Dark Matter Detector Finds the Rarest Event Ever Seen in the Universe

Since the 1960s, scientists have theorized that the Universe is filled with a mysterious, invisible mass. Known as “dark matter“, this mass is estimated to make up roughly 85% of the matter in the Universe and a quarter of its energy density. While this mass has been indirectly observed and studied, all attempts at determining its true nature have so far failed.

To address this, multiple experiments are being carried out that rely on immensely sophisticated instruments. One of these, called XENON, recently observed a process that had previously avoided multiple attempts at detection. These results could help scientists to improve their understanding of neutrinos, which some scientists believe is what dark matter is made up of.

The results (XENON1T) appeared as part of a studythat was recently published in the journal Nature. XENON is a joint experimental project of about 160 scientists from Europe, the US and the Middle East. It is currently led by Prof. Elena Aprile from Columbia University and operated by the Gran Sasso National Laboratory (LNGS) in Italy.

Like other dark matter experiments, it aims to detect the candidate dark matter particles known as weakly interacting massive particles (WIMPS). For this purpose, the facility is located deep underground so as to avoid interference from other neutrino sources (which include the Solar neutrinos created regularly by our Sun and cosmic neutrinos).

In the case of the XENON experiment, this involves observing a chamber filled with liquid Xenon-124 for signs of particle interactions. These signs would provide the first-ever direct experimental evidence of dark matter candidate particles. And while their first set of results did not confirm the existence of dark matter, it did observe the decay of Xenon-124 atomic nuclei for the first time.

For a number of reasons, this was an immense accomplishment. Aside from being a historic first, the half-life measured for Xenon-124 is about one trillion times longer than the age of the Universe itself (13.8 billion years). This makes the radioactive decay they observed – the so-called double electron capture of Xenon-124 – the rarest process ever observed in a detector.

As Prof. Christian Weinheimer – a from the University of Münster, whose group led the study – explained in a XENON press release:

“The fact that we managed to observe this process directly demonstrates how powerful our detection method actually is – also for signals which are not from dark matter.”

To break this process down, an atom of Xenon-124 is made up of 54 protons and 70 neutrons which are surrounded by atomic shells with 54 electrons. In the process known as double electron capture, two protons in the nucleus simultaneously “catch” two electrons from the innermost shell, transform them into two neutrons, and spit out two neutrinos.

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