Physicists want to drill a 5-kilometre-deep hole on the moon

Going deep into lunar rock could give us an opportunity to see if protons can decay into something else – a finding that could help us unify conflicting physics theories

Drilling a 5-kilometre-deep hole in the moon could finally provide evidence that protons can fall apart, claim a group of physicists. If this previously unseen behaviour is spotted, it would help efforts to unify incompatible physics theories.

Going below the surface of the moon could help us probe particle physics
Milan Rademakers/Shutterstock

The standard model of particle physics, which is our best working picture of how the forces and particles that make up our universe interact, says that protons are stable, living forever. This model has shortcomings, however, most notably a failure to unite quantum mechanics with Albert Einstein’s theory of relativity, which describes gravity. In contrast, some proposed unified theories stipulate that protons can decay into other particles, albeit rarely.

That means if we were to find evidence of a proton decay, these more complete models could be correct, but experiments on Earth have failed to see this happening.

Now, Patrick Stengel at the National Institute of Nuclear Physics in Ferrara, Italy, and his colleagues are proposing digging deep into the moon to search for signs of ancient protons decaying into kaons, which are made up of two elementary particles called quarks.

They say the dense lunar rock could preserve evidence of such decay in chemical changes in its mineral structure. Being deep down, and hence well shielded, would also mean that such evidence wouldn’t be confused with lookalike reactions caused by things like high-energy neutrinos. On Earth, many of these are produced by cosmic rays smashing into the atmosphere, so they are rarer in any event on the atmosphere-less moon.

Stengel and his team calculate that the shielding would only be sufficient at least 5 kilometres under the lunar surface, so a powerful drill would need to be transported to the moon to obtain the sample. Drills can go way deeper than that on Earth – the Kola Superdeep Borehole in Russia, for example, goes down more than 12 kilometres.

The rock sample would then be analysed on the moon with advanced microscopy equipment because transport to Earth could contaminate it with cosmic rays. “The idea is very speculative,” says Stengel. “You need to go to 5 kilometres depth, pull out moon rocks and use these cutting-edge microscopy techniques – all that’s very hard.”

Despite the difficulty, assessing just 100 grams of lunar rock could be enough to search for proton decays with as much or more sensitivity than projects on Earth, such as the Super-Kamioka neutrino detection experiment in Japan, which have already cost hundreds of millions of dollars and can’t be made much larger.

“Fundamentally, the idea is appealing,” says David Waters at University College London. “Instead of having detectors that weigh thousands of tonnes and operating them for a few years, you look at small mineral samples, perhaps only tens or hundreds of grams, and they might have been recording in their structure particle interactions over hundreds of millions of years.”


arXiv DOI: 10.48550/arXiv.2405.15845

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