Edit pams/gen/pythia_6410/pyevnt.F as follows
1) rename existing SUBROUTINE PYEVNT in to SUBROUTINE PYEVNT_ORG
2) add your own version of SUBROUTINE PYEVNT, which calls PYEVNT_ORG in the infinite loop until the event you want is generated. Below is an example requiering 20 GeV Et electron in the approximate Endcap boundaries
C*********************************************************************
C...PYEVNT Filter, following Qinghua example.
C.. imitates the oryginal PYEVBT , continue to generate new events until satysfying event if found
C.. WARN: common block have been copy/paste, do NOT assume they are correct for an new Pythia version, JB
SUBROUTINE PYEVNT
C...Double precision and integer declarations.
IMPLICIT DOUBLE PRECISION(A-H, O-Z)
IMPLICIT INTEGER(I-N)
C...Commonblocks.
COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
C... event filtering, following Qinghua example.
do while (1.eq.1)
CALL PYEVNT_ORG ! oryginal method
DO I=9,N
IF(ABS(K(I,2)).EQ.24 ) THEN
px=P(i,1)
py=P(i,2)
pz=P(i,3)
print *,"W=", K(i,2), px,py,pz
ENDIF
IF(ABS(K(I,2)).NE.11) goto 555
px=P(i,1)
py=P(i,2)
pz=P(i,3)
pt=sqrt(px**2 + py**2)
tgth=pt/abs(pz)
print *,"e",K(i,2), px,py,pz, pt
if ( pt<20. ) goto 555
if ( pz<0. ) goto 555
if ( tgth<0.18 ) goto 555
if ( tgth>0.93 ) goto 555
if( abs(px) > abs(py)/3 ) goto 555
return ! the event has been accepted
555 continue
enddo
enddo ! closing 'while'
END