Exotic Hadron Particles Detected at CERN

Part of the LHCb Detector (ExtremeTech)

Scientists at CERN have now confirmed the existence of exotic hadrons, a type of matter that doesn’t fit within the Standard Model of particle physics. Hadrons are subatomic particles made up of quarks and antiquarks (which have the same mass as their quark counterparts but opposite charge), which are held together by the strong nuclear force that binds protons together in the nuclei of atoms. (Fun fact: The term “quark” comes from a line in James Joyce’s 1939 novel Finnegans Wake.) There are six kinds of quarks, usually described in three pairs: up/down, strange/charm, and top/bottom. For each of these six quarks, there is a corresponding antiquark. Quarks are unique in having fractional electric charge, unlike protons and electrons, which have integer charges of +1 and -1, respectively.

Until now, there were two basic classes of hadrons: baryons (such as protons and neutrons), which consist of three quarks, and mesons, which consist of one quark and one antiquark. Exotic hadrons, so named because their existence doesn’t fit within the traditional theory particle physics, are particles composed of two quarks and two antiquarks. The composition is thought to consist of a charm quark, a charm antiquark, a down quark, and an up antiquark.

The Belle Collaboration in Japan reported the first evidence for this exotic particle, officially known as Z(4430), in 2008. The name Z(4430) means the particle has a mass of 4430 MeV/c2, about four times that of a proton (because of mass-energy equivalence described by the famous equation E = mc2, it is common in particle physics to express masses in units of eV/c2, where is the speed of light in a vacuum).

The Large Hadron Collider accelerates beams of protons at close to the speed of light and then collides them at very high energies, smashing them back into quarks. There are four major detectors around the LHC’s 17-mile circumference: ATLAS and CMS (which discovered the Higgs boson), ALICE (which studies quark-gluon plasma), and LHCb (which studies the relationship between matter and antimatter). Each detector weighs over 5,000 metric tons a piece. LHCb was responsible for finding the exotic hadrons (the b stands for the bottom quarks, also known as beauty quarks, that LHCb is designed to detect).

LHC diagram, showing the four major detectors (ExtremeTech)

LHCb is reporting their discovery with a confidence of 13.9 sigma (particle physics typically uses a standard of five sigma for the declaration of the discovery). Nonetheless, as ExtremeTech notes, the chance of a 13.9 sigma discovery being wrong is on the order of winning the lottery multiple times in a row. While the significance of this finding isn’t immediately apparent, the tetraquark’s discovery nevertheless has the potential to influence our understanding of matter at its most fundamental level.


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