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We know what you're thinking: Where the hell is all the antimatter?

Discovery could be key to cracking universe's baffling particle drought puzzle

The mystery behind why there is an imbalance between matter and antimatter in the universe could be one step closer to being solved.

First, let's kick off with some gentle particle physics to set the scene. Scientists are hunting for something called neutrinoless double beta decay: in normal double beta decay, two neutrons convert to protons and two electrons and two electron antineutrinos are emitted. In neutrinoless double beta decay, just the two electrons are emitted. This neutrinoless decay has yet to be observed, and there are ongoing efforts to catch a glimpse of it.

Recently, physicists have successfully increased the sensitivity of the GERmanium Detector Array (GERDA) so that attempts to detect this elusive neutrinoless reaction are free from background noise. That ought to make it easier to spot what exactly happens in a neutrino-free breakdown. For example, it is possible that the neutrinos are Majorana particles, meaning the antineutrinos and neutrinos are the same particles.

“The potential of an essentially background-free search for neutrinoless double-β decay will facilitate a larger germanium experiment with sensitivity levels that will bring us closer to clarifying whether neutrinos are their own antiparticles,” the researchers wrote in a paper published in Nature this week (or for free on arXiv.)

A standard double-beta decay has been observed for several isotopes with unstable nuclei. But the neutrinoless version has avoided our detectors: it’s no wonder, considering the half-life of such a reaction is roughly at least 5.3 × 1025 years – a whopping 15 orders of magnitude longer than the age of the universe. In other words, we'll be potentially waiting a long time to see it happen.

To have any chance of spotting such a reaction, the GERDA experiment must be kept spotless – free from any other disruptions caused by cosmic rays or any other reactions happening outside detectors.

Buried deep beneath thousands of metres of rock near the Gran Sasso mountain in Italy, the Laboratori Nazionali del Gran Sasso houses a large cylindrical water tank encased with a vat of liquid argon kept at -185°C (-301°F).

A layer of sensitive, pure semiconductors containing the isotope germanium-76 sits inside. Germanium-76 is slightly radioactive, and is both the source and detector for the neutrinoless double beta decay.

Should the desired reaction occur, two neutrons from a germanium nucleus will convert into two protons. Two electrons will be ejected but no antineutrinos, producing a positive charge inside the detector.

The idea is that if the neutrino particle is its own antiparticle – an antineutrino – they would annihilate each other, leaving only a tiny smidgen of energy behind.

But gamma rays outside the detectors can scatter off the germanium, also producing a positive charge. Researchers from the GERDA Collaboration have designed the detectors in a way that maximizes the time it takes for this charge to reach the attached electrodes. Since the gamma rays can bounce around the tank several times, the detection of the radiation's effects will be spread out over time, allowing the scientists to pick out these events and reject them.

They call the experiment a “background free” search. Other similar experiments looking for the disappearance of antineutrinos, such as the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND), Japan and the Enriched Xenon Observatory, United States, still suffer from slight noise from background reactions.

Scientists found no trace of the neutrinoless double-beta reaction during the first phase of the GERDA experiment, which ran from November 2011 to May 2013. Nothing interesting popped up in the second phase of the experiment, which began in 2015, either.

But scientists are hopeful, clutching onto the idea that it could explain the asymmetry between matter and antimatter in our universe.

Along with protons, neutrinos are the most abundant type of matter particle found scattered in the far-flung reaches of our cosmos. After the Big Bang, theories predict that the proportion of matter and antimatter produced should be equal, but there is almost no antimatter left today.

Leptogenesis describes a series of hypothetical reactions in which more leptons – a class of particle including electrons and neutrinos – are created than anti-leptons. But this violates the lepton number, an essential property – like energy or charge – that must be conserved in nuclear reactions. If neutrinos and antineutrinos were the same particle, this violation wouldn’t occur, making reactions like the neutrinoless double-beta reaction possible.

And that may explain why we have oodles of matter all around us and no sign of any antimatter. ®

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