Question about Particle and Selecting Certain Events

[usernew]

Hello,

I am generating a collision event.

I would like to know how can I select events that contain z bosons and furthermore only select events that have three z bosons and then select only a pair of z bosons at a time.

I tried looking at Example1.C to get an idea. I have written this down so far:

if(branchParticle->GetEntries() >= 3)

{

if(TMath::Abs(gen->PID==23))
{

z1 = (GenParticle *) branchParticle->At(0);
z2 = (GenParticle *) branchParticle->At(2);

histzmass->Fill(((z1->P4()) + (z2->P4())).M());

}
}

However, one mistake I notice is that I do not think this only selects a pair of z bosons at a time because I can write At(0) and At(5) even though I thought I specified with
if(branchParticle->GetEntries() >= 3) that I am looking at an event with three bosons. What does At() actually do then and how can I properly achieve what I am looking for?

Thank you very much!

[usernew]

Also, how would you match mother particle with daughter particle. I.e. how do you know which daughter particle came from which mother particle.