Rare glimpse of ‘ghostly’ neutrino fog detected by dark matter experiments in a first

November 11

Do you know which are the most abundant particles in the universe? It is neutrinos — small, chargeless, and nearly massless subatomic particles that either don’t interact with matter at all or interact very weakly.

They are so lightweight that even an electron is six million times heavier than a neutrino. Also, since they barely interact with matter, they have the uncanny ability to pass through any object without being detected. This is why they’re also called ghost particles.

However, for the first time, two dark matter experiments have detected a neutrino fog, a dense cloud of neutrinos. This discovery is reported by researchers from XENON and PandaX — two scientific experiments that aim to detect dark matter, operating independently in Italy and China respectively.

“This is the first measurement of astrophysical neutrinos with a dark matter experiment,” Fei Gao, a scientist involved in the Xenon experiment, said.

Spotting a cloud of neutrinos

Neutrinos are typically detected through coherent elastic neutrino-nucleus scattering (CEvNS), a process in which neutrinos interact with the entire nucleus rather than just a proton or electron.

This wholesome interaction distributes the neutrino’s energy evenly across the nucleus, causing it to recoil. Scientists then measure the energy from the CEvNS process to detect the presence of neutrinos.

Generally, CEvNS detection involves the use of particle accelerators. However, a much weaker CEνNS signal from solar neutrinos, rather than those produced by accelerators, is observed by both the PandaX and XENON experiments.

Both these dark matter experiments use liquid xenon detectors to look for dark matter particles or neutrinos by studying how these particles interact with xenon atoms within the liquid. This allows them to potentially detect even faint signals from these elusive particles.

During their study, the researchers went through two years of data from the experiments. Their analysis revealed CEvNS signals from the radioactive beta decay of Boron-8 in the Sun’s core.

While the XENON team reported 11 CEvNS signals, PandaX identified 75 signals. However, the statistical confidence (reliability of the results) for both experiments was almost similar i.e. 2.64 sigma (for PandaX) and 2.73 sigma (for XENON).

“I think that most people, including myself, are pretty confident that both collaborations have measured the neutrino fog,” Kate Scholberg, a professor of physics at Duke University, said.

This makes the search for dark matter more challenging

While neutrino fog detection via dark matter experiments is fascinating, the presence of dense neutrino clouds around the dark matter makes our quest for the mysterious and elusive dark matter more difficult.

This is because neutrinos are themselves hardly detectable. Their abundant presence in the universe can create background noise that can derail the attempts to detect dark matter.

For instance, in experiments designed to find dark matter, such as those looking for rare interactions with nuclei, the neutrinos in the fog can produce signals that resemble those from dark matter interactions, making it harder to distinguish between the two.

However, “The perceived ‘existential threat’ posed by the neutrino fog is likely overstated. There is still a lot to be done before this background prevents us from making further progress,” Elisabetta Barberio, who wasn’t involved in any of the experiments but is an expert in dark matter particle physics at the University of Melbourne, said.

The PandaX and XENON studies are published in the journal Physical Review Letters.

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