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Comment by dandare | Hacker News Reader

Can someone please explain?

> Neutrinos are the lightest and least interacting of all known particles. They easily fly through planets, stars and whole galaxies, completely unaffected. Because of this, neutrinos can reach us from extreme environments in the cosmos, inaccessible by other messengers.

Two slides later:

> It spies for flashes of light generated when a cosmic neutrino hits an atom of the ice, sparking a cascade of secondary particles that leave their trace in the detector.

So why do neutrinos interact with atoms of ice in detectors but pass through planets and galaxies?

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Neutrinos interact extremely weakly (by the weak interaction), which means that one either needs quite a lot of neutrinos (which they do in terrestrial experiments) and/or a very large detector with a lot of mass and area. For example the ice cube neutrino observatory has a Gigaton of ice as detector mass for neutrinos to interact with. And still the event rate is only on the order of a couple hundred neutrinos _per day_. Most of which are from the Sun - which has a neutrino flux of more than 10^10 neutrinos per second per square centimeter (at the distance of Earth!).

It is not that neutrinos are not interacting with other matter, but that we cannot observe the interactions. The vast majority of neutrinos spend their lives speeding off in a particular direction (from which they started). Only a very, very, very tiny percentage of neutrinos manage to collide with anything and produce a reaction which can then be detected.

The way this detector works is that is essentially a gigantic swimming pool of water and periodically there will be an interaction someplace in it which produces a tiny flash of light/radiation which we can then observe. If we could somehow 'see' through the earth or sun, we could observe these same interactions at a much larger scale.

There are so many of them, that occasionally one must interact with other matter. One in a billion billion billion, sure, but there are trillions of trillions of trillions of neutrinos flying past over a period of time.

Neutrinos are normally created by beta decay[0]. So when they are detected, are they going through "beta reabsorbtion"? Does the (anti-)neutrino hit an electron, form a W boson, then collide with a proton to form a neutron? Or is there another interaction that causes the light?

[0]https://en.wikipedia.org/wiki/Beta_decay

The primary channel is the inverse beta decay where an anti-neutrino interacts with a proton to produce a neutron and a positron (via a W boson like you said). It can also just scatter of an electron (via either the W or the Z boson).

But the important point is that it's only via the weak interaction (meaning only via the W^\pm and Z^0 bosons).

At first, it sounds like an anti-neutrino can "spontaneously" emit a positron to form a W boson, instead of finding an electron.

On re-reading, it's like the neutrino "bumps" the positive charge out of the proton. Like the neutrino interacting with a down quark will convert them into an up quark and a positron.

Edit: Inverse beta decay, commonly abbreviated to IBD, is a nuclear reaction involving electron antineutrino scattering off a proton, creating a positron and a neutron.

... The IBD reaction can only be initiated when the antineutrino possesses at least 1.806 MeV of kinetic energy (called the threshold energy). This threshold energy is due to a difference in mass between the products ( positron and neutrino ) and the reactants ( antineutrino and proton ) and also slightly due to a relativistic mass effect on the antineutrino. Most of the antineutrino energy is distributed to the positron due to its small mass relative to the neutron.

https://en.wikipedia.org/wiki/Inverse_beta_decay

To state the obvious, since the Neutrino is massless, its energy can only come from momentum. Hence the distribution of the 1.806 MeV from the Wiki article above:

  P+ to N:  1.293 MeV
  Positron: 0.511 MeV
  total:    1.804 MeV (rounding error?)

"least interacting" is the key here. Trillions are emitted by the source, and nearly zero impact anything along the way, so most of them make it through the planets, stars, etc. Occasionally one hits an atom of ice/water (depending on which detector we're talking about), allowing us to detect it. The impacts on the detector are pretty rare, relatively speaking.

Can a neutrino going through a planet be compared to a spaceship going through space in terms of likelihood of colliding with something?

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