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Why does a particle loose energy?
If you consider the small size of a particle traveling through
a detector and that a gas filled box like a drift chamber is
mainly ``empty space'',
how can this particle interact with the gas molecules and loose energy?
Well, the reason is its charge.
As you may know,
an electric charge creates an electric field around the charged object.
This field mediates the interaction between this particle
and other charged object ``feeling'' this field.
The second important fact is that atoms are made out of
charged sub particles.
Although the gas molecules in the drift chamber are not charged overall,
they still feel the electric forces of the passing particle.
This the reason why once in a while
a gas molecule becomes ``excited'' or even ``ionized''.
Both situations need energy to change the molecule from the ``unexcited
ground state'' to the new state.
This energy has to come from the particle passing by,
so it will loose
kinetic energy.
The overall effect is like moving a tea spoon through honey.
You feel a certain drag, slowing down the spoon.
What we are interested in,
is the actual amount of energy lost by a particle,
since this is related to its mass and momentum.
Prediction of Energy Loss
For many particles in a cretain range of momentum
the energy loss can be predicted by the Bethe-Bloch equation.
The predicted value is the lost energy per length intervall,
also written as ``dE/dx''.
If you calculate the prediction,
you need the speed of the particle.
But this depends on the mass of the particle!
(See also here).
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CLEO WEB PAGES
Updated: 26. December 1995
Author: Andreas H. Wolf (ahw@mps.ohio-state.edu)