The formation of wave packets in the process of particle measurement is a delicate problem that is connected with the fundamental problem of measurement of quantum mechanics. In fact, a particle is identified using a classical trajectory in a detector, and its position and momentum are measured with finite uncertainties regardless of the dynamics of measurement. Thus, the state that has these uncertainties of momentum and position is described using a wave packet.

Short mean free path: finite spatial extensions
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Particles in matter frequently interact with atoms and lose coherence. The average distance for one particle to move freely is the mean free path in which the particle’s wave maintains coherence. Beyond the mean free path, particles lose coherence and are expressed using different wave functions. Hence, this state has a finite spatial width and its momentum is defined with a finite uncertainty. This momentum uncertainty is inversely proportional to the mean free path and becomes large if the mean free path is short. Thus, a mean free path is used as a wave packet size.

### Mean free path 

The mean free path of a particle when it propagates in matter and is scattered incoherently by scatterers is 