Neutrinos – ghostly particles that rarely interact with normal matter – are the Sun’s secret messengers. These particles are born deep within the Sun, a byproduct of the nuclear fusion process which powers all stars.

Neutrinos escape the Sun and stream through the Earth in immense quantities. These particles are imprinted with information about the inner workings of the Sun.

Two people in overalls and hardhats in a concrete-lined cavern — Authors Dr Meighen-Berger and Professor Bell recently visited the newly excavated DUNE cavern, 1470 metres below the surface in the Sanford Underground Research Facility. Picture: Supplied

Our new theoretical paper shows that the Deep Underground Neutrino Experiment (DUNE), currently under construction, will help us unlock the deepest secrets of these solar messengers.

A ‘desperate remedy’

Neutrinos are the most mysterious of the known fundamental particles. They were first proposed by Austrian theoretical physicist Wolfgang Pauli in 1930, at a time when the only known subatomic particles were the electron and the proton.

Pauli introduced the neutrino to explain puzzling observations of nuclear beta decay, in which an unstable nucleus emits an electron.

The problem was that less energy was released than expected – appearing to violate the law of conservation of energy.

Pauli’s “desperate remedy” to this dilemma was to propose that an additional particle is emitted: the neutrino.

To match the observations, this new particle would need to be very light (less than one per cent of the proton’s mass) and interact extremely weakly with regular matter.

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Pauli penned in his journal: “I have done something very bad today by proposing a particle that cannot be detected; it is something no theorist should ever do.”

Yet detect them we did.

It took 26 years before neutrinos were detected directly in 1956 (coming from nuclear reactors), but even by the end of the 1930s it was understood that the Sun must produce an immense flux of neutrinos as a byproduct of nuclear fusion.

We have now detected very large numbers of neutrinos from the Sun and elsewhere.

And, along the way, the detection of neutrinos has led to four Nobel Prizes (1988, 1995, 2002, and 2015) – and counting.

Why does the Sun shine?

In 1938, the German-American physicist Hans Bethe correctly suggested the primary source of the Sun’s power was due to the nuclear fusion of protons – finally giving an answer to the age-old question: what makes the Sun shine?

Close up image of part of the Sun — Fusion in the core of the Sun is the primary source of its power. Picture: NASA

In the core of the Sun, nuclei are forced together under immense pressure, occasionally fusing together and releasing both energy and neutrinos.

The exact details of the Sun’s fusion processes are highly sensitive to solar properties that we have no ability to measure from the Earth, like the temperature of the solar core and the precise abundance of certain elements, like boron.

To confirm our understanding of how the Sun shines, physicists needed a way to access the solar interior.

Solar neutrinos originate deep inside the Sun and, owing to their small mass and ghostly nature, quickly leave at close to the speed of light, carrying information about the reactions that birthed them.

By making measurements of these neutrinos, we can decode the messages they carry from the Sun’s interior.

However, because neutrinos don’t behave like ‘regular’ matter, and interact only via the ‘weak nuclear force’, they are notoriously difficult to detect.

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The solar neutrino problem

The Homestake (or Davis) experiment, pioneered by American chemist and physicist Ray Davis in the late ‘60s, was situated in a goldmine 1480 meters under the Black Hills of South Dakota.

This shielded the detector from ‘noise’, like potentially confounding cosmic rays.

It used an immense 380,000 litres (or 615 tonnes) of dry-cleaning fluid, rich in chlorine, as its detector. Neutrinos will occasionally convert chlorine into radioactive argon, and the number of argon atoms can then be counted.

Despite this colossal scale, the experiment initially detected only about one neutrino every two days.

This result was in stark disagreement with the theoretically predicted neutrino detection rate – about two-thirds of the neutrinos seemed to be missing.

This was dubbed the ‘solar neutrino problem’.

Old photo of an underground laboratory — Davis’s Solar Neutrino Detector detected about one neutrino every two days, one-third of the expected amount. Picture: Wikimedia

It was initially suspected that there was something wrong with the solar models. For example, the flux of boron-8 neutrinos is exquisitely sensitive to the solar core temperature (to be precise, it depends on the temperature to the 24th power).

Sceptics questioned whether there was really a ‘missing neutrino’ discrepancy at all.

However, the solar models were refined and better measurements confirmed that the solar neutrino problem was real.

It turns out that neutrinos change identity as they travel.

Neutrinos come in three types: the electron neutrino, the mu neutrino and the tau neutrino. The nuclear fusion processes in the Sun make only electron type neutrinos.

But as they move through space, they change type, and some arrive at Earth as mu or tau neutrinos – invisible to the Homestake experiment.

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This shape-shifting ability is due to a quantum mechanical interference effect, which enables neutrinos to oscillate back and forth, from one type to another, over large distances.

The ultimate confirmation of solar neutrino oscillations came three decades later, from the Sudbury Neutrino Observatory (SNO), which used two separate detection processes – charged current interactions (CC), which can detect only electron-type neutrinos; and neutral current interactions (NC), which can detect all types of neutrinos.

The comparison between the two unequivocally demonstrated that, roughly, two thirds of the electron-type neutrinos produced in the Sun have changed identity by the time they show up on Earth.

DUNE (but not Arrakis)

The Homestake mine is once again at the forefront of solar neutrino studies, with a new experiment called DUNE.

While DUNE wasn’t primarily designed to detect solar neutrinos, we have shown that DUNE will be able to do something like SNO – but much better.

Drawing of the voyage of neutrinos from the Sun to a detector on Earth — Neutrinos produced in the core of the Sun from the beta-decay of boron-8 stream through the Earth. A small fraction of them will bump into argon nuclei in the DUNE experiment, which causes them to release gamma rays that can be detected. Picture: Supplied

DUNE is a massive new neutrino detector that will consist of about 70,000 tonnes of liquid argon, located in a huge, newly excavated chamber in the Sanford Underground Research Facility (SURF), at a cost of more than US$3 billion.

This is the largest quantity of liquid argon that is ever likely to be amassed in one location.

While Pauli worried that neutrinos might never be detected, we are now entering an era of precision neutrino physics, made possible by truly enormous detectors that will detect many hundreds of thousands of neutrinos – around a hundred every day.

Our new paper shows that DUNE will be able to measure the total flux of all neutrino types using NC interactions with argon nuclei.

DUNE wasn’t expected to have this capability, but we found that these interactions are more frequent and easier to observe than previously thought.

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An earlier paper showed that DUNE will also be able to measure the flux of electron-type neutrinos using CC interactions with argon.

Together, these new-found capabilities will enable DUNE to compare the rates of CC and NC interactions to make a high-precision measurement of the fraction of neutrinos that change flavour.

This will allow very accurate measurements of those quantum shape-shifting properties of neutrinos.

It will also provide a wealth of information about the deep secrets at the core of our Sun, and, ultimately, the fundamental laws of the universe.

This article was first published on Pursuit. Read the original article.

Dr

Stephan Meighen-Berger

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