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Fangfei Shen
This “J,” representing the J/ψ meson is displayed at the MIT Museum.
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I may not remember as much as I would like from 3.091 (Introduction to Solid-State Chemistry), but I do remember the big, red “J”. And no, J here does not stand for joule.

My 3.091 recitation was located in Building 36 — across the street from Building 44 — a dreary-looking building from the World War II era. I usually sat near the window, which offered a stunning view of the drab building, complemented best by overcast days. It wasn’t a terribly interesting picture, but I often ended up staring at the building’s giant J. This J was painted on a sign the size of a doorway, placed over a set of stiff black doors and a glass pane marked “CYCLOTRON” in gold letters.

I was quite curious about the J. Why J? What did it mean? I couldn’t make much sense of it then. While I was curious, I apparently wasn’t curious enough to search for the answer.

I learned the answer about two years later in 8.276 (Nuclear and Particle Physics). J is one of the names of a famous particle discovered by a group under MIT’s Laboratory for Nuclear Science physicist Samuel C. C. Ting. The full name of the particle is the J/ψ meson, the ψ (Greek letter psi) coming from Burton Richter ’52’s group at Stanford, which made its discovery nearly concurrently with Ting’s group.

What was especially noteworthy about the J/ψ meson was that it was made out of two previously hypothetical particles: the charm quark and its antiparticle, the anticharm. We know today that there are six quarks total (up, down, strange, charm, top, and bottom), and that they are the buildings blocks of matter, making up protons and neutrons. However, prior to November 1974, when the J/ψ discovery was announced, the known quark family comprised only of the up, down, and strange. Evidence of the charm meant the three quarks became four quarks, which was a big deal, to say the very least.

Physicists returned to the drawing board for particle physics theory so enthusiastically that the announcement of the J/ψ meson discovery set into motion what became known as the “November Revolution” in particle physics. Just a short two years later, both Ting and Richter were awarded the Nobel Prize for their work on the J/ψ meson. Once news of Ting’s Nobel Prize reached Cambridge, letter Js started to appear around the MIT campus, including the one on the Laboratory for Nuclear Science’s cyclotron, which I would stare at from 3.091 recitation.

The reason Ting named his discovery “J” varies by source. Some say that the J resembles the Chinese character for Ting’s last name. Others sources, like the MIT Museum’s MIT 150 Exhibit where another J is displayed, say that the previous meson discovered was named K, and J is next to K, hence the J. And finally, Ting himself said in his Nobel Lecture that “the really exciting stable particles are designated by Roman characters — like the postulated W0, the intermediate vector boson, the Z0, etc … [both since discovered] — whereas the ‘classical’ particles have Greek designations like ρ, ω, etc. This, combined with the fact that our work in the last decade had been concentrated on the electromagnetic current jμ(x), gave us the idea to call this particle the J particle.”

Regardless of the actual origin, each explanation of the naming of the J carries its own charm.