A Large Detector
The detector is so big for two main reasons, both coming from the need
to measure particle momenta and energies precisely.
- In the central tracking detectors, we measure the momentum of
charged particles by how much the particle paths bend in the central
magnetic field.
For the highest energy particles produced at LEP1
the total bending over the 2m radius of the jet chamber is
only about 2.6 cm.
We need to measure this distance well to measure the particle momentum
well.
- When a hadron interacts in the detector, the typical length of the
resulting shower depends on energy, and the density of the material.
We can measure the energy of a hadron properly only if we detect most
of the shower.
We use dense materials in the calorimeters (lead-glass, iron) but even so
the highest energy showers need about a metre depth of material to
completely absorb them.
The OPAL Webweavers 7 April 1998