English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Direct photon production in central Pb+Pb collisions at CERN-SPS energy iscalculated within the relativistic microscopic transport model UrQMD, andwithin distinctly different versions of relativistic hydrodynamics. We findthat in UrQMD the local momentum distributions of the secondaries are stronglyelongated along the beam axis initially. Therefore, the pre-equilibriumcontribution dominates the photon spectrum at transverse momenta above $\approx1.5$ GeV. The hydrodynamics prediction of a strong correlation between thetemperature and radial expansion velocities on the one hand and the slope ofthe transverse momentum distribution of direct photons on the other hand thusis not recovered in UrQMD. The rapidity distribution of direct photons in UrQMDreveals that the initial conditions for the longitudinal expansion of thephoton source (the meson ``fluid'') resemble rather boostinvariance thanLandau-like flow.Comment: 14 pages, RevTex, 5 Encapsulated-PostScript Figure

Nonthermal direct photons in Pb+Pb at 160A GeV from microscopic transport theory