Primer
Neutrinos in One Page
Everything in this course follows from one number being very, very small. This page establishes what a neutrino is and how small that number is, so the rest can be derived.
What it is
A neutrino is an elementary particle with no electric charge, no colour charge, a mass so small it has never been measured directly, and therefore no way to interact except through the weak force and gravity. It comes in three flavours — electron, muon and tau — each paired with its charged partner.
Because it is neutral it is not bent by magnetic fields, and because it barely interacts it is not absorbed. Those two facts are what make it useful for astronomy: unlike a cosmic-ray proton, it arrives pointing back at wherever it was made.
The one number that matters
A neutrino's chance of interacting as it crosses a thickness of material is set by the cross-section σ, the target number density n, and the path length L. For a small probability:
\[ P \simeq n\,\sigma\,L \]
At a few GeV the neutrino–nucleon cross-section is around 10−38 cm². A nuclear cross-section is of order 10−24cm². That is fourteen orders of magnitude, and it is the entire design problem.
The saving grace is that σ grows with energy — roughly linearly up to a TeV, and more slowly above it. By a PeV it has climbed to about 10−33cm², which is why high-energy neutrino astronomy is possible at all while low-energy neutrino astronomy needs a mine full of cleaning fluid.
Cross-section values here are approximate teaching figures, good to a factor of a few; Part 2 uses them to derive an event rate and the answer lands where IceCube actually sits.
Why the Earth is part of the detector
A detector at the South Pole is swamped by muons made in the atmosphere above it — millions for every neutrino. But those muons cannot pass through the planet.
So the trick is to look down. An upward-going track must have come through the Earth, and only a neutrino can do that. Eight thousand kilometres of rock becomes a filter that nothing else survives, and the sky IceCube watches best is the northern one, seen through the planet.
What a neutrino leaves behind
When a neutrino does interact with a nucleon, it does so in one of two ways. In a charged-current interaction it turns into its charged partner — a muon neutrino makes a muon — which then travels on and can be tracked. In a neutral-current interaction it stays a neutrino, kicks the nucleon, and leaves only a localised spray of particles.
That distinction produces the two event shapes the whole analysis rests on, and Part 3 shows why neither gives you direction and energy at the same time.