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Exotic hadrons are subatomic particles composed of quarks and gluons, but which – unlike "well-known" hadrons such as protons, neutrons and mesons – consist of more than three valence quarks. By contrast, "ordinary" hadrons contain just two or three quarks. Hadrons with explicit valence gluon content would also be considered exotic. [1]
Baryons and mesons are both hadrons, which are particles composed solely of quarks or both quarks and antiquarks. The term baryon is derived from the Greek "βαρύς" ( barys ), meaning "heavy", because, at the time of their naming, it was believed that baryons were characterized by having greater masses than other particles that were classed ...
An example would be pentaquarks, consisting of four quarks and one antiquark (qqqqq̅). So far, the only observed exotic baryons are the pentaquarks P c (4380) +, P c (4450) + discovered in 2015, [1] P c (4312) + in 2019 [2] and P Λ ψs (4338) 0 in 2022 by the LHCb collaboration. [3]
Six constituent quarks in any of several combinations could yield a colour charge of zero; for example a hexaquark might contain either six quarks, resembling two baryons bound together (a dibaryon), or three quarks and three antiquarks. [2] Once formed, dibaryons are predicted to be fairly stable by the standards of particle physics.
All quarks are assigned a baryon number of 1 / 3 . Up, charm and top quarks have an electric charge of + 2 / 3 , while the down, strange, and bottom quarks have an electric charge of − 1 / 3 . Antiquarks have the opposite quantum numbers. Quarks are spin- 1 / 2 particles, and thus fermions. Each quark or antiquark ...
In 2003, a particle temporarily called X(3872), by the Belle experiment in Japan, was proposed to be a tetraquark candidate, [6] as originally theorized. [7] The name X is a temporary name, indicating that there are still some questions about its properties to be tested. The number following is the mass of the particle in MeV/c 2.
One model predicts that the lowest-energy heptaquark state would be a spin-1/2 or spin-3/2 state of energy roughly 2.5 GeV. [5] Another study found that the most stable heptaquark would include three strange quarks and two strange antiquarks. [6]
Quarks have fractional electric charge values – either (− 1 / 3 ) or (+ 2 / 3 ) times the elementary charge (e), depending on flavor. Up, charm, and top quarks (collectively referred to as up-type quarks) have a charge of + 2 / 3 e; down, strange, and bottom quarks (down-type quarks) have a charge of − 1 / 3 e.