An international team of researchers with key participation from the
PRISMA+Cluster of Excellence at Johannes Gutenberg University Mainz (JGU)
and the Helmholtz Institute Mainz (HIM) has published for the first time
comprehensive data on the search for dark matter using a worldwide network
of optical magnetometers. According to the scientists, dark matter fields
should produce a characteristic signal pattern that can be detected by
correlated measurements at multiple stations of the GNOME network. Analysis
of data from a one-month continuous GNOME operation has not yet yielded a
corresponding indication. However, the measurement allows the formulation of
constraints on the characteristics of dark matter, as the researchers report
in the journal Nature Physics.
GNOME stands for Global Network of Optical Magnetometers for Exotic Physics
Searches. Behind it are magnetometers distributed around the world in
Germany, Serbia, Poland, Israel, South Korea, China, Australia, and the
United States. With GNOME, the researchers particularly want to advance the
search for dark matter—one of the most exciting challenges of fundamental
physics in the 21st century. After all, it has long been known that many
puzzling astronomical observations, such as the rotation speed of stars in
galaxies or the spectrum of the cosmic background radiation, can best be
explained by dark matter.
"Extremely light bosonic particles are considered one of the most promising
candidates for dark matter today. These include so-called axion-like
particles—ALPs for short," said ProfessorDr. Dmitry Budker, professor at
PRISMA+and at HIM, an institutional collaboration of Johannes Gutenberg
University Mainz and the GSI Helmholtzzentrum für Schwerionenforschung in
Darmstadt. "They can also be considered as a classical field oscillating
with a certain frequency. A peculiarity of such bosonic fields is
that—according to a possible theoretical scenario—they can form patterns and
structures. As a result, the density of dark matter could be concentrated in
many different regions—discrete domain walls smaller than a galaxy but much
larger than Earth could form, for example."
"If such a wall encounters the Earth, it is gradually detected by the GNOME
network and can cause transient characteristic signal patterns in the
magnetometers," explained Dr. Arne Wickenbrock, one of the study's
co-authors. "Even more, the signals are correlated with each other in
certain ways—depending on how fast the wall is moving and when it reaches
each location."
The network meanwhile consists of 14 magnetometers distributed over eight
countries worldwide. Nine of them provided data for the current analysis.
The measurement principle is based on an interaction of dark matter with the
nuclear spins of the atoms in the magnetometer. The atoms are excited with a
laser at a specific frequency, orienting the nuclear spins in one direction.
A potential dark matter field can disturb this direction, which is
measurable.
Figuratively speaking, one can imagine that the atoms in the magnetometer
initially dance around in confusion, as clarified byHector Masia-Roig, a
doctoral student in the Budker group and also an author of the current
study. "When they 'hear' the right frequency of laser light, they all spin
together. Dark matter particles can throw the dancing atoms out of balance.
We can measure this perturbation very precisely." Now the network of
magnetometers becomes important: When the Earth moves through a spatially
limited wall of dark matter, the dancing atoms in all stations are gradually
disturbed.One of these stations is located in a laboratory at the Helmholtz
Institute in Mainz. "Only when we match the signals from all the stations
can we assess what triggered the disturbance,"said Masia-Roig. "Applied to
the image of the dancing atoms, this means: If we compare the measurement
results from all the stations, we can decide whether it was just one brave
dancer dancing out of line or actually a global dark matter disturbance."
In the current study, the research team analyzes data from a one-month
continuous operation of GNOME. The result: Statistically significant signals
did not appear in the investigated mass range from one femtoelectronvolt
(feV) to 100,000 feV. Conversely, this means that the researchers can narrow
down the range in which such signals could theoretically be found even
further than before. For scenarios that rely on discrete dark matter walls,
this is an important result—"even though we have not yet been able to detect
such a domain wall with our global ring search," added Joseph Smiga, another
Ph.D. student in Mainz and author of the study.
Future work of the GNOME collaboration will focus on improving both the
magnetometers themselves and the data analysis. In particular, continuous
operation should be even more stable. This is important to reliably search
for signals that last longer than an hour. In addition, the previous alkali
atoms in the magnetometers are to be replaced by noble gasses. Under the
title Advanced GNOME, the researchers expect this to result in considerably
better sensitivity for future measurements in the search for ALPs and dark
matter.
Reference:
Samer Afach et al, Search for topological defect dark matter with a global
network of optical magnetometers, Nature Physics (2021).
DOI: 10.1038/s41567-021-01393-y
Tags:
Physics