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Welcome to the Virtual Visitor Center at SLAC

Virtual Visitor Center at SLAC

Particle Detectors

After particles have been produced by colliding electrons and positrons, we need to track and identify them. A particle can be fully identified when we know its charge and its mass.

In principle we can calculate the mass of a particle if we know its momentum and either its speed or its energy. However, for a particle moving close to the speed of light any small uncertainty in momentum or energy makes it difficult to determine its mass from these two, so we need to measure speed too.

A multi-layer detector is used to identify particles. Each layer gives different information about the "event." Computer calculations based on the information from all the layers reconstruct the positions of particle tracks and identify the momentum, energy, and speed of as many as possible of the particles produced in the event.

The Many Layers of the Detector Surround the Collision Point

This cutaway schematic shows all the SLAC Large Detector elements installed inside the massive steel barrel and end caps. The complete detector weights 4,000 tons and stands six stories tall.

SLAC Large Detector Schematic
Note: Beam pipe size is exaggerated to show particle interaction (positron and electron).
Pink - color coding for SLD schematic diagram above The innermost layer, the vertex detector, about the size of a Coke can, gives the most accurate information on the position of the tracks.
Green - color coding for SLD schematic diagram above The next layer, the drift chamber, detects the positions of charged particles at several points along the track. The curvature of the track in the magnetic field reveals the particle's momentum.
Brown - color coding for SLD schematic diagram above The middle layer, the Cerenkov detector, measures particle velocity.
Purple - color coding for SLD schematic diagram above The next layer, the liquid argon calorimeter, stops most of the particles and measures their energy. This is the first layer that records neutral particles.
Red - color coding for SLD schematic diagram above The large magnet coil separates the calorimeter and the next layer.
Yellow - color coding for SLD schematic diagram above The outermost layer (magnet iron and warm iron calorimeter) detects muons.

Cross-Sectional View of the SLD Detector

Cross Sectional View of the SLD Detector

This view shows a cross section of the elements of the SLD detector.

The BaBar detector was built specifically for the B Factory project.

For an introduction to particle detection, tour the experimental evidence track of The Particle Adventure. Also, the Particle Detector BriefBook includes more information about particle detectors.
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