# Scattering amplitude

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. At large distances from the centrally symmetric scattering center, the plane wave is described by the wavefunction
    $\psi (\mathbf {r} )=e^{ikz}+f(\theta ){\frac {e^{ikr}}{r}}\;,$
where $\mathbf {r} \equiv (x,y,z)$ is the position vector; $r\equiv |\mathbf {r} |$; $e^{ikz}$ is the incoming plane wave with the wavenumber k along the z axis; $e^{ikr}/r$ is the outgoing spherical wave; θ is the scattering angle (angle between the incident and scattered direction); and $f(\theta )$ 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,
    $d\sigma =|f(\theta )|^{2}\;d\Omega .$
The asymptotic form of the wave function in arbitrary external field takes the form
    $\psi =e^{ikr\mathbf {n} \cdot \mathbf {n} '}+f(\mathbf {n} ,\mathbf {n} '){\frac {e^{ikr}}{r}}$
where $\mathbf {n}$ is the direction of incidient particles and $\mathbf {n} '$ 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
    $f(\mathbf {n} ,\mathbf {n} ')-f^{*}(\mathbf {n} ',\mathbf {n} )={\frac {ik}{2\pi }}\int f(\mathbf {n} ,\mathbf {n} '')f^{*}(\mathbf {n} ,\mathbf {n} '')\,d\Omega ''$
Optical theorem follows from here by setting $\mathbf {n} =\mathbf {n} '.$
In the centrally symmetric field, the unitary condition becomes
    $\mathrm {Im} f(\theta )={\frac {k}{4\pi }}\int f(\gamma )f(\gamma ')\,d\Omega ''$
where $\gamma$ and $\gamma '$ are the angles between $\mathbf {n}$ and $\mathbf {n} '$ and some direction $\mathbf {n} ''$. This condition puts a constraint on the allowed form for $f(\theta )$, i.e., the real and imaginary part of the scattering amplitude are not independent in this case. For example, if $|f(\theta )|$ in $f=|f|e^{2i\alpha }$ is known (say, from the measurement of the cross section), then $\alpha (\theta )$ can be determined such that $f(\theta )$ is uniquely determined within the alternative $f(\theta )\rightarrow -f^{*}(\theta )$.

## Partial wave expansion

In the partial wave expansion the scattering amplitude is represented as a sum over the partial waves,
    $f=\sum _{\ell =0}^{\infty }(2\ell +1)f_{\ell }P_{\ell }(\cos \theta )$,
where f$_ℓ$ is the partial scattering amplitude and P$_ℓ$ are the Legendre polynomials. The partial amplitude can be expressed via the partial wave S-matrix element S$_ℓ$ ($=e^{2i\delta _{\ell }}$) and the scattering phase shift δ$_ℓ$ as
    $f_{\ell }={\frac {S_{\ell }-1}{2ik}}={\frac {e^{2i\delta _{\ell }}-1}{2ik}}={\frac {e^{i\delta _{\ell }}\sin \delta _{\ell }}{k}}={\frac {1}{k\cot \delta _{\ell }-ik}}\;.$
Then the total cross section
    $\sigma =\int |f(\theta )|^{2}d\Omega$,
can be expanded as
    $\sigma =\sum _{l=0}^{\infty }\sigma _{l},\quad {\text{where}}\quad \sigma _{l}=4\pi (2l+1)|f_{l}|^{2}={\frac {4\pi }{k^{2}}}(2l+1)\sin ^{2}\delta _{l}$
is the partial cross section. The total cross section is also equal to $\sigma =(4\pi /k)\,\mathrm {Im} f(0)$ due to optical theorem.
For $\theta \neq 0$, we can write
    $f={\frac {1}{2ik}}\sum _{\ell =0}^{\infty }(2\ell +1)e^{2i\delta _{l}}P_{\ell }(\cos \theta ).$

## X-rays

The scattering length for X-rays is the Thomson scattering length or classical electron radius, r$_0$.

## 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.