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(Nanowerk NewsDark matter so far has defied every type of detector designed to find it. Due to its huge gravitational footprint in space, we know that dark matter must account for about 85 percent of the total mass of the universe, but we still don’t know what it’s made of.
Several large dark matter-seeking experiments have looked for signs of dark matter particles hitting atomic nuclei through a process known as scattering, which can produce small flashes of light and other signals in these interactions.
Now, a new study, led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley, suggests new ways to pick up the signals from dark matter particles that absorb their energy from these nuclei.
The absorption process could kick off the affected atom causing it to eject a lighter energized particle, such as an electron, and can also produce other types of signals, depending on the nature of the dark matter particle.
The study focuses mainly on those cases in which an electron or neutrino is ejected when the dark matter particle hits the nucleus of an atom.
Published in Physical Review Letters (“Direct detection of fermionic dark matter absorption signals”), the study proposes that some existing experiments, including those looking for dark matter particles and processes related to neutrinos, ghostly and detectable particles that can pass through most matter and have the ability to switch to different shapes – can easily be expanded to also search for these types of telltale dark matter signals related to absorption.
Furthermore, the researchers propose that new searches of previously collected particle detector data could possibly generate these overlooked dark matter signals.
“In this field, we have had a certain idea in mind about well-motivated dark matter candidates, such as WIMP,” or weakly interacting massive particles, said Jeff Dror, lead author of the study, a postdoctoral researcher. at the Berkeley Lab Theory Group and UC Berkeley Berkeley Center for Theoretical Physics.
Dark matter pushes the boundaries of the known fundamental laws of physics, encapsulated in the Standard Model of Particle Physics, and “The WIMP paradigm is very easy to incorporate into the Standard Model, but we haven’t found it in a long time,” Dror noticed.
Therefore, physicists are now considering other places where dark matter particles may be hidden, and other possibilities for particles like theorized “sterile neutrinos” that could also be incorporated into the family of particles known as fermions, which includes electrons, protons and neutrinos.
“It is easy, with minor modifications to the WIMP paradigm, to accommodate a completely different type of signal,” said Dror. “You can make tremendous progress at very little cost if you go back a bit in the way we’ve been thinking about dark matter.”
Robert McGehee, a graduate student at UC Berkeley, and Gilly Elor of the University of Washington were co-authors of the study.
The range of new signals they are focusing on opens up an “ocean” of possibilities for dark matter particles, the researchers said, namely undiscovered fermions with masses lighter than the typical range considered for WIMP. They could be close cousins of sterile neutrinos, for example.
The study team considered the absorption processes known as “neutral current”, in which the nuclei in the detector material recoil or are shaken by their collision with dark matter particles, producing different energy signatures that the detector can pick up; and also those known as “charged current,” which can produce multiple signals when a dark matter particle hits a nucleus, causing a recoil and ejection of an electron.
The charging current process may also involve nuclear decay, in which other particles are ejected from a nucleus as a kind of domino effect triggered by the absorption of dark matter.
Searching for signatures suggested by the study of neutral current and charge current processes could open up “orders of magnitude of unexplored parameter space,” the researchers note. They focus on the energy signals in the MeV, which means millions of electron volts. An electron volt is a measure of energy that physicists use to describe the masses of particles. Meanwhile, typical WIMP searches are now sensitive to interactions of particles with energies in the keV range, or thousands of electron volts.
For the various particle interactions that the researchers explored in the study, “you can predict what the energy spectrum of the particle coming out or the nucleon getting the ‘kick,'” Dror said. Nucleon refers to the positively charged proton or the uncharged neutron that resides in the nucleus of an atom and that could absorb energy when hit by a dark matter particle. He added that these absorption signals could be more common than the other types of signals that dark matter detectors are designed to find, we just don’t know yet.
Experiments that have large volumes of detector material, with high sensitivity and very low background “noise”, or unwanted interference from other types of particle signals, are particularly well suited for this expanded search for different types of dark matter signals, Dror said.
LUX-ZEPLIN (LZ), for example, an ultrasensitive dark matter search project led by Berkeley Lab under construction at a former South Dakota mine, is a potential candidate, as it will use approximately 10 metric tons of liquid xenon as a medium. detector and is designed to be strongly protected from other types of particle noise.
The team of researchers who participated in the study has already worked with the team operating the Xenon Rich Observatory (EXO), an underground experiment looking for a theorized process known as neutrino-free beta double decay using liquid xenon, to open their search to these other types of dark matter signals.
And for the similar types of experiments that are up and running, “the data is basically there already. It’s just a matter of looking at it,” Dror said.
The researchers name a long list of candidate experiments around the world that could have relevant data and search capabilities that could be used to find their target signals, including: CUORE, predecessor LZ LUX, PandaX-II, XENON1T, KamLAND-Zen, SuperKamiokande, CDMS-II, DarkSide-50 and Borexino among them.
As a next step, the research team hopes to work with experimental collaborations to analyze existing data and discover whether the search parameters of active experiments can be adjusted to search for other signals.
“I think the community is beginning to realize this,” Dror said, adding: “One of the most important questions in the field is the nature of dark matter. We don’t know what it’s made of, but answering these questions could be within our grasp in the near future. For me, that’s a great motivation to keep pushing: there’s a new physics out there. ”
