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Menu 5.

This menu appears on the screen just after construction of Feynman diagrams and together with the information about numbers of diagrams and subprocesses generated.

The first function of this menu (Squaring) is the instruction for CompHEP  to create squared diagrams. CompHEP  uses the squared diagram technique for evaluation of squared matrix elements. See Section for details.

The View diagrams function gives you a tool to view a graphic representation of generated Feynman diagrams, to remove some diagrams before the squaring and to create the L^ATEX  output for undeleted diagrams.

If a few subprocesses have been generated then the subprocess menu appears after an activation of the View diagrams menu function. There is a possibility to remove all diagrams in the highlighted subprocess by pressing the F7 button. In its turn the F8 key restores all diagrams of the highlighted subprocess deleted before.

When you choose a subprocess the diagram graphic viewer is launched. See Section for details or use the $F1$ and $F2$ functional keys for online help.

Here we would like to note some peculiarities of constructed diagrams.

• Incoming particles are drawn on the left side of diagrams, whereas the outgoing ones are shown on the right.
• Particles with spins $0,\,\frac{1}{2}\,,1$ are represented by the dotted, solid, and dashed line correspondingly.
• Charged particles are represented by arrow lines. The arrow indicates the direction of particle (not anti-particle) propagation.
• Incoming and outgoing particles are labeled by relevant names in the end of the line. Virtual particles are labeled by their names at the mid-line. If a particle is not self-conjugated, the particle's name is used for the labeling (not the anti-particle's one).
• In the case of collision process a first colliding particle is disposed in the top part of picture, whereas the second one is drawn in the bottom part.
• CompHEP  produces only one representative of a set of diagrams which can be transformed one to another by replacing the identical outgoing particles. For example, CompHEP  creates only one diagram for the $e1,E1$ -> $A,A$ process, whereas all textbooks present two diagrams in this case. In other words, momenta are not assigned to the outgoing particles on the step of diagrams generation.

• CompHEP  does not generate diagrams with Faddeev-Popov and Goldstone ghosts¹. Such diagrams are restored on the step of squared diagram evaluation. Any ghost has a real particle as a prototype (see Section ). As a rule, a diagram with ghosts has got the parent diagram where ghosts are replaced by real particles. During the calculation of these parent diagrams the contributions of the corresponding ghost diagrams are also calculated and added to the contribution of the parent one (see Section ). There exist some exceptions from this rule. For example, the Standard Model contains the vertex with four Goldstone bosons associated with the $Z$-boson, however the $Z^4$ vertex is absent in the theory. To provide a possibility to take into account the ghost diagrams with a four-Goldstone vertex CompHEP  generates false diagrams with $Z^4$ interaction.

• Vertices with a complicated color structure, for example, the four-gluon one, are implemented by means of unphysical tensor field. This field is treated by CompHEP  as a special gluon ghost and does not appear in the constructed diagrams. A contribution of the four-gluon vertex is restored when squared diagrams are evaluated, in the same manner as the contribution of Faddeev-Popov and Goldstone ghosts. See Sections , , for further explanations.

Footnote

1  In the following text we use Goldstone ghost instead of commonly used Goldstone boson. For our convenience we name both of these two kinds of fields and the auxiliary tensor field as ghosts.