Corrections to the Smoothness and On-Shell Approximations in Femtoscopy and Coalescence

TL;DR

This paper investigates the limitations of common approximations in femtoscopy and coalescence analyses of heavy-ion collisions, deriving correction terms and demonstrating their small impact at LHC energies.

Contribution

It introduces a model-independent framework to quantify corrections to the smoothness and on-shell approximations in femtoscopy and coalescence.

Findings

Corrections are at or below the percent level for pp correlations.

First-order corrections vanish for angle-averaged correlations.

Framework is applicable to arbitrary sources and interactions.

Abstract

Relativistic heavy-ion collisions produce femtometer-scale sources whose space-time structure can be constrained using two-particle femtoscopic correlations. Standard implementations rely on the smoothness and on-shell approximations, which effectively remove the relative momentum dependence of the particle emission function. We explore the validity of these approximations by deriving model-independent expansions that quantify the leading corrections for femtoscopy and coalescence with arbitrary sources and final-state interactions. The resulting first- and second-order correction terms can be evaluated with essentially the same numerical complexity as the usual Koonin-Pratt expressions; for angle-averaged correlations the first-order contributions vanish by symmetry. We illustrate the framework with explicit calculations in a blast-wave source model. For parameter sets representative of pp and PbPb fits at LHC energies, the corrections are at or below the percent level for pp correlations and deuteron coalescence.