The particle, a baryon dubbed Xicc++, contains two heavy "charm" quarks and one "up" quark, and has about four times the mass of a more familiar baryon — the proton.
The particle is predicted in the Standard Model of particle physics, and its discovery was "not a shock," said Matthew Charles of the LPNHE physics lab in Paris.
He is one of about 800 scientists to attach their names to the discovery by the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN).
The collider is most famous for discovering the Higgs boson, which confers mass on matter.
The new particle is the first seen with two such heavy quarks, said the team.
There are six types of quark, with exotic names such as "charm," "strange," and "beauty."
The "charm," "top," and "bottom" quarks are the heaviest types.
Quarks make up baryons such as protons and neutrons that comprise most of the mass in the known universe.
Baryons gather together in atoms, which form the molecules that constitute matter.
"This type of particle, these doubly-charmed baryons... they've been quite elusive," Charles told AFP.
From their short-lived existence in the early Universe, none are left today. And to produce them in the lab requires an extreme concentration of energy, such as can be generated by the new, upgraded LHC.
The Xicc++ is an unstable baryon, said Charles. It lives for "a very small fraction of a second" before decaying into other, lighter particles.
Its discovery will allow scientists to continue testing the Standard Model of physics — the mainstream theory of the fundamental particles that make up matter, and the forces that govern them.
It does not, however, explain dark matter, or why there is more matter than anti-matter in the universe.
Critically, the model is incompatible with Einstein's theory of general relativity — the force of gravity as we know it does not seem to work at the subatomic quantum scale.
"A big part of our work as a field is trying to put our finger on the place where the Standard Model breaks down," to eventually find alternative explanations, said Charles.
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