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Дата индексирования: Mon Oct 1 22:42:26 2012
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QCD scale

CompHEP  substitutes the QCD strong coupling constant as a function of the ${\Lambda}^{(6)}$ parameter and the $Q^2$ scale value. It is calculated in the next-to-next-to-leading order (NLLO) and depends on a number of quark flavors (nf) with masses less than Q (see Eq.9.5a of [Particles&Fields]). Relevant constants $\Lambda^{(nf)}$ are recalculated from ${\Lambda}^{(6)}$ according to the matching equation (9.7) of [Particles&Fields].

If the user is going to change a numerical value of $\Lambda^{(6)}$ or the $Q^2$ scale definition he should call the necessary function of Menu 4. For the $Q^2$ choice he will be prompted what kind of scale he would like to implement, namely, the constant scale or the running one. In the first case a subsequent input of number in GeV units is expected. In the second case CompHEP  provides the user with a possibility to define the $Q^2$ scale as a squared sum of particle momenta. Digits on input will be transformed into ordinal momentum numbers. For example: the input '13' will be treated as $Q^2=\vert(p1-p3)^2\vert$.

If $Q^2$ is defined as a constant, the corresponding value of $\alpha = GG^2 / ( 4 \pi )$ is displayed on the screen. The $\alpha(s)$ plot for a user-defined region of s is available also by means of the corresponding function of Menu 4. The same $Q^2$ scale is used as an argument of parton distribution functions.