WIP: Pythia DIS

	new file:   README.md
	modified:   dis.sh
	new file:   src/pythia_dis.cc

dis/README.md

+# DIS Benchmarks
+
+## Compiling Pythia
+
+```
+g++ src/pythia_dis.cc -o pythia_dis  \
+   -I/home/whit/stow/hepmc3/include  \
+   -I../include -O2 -std=c++11 -pedantic -W -Wall -Wshadow -fPIC  \
+   -L../lib -Wl,-rpath,../lib -lpythia8 -ldl \
+   -L/home/whit/stow/hepmc3/lib -Wl,-rpath,/home/whit/stow/hepmc3/lib -lHepMC3
+```
+

dis/dis.sh

@@ -46,6 +46,13 @@ export JUGGLER_REC_FILE="rec_${JUGGLER_FILE_NAME_TAG}.root"
 ## TODO remove this
 #bash util/build_detector.sh
 
+g++ dis/src/pythia_dis.cc -o pythia_dis  \
+   -I/usr/local/include  \
+   -O2 -std=c++11 -pedantic -W -Wall -Wshadow -fPIC  \
+   -L/usr/local/lib -Wl,-rpath,/usr/local/lib -lpythia8 -ldl \
+   -L/usr/local/lib -Wl,-rpath,/usr/local/lib -lHepMC3
+
+
 ## =============================================================================
 ## Step 2: Run the simulation
 echo "Running Geant4 simulation"

dis/src/pythia_dis.cc

+// main45.cc is a part of the PYTHIA event generator.
+// Copyright (C) 2020 Torbjorn Sjostrand.
+// PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
+// Please respect the MCnet Guidelines, see GUIDELINES for details.
+
+// Author: Stefan Prestel <stefan.prestel@thep.lu.se>.
+
+// Keywords: LHE file; hepmc;
+
+// This program (main45.cc) illustrates how a file with HepMC3 events can be
+// generated by Pythia8. See main44.cc for how to ouput HepMC2 events instead.
+// Note: both main44.cc and main45.cc can use the same main44.cmnd input card.
+
+#include "Pythia8/Pythia.h"
+#include "Pythia8Plugins/HepMC3.h"
+#include <unistd.h>
+
+using namespace Pythia8;
+
+//==========================================================================
+
+// Example main programm to illustrate merging.
+
+int main( int argc, char* argv[] ){
+
+  // Check that correct number of command-line arguments
+  if (argc != 2) {
+    cerr << " Unexpected number of command-line arguments ("<<argc<<"). \n"
+         << " You are expected to provide the arguments" << endl
+         << " 1. Output file for HepMC events" << endl
+         << " Program stopped. " << endl;
+    return 1;
+  }
+
+  // Beam energies, minimal Q2, number of events to generate.
+  double eProton   = 250.;
+  double eElectron = 10.0;
+  double Q2min     = 5.;
+  int    nEvent    = 10000;
+
+  // Generator. Shorthand for event.
+  Pythia pythia;
+  Event& event = pythia.event;
+
+  // Set up incoming beams, for frame with unequal beam energies.
+  pythia.readString("Beams:frameType = 2");
+  // BeamA = proton.
+  pythia.readString("Beams:idA = 2212");
+  pythia.settings.parm("Beams:eA", eProton);
+  // BeamB = electron.
+  pythia.readString("Beams:idB = 11");
+  pythia.settings.parm("Beams:eB", eElectron);
+
+  // Set up DIS process within some phase space.
+  // Neutral current (with gamma/Z interference).
+  pythia.readString("WeakBosonExchange:ff2ff(t:gmZ) = on");
+  // Uncomment to allow charged current.
+  //pythia.readString("WeakBosonExchange:ff2ff(t:W) = on");
+  // Phase-space cut: minimal Q2 of process.
+  pythia.settings.parm("PhaseSpace:Q2Min", Q2min);
+
+  // Set dipole recoil on. Necessary for DIS + shower.
+  pythia.readString("SpaceShower:dipoleRecoil = on");
+
+  // Allow emissions up to the kinematical limit,
+  // since rate known to match well to matrix elements everywhere.
+  pythia.readString("SpaceShower:pTmaxMatch = 2");
+
+  // QED radiation off lepton not handled yet by the new procedure.
+  pythia.readString("PDF:lepton = off");
+  pythia.readString("TimeShower:QEDshowerByL = off");
+
+  // Initialize.
+  pythia.init();
+
+  // Interface for conversion from Pythia8::Event to HepMC one.
+  HepMC3::Pythia8ToHepMC3 toHepMC;
+  // Specify file where HepMC events will be stored.
+  HepMC3::WriterAscii ascii_io(argv[1]);
+  cout << endl << endl << endl;
+
+  // Histograms.
+  double Wmax = sqrt(4.* eProton * eElectron);
+  Hist Qhist("Q [GeV]", 100, 0., 50.);
+  Hist Whist("W [GeV]", 100, 0., Wmax);
+  Hist xhist("x", 100, 0., 1.);
+  Hist yhist("y", 100, 0., 1.);
+  Hist pTehist("pT of scattered electron [GeV]", 100, 0., 50.);
+  Hist pTrhist("pT of radiated parton [GeV]", 100, 0., 50.);
+  Hist pTdhist("ratio pT_parton/pT_electron", 100, 0., 5.);
+
+  double sigmaTotal(0.), errorTotal(0.);
+  bool wroteRunInfo = false;
+    // Get the inclusive x-section by summing over all process x-sections.
+    double xs = 0.;
+    for (int i=0; i < pythia.info.nProcessesLHEF(); ++i)
+      xs += pythia.info.sigmaLHEF(i);
+
+  // Begin event loop.
+  for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
+    if (!pythia.next()) continue;
+
+    double sigmaSample = 0., errorSample = 0.;
+
+    // Four-momenta of proton, electron, virtual photon/Z^0/W^+-.
+    Vec4 pProton = event[1].p();
+    Vec4 peIn    = event[4].p();
+    Vec4 peOut   = event[6].p();
+    Vec4 pPhoton = peIn - peOut;
+
+    // Q2, W2, Bjorken x, y.
+    double Q2    = - pPhoton.m2Calc();
+    double W2    = (pProton + pPhoton).m2Calc();
+    double x     = Q2 / (2. * pProton * pPhoton);
+    double y     = (pProton * pPhoton) / (pProton * peIn);
+
+    // Fill kinematics histograms.
+    Qhist.fill( sqrt(Q2) );
+    Whist.fill( sqrt(W2) );
+    xhist.fill( x );
+    yhist.fill( y );
+    pTehist.fill( event[6].pT() );
+
+    // pT spectrum of partons being radiated in shower.
+    for (int i = 0; i < event.size(); ++i) if (event[i].statusAbs() == 43) {
+      pTrhist.fill( event[i].pT() );
+      pTdhist.fill( event[i].pT() / event[6].pT() );
+    }
+
+    // Get event weight(s).
+    double evtweight         = pythia.info.weight();
+
+    // Do not print zero-weight events.
+    if ( evtweight == 0. ) continue;
+
+    // Create a GenRunInfo object with the necessary weight names and write
+    // them to the HepMC3 file only once.
+    if (!wroteRunInfo) {
+      shared_ptr<HepMC3::GenRunInfo> genRunInfo;
+      genRunInfo = make_shared<HepMC3::GenRunInfo>();
+      vector<string> weight_names = pythia.info.weightNameVector();
+      genRunInfo->set_weight_names(weight_names);
+      ascii_io.set_run_info(genRunInfo);
+      ascii_io.write_run_info();
+      wroteRunInfo = true;
+    }
+
+
+    // Construct new empty HepMC event.
+    HepMC3::GenEvent hepmcevt;
+
+    // Work with weighted (LHA strategy=-4) events.
+    double normhepmc = 1.;
+    if (abs(pythia.info.lhaStrategy()) == 4)
+      normhepmc = 1. / double(1e9*nEvent);
+    // Work with unweighted events.
+    else
+      normhepmc = xs / double(1e9*nEvent);
+
+    // Set event weight
+    //hepmcevt.weights().push_back(evtweight*normhepmc);
+    // Fill HepMC event
+    toHepMC.fill_next_event( pythia, &hepmcevt );
+    // Add the weight of the current event to the cross section.
+    sigmaTotal  += evtweight*normhepmc;
+    sigmaSample += evtweight*normhepmc;
+    errorTotal  += pow2(evtweight*normhepmc);
+    errorSample += pow2(evtweight*normhepmc);
+    // Report cross section to hepmc
+    shared_ptr<HepMC3::GenCrossSection> xsec;
+    xsec = make_shared<HepMC3::GenCrossSection>();
+    // First add object to event, then set cross section. This order ensures
+    // that the lengths of the cross section and the weight vector agree.
+    hepmcevt.set_cross_section( xsec );
+    xsec->set_cross_section( sigmaTotal*1e9, pythia.info.sigmaErr()*1e9 );
+    // Write the HepMC event to file. Done with it.
+    ascii_io.write_event(hepmcevt);
+
+
+
+    // End of event loop. Statistics and histograms.
+  }
+  pythia.stat();
+  cout << Qhist << Whist << xhist << yhist << pTehist << pTrhist << pTdhist;
+
+
+  return 0;
+
+}