Physical interpretation and limits of photon--dilepton radial-flow tomography

Abstract

Thermal photons and dileptons provide complementary information on the temperature and collective expansion of matter created in relativistic heavy-ion collisions. Their combination has been proposed as an electromagnetic probe of radial flow: a comparatively flow-insensitive dilepton invariant-mass inverse slope is used to infer the photon inverse slope expected without transverse flow and compared with the observed photon slope. Here the physical basis and leading limitation of this construction are examined using a controlled expanding-fireball calculation. The dilepton and flow-free photon inverse slopes are tightly correlated because they respond similarly to changes of the underlying thermal scale, although the numerical relation depends on the spectral window and electromagnetic source. The flow-induced change of the photon inverse slope correlates more strongly with the photon-emission-weighted velocity component along the photon momentum than with a bulk-like radial-flow average. Prethermal source variations can bias the inferred flow-free photon reference and generate apparent flow even when the direct photon spectral-flow signal vanishes, while multiple photon and dilepton windows provide independent response patterns that can help diagnose this source ambiguity. These results clarify both what photon--dilepton radial-flow tomography measures and the source consistency required for its interpretation.

Comments: 13 pages, 5 figures