W and Z bosons weak-force boson particle. Thus, this interaction requires a Z boson. These bosons are among the heavyweights of the elementary particles. With masses of and , respectively, the W and Z bosons are almost 80 times as massive as the proton – heavier, even, than entire iron atoms. Their high masses limit the range of the weak interaction. By way of contrast, the photon is the force carrier of the electromagnetic force and has zero mass, consistent with the infinite range of electromagnetism; the hypothetical graviton is also expected to have zero mass. (Although gluons are also presumed to have zero
Atom In the Standard Model of physics, electrons are truly elementary particles with no internal structure. However, both protons and neutrons are composite particles composed of elementary particles called quarks. There are two types of quarks in atoms, each having a fractional electric charge. Protons are composed of two up quarks (each with charge +) and one down quark (with a charge of −). Neutrons consist of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles. The quarks are held together by the strong interaction (or strong force), which