The tiniest measurement of the mass of a neutrino has just been made using decaying hydrogen isotopes.
Physicists
have found that the upper limit for the mass of the electron antineutrino is
just 0.8 electronvolts by analysing the energy distribution of electrons
emitted during tritium beta decay. In metric units, that's 1.6x10–36 kg, and in
imperial units, that's very, very small.
Although
we don't have a precise measurement yet, narrowing it down brings us closer to
understanding these unusual particles, their role in the Universe, and the
influence they may have on current physics theories. The breakthrough was made
at Germany's Karlsruhe Tritium Neutrino Experiment (KATRIN).
Theresearchers noted in their study, "The second neutrino-mass measurement
campaign of KATRIN, published here, reached sub-electronvolt sensitivity."
"We
enhanced the upper limit to mv0.8 electronvolts by combining the first and
second campaigns. As a result, the allowable range of quasi-degenerate
neutrino-mass models has been limited, and model-independent neutrino mass
information has been provided, allowing non-standard cosmological models to be
tested."
Neutrinos
are strange particles. They are among the most common subatomic particles in
the Universe, resembling electrons but lacking a charge and being nearly
massless. This means they only contact with ordinary stuff on a very rare
basis; in reality, billions are currently flowing through your body.
This is
why they're known as ghost particles. They're also extremely tough to detect.
We do have some detection technologies, such as Cherenkov neutrino detectors,
however they are indirect and catch the effects of passing neutrinos rather
than the neutrinos themselves.
As a
result, determining the near-zero mass of these particles is a very difficult
task. However, if we can measure this property, we will be able to discover a
lot more about the Universe. Unfortunately, it's also extremely difficult to
achieve. You can't just take a minuscule scale and slap a neutrino on top of it
and call it a day.
KATRIN
uses the beta decay of tritium, an unstable radioactive isotope of hydrogen, to
determine the mass of neutrinos. Tritium gas decays into helium, an electron,
and an electron antineutrino within the 70-meter (230-foot) chamber, while the
findings are monitored by a large, sensitive spectrometer.
It's
impossible to quantify neutrinos since they're so ethereal. However, physicists
are very certain that a particle and its antiparticle have evenly distributed
mass and energy, therefore measuring the energy of the electrons will yield the
energy of the neutrino.
This is
how, in 2019, the team came up with the upper limit of 1 electronvolt for the
mass of the neutrino. The researchers refined their upper limit by combining an
increase in the number of tritium decays with efforts to prevent contamination
from other types of radioactive decay.
PhysicistsMagnus Schlösser of the Karlsruhe Institute of Technology and Susanne Mertensof the Max Planck Institute for Physics in Germany remarked, "This
painstaking and complicated study was the only way to avoid a systematic bias
of our result due to distorting processes."
"We
are especially pleased of our analysis team, which took on this enormous
challenge with zeal and succeeded."
This is the
first time a neutrino's readings have dipped below the 1 electronvolt
threshold. It's a critical result that will allow scientists to refine physical
models of the Universe, even if the mass isn't exact.
Meanwhile,
the cooperation will continue to refine measurements of the neutrino's mass.
"Further
neutrino mass measurements will be conducted until the end of 2024," theresearchers said. "To realise the full potential of this one-of-a-kind
experiment, we'll gradually increase the number of signal occurrences and
create and deploy improvements to further minimise the background rate."
Reference:
- The KATRIN Collaboration. Direct neutrino-mass measurement with sub-electronvolt sensitivity. Nat. Phys. 18, 160–166 (2022). https://doi.org/10.1038/s41567-021-01463-1
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