Photon production can be enabled in the corresponding Photon section in the Collision_Term one of the configuration file. Remember to also activate the photon output in the output section.

2to2_Scatterings — bool, optional, default = false

Whether or not to enable photon production in mesonic scattering processes.

Bremsstrahlung — bool, optional, default = false

Whether or not to enable photon production in bremsstrahlung processes.

Fractional_Photons — int, required

Number of fractional photons sampled per single perturbatively produced photon.

Example of photons configuration

The following example configures the photon production in both binary scatterings and bremsstrahlung processes, where 1000 fractional photons are sampled per single perturbatively produced photon. In addition, the binary photon output is enabled.

Output:
    Photons:
        Format: ["Binary"]
Collision_Term:
    Photons:
        Fractional_Photons: 1000
        2to2_Scatterings: True
        Bremsstrahlung: True

Photon production in SMASH

Photons are treated perturbatively and are produced from binary scattering processes. Their production follows the framework from Turbide et al. described in Turbide:2006zz [59]. 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:

Look for hadronic interactions of particles that are also incoming particles of a photon process. Currently, the latter include binar scatterings of \( \pi \) and \( \rho \) mesons in the case of photons from 2-to-2-scatterings or \( \pi \) scatterings in the case of bremsstrahlung photons.
Perform the photon action and write the results to the photon output. The final state particles are not of interest anymore as they are not propagated further in the evolution. To account for the probability that photon processes are significantly less likely than hadronic processes, the produced photons are weighted according to the ratio of the photon cross section to the hadronic cross section used to find the interaction,
\[W = \frac{\sigma_\gamma}{\sigma_\mathrm{hadronic}}\;.\]
This weight can be found in the weight element of the photon output, denoted as photon_weight there.
Perform the original hadronic action based on which the photon action was found. Propagate all final states particles throughout the hadronic evolution as if no photon action had occured.

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 for 2-to-2-scatterings is redefined as

\[ W = \frac{\frac{\mathrm{d}\sigma_\gamma}{\mathrm{d}t} \ (t_2 - t_1)}{ N_\mathrm{frac} \ \sigma_{\mathrm{had}}}. \]

Unlike for binary scatterings, the final state kinematics of bremsstrahlung processes are not entirly defined from the incoming particles. Moreover, the final state momentum of the photon and as well as the scattering angle with respect to the incoming pion collision axis are free parameters whose distribution is encapsulated in the the differential cross section \( \frac{\mathrm{d}^2\sigma_\gamma}{\mathrm{d}k\ \mathrm{d} \theta}\). For numerical reasons and as the differential cross section can be approximately factorized over the common \( k \) and \( \theta \) range, \( \frac{\mathrm{d}\sigma_\gamma}{\mathrm{d}k}\) and \( \frac{\mathrm{d}\sigma_\gamma}{\mathrm{d} \theta}\) are considered separately. Consequently, the weighting factor in the case of bremsstrahlung photons is redefined as:

\[ W = \frac{ \sqrt{\frac{\mathrm{d}\sigma_\gamma}{\mathrm{d}k} \ \Delta k \ \frac{\mathrm{d}\sigma_\gamma}{\mathrm{d}\theta}\ \Delta\theta} }{N_\mathrm{frac}\ \sigma_{\mathrm{had}}}\;, \]

where \( \Delta k \) and \( \Delta\theta \) correspond to the available \( k \) and \( \theta \) ranges.

Note: As photons are treated perturbatively, the produced photons are only written to the photon output, but neither to the usual collision output, nor to the particle lists.