J/ψ mesons (pronounced jeɪ/saɪ, English jay/sigh) are subatomic particles made of one charm quark and one charm antiquark. Mesons are a family of composite particles which are made of one quark and one antiquark. The discovery of J/ψ mesons was huge because it revealed the existence of the fourth known quark of the time, the charm quark. J/ψ mesons were used in an attempt to learn more about quark-gluon plasmas, but no conclusive results were found. The name J/ψ comes from the chosen names from the two labs that found them, one of which chose the letter J, while the other chose the Greek letter ψ (psi).