Main-Program and Related Settings
Introduction
The main program is up to the user to write. However,
sample main programs
are provided. In one such class of programs, key settings of the run
are read in from a "cards file". For experimental collaborations
this is actually the most common way to run a program like PYTHIA.
The commands in such a file may be of two types
(a) instructions directly to Pythia, like which
processes to generate, and
(b) instructions to the main program for what it should do,
like how many events to generate, and how many events should
be listed.
In principle these two kinds could be kept completely separate.
However, to make life simpler, a few useful main-program
settings are defined on this page, so that they are recognized by
the Settings machinery. They can thus be put among
the other cards without distinction. It is up to you to decide which
ones, if any, you actually want to use when you write your main program.
To further reduce the necessary amount of main-program code, some of
the tasks that you can steer from your main program can also be done
internally. This in particular relates to some information printing.
To give an example, pythia.event.list() can be inserted
to print an event, i.e. all the particles belonging to it. Given the
length of these listings one would list only a few events at the
beginning of the run, to get some feeling for the character of events.
This could be achieved e.g. with a main-program statement
if (iEvent < 3) pythia.event.list()
to list the first three events in a loop over iEvent,
after pythia.next() has been used to generate the next
event. Alternatively a Next:numberListEvent = 3
setting, e.g. in a command file, would achieve the same, by an
internal call at the end of pythia.next().
The settings names on this page thus fall into four main groups
Init:... denote actions that automatically may be
taken during the pythia.init() call.Next:... denote actions that automatically may be
taken during the pythia.next() call.Stat:... denote actions that automatically may be
taken during the pythia.stat() call.Main:... denote actions that you yourself
have the freedom to make use of in your main program.
Initialization settings
Print a list of all processes that will be simulated, with
their estimated cross section maxima, as used for the
subsequent Monte Carlo selection. Also print corresponding
Les Houches initialization data, where relevant.
Print initialization information for the multiparton interactions
machinery.
Print a list of the changed flag/mode/parameter/word settings.
Print a list of all flag/mode/parameter/word settings.
Warning: this will be a long list.
Print a list of particle and decay data for those particles
that were changed (one way or another).
In the previous listing also include the resonances that are
initialized at the beginning of a run and thus get new particle
data, even if these may well agree with the default ones.
Warning: this will be a rather long list.
Print a list of all particle and decay data.
Warning: this will be a long list.
Print particle and decay data for the particle with this particular
identity code. Default means that no particle is printed.
Event-generation settings
Print a line telling how many events have been generated so far,
once every numberCount events. If set zero then no
lines are ever printed.
The number of events to list the Les Houches input information for,
where relevant.
The number of events to list the Info information for,
where relevant.
The number of events to list the process record for,
where relevant.
The number of events to list the event record for,
where relevant.
In addition to the normal information in the listing of the
process and event records, a second line
per particle provides information on the production scale,
particle polarization and production vertex.
In addition to the normal information in the listing of the
process and event records, further lines
list all the mothers and daughters of each particle.
Statistics
Print the available statistics on number of generated events and
cross sections, where relevant.
Print the available statistics on number and types of multiparton
interactions, where relevant.
Print the available statistics on number and types of
aborts, errors and warnings.
Reset the statistics of the above three kinds. The default is that
all stored statistics information is unaffected by the
pythia.stat() call. Counters are automatically reset
in each new pythia.init() call, however, so the only time
the reset option makes a difference is if stat()
is called several times in a (sub)run.
Main-program settings
The settings in this section must be under the control of the
user, i.e. there are no internal equivalents. The first one is especially
important and would be a standard feature of any separate command file.
The number of events to be generated.
The number of events to be tried during generation. Any number smaller than one
means that the setting will be ignored.
The number of events to be selected during generation. Any number smaller than
one means that the setting will be ignored.
The number of events to be accepted during generation. Any number smaller than
one means that the setting will be ignored.
Allow this many times that pythia.next() returns false,
i.e. that an event is flawed, before aborting the run.
The Main:... options works like this. Once you have used
the pythia.readFile(fileName) method to read in the cards
file, where the values have been set, you can interrogate the
Settings database to make the values available in your
main program. A slight complication is that you need to use a different
Settings method for each of the four possible return types
that you want to extract. To save some typing the same method names are
found directly in the Pythia class, and these just send on
to the Settings ones to do the job, e.g.
int nEvent = pythia.mode("Main:numberOfEvents");
The area of subruns is covered separately below. A few spares are also
defined after that, for unforeseen applications.
Subruns
You can use subruns to carry out
several tasks in the same run. In that case you will need repeated
instances of the first setting below in your command file, and could
additionally use the second and third as well.
The number of the current subrun, a non-negative integer, put as
first line in a section of lines to be read for this particular subrun.
If you read several Les Houches Event Files that you want to see
considered as one single combined event sample you can set this flag
on after the first subrun to skip (most of) the
(re-)initialization step.
The number of subruns you intend to use in your current run.
Unlike the two settings above, Pythia itself will not
interpret this number, but you could e.g. have a loop in your main
program to loop over subruns from 0 through
numberOfSubruns - 1.
Spares
For currently unforeseen purposes, a few dummy settings are made
available here. The user can set the desired value in a "cards file"
and then use that value in the main program as desired.