An Introduction to the Standard Model of Particle Physics

1.10 Units

In particle physics it is usual to simplify the appearance of equations by using units in which hbar = 1 and c = 1. In electromagnetism we set epsilon ₀ = 1 (so that the force between charges q₁ and q₂ is q₁q₂ / 4r²), and µ₀ = 1, to give c² = (µ₀ epsilon ₀)^-1 = 1. We shall occasionally reinsert factors of hbar and c where it may be reassuring or illuminating, or for the purposes of calculation. It is useful to remember that

hbar c approx 197 MeV fm, e² / 4 approx 1.44 MeV fm,
alpha = e² / 4 hbar c approx (1/137), c approx 3 x 10²³ fm s^-1.

Energies, masses, and momenta are usually quoted in MeV or GeV, and we shall follow this convention.

c 197 MeV fm,	e² / 4 1.44 MeV fm,
= e² / 4 c (1/137),	c 3 x 10²³ fm s^-1.