How to run a QED fit

It is possible to perform a QED fit adding the key fiatlux to the runcard. In this way a photon PDF will be generated using the FiatLux public library that implements the LuxQED (see [MNSZ16] and [MNSZ17]) approach. The parameters to be added are the following:

fiatlux:
  luxset: NNPDF40_nnlo_as_01180
  additional_errors: true
  luxseed: 1234567890

luxset is the PDF set used to generate the photon PDF with FiatLux <https://github.com/scarrazza/fiatlux/>. The code will generate as many photon replicas as the number of replicas contained in the luxset. Therefore, if the user tries to generate a replica with ID higher than the maximum ID of the luxset, the code will raise an error. Moreover, being the LuxQED approach an iterated prcedure, and given that some replicas do not pass the postfit selection criteria, the user should make sure that the number of replicas in the luxset is high enough such that in the final iteration there will be a number of replicas higher than the final replicas desired. additional_errors is the parameter that switches on and off the additional errors of the LuxQED approach, while luxseed is the seed used to generate such errors. This errors should be switched on only in the very last iteration of the procedure.

Whenever the photon PDF is generated, it will remain constant during the fit and will enter in the MSR.