Exotic particles with four or more quarks

Researchers at the LHC say they have discovered four new subatomic particles that are each made up of four quarks. Protons and neutrons are made of three quarks; lots of other particles are made of two. That's what makes the new "tetraquarks," and others like them that have been discovered recently, so strange. This article describes the six varieties of quarks and explains why they usually prefer to combine into pairs or triplets rather than quartets.

Read the article:http://scitation.aip.org/content/aip/magazine/physicstoday/article/67/9/10.1063/PT.3.2525

Photo:
Conventional and exotic quark combinations.
(a) Three differently colored quarks form baryons such as the proton; three antiquarks combine to form antibaryons. Quarks couple with antiquarks of the corresponding anticolor in mesons such as the pion.

(b) Combining two quarks of different colors produces anticolored diquarks.

(c) Any number of quarks can combine to form a particle as long as the resulting color is white. Examples are pentaquark baryons, dibaryons, and tetraquark mesons. In such cases, “baryon” means that the number of quarks minus the number of antiquarks is three, and “meson” means that quarks and antiquarks appear in equal number.

Prefixes like “penta” or “tetra” refer to the total amount of quarks and antiquarks, without distinction. (d) In 2007 the Belle collaboration at KEK studied the reaction B → Kπ + ψ′. The plot here shows the invariant mass (defined in the text) of the π + ψ′ products; the peak indicates that those products arose from the decay of an intermediate Z meson, as illustrated in the inset, and the nature of the products suggests that the Z is a four-quark state.