One important function of the detector is to measure a particle's
charge and momentum.
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. A particle with greater momentum will spend less time
in the magnetic field and has greater inertia than the particle
with lesser momentum, so it's path will bend less than that of
the lesser-momentum particle.
