The search for Nothing could explain why there’s anything at all

The existence of a type of nuclear decay in which neutrinos are absent — neutrinoless double beta decay — could suggest why there is anything at all in the Universe.

Bill Fairbank, a Colorado State University professor of physics, studies the fundamental matter particles known as neutrinos. More specifically he looks for an exceedingly rare instance of radioactive decay in which neutrinos — otherwise present in such decays — are absent.

Colorado State University physics professor Bill Fairbank with his lab’s single-atom imaging apparatus ( John Eisele/Colorado State University)

The discovery is of some significance, as it could lead to the overhaul of the standard model of particle physics.

This decay — ‘neutrinoless double-beta decay’ — has been theorised but never before observed. Thus its discovery would likely rock the world of particle physics. It could also potentially solve longstanding mysteries about the basic properties of neutrinos, possibly the most abundant but least understood particles in the universe.

In addition to this, it could answer the fundamental question of why matter dominates anti-matter in the universe.

Barium tagging — a new investigative tool.

Fairbank’s team — which has been part of the international EXO-200 (Enriched Xenon Observatory) scientific collaboration, hunting for neutrinoless double-beta decay using a particle detector filled with super-cold liquid xenon since 2005 — laid the foundation for a single-atom illumination strategy called barium tagging. Their achievement is the first known imaging of single atoms in a solid noble gas.

Fairbank explains why Barium tagging stands a better chance at spotting neutrinoless double beta-decay than previous methods: “In current leading experiments only the electrons are detected and have their energy measured. This means there are a few events that you can't distinguish from neutrinoless double beta decay.”

So far, the EXO-200 detector has produced decay events of the correct energy, but no definitive excess over what’s expected from the measured detector background.

Fairbank goes on to explain that these faux-events don’t produce a daughter barium atom, whilst neutrinoless double beta decay does.

Fairbank continues: “If we could detect this barium atom, we could say ‘yes, this is really is neutrinoless double beta decay’ and rule these other events out.”

The nEXO experiment could use barium tagging to do just this — boosting the detector’s sensitivity to neutrinoless double-beta decay by up to a factor of 4 — thus empowering scientists to clearly pinpoint single-atom byproducts of double-beta decay by separating real events from background imposter signals.

The EXO-200 particle detector — located half-a-mile underground in Carlsbad, New Mexico — is filled with 170 kilograms of isotopically enriched xenon atoms in liquid form. The unstable xenon isotopes undergo radioactive decay occasionally, releasing two electrons and two neutrinos which change the xenon atoms into barium atoms.

Fairbank describes the painstaking measures taken to ensure no components of the EXO-200 ‘contaminated’ their experiment: “Every wire, screw, nut and bolt had to be studied for the tiniest traces of uranium and thorium. That might produce backgrounds that we rarely see.”

If the decay produces just two electrons and a barium atom, it signals that a neutrinoless double-beta decay could well have occurred. This is significant because it can only occur if the neutrino is its own equal, opposite antiparticle.

Neutrinos — their own antiparticle?

The standard model of particle physics states that every particle has its own antiparticle. So electrons have positrons; the two share the same mass but equal and opposite charge.

For neutrinos, which carry no electromagnetic charge the picture is more complicated. They could be their own antiparticle. This is significant because if neutrinos are indeed their own antiparticles then they annihilate themselves — this leads to the violation of conservation of ‘lepton number’.

Fairbank says: “If these particles (neutrinos and antineutrinos) are the same, lepton number wouldn’t have a precise meaning anymore.”

If lepton numbers aren’t conserved — so the number of leptons before and after an interaction can be different — this could explain why equal amounts of matter and antimatter didn’t annihilate each other immediately after the big bang.

Fairbank points outs: “The breaking of lepton number conservation provides a mechanism to explain this early disparity between matter and antimatter.”

Thus the confirmation of such a neutrinoless decay would cause something of a paradigm shift in physics — requiring updates to the Standard Model of Particle Physics. Something of a paradigm shift in physics.

Fairbanks explains: “Lepton number conservation has been a verified principle for some time and we haven’t thus far found experimental evidence against it. But if we were to discover neutrinos and antineutrinos are the same it changes our thinking.”

In addition to this, the measured half-life of the decay would help scientists indirectly measure the absolute masses of neutrinos — a feat never before accomplished.

The barium tagging work was supported by the National Science Foundation INSPIRE program.

John Gillaspy, a physicist at the National Science Foundation, says about the method: “It’s amazing to think of how sensitive these experiments are.

“In experiments 30 years ago, I found it challenging to look for ‘one in a million’ exotic atoms. This new study searched for atoms that were 10 million times rarer. Physics and chemistry have come a long way. I’m excited to think about what Fairbank and his colleagues may eventually find using this new technique, as it holds the potential to really shake up what we know about the fundamental nature of reality.”

In their study — to be published in the journal Nature — Fairbank’s team describes using a cryogenic probe to freeze the barium “daughter” atom — produced by radioactive decay of the isotope xenon-136 — in solid xenon on the end of the probe. Then, they use laser fluorescence to illuminate individual barium atoms within the now-solid xenon.

Fairbank comments: “Our group was pretty excited when we got images of single barium atoms.”

The future for the standard model and the ‘tagging method’

In the lab at Colorado State University: Alec Iverson, James Todd, David Fairbank, Chris Chambers and Bill Fairbank

When asked what future avenues of research the concept of neutrinos and antineutrinos being the same particle opened open, Fairbank gives an answer that reflects just how groundbreaking this discovery would be.

He says: “That’s a good question. I’m not sure there’s a clear answer. The immediate impact on other experiments isn’t clear as this is the only way we know how to investigate this.

“It does open the possibility of investigating the reason that neutrinos have such small masses via a mechanism called the ‘seesaw mechanism.”

This is the idea that ‘left-handed’ neutrinos, which have a low-mass, have a hidden, hitherto undiscovered high mass right-handed partner.

Fairbank also sees an immediate application for the tagging method in astrophysics — using magnesium rather than barium. Such research is currently underway at Michigan State University.

Fairbank’s single-atom tagging technique could also be generalized for other applications, with implications for fields including nuclear physics, optical physics and chemistry.

Fairbank is clear that this was very much a team endeavour and that without his co-workers in the lab at Colorado State University — Alec Iverson, James Todd, David Fairbank and Chris Chambers — the development of barium tagging and the knowledge it opens up simply wouldn’t have been possible.