Into the fire and the fog

Scientists gathered in anticipation after new data was received from dark matter detectors around the world. These detectors used xenon in a novel way to detect neutrinos in an effort to deepen their understanding of dark matter in the universe. However, the anticipation proved ill-fated. As it turns out, this method is naturally flawed by a formidable “fog” of neutrinos. 

Surprisingly, the “normal” matter of the universe — stars, planets, and dust — make up only a fraction of the total universe while the other part resides in a “dark” state, undetectable to us. Dark matter makes up nearly a quarter of the universe, and dark energy takes up about three-quarters of the total energy in the universe. So, almost all of what we know about space pertains to “normal” matter, and we are lacking crucial information about, frankly, a majority of what makes up the universe.

“Almost all of what we know about space pertains to ‘normal’ matter, and we are lacking crucial information about, frankly, a majority of what makes up the universe.”

There have been recent pushes in astronomy to search for the particles which would fill these mysterious holes in our understanding. It is believed that these particles may be identifiable in the form of WIMPs, weakly interacting massive particles. PandaX-4T at the China Jinping Underground Laboratory in Liangshan utilizes incredible amounts of liquid xenon, similar to other leading detectors of dark matter. These xenon baths rely on the principle of dark matter particles crashing into the nucleus of the xenon atom. The collisions will theoretically be observed by the detector, revealing the presence of dark matter particles.

In July of this year, two detectors, including PandaX-4T, announced observations of atomic collisions in their xenon detectors. This sounded like a cause for celebration, but, under further analysis, this data was attributed to neutrino collisions with the xenon atoms rather than dark matter presence. 

The particles in these collisions, neutrinos, are lightweight subatomic particles that are abundant in the universe, and are often created in particle decay. Earth’s steady supply comes from the nuclear fusion reactions inside the Sun. The Sun’s nuclear furnace is powered by these reactions,  which are powered by collisions of hydrogen atoms slamming together at extremely high speeds and pressures. The result is, unsurprisingly, a heavier element, but along with this, a huge amount of energy is released. In this vein, a highly radioactive isotope of boron is produced – boron-8. During its radioactive decay, boron-8 emits highly energetic neutrinos. These neutrinos are ejected by the sun in all directions, launching billions of neutrinos towards Earth every second. Due to their light weight and electrically neutral charge, neutrinos are unaffected by electromagnetic interactions and can largely pass through matter unimpeded and undetected. However, they carry enough energy with enough volume to be observed in dark matter detectors.

The dilemma the dark matter detectors now face is overcoming these neutrino false positives. Due to the nature of atomic-collision detection, there is no distinction between a neutrino collision from a dark matter interaction. This fog calls into question all instances of detection, potentially masking future attempts at understanding dark matter.  

“This fog calls into question all instances of detection, potentially masking future attempts at understanding dark matter. ”

However, scientists are still holding out hope. Alterations to methods of detection are in development in an effort to overcome this newfound obstacle. If detection was modified to account for the directionality of the particles, researchers could discard data showing particles that seem to be coming from the Sun. This could eliminate the detection of those confounding neutrinos that come from the Sun. 

Although neutrinos seem to be underhandedly polluting the search for dark matter, efforts for detection have, by no means, been a failure. Even though they are not quite the particles scientists intended to measure, the discovery will soon lead to an improvement in performance of the detectors’ ability to collect accurate data about WIMPs. In the meantime, astronomers are holding their heads high, sailing into the fog, and continuing their search for those precious indications of dark matter.