The up quark is one of the most abundant quarks in the universe: together with the down quark, they form protons and neutrons and are thus the basic building blocks of all the matter that surrounds us in daily life.
Quarks are electrically charged, but only with a fraction of the elementary charge of the electron (1 e). While the positive up quark carries +⅔ of an elementary charge, the negatively charged down quark carries -⅓ e. A proton consists of two up quarks and one down quark, summing up to a total charge of 1 e. The neutron, on the other hand, is a combination of two down quarks and one up quark, this is why it is overall electrically neutral.
Surprisingly, the proton is about 100 times heavier than the sum of the masses of two up quarks and a down quark. The mass of the proton, in fact, is largely determined by the so-called binding energy, which holds the quarks together. Protons, like neutrons, are held together by the strong force, mediated by the gluon. In each proton the three quarks stay together by constantly exchanging gluons.
The sign shows the proton (left) and neutron (right) structure. The quarks are bound by a constant exchange of gluons.
