Conceptual fractal illustration of quantum chromodynamics (QCD), the theory of strong interactions, a fundamental force describing the interactions between quarks and gluons which make up hadrons such as the proton, neutron and pion. QCD is a type of quantum field theory called a non-abelian gauge theory with symmetry group SU (special unitary group). The QCD analog of electric charge is a property called colour. Gluons are the force carrier of the theory, like photons are for the electromagnetic force in quantum electrodynamics. The theory is an important part of the Standard Model of particle physics. A huge body of experimental evidence for QCD has been gathered over the years. Fractals are infinitely complex mathematical patterns that are self-similar across different scales. They are created by repeating a simple process over and over in an ongoing feedback loop.