Where Is All The Antimatter? Experiment Hunting For Hypothesized Radioactive Decay Could Provide Answers

A team of scientists has embarked on an ambitious experiment that aims to answer what is truly the oldest question in the universe — why is there something rather than nothing?

On Wednesday, in a study published in the journal Nature, the researchers associated with the experiment, named GERmanium Detector Array (GERDA), announced that it was free from background noise.

Read: CERN Experiment Sheds Light On Antimatter

When the Big Bang occurred roughly 13.8 billion years ago, equal quantities of matter and antimatter were created — at least that’s what our current theoretical models tell us. The problem is, matter and antimatter particles annihilate each other when they come in contact. This means if our understanding of the universe is correct, it should not exist at all.

Obviously, as is attested by the fact that the universe exists, there is something fundamentally incomplete — if not outright wrong — in our theories that describe reality. Either significantly more matter was created by the Big Bang, or there is an as-of-yet-undiscovered asymmetry between matter particles and their antimatter counterparts.

It is the first possibility that the GERDA experiment, which consists of a giant particle detector located a mile below a mountain in central Italy, aims to examine. Are there any circumstances in which more matter can be produced than antimatter?

The answer, at least in theory, is yes. Physicists say that if a special kind of radioactive decay called “neutrinoless double-beta decay” is observed, it would prove two things — one, that in certain circumstances more matter is in fact created than antimatter; and two, neutrinos (which are virtually massless particles produced during radioactive decay) can behave as their own antiparticles.

“If neutrinoless double-beta decay is observed, it helps solve a couple of problems,” Philip Barbeau, an assistant professor of physics at Duke University who wrote a commentary on the Nature paper, told Live Science. “For one, it helps to explain the matter-antimatter asymmetry in the universe. It also helps explain why neutrino masses are so surprisingly small. We would also get an idea of the neutrino masses as well, as the decay rate is related to the mass scale of the neutrinos.”

So what exactly is neutrinoless double-beta decay?

Normally, when the atom of a radioactive particle decays, a neutron breaks down into a proton, an electron, and an antineutrino (or a neutrino). This process is called beta-decay. Double-beta decay is when beta-decay happens twice — two neutrons in an element like germanium decay simultaneously into two electrons and two antineutrinos (or neutrinos).

Scientists have long theorized that a “neutrinoless” version — wherein two antineutrinos (or neutrinos) annihilate each other — also exists. If it does, then such an event would produce two electrons, but no antimatter particles.

Simply put, scientists are looking for events in which neutrinos should be produced, but are not.

So far, in its seven-month-long run, the GERDA experiment — which consists of 78.5 pounds of germanium held in a vat of liquid argon — has failed to detect any signs of such decay. But, at the same time, it has placed a lower limit on how often such an event happens, and, more importantly, shown that the experiment is incredibly successful in eliminating background noise.

“No signal of neutrinoless double-β decay was found when Phase I and Phase II data were combined, and we deduce a lower-limit half-life of 5.3 × 10²⁵ years at the 90 per cent confidence level,” the authors of the study wrote in their paper. “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.”

For now, all we can do is wait and watch, as GERDA — and other experiments with a similar goal — hunt for this elusive particle decay.