Timeline: Discovering quarks and quarkstars

1960s: theoretical physicists, trying to account for the ever-growing number of subatomic particles observed in experiments, consider the possibility that protons and neutrons are composed of smaller units of matter.

1961: physicists, Murray Gell-Mann of the United States and Yuval Ne'eman of Israel, propose a particle classification scheme called the Eightfold Way.

1964: Gell-Mann introduces the concept of quarks as a physical basis for the scheme.

1970s: The discovery of the charmed and bottom quarks and their associated antiquarks, strongly suggests that quarks occur in pairs.

1993: 22 October: researchers claim that something entered the Earth off Antarctica and left it south of India 0.73 of a second later.

1993: 24 November: researchers claim an object entered south of Australia and exited the Earth near Antarctica 0.15 of a second later.

1995: two independent groups of scientists at Fermi National Accelerator Laboratory, Batavia, Illinois, reported that they had found the top quark.

2001: Physicists create atomic nuclei containing two strange quarks at the Brookhaven National Laboratory in the United States.

2002: Astronomers believe they have found their first quark stars — super-dense objects that are formed when the remnants of old stars collapse in on themselves.

2003: Physicists discover pentaquark.

Quark to pentaquark

Physicists discover a new class of subatomic particle that should provide unexpected insights into the fundamental building blocks of matter.

The discovery involves quarks — particles that make up the protons and neutrons usually found in the nuclei of atoms. The new particle, the pentaquark, is five quarks in formation. Until now, physicists had only seen quarks packed into two- or three-quark combinations. Scientists feel that this discovery should have far-reaching consequences for understanding the structure of matter.

Context: Quarks

Quarks are any of a group of subatomic particles believed to be among the fundamental constituents of matter. In much the same way that protons and neutrons make up atomic nuclei, these particles themselves are thought to consist of quarks. Quarks constitute all hadrons (baryons and mesons), that is, all particles that interact by means of the strong force, the force that binds the components of the nucleus.

According to prevailing theory, quarks have mass and exhibit a spin (type of intrinsic angular momentum corresponding to a rotation around an axis through the particle) equal to one-half the basic quantum mechanical unit of angular momentum. The latter property implies that they obey the Pauli exclusion principle, which states that no two particles having half-integral spin can exist in exactly the same quantum state. Quarks appear to be truly fundamental. They have no apparent structure; that is, they cannot be resolved into something smaller. Quarks always seem to occur in combination with other quarks or antiquarks, never alone. For years physicists have attempted to knock a quark out of a baryon in experiments with particle accelerators to observe it in a free state.