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Scattering amplitude

## Summary

In quantum physics, the scattering amplitude is the probability amplitude of the outgoing spherical wave relative to the incoming plane wave in a stationary-state scattering process.[1] At large distances from the centrally symmetric scattering center, the plane wave is described by the wavefunction[2]

${\displaystyle \psi (\mathbf {r} )=e^{ikz}+f(\theta ){\frac {e^{ikr}}{r}}\;,}$

where ${\displaystyle \mathbf {r} \equiv (x,y,z)}$ is the position vector; ${\displaystyle r\equiv |\mathbf {r} |}$; ${\displaystyle e^{ikz}}$ is the incoming plane wave with the wavenumber k along the z axis; ${\displaystyle e^{ikr}/r}$ is the outgoing spherical wave; θ is the scattering angle (angle between the incident and scattered direction); and ${\displaystyle f(\theta )}$ is the scattering amplitude. The dimension of the scattering amplitude is length. The scattering amplitude is a probability amplitude; the differential cross-section as a function of scattering angle is given as its modulus squared,

${\displaystyle d\sigma =|f(\theta )|^{2}\;d\Omega .}$

The asymptotic form of the wave function in arbitrary external field takes the form[2]

${\displaystyle \psi =e^{ikr\mathbf {n} \cdot \mathbf {n} '}+f(\mathbf {n} ,\mathbf {n} '){\frac {e^{ikr}}{r}}}$

where ${\displaystyle \mathbf {n} }$ is the direction of incidient particles and ${\displaystyle \mathbf {n} '}$ is the direction of scattered particles.

## Unitary condition

When conservation of number of particles holds true during scattering, it leads to a unitary condition for the scattering amplitude. In the general case, we have[2]

${\displaystyle f(\mathbf {n} ,\mathbf {n} ')-f^{*}(\mathbf {n} ',\mathbf {n} )={\frac {ik}{2\pi }}\int f(\mathbf {n} ,\mathbf {n} '')f^{*}(\mathbf {n} ,\mathbf {n} '')\,d\Omega ''}$

Optical theorem follows from here by setting ${\displaystyle \mathbf {n} =\mathbf {n} '.}$

In the centrally symmetric field, the unitary condition becomes

${\displaystyle \mathrm {Im} f(\theta )={\frac {k}{4\pi }}\int f(\gamma )f(\gamma ')\,d\Omega ''}$

where ${\displaystyle \gamma }$  and ${\displaystyle \gamma '}$  are the angles between ${\displaystyle \mathbf {n} }$  and ${\displaystyle \mathbf {n} '}$  and some direction ${\displaystyle \mathbf {n} ''}$ . This condition puts a constraint on the allowed form for ${\displaystyle f(\theta )}$ , i.e., the real and imaginary part of the scattering amplitude are not independent in this case. For example, if ${\displaystyle |f(\theta )|}$  in ${\displaystyle f=|f|e^{2i\alpha }}$  is known (say, from the measurement of the cross section), then ${\displaystyle \alpha (\theta )}$  can be determined such that ${\displaystyle f(\theta )}$  is uniquely determined within the alternative ${\displaystyle f(\theta )\rightarrow -f^{*}(\theta )}$ .[2]

## Partial wave expansion

In the partial wave expansion the scattering amplitude is represented as a sum over the partial waves,[3]

${\displaystyle f=\sum _{\ell =0}^{\infty }(2\ell +1)f_{\ell }P_{\ell }(\cos \theta )}$ ,

where f is the partial scattering amplitude and P are the Legendre polynomials. The partial amplitude can be expressed via the partial wave S-matrix element S (${\displaystyle =e^{2i\delta _{\ell }}}$ ) and the scattering phase shift δ as

${\displaystyle f_{\ell }={\frac {S_{\ell }-1}{2ik}}={\frac {e^{2i\delta _{\ell }}-1}{2ik}}={\frac {e^{i\delta _{\ell }}\sin \delta _{\ell }}{k}}={\frac {1}{k\cot \delta _{\ell }-ik}}\;.}$

Then the total cross section[4]

${\displaystyle \sigma =\int |f(\theta )|^{2}d\Omega }$ ,

can be expanded as[2]

${\displaystyle \sigma =\sum _{l=0}^{\infty }\sigma _{l},\quad {\text{where}}\quad \sigma _{l}=4\pi (2l+1)|f_{l}|^{2}={\frac {4\pi }{k^{2}}}(2l+1)\sin ^{2}\delta _{l}}$

is the partial cross section. The total cross section is also equal to ${\displaystyle \sigma =(4\pi /k)\,\mathrm {Im} f(0)}$  due to optical theorem.

For ${\displaystyle \theta \neq 0}$ , we can write[2]

${\displaystyle f={\frac {1}{2ik}}\sum _{\ell =0}^{\infty }(2\ell +1)e^{2i\delta _{l}}P_{\ell }(\cos \theta ).}$

## X-rays

The scattering length for X-rays is the Thomson scattering length or classical electron radius, r0.

## Neutrons

The nuclear neutron scattering process involves the coherent neutron scattering length, often described by b.

## Quantum mechanical formalism

A quantum mechanical approach is given by the S matrix formalism.

## Measurement

The scattering amplitude can be determined by the scattering length in the low-energy regime.