A quark is an elementary particle, meaning it can’t be divided into smaller constituents. All matter in the Universe is made of quarks, but we haven’t been able to observe them directly. They are combined into other particles which we can observe, like protons and neutrons.

Particles made of quarks are called hadrons and hadrons can be divided into two groups. Protons and neutrons are hadrons made of three quarks each, so that makes them baryons as opposed to mesons, which are made of a quark-antiquark pair.

Quarks come in six flavours - up, down, charm, strange, top, bottom. Name flavour has nothing to do with tasting quarks, it’s just a way to differentiate between them with respect to electric charge, mass, colour charge and weak isospin.

Only up and down quarks are found in nature because only these two are stable. Other four can be created in large particle colliders. Briefly after they are created, they decay to either up or down quarks. They do this by means of weak interaction or weak force responsible for other types of decay as well. Heavier down quark can decay into lighter up quark the same way. Neutrons are made of two down quarks and one up quark while protons are made of one down quark and two up quarks. As neutrons decay into protons, what actually happens is that one of two down quarks in a neutron decays into an up quark, creating a proton. A virtual W- boson is emitted and shortly after it decays into electron and antineutrino. Quarks carry an isobaric spin or weak isospin, which is weak force equivalent to electric charge in electromagnetism. Up and down quarks have +½ and -½ isospin respectively, while other four quarks have none.

As you see and as it is expected from the particle that makes up all the matter, quarks have mass. Up quark is the lightest, the down quark second lightest. Then come strange and charm quarks, while bottom and especially top quarks are heavier than proton itself. By having mass, they naturally interact with other massive particles by another fundamental force - gravity. Although a single quark is too light to interact with other particles via gravity in a measurable way, when they are grouped into nuclei, atoms, molecules and finally celestial bodies, the effects of gravity are measurable.

Another force quarks experience is electromagnetism. Electric charge of a quark can be either ⅓ or ⅔ of the elementary charge. All other particles have integer multiples of the elementary charge. If we group quarks by electric charge, there are up-type quarks with +⅔ electric charge (up, charm and top quarks) and down-type quarks with -⅓ electric charge (down, strange and bottom quarks). If you consider that proton is made of two up quarks with total charge of +1⅓ and one down quark with charge of -⅓, by addition you have +1 electric charge for proton. Same can be applied for neutrons which have no charge since their quarks have +⅔, -⅓ and -⅓ charges.

Of course, quarks also experience strong interaction or strong force. They are inseparable from one another because between them, the strong force is carried by gluons, creating gluon field. Name gluon fits nicely because the particle “glues” quarks together. Quarks also have colour charge, which similarly like with flavours, has nothing to do with colours we see, but is rather a way to differentiate quarks by their strong interaction equivalent to the electric charge in electromagnetism. There are red, blue and green color charges and antired, antiblue and antigreen charges for antiquarks. A red quark can form a bound with two other quarks (blue and green) and build a baryon, or it can form a bound with an antired antiquark and build a meson. It can’t bind with another red quark just like two particles with positive electric charge can’t be bound by electromagnetic force.

As you could have guessed, every quark has its own antiquark, which is identical in every way except certain charges have opposite sign.

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