Science

Physicists Could Have to Wait 66,000 Yottayears to See an Electron Decay

Inside the Borexino neutrino detector, during construction, in 2001. The photomultiplier tubes are visible. Credit: Borexino

Inside the Borexino neutrino detector, during construction, in 2001. The photomultiplier tubes are visible. Credit: Borexino

The longest coherently described span of time I’ve encountered is from Hindu cosmology. It concerns the age of Brahma, one of Hinduism’s principal deities, who is described as being 51 years old (with 49 more to go). But these are no simple years. Each day in Brahma’s life lasts for a period called the kalpa: 4.32 billion Earth-years. In 51 years, he will actually have lived for almost 80 trillion Earth-years. In a 100, he will have lived 157 trillion Earth-years.

157,000,000,000,000. That’s stupidly huge.

On December 3, scientists announced that we’ve all known something that will live for even longer: the electron.

Yup, the same tiny lepton that zips around inside atoms with gay abandon, that’s swimming through the power lines in your home, has been found to be stable for at least 66,000 yottayears – yotta- being the largest available prefix in the decimal system.

In stupidly huge terms, that’s 66,000,000,000,000,000,000,000,000,000 (66,000 trillion trillion) years. Brahma just slipped to second place among the mortals.

But why were scientists making this measurement in the first place?

Because they’re desperately trying to disprove a prevailing theory in physics. Called the Standard Model, it describes how fundamental particles interact with each other. Though it was meticulously studied and built over a period of more than 30 years to explain a variety of phenomena, the Standard Model hasn’t been able to answer few of the more important questions. For example, why is gravity among the four fundamental forces so much weaker than the rest? Or why is there more matter than antimatter in the universe? Or why does the Higgs boson not weigh more than it does? Or what is dark matter?

Silence.

The electron belongs to a class of particles called leptons, which in turn is well described by the Standard Model. So if physicists are able to find that an electron is less stable the model predicts, it’d be a breakthrough. But despite multiple attempts to find an equally freak event, physicists haven’t succeeded – not even with the LHC (though hopeful rumours are doing the rounds that that could change soon).

The measurement of 66,000 yottayears was published in the journal Physical Review Letters on December 3 (a preprint copy is available on the arXiv server dated November 11). It was made at the Borexino neutrino experiment buried under the Gran Sasso mountain in Italy. The value itself is hinged on a simple idea: the conservation of charge.

If an electron becomes unstable and has to break down, it’ll break down into a photon and a neutrino. There are almost no other options because the electron is the lightest charged particle and whatever it breaks down into has to be even lighter. However, neither the photon nor the neutrino has an electric charge so the breaking-down would violate a fundamental law of nature – and definitely overturn the Standard Model.

The Borexino experiment is actually a solar neutrino detector, using 300 tonnes of a petroleum-based liquid to detect and study neutrinos streaming in from the Sun. When a neutrino strikes the liquid, it knocks out an electron in a tiny flash of energy. Some 2,210 photomultiplier tubes surrounding the tank amplify this flash for examination. The energy released is about 256 keV (by the mass-energy equivalence, corresponding to about a 4,000th the mass of a proton).

However, the innards of the mountain where the detector is located also produce photons thanks to the radioactive decay of bismuth and polonium in it. So the team making the measurement used a simulator to calculate how often photons of 256 keV are logged by the detector against the ‘background’ of all the photons striking the detector (like a filter). They used data logged over 408 days (January 2012 to May 2013).

The answer: once every 66,000 yotta-years (that’s 420 trillion Brahma-years).

Physics World reports that if photons from the ‘background’ radiation could be eliminated further, the electron’s lifetime could probably be increased by a thousand times. But there’s historical precedent that to some extent encourages stronger probes of the humble electron’s properties.

In 2006, another experiment situated under the Gran Sasso mountain tried to measure the rate at which electrons violated a defining rule in particle physics called Pauli’s exclusion principle. All electrons can be described by four distinct attributes called their quantum numbers, and the principle holds that no two electrons can have the same four numbers at any given time.

The experiment was called DEAR (DAΦNE [pronounced ‘Daphne’] Exotic Atom Research). It energised electrons and then measured how much of it was released when the particles returned to a lower-energy state. After three years of data-taking, its team announced in 2009 that the principle was being violated once every 570 trillion trillion measurements (another stupidly large number).

That’s a violation 0.0000000000000000000000001% of the time – but it’s still something. And it could amount to more when compared to the Borexino measurement of an electron’s stability. In March 2013, the team that worked DEAR submitted a proposal for building an instrument that improve the measurement by a 100-times, and in May 2015, reported that such an instrument was under construction.

Here’s hoping they don’t find what they were looking for?