It is an astounding fact that modern-day physics can quantitatively describe phenomena from the
small scale of quarks and leptons (10−18 cm) to the large scale of the whole present-day Universe
(1028 cm) using the four fundamental forces: gravity, electromagnetism, strong and weak interactions.
While gravity is governed by Einstein’s general relativity, the other three forces can be
described to an excellent degree by a quantum field theory of quarks and leptons based on a framework
consistent with Einstein’s special theory of relativity and quantum mechanics: the so-called
standard model (SM). This book is mainly about the physics of strong interactions, the associated
structure and interactions of hadronic matter, and its role in the evolution of the Universe from a
very early stage (microseconds young) to the present day (13 billion years old). However, as the
reader will discover, it is not possible to tell the story without including the other forces. Therefore
in this beginning chapter, we give an introduction to the standard model, with an emphasis on the
fundamental theory of strong interactions—quantum chromodynamics (QCD).