Monte Carlo generation using Pythia and QQ

Kin Yip - March 13, 2000
 

I describe below some steps in producing Monte Carlo samples, mainly for B physics studies, using   Pythia and QQ   which have been incorporated in the Dzero Run II software (mcpp/d0_mcpp). 

(A) Making Generator File:

(1)  There is already a  MCpythia.x  in every release located in
     $BFDIST/releases/$BFCURRENT/bin/$BFARCH
(all these variables are defined after one does "setup D0RunII xx.xx.xx").

One can run as    MCpythia.x -rcp runMCpythia.rcp    in which runMCpythia.rcp ^* may have

string Packages = "p1 MCpythia p3 p4"

RCP p1 = < d0_mcpp MCNewEvent >
RCP MCpythia = < d0_mcpp MCpythia >
RCP p3 = < d0_mcpp MCDumpEvent >
RCP p4 = < d0_mcpp MCWriteEvent >

assuming that you put the rcp files in the directory d0_mcpp/rcp.

^* You have to create runMCpythia.rcp yourself and you may modify from d0_mcpp/rcp/runMCisagen.rcp .

(2) The above rcp files can be found in the release area of d0_mcpp/rcp. The other rcp files which may be changed are mcvertex.rcp and seed.rcp and they can also be found in the same directory.

(3) The other files necessary for Pythia are   pythia.cards   and pythia.cmd.   Both files can be found in the release area of mcpp/doc.   pythia.cmd is customarily written so that mcpp can call pythia codes.   pythia.cards is the standard file to set all the optional parameters for Pythia.

(4) One has to stop the B's generated in Pythia from decaying and let QQ decay the B's.   This is set in pythia.cards.   For example, if you want QQ to decay B_d ,   you should set

MDCY(C511,1) = 0     in pythia.cards

(5) Another important file is "user.dec", which control how the B's would decay.   The default decays are set in "$QQ_DECAY_FILE", which is also defined after you setup D0RunII --- that in turn "setup qq" for you.   "user.dec" would overwrite the information in "$QQ_DECAY_FILE".

In order to activate "user.dec", one needs to define the variable "QQ_USER_FILE" such as
      setenv QQ_USER_FILE   ./user.dec   (in csh).

One example of "user.dec" can be found in the release area of mcpp/doc.   For example, to force B0 to decay to pi ⁺ pi ^- , one can have the following in the file "user.dec":

DECAY B0
CHANNEL 0 1.000 PI+ PI-
ENDDECAY

You probably want to look at the file "$QQ_DECAY_FILE" in order to write your own "user.dec" ("$QQ_USER_FILE").

In certain complicated (and probably rare) cases, if you really want to create your own particle in QQ, read these email correspondences.

(6) Succeeding all these steps, you would have a data file in which each event should have one MCKineChunk (from the generator Pythia/QQ).

(B) Select those events which contain the type of B's that you wish to have in your sample.

// Event tagging
string WriteEventTags = "save"
string VetoEventTags = "delete"

list< THandle<MCKineChunk> > mcchunks=_mcKey.findAll(event);
list< THandle<MCKineChunk> >::iterator  kinechunk ;
int iichunk=0;

myBflavor = -250 ;

for ( kinechunk=mcchunks.begin(); kinechunk != mcchunks.end() ; ++kinechunk ) {

  iichunk++;

  if ( iichunk == 1 ) {  // To look at the first MCKineChunk from generator, not d0gstar

    MCevt* ptevt=(*kinechunk)->ptrMCevt();  //  get pointer to MC event

    list listp;
    ptevt->getParticles(listp);   //  get complete list of particles
    list::iterator jj;

    int num_B = 0 ;
    for ( jj=listp.begin(); jj != listp.end()  ; ++jj ) {

      if ( (*jj)->isaId() == myBflavor ) {
        num_B++ ;
	// do something ...
      }
    }	

    // "save" and "delete" are used in consistent with what it is put in the RCP file above
    if ( num_B > 0 ) {
      event.tag("save") ;
      // do something ...
    }
    else {
      event.tag("delete") ;
      // do something ...
    }

  }

}