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The genEvents command

To restore events from the events_N file the user should use the
$COPMHEP/genEvents command. In being launched it is waiting for user commands. These instructions are read from the standard input and the outcome is directed to the standard output. Any instruction should be terminated by the end-of-line symbol (the Enter  key).

The first instruction should contain an event file name, say, event_1. If such a file exists and has an appropriate format, genEvents writes the process name, e.g.
PROCESS: e1,E1 -> e2,E2
Then the line(s) with a physical cut definition could follow. The format of these lines is:
[function identifier] [min.limit] [max.limit]
See Section for a format of the function identifier. The limits are just numbers. For the case of the S function instead of the values of squared momentum Val the user has to put in the quantity $Val/\sqrt{\vert Val\vert}$, similar to the case of cut definition in Section .

The last line of input may be empty or contain a desired distribution specification. For the first case the weighted event flow will be created. Otherwise the corresponding distribution will be produced. The format of input which specifies a distribution is the same as that for cuts, but contains an addition field which defines the number of bins:
[function identifier] [min.limit] [max.limit] [number of bins]
The number of bins has not to exceed 300.

The genEvents command verifies the input. But the user-friendly interface is not implemented here. For the case of wrong input the error message appears and the program terminates. So, in order to escape the repetition of typing we recommend to store the input in a file and pass it to genEvents by
$COMPHEP/genEvents < user_input_file
or
cat user_input_file | $COMPHEP/genEvents

Let us give an example of user_input_file:

event_1

A13 5 175

A15 5 175

M35 1 100 50

It is an instruction to build a 50 bin distribution for the invariant mass of $3^{rd}$ and $5^{th}$ particles under condition of a 5 degree cut for the angle between the momenta of these particles and the collision axis.

To get a matching of particles with their momenta see the process string in the corresponding protocol file f_prt_N.

The output for the distribution has got the following formats.
1. The process string:
PROCESS: e1 , E1 -> e2 , E2
2. The name of the X-axis in double quotas, limits for the axis and a number of bins:
"Cosine(p1,p3)" from $-1.0$ to $1.00$ $N\_bins= 10$
3. The name of the Y-axis:
Diff. cross section [pb]
4. The sequence of $N\_bins$ lines for the averaged differential cross section for each bin and its statistical uncertainty separated by "+/-":
..............
8.0978E+02 +/- 9.2786E+00
...............

The following normalization of data is assumed: a sum of all values of the first column multiplied by the bin size is equal to the cross section.

The graphical image of the distribution may be displayed by the
$COMPHEP/tab_view command. One of the ways to use it is the long pipe:

cat user_input_file | $COMPHEP/genEvents |$COMPHEP/tab_view

The options of the tab_view program are described in Section . See Fig. as an example of plot representation. One more possibility is to save the intermediate result and then use it:
cat user_input_file | $COMPHEP/genEvents > hist_file
$COMPHEP/view_tab < hist_file.

To generate the event flow the user has to enter an empty line instead of the distribution definition. After that the genEvents program writes down the weighted events in the standard output. The first six lines of output contain general information about the process, namely the process name, masses of particles, structure function records, the QCD scale, and values of center-of-mass energy and rapidity. For example,

 PROCESS: e1 , E1    ->  e2 , E2
 MASSES:   0.000000E+00  0.000000E+00  1.057000E-01  1.057000E-01
 StructFun1: OFF
 StructFun2: OFF
 QCD constant scale: 9.12E+01, Lambda6=1.18E-01 GeV
 SQRT(S):   9.000E+01
 Rapidity:   0.000E+00

The sign of rapidity is defined in such a way that its change towards a positive value corresponds to an acceleration of the first incoming particle.

The seventh line contains the column titles. The following lines contain columns of numerical information about events. In the first column the event weights are written down. They are normalized so that the sum of all numbers of the first column gives the calculated cross section or decay width. The other columns contain the components of three-momenta of particles. The corresponding momentum and component numbers are described by the seventh (title) line of the output. For example, P2_3 means the third component of the second momentum. Note that the $3^{rd}$ component is assigned to the collision axis. The first incoming particle has this component positive while the second one has it negative. There are no columns for the momenta of incoming particles which are equal to zero by definition, for example, for the $1^{st}$ and $2^{nd}$ components of momenta of colliding particles.

The event flow output is normally very large, so we recommend to redirect the output to the subsequent program for some processing

cat user_input_file | $COMPHEP/genEvents | userProcessing

A few such programs were created in the framework of CompHEP  project. They are disposed in the $COMPHEP  directory.

I. unweight. It transforms weighted events into unweighted ones. It needs a parameter $maxw$ which specifies the maximum value of weights. Indeed this program transforms the floating number weight into the integer number one according to the following expression

$$floor(\frac{weight}{maxw}) + p\left(\frac{weight}{maxw} - floor(\frac{weight}{maxw})\right)\;\;,$$

where $floor(x)$ function rounds $x$ downwards to the nearest integer and $p(x)$ is the random function which equals 1 with probability $x$ and zero otherwise. If the integer weight does not equal zero then this event is written down into the output in the same format as the unweighted event flow. The true maximum weight may be found in the protocol file.

II. filterD2. This program generates decays of one of outgoing particles into two others. It needs 6 arguments:
1) name of disintegrated particle;
2) name of the first decay particle;
3) numerical value of the mass of the first particle;
4) name of the second decay particle;
5) numerical value of the mass of the second particle;
6) branching fraction for this decay mode. For example,

filterD2 Z e2 0.1 E2 0.1 0.034

will generate decays of the Z boson into muons. This program reads event flow from the standard input and writes down generated events into the standard output. It can be used just after genEvents and after unweight.

III. filterDN This function can generate more complicated sub-decays, for example, 1->3 ones. It is assumed that before its usage the events of corresponding decay have been generated by CompHEP, transformed into the unweighted format, and stored in some file. The name of this file must be passed to filterDN as the first argument. Numerical value of the corresponding branching fraction must be passed as the second argument.

You can obtain a wrong result using some part of the whole event sample because Vegas  generates strongly correlated events according to the Stratified Sampling algorithm (see Section ). If events are written down in some file they may be mixed up by means of the $COMPHEP/randomize program. The first argument of this function is a name of the file where events are stored. The randomized event flow is directed to the standard output.

The programs mentioned above, namely unweight, filterD2, filterDN, randomize, can have three optional arguments. They must be some integers which are used to re-initialize the starting point of the random number generator ¹.

The generated event flow can be transformed to the table of distribution of some physical variable by means of the mk_tab program. This program needs four arguments, namely, a name of variable in format of Section , minimum and maximum limits of this variable and a number of bins, which cannot exceed 300. The mk_tab program reads the event flow in the standard input and writes the generated table down to the standard output. This table can be transformed into a plot by means of the tab_view utility.

Footnote

1  We use standard random number generator drand48.