[chris (OP)]

Could someone please give me an idiot's guide to the neutrino?

Thanks

[evan_au]

History: Around 1930, physicists knew that in a radioactive process called Beta Decay, the neutron (no charge) decayed into an electron (negative charge) and a proton (equal positive charge). However, the momentum did not add up, and physicists believe that momentum should be conserved in nuclear reactions. Pauli predicted that there was an invisible third particle that was emitted in the decay of a neutron. To conserve electrical charge in neutron decay, it had to be electrically neutral; Fermi called it the "neutrino", or "little neutral one" to contrast it with the neutron, a more massive neutral particle. Neutrinos were first experimentally detected in 1956.

Sources: Neutrinos are generated in massive quantities in the Sun - billions pass through every square centimeter of your body every second. In 1987, a supernova in a nearby galaxy in 1 second hit the Earth with far more neutrinos than the Sun, despite its extreme distance. Neutrinos are also produced in smaller quantities in nuclear reactors and particle accelerators.

Detection: Of the four known forces, neutrinos have no electrical charge, they have a vanishingly small mass, they do not respond to the strong nuclear force. The only way to detect them is through the "weak nuclear force", which as the name suggests, does not react strongly. So they are very ghostly particles, which mostly pass through the whole Earth without reacting.

Since neutrinos hardly ever interact with matter, this makes them very hard to detect - you need a very large detector, and you have to wait a very long time. Most detectors consist of very large volume of a transparent substance which emits a faint flash when an atom happens to interact with one of the billions of passing neutrinos. The tank is surrounded by sensitive light detectors to detect the flash.

You need to screen out natural radioactivity, which triggers the detector far more often than neutrinos. The detectors are made of very pure, non-radioactive materials. They are usually buried far underground, as this filters out cosmic rays. Computers process the data to determine the type of particle, its energy and direction. In the case of Ice Cube, they only accept particles that have passed through the entire Earth.

Neutrinos are really hard to detect, and that makes it hard to measure many of their properties, including mass, speed and spin. It is not even certain whether the neutrino is the same or a different particle than the anti-neutrino. It is believed that many low-energy neutrinos are left over from the big bang, but they have never been detected; at one time they were a candidate for Dark Matter, but recent estimates have ruled out this hypothesis.

There are three known "flavours" of neutrino, and the neutrinos switch between all three types as they pass through space. Some theories allow for a fourth flavour, which has not yet been detected.

More at: http://en.wikipedia.org/wiki/Neutrino

[chris (OP)]

Absolutely brilliant; thanks so much for this really nice intro to the subject!

[acsinuk]

Happy New Year Evan
You really are the expert on neutrinos. Now I have heard that we are missing the expected amount of neutrinos from the sun if it is fusing hydrogen into helium.
What I would like to know is if the neutrino count would be correct if in fact the sun was burning or reducing helium into hydrogen; instead of the other way round?
CliveS

[evan_au]

we are missing the expected amount of neutrinos from the sun

The missing Solar neutrinos were discovered by the earliest neutrino "telescope" experiments, which relied on transmutation of chlorine atoms in dry-cleaning fluid. The number of neutrinos detected were not enough to sustain the Sun's current power output. There were concerns that perhaps the Sun was "going out".

This was resolved with the discovery of neutrino oscillation: Neutrinos generated by nuclear fusion in the Sun turn into other types of neutrinos on their way to the Earth.

It turns out that these early detectors were only sensitive to one of the three types of neutrino.

if in fact the sun was burning or reducing helium into hydrogen

Reducing helium into hydrogen is energetically unfavourable inside a star. It also would not sustain the Sun's current power output.

[evan_au]

The recently completed Ice Cube neutrino observatory at the South Pole has reported detection of extremely energetic neutrinos, probably originating in cosmic events like supernovas.

The detector achieves its high sensitivity to rare events by collecting light from neutrinos striking a cubic kilometer of ice.

A description of Ice Cube and neutrino astronomy in general (1 hour podcast):
http://omegataupodcast.net/2013/11/134-high-energy-neutrinos-and-the-icecube-neutrino-observatory/

[Alan McDougall]

Hi,

Nuetrinoe's might account for dark matter of the universe, does anyone have any knowledge around this possibility

[acsinuk]

Enjoyed the podcast but the ice cube appears to be more in receipt of high energy neutrinos from far away galaxies rather than anything in the solar system or even milky way.  There was no mention of the suns lost neutrinos being sensed.
I am hugely sceptical about the existence of dark matter and believe that what we see and can sense is what we've got. Could be there is a 3D dark force in deep space that we have yet to discover?
CliveS

[evan_au]

Neutrinos might account for dark matter of the universe, does anyone have any knowledge around this possibility

It is certain that neutrinos make up some of the invisible matter of our universe; Neutrinos would form "Hot dark matter". Estimates of the mass of neutrinos from all known sources does not seem to be enough to account for the effects attributed to dark matter.

There is a theoretical form of neutrino called the sterile neutrino, which is even harder to detect than the 3 regular kinds of neutrino - but nobody is sure if it exists, let alone how much of it exists.

Most theoreticians are currently looking for "Cold Dark Matter". Candidates include massive super-partners of the known sub-atomic particles. Like neutrinos, they are thought to interact very rarely with matter, but unlike neutrinos have a lot of mass, move much more slowly, and could be captured by the gravitational field of galaxies, forming a "halo" around the galaxy, preventing individual galaxies from flying apart, and holding together clusters of galaxies.

Could be there is a 3D dark force in deep space that we have yet to discover?

There are some theories about galactic-scale deviations from an inverse square law for gravity which might explain dark matter.