# 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 mv2, 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/c2.