

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Neutrinos are ghostly particle that have no charge, at most very little mass, and travel at speed close to that of light. They only interact with other particles through the weak nuclear force, which can change a neutron to a proton, ejecting an electron, or (for an antineutron) change a proton into a neutron, ejecting a positron (antielectron). Because the weak force is so weak, such events happen rarely, so observing these particles is quite difficult. Trillions of neutrinos are passing through your body every second, but you’d never know it because so few actually interact with the nuclei in your body.

There are three known types of neutrinos, each linked to electron-like particles. The electron has the electron neutrino; the heavier muon has the muon neutrino; and much heavier tau particle has the tau neutrino. One discovery regarding these various neutrinos is that each neutrino interconverts to the other types while it propagates through space. The fact that neutrinos are not static as they move means that they are not moving at the speed of light (but at a speed slightly slower than that), which in turn means that they have mass. A natural question to ask, then, is what are the various masses of the known neutrinos? And if it isn’t possible to actually measure these masses, is it at least possible to determine what the ordering of the masses is?

An experiment due to go online shortly in China hopes to address this question, if not answer it outright. The experiment consists of an acrylic sphere 35 meters in diameter, to be filled with an organic solution that emits a flash of light (scintillates) when a nuclear event takes place. An array of 43,000 photomultiplier tubes will surround the sphere to detect such events. There are 8 nuclear reactors 53 kilometers away from this sphere that produce a steady stream of electron antineutrinos. Scientists will be measuring how many of these electron antineutrinos have not converted to one of the other types on their journey from the reactor to the detector. (The detector is blind to muon and tau neutrinos.)

Here’s where things get a little bit confusing. Theorists have determined that there are three mass states of the neutrino, which they call m1, m2 and m3. It is natural to immediately assume that each of these three masses corresponds to the mass of one of the three types of neutrino, but in fact, the mass states to not correlate to the actual neutrino types in a one-to-one fashion. Each of the neutrino types is a quantum mechanical superposition of the three mass states, in fact. It has already been determined that m2 is larger than m1. The question that remains is whether m3 is heavier than m2 (what’s called “normal ordering”) or lighter than m1 (“inverted ordering”).

When a non-converted electron antineutrino makes it to the detector and interacts with a proton, it will convert the proton to a neutron, and release an energetic positron. That positron will produce a flash seen by the detector. But also, the neutron product will eventually be absorbed by a nucleus, which will produce a second flash about 200 milliseconds later. This sequence of two flashes (combined with being far underground and thus shielded by the Earth itself) will allow distinguishing an event induced by an antineutrino from the reactors from an even produced by a random cosmic ray. The pattern of how many electron antineutrinos survive their journey will hold the signature for whether the mass spectrum ordering is normal or inverted.

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Highlighted by J Graham:

This comment by FishOutofWater in Egberto Willies recommended post regarding kos’ remarks about No Labels on Meet the Press.

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