The initial momentum of a charged particle can be calculated from the

*curvature* of its
track whilst it is moving in a magnetic field. This is because of the

**Lorentz
force**.

As the diagram of the **DØ
detector** in section 3.2 shows, there is a solenoid
around the tracking chamber. The *magnetic field* from the solenoid
exerts the Lorentz Force on any charged particle within the detector,
curving its path.

The magnetic field in DØ is 2 Tesla. In contrast, the magnetic field
of
the Earth is only about 0.05 milliTesla so this field is huge!

In the DØ experiment the magnetic field is orthogonal to the direction
of motion of the particles. Therefore the magnetic force exerted on a particle
is given by;

If we equate this with the * centripetal force* of circular motion, and use *p = mv*, we can find the momentum of a particle in terms of its * radius of curvature*.

The greater the curvature of the path, the smaller the momentum of the
particle, and vice-versa. Positively and negatively charged particles curve in *opposite* directions in the field.