Cherenkov radiation and muons in IceCube

Fig. 3: IceCube data for the Glashow event: (a) Schematic of an escaping muon travelling faster than the speed that light propagates in ice, resulting in a cone of Cherenkov light (orange). The muons reach the nearest IceCube sensors ahead of the Cherenkov photons produced by the electromagnetic component of the hadronic shower (blue) since these photons travel at the same speed as light in ice. (b) An event view showing sensors that triggered across IceCube. Each bubble represents a sensor; its size is proportional to the deposited charge. The colours indicate the time each sensor first triggered.

WHEN a charged particle passes through any medium at a velocity greater than the velocity of light in the medium, it emits a short flash of blue light known as Cherenkov radiation in its wake. This is the electromagnetic equivalent of a sonic boom. This flash is detectable at tens of metres by light sensors in water or ice, which are transparent to light. The velocity of light in clear water or ice is 2.19 × 10 ⁸ m/s, which is about three-fourths of its velocity in vacuum (3 × 10 ⁸ m/s). Muons, in fact, travel with this velocity in ice. An interesting consequence of this is that muons produced by neutrino interactions in ice overtake the main wave front of Cherenkov light from other charged particles produced in the event (by 1.23 ns/m) and deposit Cherenkov pulses in digital optical modules slightly earlier than the other charged particles (Fig. 3). This allows IceCube to unambiguously detect muons.