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Sketch of the menu system

A general scheme of CompHEP  numerical calculations is presented in Fig.. The CompHEP  screen with the menu number 1 is presented in Figs. and . Three functions of this menu launch integration programs. One of them is Monte Carlo program Vegas  [Lepage-1978,Press-1992]. The other one is the ordinary Simpson integration routine which can operate only for the simplest 2 -> 2 type of reactions. The Batch menu function is used to organize a set of Vegas  evaluations in the non-interactive mode (see Section ). Other menu items are needed to set an environment for integration. Below we briefly describe all of them. More detailed explanation is given in the following sections.

By means of the Subprocess function the user may select one subprocess for a further processing. This function is active only if several subprocesses have been evaluated symbolically before. The name of current subprocess is displayed on the screen (Fig. and Fig.). Subprocess summation is not implemented in the current version.

The In_state function gives the user an option to choose structure functions for incoming particles and assign some values for their momenta. The GeV units are assumed for the momenta. This menu function is available for collision processes only.

The Model_parameters function allows the user to assign a new value to any independent parameter involved in the evaluation. After an activation of this function the corresponding menu of parameters with information about their values is displayed. As a result of changing the independent parameters some constrained parameter can get non-evaluated as a result of a negative square root argument or division by zero in the relevant expressions. In this case an error message appears on the screen and the user is required to choose an admissible set of parameter values.

Although the user may assign some value to the QCD strong coupling parameter GG it does not influence on results of calculation. This parameter will be reset according to the rules defined by the QCD scale menu function.

CompHEP  substitutes a Breit-Wigner exact propagator for the s-channel virtual particles. This trick is needed to avoid the integral divergence near the mass pole and is motivated by high order corrections. The way of implementation of the Breit-Wigner propagator is driven by the Breit-Wigner menu function. See Section for details.

The Cuts menu function provides the user with a possibility to cut a phase space volume. A set of physical variables which could be used to construct a cut function is described in Section . Details of the Cuts menu processing are given in Section .

The Kinematics and Regularization menu functions are used to construct an appropriate mapping of phase space onto the Vegas  integration volume. This is important for successful Monte Carlo integration.