13.2: Scattering of distinguishable particles and identical particles

Last updated
Save as PDF

Page ID: 28691

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

Consider two beams of distinguishable particles with the same mass colliding, and scattering through some angle \(\theta\). Let the intensity of the scattered particles have angular dependence \(|f(\theta )|^2\). Conservation of energy and momentum ensure that the scattering angles are the same for both particles in the COM frame. As usual, the radial part of the wavefunction far from the region of interaction is simply a plane wave so the wavefunction can be written as a function of \(\theta\).

The intensity for the process in which both particles are scattered through an angle \((\pi − \theta )\) is \(|f(\pi − \theta )|^2\). Note that this process results in particles arriving in the same places as with \(f(\theta )\) - it is just the other particles (see diagram).

If the two particle beams are distinguishable they cannot interfere and differential cross section for either particle to be detected at \(\theta\) is:

\[I_{dis} = |f(\theta )|^2 + |f(\pi − \theta )|^2 \nonumber\]

If, however, the particles are indistinguishable bosons(fermions), they can interfere and the combined wavefunction must (anti)symmetric under exchange of labels:

\[\Phi^{bos}_{fer} = f(\theta ) \pm f(\pi − \theta ) \\ I^{bos}_{fer} = |f(\theta ) \pm f(\pi − \theta )|^2 \nonumber\]

Taking the specific extreme example of scattering through \(\pi /2\), the differential cross section is \(2|f(\pi /2)|^2\) for distinguishable particles, \(4|f(\pi /2)|^2\) for identical bosons, and 0 for identical fermions.

13.2.PNG — Figure \(\PageIndex{1}\): Two indistinguishable scattering processes.