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After the COT comes the magnet— in this case, the solenoid. The solenoid creates a strong magnetic field in the volume occupied by the COT and silicon systems. This magnetic field bends the charged particles that pass through it, allowing experimenters to use the direction and the amount by which the particle's track bends to identify its charge and momentum. The higher the particle's momentum, the smaller the curvature.

CDF solenoid
The CDF solenoid (white tube at right) waiting to be inserted into the metal shell of the CDF detector. The magnetic field is created by running an electrical current of almost 5000 Amperes (about 2500 times the current that flows through your computer) through a coil of superconducting wire wrapped around the large metal tube that makes up the body of the magnet. When the magnet is turned on, it pulls each side of the detector with a force of over 600 tons.
It takes a very strong magnet to bend the paths of high-energy particles. To achieve this, scientists designed the solenoid to be a superconducting magnet, one which is cooled using liquid helium to get it down to a temperature just 4.7 degrees above absolute zero (about -459 degrees F). The strength of a magnet is limited by the amount of heat generated as the current flows through it. Coils that have a high resistance — meaning they produce a lot of heat — make poor magnets; in extreme cases, the coils can heat up until they melt.

Superconducting materials allow current to flow with no resistance (heat loss), but only if they are kept very cold. The cooler you keep the magnet, the more efficiently it conducts electric current.

Why a magnetic field?
A word about momentum
Physicists can tell a lot about a particle based on the curvature of its track in a magnetic field. They can tell if its charge is negative or positive depending on whether its track bends to the left or right as it moves outward through the silicon detector and COT. The amount by which a particle's track curves is a measure of its momentum. Momentum describes an objectís tendency to continue moving in a certain direction at a certain speed, and it is determined by the particleís mass and velocity. The straighter the track, the greater the momentum.

tracks
The straighter tracks in
this event display indicate
the creation of higher
energy particles.
Momentum helps you piece together a process you canít see directly. For example: we canít “see” a massive particle like the top quark directly, because it decays too quickly to interact with the detector. But we can see its more stable, longer-living daughter particles. These particles, created when a top quark decays, inherit the energy that went into the top quark's mass. High energy means high momentum, which means the particle will leave a straighter track. Scientists can use these straighter tracks as indications that a very heavy particle like the top quark has been created.

The magnet used in the CDF detector has a field that effectively ends at a radius of 1.5 meters from the center of the detector— just large enough to include the silicon detector and central outer tracker. This is fine, because only the silicon detector and COT use the magnetic field. The next layer of detectors, the calorimeter, doesn't track a particle's path, so it doesn't need to see how the particle reacts to a magnetic field.


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Created by Jennifer Lauren Lee; updated January 2008 by JLL.