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Дата изменения: Wed Aug 9 20:40:47 2000
Дата индексирования: Mon Oct 1 22:36:42 2012
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Input of the process

After activating the Enter process function of Menu 3 the list of particles together with their notation conventions is displayed. The notation of anti-particle is shown in parentheses after that of particle. In the case of the Standard Model the corresponding screen is shown in Fig.. If the list is too long one may use the PgUp  and PgDn  buttons to scroll it.

In the bottom part of the screen the prompt 'Enter process:' appears. The syntax for the input is:
P1[,P2] -> P3,P4 [,...,[N*x]]

These 'P1'..'P4' are particle names, N is a number of inclusive particles. The sets of in- and out-particles are separated by the arrow '->' formed of two printing symbols. The particles inside of each set are separated by commas. The total number of particles should not exceed 6.

For example, the input $u,U$ -> $G,G$ denotes the process of annihilation of the $u$-quark and $\bar{u}$-quark into two gluons.

One can also construct inclusive processes. For example, the input
u,U -> G,G,2*x
is a request to construct all processes of annihilation of the $u$-quark and $\bar{u}$-quark into two gluons accompanied by two arbitrary particles.

If the program finds an unknown name among the in-particles it will try to consider it as a name of composite particle and will ask you about its parton contents. For instance, after the input
e,p -> 3*x
the question appears:
Is 'p' a composite particle Y/N ?
If you choose 'Y' you will be prompted to specify the parton structure of 'p'. A possible input is
'p' consists of: $u,U,d,D,G$

If one enters a collision process, the information on total energy of colliding particles in the center-of-mass system is demanded:
Enter Sqrt(S) in GeV: 300

CompHEP  generates only those channels where the total mass of incoming particles and the total mass of outgoing particles are smaller than Sqrt(S).

On the next step of input you are prompted to exclude diagrams with specified virtual particles. The input should be
Exclude diagrams with : $P1>n1$ [$,P2>n2$ ...]
where $P1,P2,...$ are particle names, $n1,n2,..$ are the quantity limits. Such an input means that diagrams where the number of virtual particles $P_i$ is more than $n_i$ will not be constructed. For example:
Exclude diagrams with : $W+>1$
means that only those diagrams will be generated which contain less than two virtual W-bosons.

Several restrictions separated by commas are allowed. If one has a restriction for a particle, the restriction for the corresponding anti-particle is not needed. If one would like to forbid the appearance of some virtual particle $P$ at all, the input "$P>0$" may be shortened to the "$P$".

This option may be used to exclude diagrams which are suppressed due to a large virtual particle mass, or a small coupling constant, or for some other reasons. Use the empty input to get a full set of diagrams.

Use the Esc  key to return to the previous level of input and the F1 key to get the online help.

After the input is completed CompHEP  starts the Feynman diagram generation. If the number of generated diagrams is zero the corresponding warning appears and you return to the beginning of process input, otherwise the next menu appears.