Photon production can be enabled in the corresponding Photon
section of the configuration file. There are the following options:
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.
Remember to also activate the photon output in the output section.
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 [50]. 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 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.
Examples: Configuring Photons
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: 2to2_Scatterings: True Bremsstrahlung: True Fractional_Photons: 1000