double invMass = invMass(t1,t2);
histogram("Inv Mass").fill(invMass);

if (Math.abs(invMass-mZ0) < 5)
{
    getDebugStream().println("Found a Z, mass="+invmass);
}

if (Math.abs(invMass-mZ0) < 5)
{
    getDebugStream().println("Found a Z, mass="+invmass);
    throw new RuntimeException("Stop! invmass="+invmass);
}