Dilepton production can be enabled in the corresponding Dileptons section in the Collision_Term one of the configuration file. Remember to also activate the dilepton output in the output section.

Decays — bool, optional, default = false

Whether or not to enable dilepton production from hadron decays. This includes direct decays as well as Dalitz decays. Dilepton decays additionally have to be uncommented in the used decaymodes.txt file (see also this note).

Example of dileptons configuration

The following example configures the dilepton production for dileptons originating from resonance decays. In addition, the extended OSCAR2013 dilepton output is enabled.

Output:
    Dileptons:
        Format: ["Oscar2013"]
        Extended: True
Collision_Term:
    Dileptons:
        Decays: True

Dilepton production in SMASH

The treatment of Dilepton Decays is special:

Dileptons are treated via the time integration method, also called shining, as e.g. described in Schmidt:2008hm [48], chapter 2D. This means that, because dilepton decays are so rare, possible decays are written in the output at every hadron propagation without ever performing them. The are weighted with a "shining weight" to compensate for the over-production.
The shining weight can be found in the weight element of the output.
The shining method is implemented in the DecayActionsFinderDilepton, which is automatically enabled together with the dilepton output.

Note: If you want dilepton decays, you have to modify the decaymodes.txt file of your choice, which you then specify as the input with the -d command line option. Without this decay modes modification the dilepton output will be empty. Dilepton decays are commented out by default. Therefore, you need to uncomment them. For the N(1520) Dalitz decay, two treatments are available: Either by proxy of the \(\rho N\) decay, which is enabled by default (and leads to a dilepton Dalitz decay, if \(\rho \rightarrow e^+e^-\) is also enabled) or as a direct Dalitz decay to \(e^+e^- N\). If using the latter comment-out the \(\rho N\) decay to avoid double counting. The form factor in the direct case, is constant and fixed at the real photon point. Furthermore note, that for dilepton decays, new decay channels can not simply be added to the decaymodes.txt file. You also have to modify the decay width formulas TwoBodyDecayDilepton::width and ThreeBodyDecayDilepton::diff_width in decaytype.cc file.