Sunday, February 5, 2023

3861 - NEUTRINO SCIENCE - what is its mass?

 

     -  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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