RAMBO Generator

The Rambo algorithm for flat phase-space generation is a standalone utility which is not used by VINCIA internally (VINCIA uses the antenna shower as its phase space generator instead). We (the authors) use the Rambo implementation mostly to make (un)weighted phase-space scans for comparisons between matrix elements and antenna shower expansions, but it is obviously a tool with a great variety of possible uses. If you use it, please add a reference to


When the VINCIA plugin is constructed, a member object called rambo is automatically constructed and initialized inside the plugin. As mentioned, this object is not actually used by VINCIA, but is available to the user if needed.

A uniformly distributed (flat) n-particle phase-space point can then be generated by using the member function
rambo.genPoint(double S,int nPartons,double p[4][8]);
where S is the desired center-of-mass energy squared, nPartons <= 8 sets the multiplicity, and the momenta are returned in the double array p, whose entries can be converted to PYTHIA 8's Vec4 format in the following way:
Vec4 VecP;
where 0 <= iParton <= nPartons-1.

If the particles are massive, the syntax is
double rambo.genPoint(double S, int nPartons, double m[8], double p[4][8]);
where m contains the masses of the particles. The return value is the weight of the phase space point, which is NOT unity for nonvanishing masses:
w = (Σi|pi|/ECM) (2*nPartons-3)i |pi|/Ei) ECM/( Σi (pi)2/Ei )

If desired, further Rambo generators could in principle be created using the following Rambo constructor
Vincia::Rambo myRambo(myRndmPtr);
where myRndmPtr should be a pointer to an instance of PYTHIA's Rndm class. For most purposes, however, the automatically initialized rambo object already provided inside the VINCIA plugin should be sufficient.