# Particle Physics Units

#### Energy

The fundamental unit used for most measurements in particle
physics is the electron volt, a unit of energy abbreviated eV.
An eV is the kinetic energy that an electron gets
when it is accelerated under a voltage difference of 1 Volt. So if you hold an
electron near the negative electrode of a 1 Volt battery and then let it go, it
will have a kinetic energy of 1 eV when it reaches
the positive electrode, A GeV is a giga-electron volt, or 1 billion electron volts.

The eV is a very small unit of energy. For
example, consider the 10.58 GeV available in the
collisions at the BaBar detector. Converted to SI units, this is 1.7 × 10^{-9} Joules of energy. For
comparison, calculate in Joules the kinetic energy of a baseball (mass = 0.145
kg) thrown at 90 mph = 40 m/s. [Solution]

#### Momentum

As you know, the formula for kinetic energy is
E = ½ mv^{2}, and the formula for momentum is
p = mv. Notice that the unit for energy is
the unit for momentum times a velocity. In particle physics, we use this
relationship to express the unit for momentum in terms of the unit for energy
divided by a velocity, namely the speed of light (denoted *c*). Since we
use the GeV to measure energy, our momentum unit is GeV/*c*.

#### Mass

We use the same argument again to derive a unit for mass.
The units of momentum divided by a velocity (the speed of light *c*) gives
a unit of mass. So for measurements of mass we use the unit GeV/*c*^{2}.

Last
modified: Tue Nov 2 14:55:34 PST 2004