The international KArlsruhe TRItium Neutrino Experiment (KATRIN), located at
Karlsruhe Institute of Technology (KIT), has broken an important barrier in
neutrino physics that is relevant for both particle physics and cosmology.
Based on data published in the journal Nature Physics, a new upper limit of
0.8 electronvolt (eV) for the mass of the neutrino has been obtained. This
first push into the sub-eV mass scale of neutrinos by a model-independent
laboratory method allows KATRIN to constrain the mass of these "lightweights
of the universe" with unprecedented precision.
Neutrinos are arguably the most fascinating elementary particle in our
universe. In cosmology they play an important role in the formation of
large-scale structures, while in particle physics their tiny but non-zero
mass sets them apart, pointing to new physics phenomena beyond our current
theories. Without a measurement of the mass scale of neutrinos our
understanding of the universe will remain incomplete.
This is the challenge the international KATRIN experiment at Karlsruhe
Institute of Technology (KIT) with partners from six countries has taken up
as the world's most sensitive scale for neutrinos. It makes use of the beta
decay of tritium, an unstable hydrogen isotope, to determine the mass of the
neutrino via the energy distribution of electrons released in the decay
process. This necessitates a major technological effort: the 70 meter long
experiment houses the world's most intense tritium source as well as a giant
spectrometer to measure the energy of decay electrons with unprecedented
precision.
The high quality of the data after starting scientific measurements in 2019
has continuously been improved over the last two years. "KATRIN is an
experiment with the highest technological requirements and is now running
like perfect clockwork" enthuses Guido Drexlin (KIT), the project leader and
one of the two co-spokespersons of the experiment. Christian Weinheimer
(University of Münster), the other co-spokesperson, adds that "the increase
of the signal rate and the reduction of background rate were decisive for
the new result."
Data analysis
The in-depth analysis of this data was demanding everything from the
international analysis team led by its two coordinators, Susanne Mertens
(Max Planck Institute for Physics and TU Munich) and Magnus Schlösser (KIT).
Each and every effect, no matter how small, had to be investigated in
detail. "Only by this laborious and intricate method we were able to exclude
a systematic bias of our result due to distorting processes. We are
particularly proud of our analysis team, which successfully took up this
huge challenge with great commitment," say the two analysis coordinators.
The experimental data from the first year of measurements and the modeling
based on a vanishingly small neutrino mass match perfectly: from this, a new
upper limit on the neutrino mass of 0.8 eV can be determined (Nature
Physics, July 2021). This is the first time that a direct neutrino mass
experiment has entered the cosmologically and particle-physically important
sub-eV mass range, where the fundamental mass scale of neutrinos is
suspected to be. "The particle physics community is excited that the
1-eV-barrier has been broken by KATRIN," says neutrino expert John Wilkerson
(University of North Carolina, chair of the executive board).
Susanne Mertens explains the path to the new record: "Our team at the MPP in
Munich has developed a new analysis method for KATRIN that is specially
optimized for the requirements of this high-precision measurement. This
strategy has been successfully used for past and current results. My group
is highly motivated: We will continue to meet the future challenges of
KATRIN analysis with new creative ideas and meticulous accuracy."
Further measurements should improve sensitivity
The co-spokespersons and analysis coordinators of KATRIN are very optimistic
about the future: "Further measurements of the neutrino mass will continue
until the end of 2024. To realise the full potential of this unique
experiment, we will not only steadily increase the statistics of signal
events, we are continuously developing and installing improvements to
further lower the background rate."
The development of a new detector system (TRISTAN) plays a specific role in
this, allowing KATRIN from 2025 on to embark on a search for "sterile"
neutrinos with masses in the kiloelectronvolt-range, a candidate for the
mysterious dark matter in the cosmos that has already manifested itself in
many astrophysical and cosmological observations, but whose
particle-physical nature is still unknown.
The research was published in Nature Physics.
Reference:
Magnus Schlösser, Direct neutrino-mass measurement with sub-electronvolt
sensitivity, Nature Physics (2022).
DOI: 10.1038/s41567-021-01463-1.
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Physics