Measuring charge and momentum
One important function of the detector is to measure a particle's charge and
For this reason, the inner parts of the detector,
especially the tracking device,
are in a strong magnetic field.
The signs of the charged particles can easily be
read from their paths, since
positive and negative particles curve in opposite
directions in the same magnetic field.
The momenta of particles can be calculated since the paths of particles with
greater momentum bend less than those of lesser momentum. This is
because a particle with greater momentum will spend less time in the
magnetic field or have
greater inertia than the particle
with lesser momentum, and thus bends less in a magnetic field.