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Дата изменения: Wed Aug 9 20:40:47 2000
Дата индексирования: Mon Oct 1 22:43:27 2012
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Integration by means of the Simpson method

In the case of 2->2 type of process we provide the user with a possibility to calculate the cross-section using the adaptive Simpson algorithm. It allows to get the results instantly and with a high precision.

Note that some user's settings are ignored when you choose this option. Namely, the structure functions are switched off, the center-of-mass rapidity is assigned to zero, regularizations are also ignored and only angle and cosine cuts for incoming and outgoing particles are taken into account.

When the Simpson menu function is activated CompHEP  tries to evaluate the cross-section. Sometimes it is not possible, for example, if the corresponding value is infinite, as we have in the case of $e^+,e^+ \to e^+,e^+$ reaction. When program detects a pole inside of the integration region it produces an error message like

Division by zero

Press any key

In the above case you should remove the region of small angle scattering by applying the corresponding cuts and after that activate the Simpson function once more.

If the cross-section is successfully integrated the result is displayed on the screen. Menu 6 which is also displayed on the screen (see Fig.) has the following options:

a) to set relative precision of integral evaluation. A default value is $10^{-4}$;

b) to display dependence of the differential cross-section on the cosine of the scattering angle;

c) to calculate and display on the screen the total cross-section or the forward-backward asymmetry as a function of any parameter involved in the calculation.