Quarks are elementary particles that combine to form composite particles, such as the protons and neutrons found in the atomic nucleus. These fundamental constituents of matter do possess an intrinsic mass. However, this mass accounts for only a tiny percentage of the overall mass of the larger particles they create. This counter-intuitive fact is central to understanding the origin of nearly all visible matter in the universe.
The Six Flavors and Intrinsic Mass
Quarks are categorized into six distinct types, or “flavors”: up, down, charm, strange, top, and bottom. These flavors are grouped into three generations. The first generation, consisting of up and down quarks, are the lightest and form ordinary matter. The remaining four flavors are much heavier and are only produced in high-energy collisions, quickly decaying into their lighter counterparts.
Physicists refer to a quark’s inherent mass as its intrinsic mass, which is a fundamental property of the particle. The masses of the six flavors vary enormously, ranging from a few million electron volts (MeV) for the up and down quarks to many billions of electron volts (GeV) for the heaviest flavors. The up quark is the lightest, with a mass of about 2.2 MeV, while the top quark weighs approximately 173 GeV.
The Source of Intrinsic Quark Mass
The mechanism that grants quarks their intrinsic mass involves the Higgs field, a universal energy field that permeates all of space. A quark’s interaction with this field determines its mass. The Higgs boson is the particle associated with this field, and its discovery confirmed the existence of this mass-generating mechanism.
The Higgs field can be imagined as a cosmic molasses that resists the movement of particles. Quarks interact with the Higgs field with varying degrees of strength, determined by the Yukawa coupling parameter. A particle with a weak coupling, such as the light up quark, acquires very little mass. Conversely, the top quark has an exceptionally strong coupling, causing it to possess a colossal intrinsic mass.
The Origin of Visible Matter’s Mass
While the Higgs mechanism explains the intrinsic mass of quarks, it is not the primary source of mass for the visible universe. A proton is composed of two up quarks and one down quark, with a total mass of about 938 MeV. If the intrinsic masses of these three constituent quarks are summed, the total is only around 10 to 12 MeV, or roughly 1% of the proton’s total mass.
The overwhelming majority, nearly 99%, of a proton’s mass comes from the energy of the strong nuclear force binding the quarks together. This force is carried by massless particles called gluons, which constantly exchange between the quarks. According to Einstein’s principle of mass-energy equivalence, \(E=mc^2\), this intense, concentrated energy of motion and interaction is converted directly into mass.
Within the proton, the quarks move at speeds approaching the speed of light, and gluons continuously pop in and out of existence, creating a sea of virtual particles. The kinetic energy of the rapidly moving quarks and the immense binding energy of the gluon field primarily define the proton’s mass. This emergent mass, derived from pure energy, is the reason a proton is so much heavier than the sum of its parts. This phenomenon, often called confinement, means that the mass of all visible matter is fundamentally a manifestation of energy from the strong force.
Determining Quark Mass
Measuring the mass of a quark is a complex task because quarks can never be observed in isolation due to color confinement. The strong force becomes exponentially stronger as the distance between quarks increases, making it impossible to separate a single quark from its hadron. Consequently, their masses cannot be determined by simply weighing them.
Physicists rely on advanced theoretical calculations and indirect measurements from particle collision experiments. One successful method is Lattice Quantum Chromodynamics (Lattice QCD), a computational approach that models the behavior of quarks and gluons on a discrete spacetime grid. By adjusting the theoretical quark masses within the model until the calculated masses of the composite particles, such as the proton, match the experimentally measured values, scientists can accurately estimate the intrinsic mass of each quark flavor. These complex calculations provide confirmation for the minute intrinsic masses of the light quarks and the enormous masses of the heavy quarks.