A telescope made of ice: catching neutrinos from deep space

Neutrinos are tiny particles that fly through almost everything. Right now, billions of them from the Sun are passing through your fingernail every second, and you feel nothing. Some neutrinos come from much farther away, from wild places in deep space where matter is thrown about with enormous force.

To catch one, you need a lot of stuff, because a neutrino only bumps into an atom once in a very long while. When it does, it makes a tiny flash of blue light. Think of a huge dark room with cameras hanging everywhere, waiting for one firefly to blink. If enough cameras see the blink, you can tell where the firefly flew from.

This giant ice detector is called IceCube. It caught neutrinos from far outside our solar system, so we can now study space with a new kind of messenger.

The cosmos has natural particle accelerators. The press release says they fire out particles with up to a million times more energy than any lab on Earth can reach. We have measured these cosmic rays for over a century, but we still do not know where the most energetic ones are made. They are mostly protons, so magnetic fields in space bend their paths and scramble their directions.

A neutrino offers a way around this. When fast protons crash into gas or light near their source, they make short-lived particles called pions. Charged pions decay into neutrinos. A neutrino has no charge and hardly ever interacts, so it travels in a straight line, does not lose energy, and escapes even from places too thick for light to leave.

IceCube hangs 86 strings of 60 sensors each, at depths between 1450 and 2450 metres, filling a cubic kilometre of ice. A muon neutrino leaves a long straight track that is good for direction. Other events make a round burst called a cascade, which is good for measuring energy. The hard part is the background: muons and neutrinos made by cosmic rays in our own atmosphere. Astrophysical neutrinos stand out because they reach much higher energies.

The physics case rests on hadronic production at cosmic-ray accelerators. Accelerated protons hit ambient gas (pp) or photon fields (pγ). In pγ the Δ⁺ resonance decays to pπ⁰ or nπ⁺; the π⁰ gives two gamma rays and the π⁺ gives ν_μ plus μ⁺, which then decays to e⁺ν_eν̄_μ. The pion takes about 20% of the proton energy and each neutrino ends up with roughly 5%. So protons at 10¹⁵ to 10¹⁸ eV map onto neutrinos from tens of TeV to tens of PeV, and the gamma-ray and neutrino energy fluxes are tied by the charged-to-neutral pion ratio K_π (about 2 for pp, about 1 for pγ).

Detection is deep inelastic scattering on nucleons, followed by Cherenkov emission from the charged secondaries (ice index about 1.31). Below about 2100 m the absorption length is about 200 m and the scattering length about 50 m. Charged-current ν_μ events give tracks with 0.3° angular resolution at 100 TeV, but energy known only to a factor of about 2, since the muon may be born far outside the array. Cascades from ν_e, ν_τ and neutral-current events are under 10 m long, so direction degrades to about 5° while contained energy resolution reaches about 8% at 100 TeV.

Two backgrounds must be beaten. Down-going atmospheric muons arrive at about 3 kHz; selecting up-going tracks, with the Earth as a shield, cuts them to the μHz level. The roughly 100,000 atmospheric neutrinos per year above 0.1 TeV fall as about E^-3.7, so the astrophysical flux of about E^-2.5 rises above them beyond a few tens of TeV, for an expected ~100 astrophysical events a year. The starting-event method adds an outer veto shell around a ~500 Mt inner volume. It found two PeV events (1.04 and 1.14 PeV) by chance in 2010 to 2012 data during a search for EeV neutrinos, then 28 events from 30 TeV to 1.14 PeV. In 2014 a purely atmospheric origin was rejected at 5.7σ.