The existance of a photon subsection in the output section of the configuration file enables the photon output together with the possible photon producing scattering processes. If photons are enabled, the output file named Photons (followed by the appropriate suffix) is generated when SMASH is executed. It's format is identical to the collision output (see OSCAR Collisions Format), it does however only contain information about all particles participating in the photon producing interactions at each timestep.
Further, the block headers differ from the usual collision output:
where
density:
Density at the interaction point photon_weight:
Weight of the photon process relative to the underlying hadonic interaction. Make sure to weigh each photon in your analysis with this value. Otherwise the photon production is highly overestimated. part_weight:
Always 0.0 for photon processes, as they are hardcoded. proc_type:
The type of the underlying process. See Process Types for possible types.Note, that "interaction", "in", "out", "rho", "weight", "partial" and "type" are no variables, but words that are printed.
The photon output is available in binary, OSCAR1999, OSCAR2013 and OSCAR2013 extended format.
Photons are treated perturbatively and are produced from binary scattering processes. Their production follows the framework from Turbide et al. described in Turbide:2006. Following the perturbative treatment, the produced photons do not contribute to the evolution of the hadronic system. They are rather direcly printed to the photon output. The mechanism for photon production is the following:
photon_weight
in the above.As photons are produced very rarely, a lot of statistics is necessery to yield useful results. Alternatively, it it possible to use fractional photons (see Content-specific output options on how to activate them). This means that for each produced photon, \( N_{\text{Frac}} \) photons are actually sampled with different kinematic properties so that more phase space is covered. In case fractional photons are used, the weight es redefined as \( W = \frac{\frac{\mathrm{d}\sigma_\gamma}{\mathrm{d}t} \ (t_2 - t_1)}{ N_\mathrm{frac} \ \sigma_{\mathrm{had}}} \).