Error message for WHIZARD FSR shower

[Juergen Reuter]

Reported by Jan Strube: The following SINDARIN input script

model = SM_CKM

mb = 0
process eebb = e1, E1 => b, B
?ps_fsr_active = true
?ps_isr_active = false
?hadronization_active = true
?ps_use_PYTHIA_shower = false

ps_max_n_flavors = 5
ps_mass_cutoff = 1 GeV

cuts = all M > 10 GeV [b,B]

sqrts = 250 GeV
beams = e1, E1 => isr
beams_pol_density = @(1), @(-1)

integrate(eebb) { iterations = 10:50000 }
simulate(eebb) {
    n_events = 10000
}

leads to the strange error message:

STOP Bug: pdf only implemented for (anti-)proton

(adopting to the new 2.2.0 syntax)

[Juergen Reuter]

This is triggered by the usage of a structure function (ISR), and presumably the presence of beam remnants in the event (the shower code tries to abuse them as protons).

[Juergen Reuter]

In Würzburg, we found out that is the usage of a structure function other than pdf_builtin or lhapdf which triggers this error. In this case, it is ISR. Some shower function tries to do something (probably color correlations) with the beam remnants and stumbles over the fact that in this case this clearly isn't a proton.