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3861 - NEUTRINO
SCIENCE - what is its mass? Neutrinos are the only fundamental particles
whose mass we still don’t know. As their name implies, neutrinos are very, very
small. But they outnumber the other fundamental particles by a factor of 10
billion.
------------- 3861 - NEUTRINO SCIENCE - what is its mass?
- Neutrinos collective abundance makes it
likely that they influenced the formation of structures in the early universe,
so knowing their mass is critical to closing gaps in our understanding of how
we ever got here.
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- But how do you measure something with a mass
so small it approaches zero. In 1987, astrophysicists interested in the mass of
neutrinos got an assist from a rare nearby supernova. The spectral data they
collected from the stellar explosion helped them make an indirect measurement
that gave them an upper limit on the neutrino mass.
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- Cosmologists have also made an indirect
measurement by looking for the imprint of neutrino mass on faint radiation in
space called the “cosmic microwave background”.
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- What we would ideally like is an
Earth-based measurement of the neutrino mass, and then we can compare that
against the cosmological measurement. Take the direct approach to searching for
the neutrino mass. The ghostly particles don’t interact with electromagnetic
fields or the nuclear strong force, and they’re so light that gravity barely
pulls on them.
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- So how does one go about building a
bathroom scale for neutrinos? In a
process called “beta decay”, a neutron in an unstable nucleus transforms into a
proton to restore balance. As the neutron becomes a proton, it emits a
negatively charged electron and a neutrino.
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- Another process that produces a neutrino is
electron capture: A proton in an unstable nucleus captures an electron from the
inner shell, converts to a neutron, and ejects a neutrino.
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- In both cases the events produce a very
specific amount of energy. That exact amount of energy is the difference
between the mass of the parent atom and the mass of the daughter atom. And that
energy is shared between the products: the neutrino and the electron in beta
decay, or the neutrino and the excited daughter atom in electron capture.
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- Experimentalists measure energies, hence
they can determine the energy taken by the neutrino. Then they take advantage
of that old reliable equation E = mc^2 and convert the neutrino’s energy to
mass.
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- Neutrinos are the only matter particles for
which the Standard Model made a prediction for what its mass would be. And that
prediction was wrong. Other examples of things that are not in the Standard
Model. Gravity is not in the Standard Model. The mass of quarks is not
predicted.
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- The upper limit for the neutrino mass, as
determined indirectly by cosmology, is roughly one millionth the mass of the
next lightest particle, the electron. That’s like the gap between one mouse,
which weighs roughly 25 grams, and five elephants, which together weigh roughly
25,000 kilograms.
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- In 1972 a Swedish physicist named Karl-Erik
Bergkvist had declared a new neutrino mass limit: 55 electronvolts. A new idea was to study beta decay using
tritium, a radioactive isotope of hydrogen.
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- The Sudbury Neutrino Observatory in 2001
demonstrated neutrino oscillation, a finding that proved neutrinos have mass
and that eventually earned the 2015 Nobel Prize in Physics.
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- Nestled in Karlsruhe, Germany, the KATRIN
experiment relies on contributions from 150 researchers from seven
countries. The KATRIN collaboration
published an exciting result in February 2022: Neutrinos must weigh less than
0.8 electronvolts. Oscillation experiments provided a floor, and the KATRIN
result provides a ceiling.
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- The supernova limit is about 5.7
electronvolts, about seven times looser than the current KATRIN limit. And, it’s close to the limit from
cosmological indirect measurements, which is somewhere in the range 0.12 to 0.5
electronvolts, depending which parameters are used in the model.
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- In 2009 the Project 8 experiment uses
tritium decay, but Project 8 determines the energy of the emitted electron
differently. They measure the frequency of the electron’s cyclotron radiation,
the microwave radiation that escapes from charged particles in circular orbit
in a magnetic field.
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- The Project 8 team hopes to measure the
neutrino mass with a sensitivity of approximately 0.04 electronvolts.
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- The third technique currently under
investigation as a way to directly measure neutrino mass uses molecules of the
holmium isotope 163Ho. The ECHo
experiment relies on electron capture events in 163Ho.
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- Scientists implant 163Ho ions in
microcalorimeters, a totally different type of detector than the ones used in KATRIN
and Project 8. The idea is that if
energy is deposited into the detector, there is an increase of temperature …
and we can measure this extremely small increase in temperature with very
precise thermometers.
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- Hundreds of other neutrino experts have
dedicated their careers to finding the mass of the neutrino. The group repeated the conference in 2018,
2020 and 2022. So many different fields
of knowledge that turn out to be necessary in order to unlock the secrets of
this incredibly lightweight, incredibly rarely interacting particle that
somehow shaped the universe.
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February 5, 2023 NEUTRINO SCIENCE
- what is its mass? 3861
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--------------------- --- Sunday, February 5, 2023 ---------------------------
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