Abstract

When a gamma or cosmic ray strikes the top of Earth’s atmosphere, a shower of secondary particles moves toward the surface. Some of these secondary particles are charged muons that subsequently enter water Cerenkov detectors (WCDs) on the ground. Many of these muons, traveling near the speed of light in vacuum, are moving faster than the speed of light in water and so trigger isotropic Cerenkov radiation in the WCDs. Inside many WCDs are photomultiplier tubes (PMTs) that detect this Cerenkov radiation. When the radial component of the speed of a muon toward a PMT drops from superluminal to subluminal, the PMT will record Cerenkov light from a little-known optical phenomenon called Relativistic Image Doubling (RID). Were the RID-detecting PMTs replaced by high resolution video recorders, they would see two Cerenkov images of the muon suddenly appear inside the tank, with one image moving with a velocity component toward the recorders, the other away. Even without a video, the RID phenomenon will cause different PMTs to record markedly different light curves for the same muon. In this paper, we present a study hoping to inspire the explicit detection and reporting of RID effects in WCDs. We consider three example cases of muon RIDs in High-Altitude Water Cerenkov (HAWC)-like systems: vertical, horizontal, and oblique. Monte Carlo simulations show that RID effects in HAWC-like systems are not rare—they occur for over 85% of all muon tracks.