How a Detector works

How a Detector works
`Stable' Particles
A number of particles produced in the CLEO experiment are relatively `stable', which means they live long enough to travel a long distance (some centimeters upto kilometers) before they decay, or they live forever anyways. Those particles can be `seen' through their interaction with some or all parts of the CLEO detector.


The next pictures are showing the data taken by the CLEO detector after they were readout from special electronic hardware. You can see, which part of the detector has delivered data and the location of this part in a two dimensional projection. The `beam pipe' is pointing out of the picture plane as well as the field lines of the CLEO magnet.


Charged particles
If the particle is charged it can easily interact with the material it travels trough. Typically it will loose a part of its `movement energy' (kinetic energy), which will be converted into `ionized atoms' or light. A very common part of a detector is the `drift chamber'. It is a `box', which contains gases that get ionized by the penetrating particles. To detect those ionized atoms along the path of the particle, a large number of metal wires are strung from one end to the other. (This computer generated picture shows you a typical drift chamber) Using an electric field, all charged residues are collected at the `signal wires' and produce a very short current pulses (less than a millionth of a second).
This picture shows one `event' inside the detector that produced a number of charge particles going through the drift chamber system. Each small circle represents a signal wire that has `fired', as it is called in HEP jargon, which means a pulse was measured on this wire in this reaction. You can clearly see the path of the particles (called `tracks').


The magnetic field inside the CLEO detector is used to `bend' the path of the particles. By doing this you can tell by the curvature what `momentum' the particle has, that means in essence, how many kinetic energy it carries. You can also immediately tell the sign of the charge, since they will be bent in different directions. If you are not familiar with the concept of a momentum vector, read the explanation of 4-vectors. You will not really need to know this, but it helps a lot to understand what's going on here...

The next image is the result of a special computer program searching for track patterns and reconstructing the path of each particle. Blue lines are positively charged particles, red are negatively charged. The green dot is positioned at the beam spot, from where the tracks should originate.


Uncharged particles
A particle that is not charged will not show up in drift chambers. They are detected by `calorimeters'. These detectors contain dense material to force the uncharged particle to induce a reaction that will create detectable charged particles. CLEO uses crystals to convert electromagnetic radiation (photons) into charged particles. Those `secondary' particles will create a dim light flash that can be seen by a device called `photodiode'. The larger the energy of the primary photon, the more energy is produced and the `brighter' is the flash.
shows an event with photons. You can find them by seeing energy in some cyrstals, without a track pointing in this direction. To give you a hint, those clusters of crystal hits that are clearly coming from tracks are marked red like the tracks in the drift chamber.

In this picture the barrel shaped overall form of the crystal calorimeter (CC) is shown in pseudo-3D projection. The hits in the barrel closer to the center are farther away from the observer. Take your time to get used to this picture.