Upsilon has one of the more obscure modern technical profiles among the Greek letters covered in this atlas, appearing mainly in a single area of particle physics: the upsilon meson, a short-lived particle composed of a bottom quark bound to its own antiquark, was discovered in 1977 by a research team at Fermilab led by Leon Lederman, who named it after the letter following the earlier convention that had already produced the tau lepton, discussed elsewhere in this atlas. The upsilon meson's discovery was significant at the time as indirect evidence for the existence of the bottom quark, one of the six fundamental quarks recognised in the Standard Model of particle physics.
Outside this specialised context, upsilon sees little general use compared with better-known letters such as pi, sigma or lambda, and it carries no comparable idiom, symbol or popular reference in English. Astronomers, following the Bayer star-naming convention of 1603, use Upsilon to mark a constellation's twentieth-brightest star, continuing the same ranked sequence used for the rest of the Greek alphabet, and this remains, for most readers, the letter's only familiar modern role. In the Milesian numeral system, discussed elsewhere in this atlas, upsilon carries the value four hundred, a use that predates the meson by roughly two and a half thousand years and shares nothing with it beyond the borrowed name.