Vertex Detector
A vertex detector gives the most accurate location of any outgoing charged particles as they pass through it.
The SLD vertex detector is an array of layers of postage-stamp-sized electronic chips surrounding the beam pipe on concentric cylinders. Each chip, called a Charge-Coupled Device (CCD), is like an electronic checkerboard with about 1000 squares on a side. (Similar CCDs form the recording elements in many video cameras.)
Cutaway drawing of the vertex detector.
Particles traveling through the thin chips leave behind small electric charges in the squares they cross. The location of these deposits can be recorded electronically and a computer can "connect the dots" to reconstruct the tracks of all the particles through the layers. Since the electronic squares are so small, they allow measurement of the charged particle position with microscopic accuracy (about 200 millionths of an inch).
Two halves of the old SLD Vertex detector, open to display
the sensitive CCD regions
By drawing each path back to where it meets with one or more other paths we can find the position where any given charged particle is created (the vertex), since charged particles are never created alone, but always in pairs of equal and opposite charges.
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In this checkerboard schematic two particle tracks leave charges in the two layers of the vertex detector. |
If the vertex is in the region where the electron and positron bunches overlapped we consider that this charged particle originated from the energy of an electron and a positron that met and "annihilated" each other.
A vertex outside the collision region indicates that a very short-lived particle was formed in the collision and then decayed at the vertex location to produce the particles whose tracks are seen emerging from this vertex. This is the way we can detect the production of unstable particles containing massive b quarks and c quarks.
