section_path: Conservation
section_name: Conservation
paragraph_text: Conservation
  
  Two quantities that can define an amount of matter in the quark–lepton sense (and antimatter in an antiquark–antilepton sense), baryon number and lepton number, are conserved in the Standard Model. A baryon such as the proton or neutron has a baryon number of one, and a quark, because there are three in a baryon, is given a baryon number of 1/3. So the net amount of matter, as measured by the number of quarks (minus the number of antiquarks, which each have a baryon number of −1/3), which is proportional to baryon number, and number of leptons (minus antileptons), which is called the lepton number, is practically impossible to change in any process. Even in a nuclear bomb, none of the baryons (protons and neutrons of which the atomic nuclei are composed) are destroyed—there are as many baryons after as before the reaction, so none of these matter particles are actually destroyed and none are even converted to non-matter particles (like photons of light or radiation). Instead, nuclear (and perhaps chromodynamic) binding energy is released, as these baryons become bound into mid-size nuclei having less energy (and, equivalently, less mass) per nucleon compared to the original small (hydrogen) and large (plutonium etc.) nuclei. Even in electron–positron annihilation, there is no net matter being destroyed, because there was zero net matter (zero total lepton number and baryon number) to begin with before the annihilation—one lepton minus one antilepton equals zero net lepton number—and this net amount matter does not change as it simply remains zero after the annihilation.
  
  In short, matter, as defined in physics, refers to baryons and leptons. The amount of matter is defined in terms of baryon and lepton number. Baryons and leptons can be created, but their creation is accompanied by antibaryons or antileptons; and they can be destroyed by annihilating them with antibaryons or antileptons. Since antibaryons/antileptons have negative baryon/lepton numbers, the overall baryon/lepton numbers are not changed, so matter is conserved. However, baryons/leptons and antibaryons/antileptons all have positive mass, so the total amount of mass is not conserved.Further, outside of natural or artificial nuclear reactions, there is almost no antimatter generally available in the universe (see baryon asymmetry and leptogenesis), so particle annihilation is rare in normal circumstances.