Monte-Carlo Generator for the Drell-Yan Events

V.Mossolov, N.Shumeiko,NC PHEP BSU, Minsk

V.Zykunov, GSTU,Gomel

IX International School-Seminar

The Actual Problems Of Microworld Physics

Belarus, Gomel, July23 - August 3, 2007

Motivation

Why we need a new generator?

The task of the automation of the computations in high energy physics has been appeared a long time ago. There are a lot of program for such automation. Several of them (such as COMHEP, PYTHYA, SANC ...) are positioned as a universal tools, i.e. they a able to perform automatic calculation of the cross sections and other observables for all processes.

Unfortunately all of them have several limitations. So time to time physicists are forced to build its own codes specialized for studied processes.

Why we need a new generator?

To solve the problem of the precise calculation of the first order radiative correction`s we need:

a) to eliminate the UV- and IR-divergencies on the analytical step;

b) to perform the numerical integration of the resulting integrands

Why we need a new generator?

When this work start we have no automatic tools to perform the first step. So the formulas from previous talk has been used.

But we try to construct the numerical integrator that could be work with the one obtained by appropriate automatic tools only.

Why we need a new generator?

The main goal is to make the program that can numerically calculate some kind of improper integral (after UV-regularization) and work wit integrand containing the sharp peaks.

One can remember some good codes like VEGAS or FOAM and the last program probably can solve this task but with a long time manual adjustment.

Why we need a new generator?

And the last but not least advantage of the proposed algorithm is his ability for perform the cross section`s calculation and event generation simultaneously.

Algorithm

The cross sections we need to calculate is

σ1(M,…)=(σ0(M,…)+σV(M,…) )+

σR(M,…),

where M – is the μ+ μ--pair mass,

… – is any other observable (y, PT etc)

Algorithm

The main Idea of the algorithm can be explained in a easiest way by this picture:

I(f) = Σ Wi

Wi= Vi * Fi

Wi ≈ Wj

Algorithm

Cross section (preliminary)

Conclusion

The algorithm of the numerical integration has been realized as FORTRAN-90 code which is able to work as a part of the automatic tool for whole cycle of precise calculation cross section and observables taking into account 1-loop contributions.