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 — Conditions on key value: None

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 thisnote").

Bremsstrahlung — bool, optional, default = false — Conditions on key value: None

Whether or not to enable dilepton production via bremsstrahlung in neutron-proton interactions. The approach follows the meson-exchange approximation depicted in Shyam:2010vr [58].

Pion_Form_Factor — string, optional, default = "Off" — Conditions on key value: Only valid strings

"Off" → Implicitly using a factor of 1.
"FF1" → Photon couples to pion direclty via \(\rho_0\) meson.
"FF2" → Photon couples 40% directly to intrinsic quark structure of pion and 60% indirectly via \(\rho_0\) meson.

This key is only relevant if Bremsstrahlung is true. Note that selecting option "FF2" leads to unphysical peaks at \(\lambda\) cutoff. Usage is therefore not recommended above the energy scales investigated by Shyam:2010vr [58].

Example of dileptons configuration

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

Output:
    Dileptons:
        Format: ["Oscar2013"]
        Extended: True
Collision_Term:
    Dileptons:
        Decays: True
        Bremsstrahlung: True
        Pion_Form_Factor: "FF1"

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 [55], 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.
String products are further weighted by their cross section scaling parameter, which depends on the formation time.
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.