S-wave scattering length
= S-wave scattering length
{title2=$k\cot\delta_0(k)=-1/a_s+O(k^2)$}
The zero-energy parameter of the angular-momentum-zero partial wave. For a short-range interaction in three dimensions, its leading elastic <scattering amplitude> is $f_0(k)\simeq- a_s/(1+ika_s)$ when effective-range corrections are negligible. For identical dilute <bosons> this parameter fixes the leading contact coupling $g_B=4\pi\hbar^2a_s/m_B$.