Upsilon
Upsilon (Υ, υ) is the 20th letter of the Greek alphabet, a vowel written y or u in Latin letters. In physics Υ is the upsilon meson, a particle made of a bottom quark and its antiquark.
Easily confused with
How to read it
Dictionary.com gives several American pronunciations: YOOP-suh-lon, UP-suh-lon and yoop-SY-luhn. The Collins English Dictionary gives British /ˈʌpsɪˌlɒn, juːpˈsaɪlən/ and describes upsilon as a vowel, transliterated as y or u. According to Dictionary.com the name is from Late Greek ŷ psīlón, “simple u”. The capital Υ has the shape of the Latin Y (U+0059), and the small υ is close to the Latin v and to Nu ν.
Also searched as: upsilon letter, upsilon symbol, υ, Υ.
Capital and small forms
| Form | Character | Code point | Unicode name |
|---|---|---|---|
| Capital | Υ | U+03A5 | GREEK CAPITAL LETTER UPSILON |
| Small | υ | U+03C5 | GREEK SMALL LETTER UPSILON |
| Symbol with hook | ϒ | U+03D2 | GREEK UPSILON WITH HOOK SYMBOL |
Unicode cross-references υ to two phonetic letters that look like it, ʊ (latin small letter upsilon) and ʋ (latin small letter v with hook). The small nu ν is U+03BD and the Latin v U+0076.
Upsilon in physics
Υ is the upsilon, a family of heavy, short-lived, neutral mesons composed of a bottom quark and its antiquark (Dictionary.com). OpenStax lists it at 9460 MeV, with a lifetime of 1.2 × 10⁻²⁰ seconds. It was the first particle of the b-quark family found: the E288 experiment at Fermilab, headed by Leon Lederman, saw first evidence in November 1976 and made the discovery in 1977 (J. Yoh, AIP Conference Proceedings).
Other uses
NIST’s Guide for the Use of the SI writes specific volume, the volume per unit mass, as υ: mass density is ρ = 1 / υ (see Rho).
Write in Greek letters
stringmash.comGreek alphabet translatorWrite English in Greek letters by sound, or turn Greek back into Latin letters as Greek passports do.Read next
- Next in Greek lettersPhiPhi (Φ, φ) is the 21st letter of the Greek alphabet, said “fye” (rhyming with pie). In physics Φ is the symbol for magnetic flux and φ for a phase shift or angle.
- The same shape in Statistics symbolsP-value (p)A p-value (p) is the probability of getting a result at least as extreme as the one observed if the null hypothesis were true. The smaller it is, the stronger the evidence against the null hypothesis; p < 0.05 has long been the usual line for calling a result “statistically significant”.
More greek letters
The same shape elsewhere
Browse symbols drawn with letters and numbers.
Sources
- upsilon (Dictionary.com entry), Dictionary.com Unabridged; Collins English Dictionary, Dictionary.com, Entry, American and British (Collins) senses
- Greek and Coptic: Range 0370–03FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Greek letters; variant letterforms
- C0 Controls and Basic Latin: Range 0000–007F (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Basic Latin
- University Physics Volume 3, 11.1 Introduction to Particle Physics, OpenStax (Rice University), Table 11.1
- The discovery of the b quark at Fermilab in 1977: The experiment coordinator’s story (J. Yoh), AIP Conference Proceedings 424 (1998), abstract, U.S. Department of Energy, OSTI.GOV, Abstract
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811 (2008), National Institute of Standards and Technology, Sec. 8.6.7 note 2
Researched, edited and approved by Rory Hansen before publishing. Last reviewed September 30, 2026. How we research
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