Low-energy scattering length
For short-range three-dimensional elastic central scattering without a threshold singularity, the s-wave scattering length is a_s=−lim_(k→0)tanδ₀/k.
Key knowledge
For short-range three-dimensional elastic central scattering without a threshold singularity, the s-wave scattering length is a_s=−lim_(k→0)tanδ₀/k. When s waves dominate and k|a_s|≪1, σ_el≈4πa_s². For an attractive spherical well of radius R and depth V₀>0, zero-energy matching gives a_s=R−tan(qR)/q, q=√(2mV₀)/ħ.
Values and equations
a_s=−lim_(k→0)tanδ₀/k; σ_el≈4πa_s²; a_s=R−tan(qR)/q (attractive finite well)
- a_s,R
- Scattering length and well radius m
- k,q
- Exterior/zero-energy interior wavenumbers m⁻¹
- δ₀
- s-wave phase shift rad
- σ_el
- Elastic cross section m²
- V₀
- Positive well depth J
- m
- Scattering/relative mass kg
- ħ
- Reduced Planck constant J s
Conditions and scope
- Finite-range real potential, regular origin, nonrelativistic elastic distinguishable spinless particles and low kR; no higher-partial-wave threshold resonance. The displayed constant cross-section approximation also needs finite a_s and k|a_s|≪1.
- Near tan(qR) poles, a shallow bound-state/threshold resonance makes a_s large and the constant-cross-section limit is not uniform; retain the energy-dependent phase shift and unitarity bound. Long-range tails and identical-particle factors are outside this formula.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Quantum Mechanics (2016): Low energy scattering: Equations 10.118–10.124: exact well/barrier radial matching; attractive-well low-energy phase 10.126 and cross section 10.127–10.129. Reexpress δ₀≈k[tan(qR)/q−R] as a_s; excludes unqualified s-wave dominance at threshold resonance and the paragraph's mistaken 10.124 reference for its attractive-well matching.
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