Dear @antonino.fulci ,
Thank you for your question. Probably you miss the muons created via pair production from photons since those go directly to FLKSTK directly, instead of GENSTK (please see the post PHOTONUC card, SDUM=MUMUPAIR, biasing, suppress the photon - #3 by msacrist).
To identify those cases in USDRAW, I suggest you filter ICODE=217 (pair production from photon). After that, you only have to differentiate electron and muon pair productions. The first will place electron and positron in GENSTK (NP=2), while the second will not. Due to the biasing you may find again in GENSTK a replica of the initial photon, so for muon pair production NP=0 or 1. The last step is simply
reading the information from the last two entries of FLKSTK (muon and antimuon). One way to achive this would be:
IF ( (ICODE .EQ. 217) .AND. (NP .LE. 1) ) THEN
DO I=NPFLKA-1,NPFLKA
WRITE (IODRAW) ILOFLK(I) , PMOFLK(I) , TXFLK(I) , TYFLK(I) , TZFLK(I) , WTFLK(I)
END DO
END IF
I hope this helps.
Best regards,
Mario Sacristan