Generic Settings
Here are collected a few scattered minor topics, controlled from the
main Pythia
class. Currently this concerns how to set the
seed of the random numbers and how to regulate error checks on the
generated event.
Random numbers
The seed of the random number generator can be set here:
flag
Pythia:setSeed
(default = off
)
Indicates whether a user-set seed should be used every time the
Pythia::init
routine is called. If off, the random number
generator is initialized with its default seed at the beginning
of the run, and never again. If on, each new Pythia::init
call (should several be made in the same run) results in the random
number being re-initialized, thereby possibly starting over with the
same sequence, if you do not watch out.
mode
Pythia:seed
(default = -1
; maximum = 900000000
)
The seed to be used, if setSeed
is on.
A negative value gives the default seed,
a value 0 gives a random seed based on the time, and
a value between 1 and 900,000,000 a unique different random number
sequence.
Error Checks
There is also a few settings related to error checking.
flag
Pythia:checkEvent
(default = on
)
When an event has been successfully generated, check that the
final event record does not contain any unphysical particles, or
nonconserved charge or energy-momentum. If this check fails, then
pythia.next()
obtains the value false
.
mode
Pythia:nErrList
(default = 0
)
The number of erroneous events, in the above check, for which
event listing and other detailed information will be printed.
After that, only the normal error messages will be issued.
Error counters are always updated, and accumulated numbers can be
shown with pythia.statistics()
at the end of the run.
parm
Pythia:epTolerance
(default = 1e-5
)
Maximum allowed summed deviation of E, p_x,
p_y and p_z between the incoming beams and the
final state, as a fraction of the initial energy. (Unfortunetely
roundoff errors do not scale linearly with the energy, and also have
a very long tail. So while most events at lower energies may be correct
to better than 1e-10, at LHC it does not have to signal any fundamental
bug if also the default tolerance above is violated occasionally.)