Power to the Pentaquark
About Quarks
What’s inside an atom? What’s inside a proton? These are questions asked by physicists, who seek to understand matter on the most fundamental level.
An atom contains a nucleus, made up of protons and neutrons, surrounded by a cloud of electrons. Experiments that probe deeper find that electrons show no structure, but protons and the neutrons do. They contain fundamental particles called quarks, which attract each other so strongly that they cannot exist as free particles under ordinary conditions. Cosmologists theorize that in the incredible conditions right after the Big Bang, quarks could exist freely in what is called the quark-gluon plasma, a state of matter that some particle physics are trying to create in the laboratory. (See Nuclei Knockdown)
Diagram of quark structure of protons and neutrons (diagram courtesy of Brookhaven National Laboratory)
Quarks come in six evocative "flavors,"—up, down, strange, charmed, bottom, and top—and for each of these there exists an antiquark. (See The Buzz about Antimatter)
Combinations of the up and down quarks make up the nuclear particles—two up quarks and one down for the proton, and one up quark and two down for the neutron, as shown in the diagram. The charges of the quarks combine to give the charge of the proton and the neutron, as shown below and in the table.
Proton = up quark + up quark + down quark
Charge of the proton:
+1 = 2/3 + 2/3 – 1/3
Neutron = up quark + down quark + down quark
Charge of the neutron:
0 = 2/3 –1/3 – 1/3
A different kind of particle, the meson, is composed of two quarks, or, more specifically, a quark and an anti-quark. For instance, the pi-plus is an up quark and an anti-down quark. That gives us two kinds of particles made of quarks—nuclear subatomic particles (the proton and neutron) and mesons. Could there be more?
