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Nicole Ackerman | SLAC | USA

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Neutrinos: A question and an answer

We often rely on photons – light – to understand the world around us. Sight is the primary faculty through which many of us navigates through the world. Additionally, light has been invaluable in understanding many phenomena, even when they are very small or very far. The way photons interact with atoms makes it possible to look at distant stars and determine their chemical makeup. We can theorize what processes are at work in distant astrophysical bodies based on the type of photons – xrays, gamma rays, radio waves – that reach us. All of this is possible due to our understanding of light.

We don’t understand the neutrino. We still have to guess about some of its most fundamental properties. But some of its properties – like how infrequently it interacts – make it very useful for studying phenomena that we can’t use light to study (completely). Photons, being the particle of electromagnetic force, interact with strong magnetic fields and most types of matter. Distant light might be deflected or absorbed by objects on their path from source to telescope. Neutrinos can pass right through gas clouds and come straight to Earth – allowing astrophysical observations where they weren’t always possible before.

A known property of the (anti)neutrino is that it is created during beta decay – this is how it was initially “found”. This means that in many types of radioactive decay, neutrinos are emitted, with specific ranges of energy depending on what is decaying. This was one of the earliest “uses” of neutrinos: looking at neutrinos from the sun to check if we understood the nuclear processes going on inside. This work is still being done, on wider energy scales, to check that our theory of the sun is correct. We know how many neutrinos of each energy we should see based on the predicted rates of different nuclear processes. Few of these processes give of photons – and many occur deep inside the sun where photons couldn’t escape – so neutrinos are the only way of verifying they occur.

Geo-neutrinos are one of the newest tools that bridge particle physics and more ‘practical’ observational science. How do we know what the core of the Earth is doing – is it a big nuclear reactor? The temperature of the Earth is a clue, but isn’t enough to know. By looking for the neutrinos coming from the center of the Earth we can understand how much radioactivity is at the core. These measurements can be done by detectors that are measuring solar neutrinos, like Borexino. They just published a paper that puts a limit on the power generated in the Earth by radioactivity. There is still a lot of room left for more experiments in this field to further improve our understanding – the Borexino result is based on about 10 neutrinos.

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