This is a simple simulation example showing how to run event simulation using the Pythia8 event generator in heavy-ion mode and use an external custom trigger.

The external trigger makes use of the `DeepTrigger` functionality to allow the user to access the internals of the event generator to define the trigger.

In this example, information about the impact parameter of the currently-generated Pythia8 heavy-ion event is used.

The trigger selection defines impact parameters to be accepted according to a min-max range that can be adjusted from the command line.

The definition and configuration of the external `DeepTrigger` is performed by the function `trigger_impactb_pythia8(double bMin = 0., double bMax = 20.)` defined in the macro `trigger_impactb_pythia8.macro`.

The macro file is specified via the argument of `--extTrgFile` whereas the specific function call to retrieve the configuration is specified via the argument of `--extTrgFunc`.

Using a trigger is not always the most efficient way to deal with event generation.

It is always advisable to find a way to bias the event-geneator process to save CPU time.

In this case, the selection of a specific impact-parameter range in Pythia8 can lead to

a large time spent in the event generation rather than in the simulation.

set -x

NEV=10

BMIN=15.

BMAX=20.

o2-sim -j 20 -n ${NEV} -g pythia8hi -m PIPE ITS TPC --trigger external --extTrgFile trigger_impactb_pythia8.macro --extTrgFunc "trigger_impactb_pythia8(${BMIN}, ${BMAX})"

/// \author R+Preghenella - May 2020

#include "Generators/Trigger.h"

#include "Pythia8/Pythia.h"

#include "Pythia8/HIUserHooks.h"

#include "FairLogger.h"

o2::eventgen::DeepTrigger

trigger_impactb_pythia8(double bmin = 5., double bmax = 10.)

{

return [bmin, bmax](void* interface, std::string name) -> bool {

if (!name.compare("pythia8")) {

auto py8 = reinterpret_cast<Pythia8::Pythia*>(interface);

#if PYTHIA_VERSION_INTEGER < 8300

auto hiinfo = py8->info.hiinfo;

#else

auto hiinfo = py8->info.hiInfo;

#endif

if (!hiinfo) {

LOG(FATAL) << "Cannot define impact parameter: is \'pythia8\' running in heavy-ion mode?";

}

auto b = hiinfo->b();

auto selected = (b > bmin && b < bmax);

LOG(INFO) << "Impact parameter = " << b << " fm: " << (selected ? "selected" : "rejected");

return selected;

} else {

LOG(FATAL) << "Cannot define impact parameter for generator interface \'" << name << "\'";

}

return false;

};

}