ALPGEN Guide

Inside and Outside CMSSW

ALPGEN Generator

ALPGEN generator - 'A collection of codes for the generation of multi-parton processes in hadronic collisions.' [1]

Reference paper: ALPGEN, a generator for hard multiparton processes in hadronic collisions, M.L. Mangano, M. Moretti, F. Piccinini, R. Pittau, A. Polosa, JHEP 0307:001,2003.

Alpha Generator - it uses the Alpha Algorithm to calculate the hard processes, instead of the Feynman Diagrams approach (that is usually slower - as an example, one can cite MadGraph generator).

To calculate the Matrix Element of the hard process, it uses perturbative QCD. It calculates each multiplicity of final state partons separately.

The Monte Carlo method is used to evaluate the integrals. However, I need to know where to calculate the integral for getting a better efficiency: if I have a phase space in which my main interest is in a small area, and I evaluate the integral using equal distributed points in my complete phase space, I am losing a lot of time and CPU effort doing a non efficient job. To solve this, first one should evaluate the phase space in a 'warm-up' run, and then give weights to each point of the phase space, in a way that when doing the further event generation, I can use this grid of weights in my phase space to better evaluate my integral. However, because of this weighted phase space, my events generated won't correspond to the proportions one should find in nature. So, to use the events as 'real' MC events in one analysis, I have to run a re-weighting process, so in the final state I'll have events in the correct proportion, and unweighted.

There are some documentation available, so one can check for further running instructions standalone and on CMSSW, as well as FAQ: