The Pool‐Table Analogy with Axion Physics

Elementary particle physicists enjoy talking about particles for which there is no experimental evidence, and of these particles the axion seems one of the strangest and least accessible. The mass of the axion is expected to be roughly 10−5 eV, a factor of a million below current limits on the neutrino mass, and the axion's couplings are suppressed by a factor of 10−12 relative to those of pions and other familiar particles. Yet there are serious claims that axions make up most of the mass of the universe and equally serious experiments to demonstrate the presence of these tenuous particles. Why should one believe in the axion? I attempt to answer this question by drawing an analogy with the physics of a pool table.