Generic Settings

Here are collected a few scattered topics, controlled from the main Pythia class.

Parton densities

There is one main physics choice to be made with the Pythia class, namely which parton densities to use, a choice that then is propagated through the program. The simplest option is to pick one of the few distributions available internally:

mode name="Pythia:pPDFset" default="2" min="1" max="2"
Parton densities to be used for proton beams (and, by implication, antiproton ones):
option value="1": GRV 94 L;
option value="2": CTEQ 5 L.

Obviously this choice is mainly intended to get going, and a much wider selection could be used by linking to an external library.

For electrons/leptons there is no need to choose, since only one implementation is available, and should be rather uncontroversial (apart from some technical details).

Random numbers

The seed of the random number generator can be set here:

flag name="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 name="Pythia:seed" default="-1" max="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 name="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.

mode name="Pythia:nErrList" default="3"
The number of erroneous events, in the above check, for which information will be printed. After that, only the error counters will be updated, and can be shown with a pythia.statistics() at the end of the run.

parameter name="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.)